Transmission structure

Abstract

To provide a transmission structure which can smoothly switch a first HMT transmission state for inputting power from a drive source and an HST to planetary first and third elements, and outputting it from a planetary second element, and a second HMT transmission state for inputting the power from the drive source to the second element, and outputting it from the first element.SOLUTION: In a transmission structure of this invention, either of input-side / output-side clutch mechanism pairs is brought into a dual transmission state after a time point at which a drive rotation force arrives at a first / second gear-stage shift-up start speed up to a time point of a finish of the shift-up, a first clutch mechanism of the other of the input-side / output-side clutch mechanism pairs is transited to a release state while making a friction plate slide during a shift-up dual transmission state, and a second clutch mechanism of the other of the clutch mechanism pairs is brought into an engagement state while making the friction part slide.SELECTED DRAWING: Figure 5

Description

【Technical Field】 【0001】 The present invention relates to a transmission structure including a hydrostatic continuously variable transmission mechanism (HST) and a hydrostatic-mechanical continuously variable transmission structure (HMT) having a planetary gear mechanism. 【Background Art】 【0002】 An HMT formed by combining an HST and a planetary gear mechanism is suitably used, for example, in the driving system transmission path of work vehicles such as combines and tractors, and various configurations for expanding the vehicle speed variable range have been proposed. 【0003】 For example, Patent Document 1 below discloses a transmission structure in which an HMT and a multi-stage transmission having three shift stages of a low speed stage, a medium speed stage, and a high speed stage are arranged in series in the driving system transmission path to expand the vehicle speed variable range. However, the transmission structure described in Patent Document 1 is planned to perform the shift operation of the multi-stage transmission in advance before the vehicle starts running. If the shift operation of the multi-stage transmission is performed while the vehicle is running, the following disadvantages occur. 【0004】 Regarding this point, taking as an example the case where the multi-stage transmission is engaged in the low speed stage and the HMT is operated to increase the running vehicle speed, and when the running vehicle speed reaches a predetermined vehicle speed, the multi-stage transmission is shifted from the low speed stage to the medium speed stage. <00用0018> In this case, when the output of the HMT reaches the maximum speed or near the maximum speed in the low speed stage engagement state of the multi-stage transmission, the multi-stage transmission is shifted from the low speed stage to the medium speed stage while the output of the HMT is maintained at the maximum speed or near the maximum speed. This results in a large vehicle speed change during shifting, deteriorating the riding comfort and imposing an excessive load on the driving system transmission path. 【0006】 In this regard, the applicant has proposed a transmission structure comprising an HST, a planetary gear mechanism having first to third elements and inputting the HST output to the third element, a speed-shifting output shaft operably driven by the planetary output section of the planetary gear mechanism, an input-side first transmission mechanism and an input-side second transmission mechanism capable of operably transmitting the rotational power of a drive source to the first and second elements of the planetary gear mechanism, respectively, an input-side first clutch mechanism and an input-side second clutch mechanism for engaging and disengaging the power transmission between the input-side first transmission mechanism and the input-side second transmission mechanism, respectively, an output-side first transmission mechanism and an output-side second transmission mechanism capable of operably transmitting the rotational power of the second element and the first element to the speed-shifting output shaft, respectively, an output-side first clutch mechanism and an output-side second clutch mechanism for engaging and disengaging the power transmission between the output-side first transmission mechanism and the output-side second transmission mechanism, respectively, a speed-shifting operating member, and a control device (see Patent Document 2 below). 【0007】 The control device, when the rotational speed of the gear shift output shaft is below a predetermined switching speed, engages the input and output side first clutch mechanisms and disengages the input and output side second clutch mechanisms, thereby creating a first transmission state in which the first element acts as a planetary input unit receiving reference power from the drive source and the second element acts as a planetary output unit, while simultaneously adjusting the output adjustment unit so that the HST output shifts from the first HST speed to the second HST speed in response to the speed increase operation of the gear shift operating member. While operating the material, in a high-speed state where the rotational speed of the speed-shifting output shaft is greater than or equal to the switching speed, the input-side and output-side first clutch mechanisms are released and the input-side and output-side second clutch mechanisms are engaged, thereby creating a second transmission state in which the first element acts as a planetary output unit and the second element acts as a planetary input unit. The output adjustment member is then operated so that the HST output shifts from the second HST speed to the first HST speed in response to the speed-increasing operation of the speed-shifting operating member. 【0008】 The gear ratio of the input-side first transmission mechanism (input-side first gear ratio) and the gear ratio of the input-side second transmission mechanism (input-side second gear ratio) are set such that the rotational speed of the second element when the HST output is set to the second HST speed in the first transmission state is the same as the rotational speed of the second element due to the rotational power transmitted via the input-side second transmission mechanism in the second transmission state, and the rotational speed of the first element when the HST output is set to the second HST speed in the second transmission state is the same as the rotational speed of the first element due to the rotational power transmitted via the input-side first transmission mechanism in the first transmission state. 【0009】 The gear ratio of the output-side first transmission mechanism (output-side first gear ratio) and the gear ratio of the output-side second transmission mechanism (output-side second gear ratio) are set so that the rotational speed displayed on the gear-shifting output shaft is the same in the first and second transmission states when the HST output is set to the second HST speed. 【0010】 The transmission structure described in Patent Document 2 is useful compared to the transmission structure described in Patent Document 1 in that it can widen the gear shift range of the gear shift output shaft without causing abrupt changes in rotational speed of the gear shift output shaft. 【0011】 However, there is room for improvement in switching between the first HMT transmission state (low-speed transmission state), in which the input and output first clutch mechanisms are engaged and the input and output second clutch mechanisms are disengaged, and the second HMT transmission state (high-speed transmission state), in which the input and output first clutch mechanisms are disengaged and the input and output second clutch mechanisms are engaged. [Prior art documents] [Patent Documents] 【0012】 [Patent Document 1] Patent No. 5822761 [Patent Document 2] Japanese Patent Publication No. 2020-152364 [Overview of the project] [Problems that the invention aims to solve] 【0013】 The present invention has been made in view of the prior art, and aims to provide a transmission structure that can smoothly switch between a first HMT transmission state in which rotational power from the drive source is input to the first element and combined rotational power is output from the second element, and a second HMT transmission state in which rotational power from the drive source is input to the second element and combined rotational power is output from the first element, wherein the present invention provides a transmission structure that can smoothly switch between the first and second HMT transmission states. [Means for solving the problem] 【0014】 To achieve the above objective, the present invention provides a transmission structure that outputs rotational power to the drive wheels as drive rotational power by continuously changing the speed of rotational power input operationally from a drive source, comprising: an HST that continuously changes the speed of rotational power input operationally from the drive source to at least between a first HST speed and a second HST speed according to the operating position of an output adjustment member and outputs it; an HST speed change actuator that operates the output adjustment member; a planetary gear mechanism having first to third elements, the third element acting as an input part for the HST output; an input-side first transmission mechanism capable of operationally transmitting the rotational power of the drive source to the first element at an input-side first speed ratio; and an input-side second speed ratio for the rotational power of the drive source at an input-side second speed ratio. The system includes an input-side second transmission mechanism capable of transmitting power to an element, a pair of input-side clutch mechanisms including friction plate type input-side first and second clutch mechanisms for engaging and disengaging power transmission between the input-side first and second transmission mechanisms, a gear shift output shaft, an output-side first transmission mechanism capable of transmitting the rotational power of the second element to the gear shift output shaft at an output-side first gear ratio, an output-side second transmission mechanism capable of transmitting the rotational power of the first element to the gear shift output shaft at an output-side second gear ratio, a pair of output-side clutch mechanisms including friction plate type output-side first and second clutch mechanisms for engaging and disengaging power transmission between the output-side first and second transmission mechanisms, and is operable at zero vehicle speed, in the first gear range, and in the second gear range which is faster than the first gear range. TaThe system comprises a gear shift operating member and a control device that receives commands from the gear shift operating member and controls the operation of the HST gear shift actuator, the input clutch mechanism, and the output clutch mechanism. When the gear shift operating member is positioned at zero vehicle speed and in the first gear range, the control device sets the input first and second clutch mechanisms to an engaged and disengaged state, respectively, so that the first element acts as a reference power input unit that receives reference rotational power from the drive source, and the second element acts as a combined power output unit that outputs combined rotational power toward the gear shift output shaft. While creating the first HMT transmission state, the rotational power of the second element is transmitted to the gear shift output shaft by engaging and disengaging the output-side first and second clutch mechanisms, respectively. When the gear shift operating member is located in the second gear range, the input-side first and second clutch mechanisms are engaged and disengaged, respectively, creating a second HMT transmission state in which the second element acts as the reference power input and the first element acts as the combined power output. The first element is engaged to transmit the rotational power of the first element to the variable speed output shaft, and the planetary gear mechanism is configured such that when the HST output is set to the first HST speed in the first HMT transmission state, the output rotational power of the second element becomes zero speed, and the input side first and second gear ratios are such that when the HST output is set to the second HST speed in the first HMT transmission state, the rotational speed of the second element due to the rotational power transmitted via the input side second transmission mechanism in the second HMT transmission state is the same, and when the HST output is set to the second HST speed in the second HMT transmission state The rotational speed of the first element when this occurs and the rotational speed of the first element due to the rotational power transmitted via the input-side first transmission mechanism when the first HMT transmission state is in operation are set to be the same, and the control device further controls the operation of the HST shift actuator so that the HST output becomes the first HST speed in response to the operation of the shift operating member to the zero vehicle speed position, and the rotational speed of the drive rotational power increases in response to the speed increase operation of the shift operating member from the zero vehicle speed position, and when the shift operating member is shifted up from the first gear range to the second gear range,At the start of the first / second gear shift, when the drive rotational power reaches a predetermined first / second gear shift start speed, the second clutch mechanism in one of the clutch mechanism pairs (the input clutch mechanism pair and the output clutch mechanism pair) is instantaneously switched from a disengaged state to an engaged state. Furthermore, at the end of the first / second gear shift, after a predetermined time has elapsed from the start of the first / second gear shift, the first clutch mechanism in the other clutch mechanism pair is instantaneously switched from an engaged state to a disengaged state. During the period from the start of the first / second gear shift to the end of the shift, the clutch mechanism in the other clutch mechanism pair... The present invention provides a transmission structure that creates a shift-up double transmission state in which both the first and second clutch mechanisms are engaged, while during the shift-up double transmission state, the first clutch mechanism in the other clutch mechanism pair (one of the input clutch mechanism pair and the other output clutch mechanism pair) is moved from an engaged state to a disengaged state by sliding the friction plates, and the second clutch mechanism in the other clutch mechanism pair is moved from a disengaged state to an engaged state by sliding the friction plates, thereby switching the other clutch mechanism pair from an engaged state of the first clutch mechanism to an engaged state of the second clutch mechanism. 【0015】 Preferably, the starting speed for shifting up to the first / second gear is the speed of the drive rotational power that is manifested on the gear shift output shaft via the output-side first transmission mechanism when the HST output is set to the second HST speed in the first HMT transmission state. 【0016】 In one embodiment, the input-side first and second clutch mechanisms and the output-side first and second clutch mechanisms are hydraulic, and their engaged and disengaged states are switched by supplying and discharging hydraulic fluid. 【0017】 In this case, the transmission structure includes an input-side solenoid valve pair, which includes input-side first and second solenoid valves that switch the supply and discharge of hydraulic fluid to the input-side first and second clutch mechanisms, respectively, in accordance with the operation control by the control device, and an output-side solenoid valve pair, which includes output-side first and second solenoid valves that switch the supply and discharge of hydraulic fluid to the output-side first and second clutch mechanisms, respectively, in accordance with the operation control by the control device. 【0018】 One of the input-side solenoid valve pair and the output-side solenoid valve pair is a solenoid proportional valve capable of gradually increasing and decreasing the hydraulic pressure of the corresponding clutch mechanism, while the other of the input-side solenoid valve pair and the output-side solenoid valve pair is a solenoid switching valve that instantly increases or decreases the hydraulic pressure of the corresponding clutch mechanism. 【0019】 In the various configurations described above, preferably, the first and second output gear ratios are set such that the rotational speed displayed on the gear shift output shaft when the HST output is set to the second HST speed in the first HMT transmission state is approximately the same as the rotational speed displayed on the gear shift output shaft when the HST output is set to the second HST speed in the second HMT transmission state. 【0020】 In the various configurations described above, for example, the control device is configured to start the switching operation of the first clutch mechanism in the other clutch mechanism pair, which moves the first clutch mechanism from an engaged state to an engaged state while sliding the friction plates, and to start the switching operation of the second clutch mechanism in the other clutch mechanism pair, which moves the second clutch mechanism from an engaged state to an engaged state while sliding the friction plates, before the start of the first / second gear shift up and after the end of the first / second gear shift up. 【0021】 In the various configurations described above, preferably, when the shift operation member is shifted down from the second speed range to the first speed range, the control device instantaneously shifts the first clutch mechanism in one of the input-side clutch mechanism pair and the output-side clutch mechanism pair from the disengaged state to the engaged state at the start point of the second / first speed shift down when the driving rotational power reaches a predetermined second / first speed shift down start speed. And at the end point of the second / first speed shift down after a lapse of a predetermined time from the start point of the second / first speed shift down, the second clutch mechanism in the one clutch mechanism pair is instantaneously shifted from the engaged state to the disengaged state, and during the period from the start point to the end point of the second / first speed shift down, a shift down double transmission state in which both the first and second clutch mechanisms in the one clutch mechanism pair are engaged is exhibited. On the other hand, during the shift down double transmission state, the first clutch mechanism in the other clutch mechanism pair of the input-side clutch mechanism pair and the output-side clutch mechanism pair is shifted from the disengaged state to the engaged state while sliding the friction plates, and the second clutch mechanism in the other clutch mechanism pair is shifted from the engaged state to the disengaged state while sliding the friction plates, and the configuration is such that the engagement state of the second clutch mechanism in the other clutch mechanism pair is switched to the engagement state of the first clutch mechanism. 【0022】 Preferably, the second / first speed shift down start speed is set to the speed of the driving rotational power that appears on the transmission output shaft via the output-side second transmission mechanism when the HST output is set to the second HST speed in the second HMT transmission state. 【0023】 For example, the control device is configured to start the switching operation of the second clutch mechanism in the other clutch mechanism pair to shift from the engaged state to the disengaged state while sliding the friction plates and the switching operation of the first clutch mechanism in the other clutch mechanism pair to shift from the disengaged state to the engaged state while sliding the friction plates before the start point of the second / first speed shift down and complete them after the end point of the second / first speed shift down. 【0024】 Preferably, the rotational speed of the driving rotational power that defines the start point of the second / first speed step downshift is substantially the same as the rotational speed of the driving rotational power that defines the start point of the first / second speed step upshift. 【0025】 Preferably, the transmission structure according to the present invention further includes a traveling output shaft that outputs driving rotational power toward the driving wheels, a forward-side transmission mechanism and a reverse-side transmission mechanism that operatively transmit the rotational power of the transmission output shaft to the traveling output shaft as driving rotational power in the forward and reverse directions, respectively, and friction plate-type forward-side clutch mechanisms and reverse-side clutch mechanisms that engage and disengage the power transmission of the forward-side transmission mechanism and the reverse-side transmission mechanism, respectively. 【0026】 In this case, the HST is configured to output rotational power in both forward and reverse directions such that the output of the first HST speed is rotational power on one side of forward and reverse, and the output of the second HST speed is rotational power on the other side of forward and reverse. 【0027】 In the planetary gear mechanism, in the first HMT transmission state, as the HST output is shifted from the first HST speed side to the second HST speed side, the combined rotational power output from the second element is increased in speed, and in the second HMT transmission state, as the HST output is shifted from the second HST speed side to the first HST speed side, the combined rotational power output from the first element is increased in speed. 【0028】 The first speed step region includes a forward-side first speed step region and a reverse-side first speed step region, the second speed step region includes a forward-side second speed step region on the higher speed side than the forward-side first speed step region and a reverse-side second speed step region on the higher speed side than the reverse-side first speed step region, and the control device is configured to bring the forward-side and reverse-side clutch mechanisms into an engaged state and a released state, respectively, in response to an operation of the shift operation member to the forward side, and to bring the forward-side and reverse-side clutch mechanisms into a released state and an engaged state, respectively, in response to an operation of the shift operation member to the reverse side. 【0029】 In one embodiment, the forward and reverse clutch mechanisms are hydraulic, with the engaged and disengaged states being switched by the supply and discharge of hydraulic fluid. In this case, the transmission structure is provided with a pair of forward and reverse switching solenoid valves, including forward and reverse solenoid valves, which switch the supply and discharge of hydraulic fluid to the forward and reverse clutch mechanisms, respectively, in accordance with the operation control by the control device. 【0030】 In a configuration comprising the aforementioned drive output shaft, the forward-side transmission mechanism, the reverse-side transmission mechanism, the forward-side clutch mechanism, and the reverse-side clutch mechanism, the transmission structure may further include an output-side third transmission mechanism capable of transmitting the rotational power of the first element to the drive output shaft as forward-direction drive rotational power, wherein the output-side third transmission mechanism has a gear ratio set such that the rotational speed of the drive output shaft when the rotational power of the first element is transmitted to the drive output shaft via the output-side third transmission mechanism is faster than the rotational speed of the drive output shaft when the rotational power of the first element is transmitted to the drive output shaft via the output-side second transmission mechanism and the forward-side transmission mechanism, and an output-side third clutch mechanism for engaging and disengaging the power transmission of the output-side third transmission mechanism. 【0031】 In this case, the gear shift operating member can be operated even in the forward third gear range, which is higher in speed than the forward second gear range. When the gear shift operating member is positioned in the forward third gear range, the control device activates the second HMT transmission state, disengages the output-side first and second clutch mechanisms, and engages the output-side third clutch mechanism, while controlling the operation of the HST gear shift actuator so that the rotational speed of the drive rotational power becomes the rotational speed corresponding to the operating position of the gear shift operating member. 【0032】 In the first configuration relating to shifting up from the forward second gear range to the forward third gear range, when the gear shifting operating member is operated to shift up from the forward second gear range to the forward third gear range, the control device instantly switches the output side second clutch mechanism and the forward side clutch mechanism from the engaged state to the disengaged state at the start of the second / third gear shift when the drive rotational power reaches a predetermined second / third gear shift start speed, and switches the output side third clutch mechanism from the disengaged state to the engaged state at the end of the second / third gear shift after a predetermined time has elapsed from the start of the second / third gear shift. The system is configured to create a shift-up coasting state in which the power transmission path to the drive output shaft is interrupted between the start of the second / third gear shift-up and the end of the second / third gear shift-up, while shifting the HST via the HST shift actuator during the shift-up coasting state so that the rotational speed of the drive output shaft, which is rotationally driven via the output-side third transmission mechanism at the end of the second / third gear shift-up, matches or approaches the rotational speed of the drive output shaft, which is rotationally driven via the output-side second transmission mechanism and the forward-side transmission mechanism at the start of the second / third gear shift-up. 【0033】 In the second configuration relating to shifting up from the forward second gear range to the forward third gear range, when the shift operating member is operated to shift up from the forward second gear range to the forward third gear range, the control device moves one of the output second clutch mechanism and the forward clutch mechanism from an engaged state to an unengaged state, and after a predetermined time has elapsed since one of the output second clutch mechanism and the forward clutch mechanism was in an unengaged state, the other of the output second clutch mechanism and the forward clutch mechanism was moved from an engaged state to an unengaged state, and after a predetermined time has elapsed since the other of the output second clutch mechanism and the forward clutch mechanism was in an unengaged state, the output third clutch By transitioning the clutch mechanism from a disengaged state to an engaged state, a shift-up coasting state is created in which the power transmission path to the travel output shaft is interrupted from the time when either the output-side second clutch mechanism or the forward-side clutch mechanism is disengaged until the output-side third clutch mechanism is engaged. At the same time, the HST is shifted via the HST shift actuator during the shift-up coasting state so that the rotational speed of the travel output shaft, which is rotationally driven via the output-side third transmission mechanism, matches or approaches the rotational speed of the travel output shaft immediately before the output-side third clutch mechanism is engaged. 【0034】 In the third configuration relating to shifting up from the forward second gear range to the forward third gear range, when the shift operating member is operated to shift up from the forward second gear range to the forward third gear range, the control device moves the output second clutch mechanism from the engaged state to the disengaged state while sliding, and moves the output third clutch mechanism from the disengaged state to the engaged state while sliding, and before the output third clutch mechanism moves to the engaged state, the forward clutch mechanism moves from the engaged state to the disengaged state, thereby causing the forward clutch mechanism to be in the disengaged state. From the time the output-side third clutch mechanism is engaged until the power transmission path to the drive output shaft is interrupted, creating a shift-up coasting state. The HST is then shifted via the HST shift actuator during the shift-up coasting state so that the rotational speed of the drive output shaft, which is rotationally driven via the output-side third transmission mechanism, matches or approaches the rotational speed of the drive output shaft immediately before the output-side third clutch mechanism is engaged. 【0035】 In one embodiment, the output-side third clutch mechanism is a hydraulic type, in which the engaged and disengaged states are switched by supplying and discharging hydraulic fluid. In this case, the transmission structure is provided with an output-side third solenoid valve that switches the supply and discharge of hydraulic fluid to the output-side third clutch mechanism in accordance with the operation control by the control device. 【0036】 In the transmission structure comprising the output-side third transmission mechanism and the output-side third clutch mechanism, in the first configuration relating to shifting down from the forward-side third gear region to the forward-side second gear region, when the shift operation member is shifted down from the forward-side third gear region to the forward-side second gear region, the control device moves the output-side third clutch mechanism from the engaged state to the disengaged state at the start of the third / second gear shift down when the drive rotational power reaches a predetermined third / second gear shift down start speed, and at the end of the third / second gear shift down after a predetermined time has elapsed from the start of the third / second gear shift down, the output-side second clutch mechanism and the forward-side clutch By transitioning the switch mechanism from the disengaged state to the engaged state, a downshift coasting state is created in which the power transmission path to the drive output shaft is interrupted from the start of the third / second gear downshift to the end of the third / second gear downshift. At the same time, the HST is shifted via the HST shift actuator during the downshift coasting state so that the rotational speed of the drive output shaft, which is rotationally driven via the output-side second transmission mechanism and the forward-side transmission mechanism at the end of the third / second gear downshift, matches or approaches the rotational speed of the drive output shaft, which is rotationally driven via the output-side third transmission mechanism at the start of the third / second gear downshift. 【0037】 Preferably, the starting speed for shifting down to the third / second gear and the starting speed for shifting up to the second / third gear are approximately the same. 【0038】 In the transmission structure comprising the output-side third transmission mechanism and the output-side third clutch mechanism, in the second configuration relating to downshifting from the forward-side third gear range to the forward-side second gear range, when the gear shifting operating member is operated to downshift from the forward-side third gear range to the forward-side second gear range, the control device performs the following in this order: transition from the engaged state to the disengaged state of the output-side third clutch mechanism, transition from the disengaged state to the engaged state of the forward-side clutch mechanism, and transition from the disengaged state to the engaged state of the output-side second clutch mechanism. Between the transition from the disengaged state to the engagement state of the output-side second clutch mechanism, a downshift coasting state is created in which the power transmission path to the travel output shaft is interrupted. The HST is then shifted via the HST shift actuator during the downshift coasting state so that the rotational speed of the travel output shaft, which is rotationally driven via the output-side second transmission mechanism and the forward-side transmission mechanism, matches or approaches the rotational speed of the travel output shaft immediately before the output-side second clutch mechanism is engaged. 【0039】 In the transmission structure comprising the output-side third transmission mechanism and the output-side third clutch mechanism, in the third configuration relating to downshifting from the forward-side third gear range to the forward-side second gear range, when the gear shifting operating member is operated to downshift from the forward-side third gear range to the forward-side second gear range, the control device performs the following in this order: transition from the engaged state to the disengaged state of the output-side third clutch mechanism, transition from the disengaged state to the engaged state of the output-side second clutch mechanism, and transition from the disengaged state to the engaged state of the forward-side clutch mechanism. This creates a downshift coasting state in which the power transmission path to the drive output shaft is interrupted between the transition from the disengaged state of the output-side third clutch mechanism to the engaged state of the forward-side clutch mechanism, while the gear shift is performed by the rotational power transmitted via the output-side second transmission mechanism at the time of the transition from the disengaged state to the engaged state of the output-side second clutch mechanism. The HST is shifted via the HST shift actuator between the time the output-side third clutch mechanism transitions from the engaged state to the disengaged state and the time the output-side second clutch mechanism transitions from the disengaged state to the engaged state, so that the rotational speed of the force shaft matches or approaches the actual rotational speed of the shift output shaft immediately before the output-side second clutch mechanism transitions to the engaged state. Subsequently, the HST is shifted via the HST shift actuator between the time the output-side second clutch mechanism transitions to the engaged state and the time the forward-side clutch mechanism transitions from the disengaged state to the engaged state, so that the rotational speed of the travel output shaft, which is rotationally driven via the output-side second transmission mechanism and the forward-side transmission mechanism, matches or approaches the rotational speed of the travel output shaft immediately before the forward-side clutch mechanism transitions to the engaged state. 【0040】 In the first to third configurations relating to downshifting from the forward third gear range to the forward second gear range, preferably, the transition of the output-side second clutch mechanism from the disengaged state to the engaged state during downshifting from the forward third gear range to the forward second gear range is performed by gradually sliding. 【0041】 When shifting up from the forward second gear to the forward third gear, preferably, the control device, during the start period from the start of the shift up from the forward second gear to the forward third gear until a predetermined time has elapsed, matches the rotational speed of the travel output shaft, which is rotationally driven via the output third transmission mechanism when the output third clutch mechanism is engaged, to the rotational speed of the travel output shaft at the start of the shift up when one of the output second clutch mechanism and the forward clutch mechanism is released. The HST shift control is configured to set the target rotational speed to an HST output rotational speed that is increased in the opposite direction to the actual HST output rotational speed at the start of the shift up, rather than the gear shifting speed, and after the end of the start period, the HST shift control is configured to set the target rotational speed to an HST output rotational speed that matches the rotational speed of the travel output shaft, which is rotationally driven via the output side third transmission mechanism at the time the output side third clutch mechanism is engaged, to the rotational speed of the travel output shaft immediately before the output side third clutch mechanism is engaged. 【0042】 More preferably, the target speed for HST shift control during the start period is the HST speed among the first and second HST speeds that is located on the opposite side of the actual HST output rotational speed at the start of the shift up, from the second / third gear switching speed on the third gear side. 【0043】 When shifting down from the forward third gear to the forward second gear, preferably, the control device, during the start period from the start of the shift down from the forward third gear to the forward second gear until a predetermined time has elapsed, sets the second gear side third / second gear switching speed so that the rotational speed of the travel output shaft, which is rotationally driven via the output side second transmission mechanism and the forward side transmission mechanism at the time the output side second clutch mechanism is engaged, matches the rotational speed of the travel output shaft, which is rotationally driven via the output side third transmission mechanism at the start of the third / second gear shift down. Rather, the HST output rotational speed is set as the target rotational speed for HST shift control, which is increased in the opposite direction to the actual HST output rotational speed at the start of the downshift. Furthermore, after the end of the start period, the HST output rotational speed is set as the target rotational speed for HST shift control, which matches the rotational speed of the travel output shaft, which is rotationally driven via the output side second transmission mechanism and the forward side transmission mechanism at the time the output side second clutch mechanism is engaged, to the rotational speed of the travel output shaft immediately before the output side second clutch mechanism is engaged. 【0044】 More preferably, the target speed for HST shift control during the initiation period is the HST speed located on the opposite side of the actual HST output rotational speed at the start of the downshift, from the target speed for the 3rd / 2nd gear shifting period on the 2nd gear side, among the first and second HST speeds. [Effects of the Invention] 【0045】 According to the present invention, in a transmission structure having an HST that receives rotational power from a drive source, and a planetary gear mechanism having first to third elements that inputs output rotational power from the HST to the third element, the transmission structure is switchable between a first HMT transmission state in which rotational power from the drive source is input to the first element and combined rotational power is output from the second element, and a second HMT transmission state in which rotational power from the drive source is input to the second element and combined rotational power is output from the first element, the switching between the first and second HMT transmission states can be made smoother. [Brief explanation of the drawing] 【0046】 [Figure 1] Figure 1 is a schematic diagram of the transmission of a work vehicle to which a transmission structure according to one embodiment of the present invention is applied. [Figure 2] Figure 2 is a hydraulic circuit diagram of a part of the transmission structure according to the above embodiment. [Figure 3] Figure 3 is a longitudinal cross-sectional side view of the transmission structure. [Figure 4] Figure 4 is a hydraulic circuit diagram of an HST, which is part of the transmission structure according to the above embodiment. [Figure 5] Figure 5 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is operated to increase the vehicle speed from zero speed to the forward direction in the transmission structure according to the above embodiment. [Figure 6] Figure 6 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in the transmission structure according to the above embodiment. [Figure 7] Figure 7 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is operated to increase the vehicle speed from zero speed to the forward direction in the transmission structure according to the first modified embodiment described above. [Figure 8] Figure 8 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in the transmission structure according to the first modified example. [Figure 9]Figure 9 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is operated to increase the vehicle speed from zero speed to the forward direction in a transmission structure according to a second modified example of the above embodiment. [Figure 10] Figure 10 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is operated to increase the vehicle speed from zero speed to the forward direction in a transmission structure according to the third modified embodiment described above. [Figure 11] Figure 11 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the fourth modified embodiment described above. [Figure 12] Figure 12 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the fifth modified example of the above embodiment. [Figure 13] Figure 13 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the sixth modified embodiment described above. [Figure 14]Figure 14 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the seventh modified example of the above embodiment. [Figure 15] Figure 15 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is operated to increase the vehicle speed from zero speed to the forward direction in the transmission structure according to the eighth modified example of the above embodiment. [Figure 16] Figure 16 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the ninth modified example of the above embodiment. [Figure 17] Figure 17 is a cross-sectional view along the line XVII-XVII in Figure 3. [Figure 18] Figure 18 is a cross-sectional view of the transmission structure. [Figure 19] Figure 19 is a cross-sectional view along the line XIX-XIX in Figure 18. [Modes for carrying out the invention] 【0047】 Hereinafter, an embodiment of the transmission structure according to the present invention will be described with reference to the attached drawings. Figure 1 shows a schematic diagram of the transmission of a work vehicle 200 to which the transmission structure 1 according to this embodiment is applied. Figure 2 also shows a hydraulic circuit diagram of a part of the transmission structure 1. Furthermore, Figure 3 shows a longitudinal cross-sectional side view of the transmission structure 1. 【0048】 As shown in Figure 1, the work vehicle 200 comprises a drive source 210, drive wheels 220, and the transmission structure 1 interposed in the drive system transmission path from the drive source 210 to the drive wheels 220. In Figures 1 and 2, reference numeral 210a denotes a flywheel included in the drive source 210. 【0049】 As shown in Figure 1, the transmission structure 1 comprises a hydrostatic continuously variable transmission (HST) 10, a planetary gear mechanism 30 that works in cooperation with the HST 10 to form an HMT (hydrostatic-mechanical continuously variable transmission structure), a gear shift output shaft 45, a gear shift operating member 90 such as a gear shift lever, and a control device 100. 【0050】 Figure 4 shows the hydraulic circuit diagram of the HST10. Furthermore, IN1 and IN2 in Figure 2 are fluid-connected to IN1 and IN2 in Figure 4, respectively. 【0051】 As shown in Figures 1 and 4, the HST 10 includes a pump shaft 12 that is operatively rotationally driven by the drive source 210, an HST pump 14 that is supported on the pump shaft 12 so as not to rotate relative to it, an HST motor 18 that is fluidly connected to the HST pump 14 via a pair of first and second HST lines 15a and 15b and is hydraulically rotationally driven by the HST pump 14, a motor shaft 16 that supports the HST motor 18 so as not to rotate relative to it, and an output adjustment member 20 that changes the volume of at least one of the HST pump 14 and the HST motor 18. 【0052】 The HST10 is configured to continuously change the ratio of the rotational speed of the HST output output from the motor shaft 16 to the rotational speed of the power input to the pump shaft 12 (i.e., the gear ratio of the HST10) according to the operating position of the output adjustment member 20. 【0053】 In other words, when the rotational speed of the rotational power operatively input from the drive source 210 to the pump shaft 12 is set as the reference input speed, the HST 10 continuously changes the rotational power at the reference input speed to a rotational power between at least the first HST speed and the second HST speed, according to the operating position of the output adjustment member 20, and outputs it from the motor shaft 16. 【0054】 In this embodiment, as shown in Figures 1 and 4, the pump shaft 12 is connected to the main drive shaft 212, which is operatively connected to the drive source 210, via an HST input gear train 214. 【0055】 In this embodiment, the HST10 is configured to allow switching between forward and reverse rotation directions of the HST output. In other words, the HST10 is configured such that, when the rotation direction of the reference input speed is set to the forward direction, when the output adjustment member 20 is in the first operating position, it outputs rotational power of a first HST speed with the rotation direction set to one side of the forward / reverse direction (for example, the reverse direction) from the motor shaft 16, and when the output adjustment member 20 is in the second operating position, it outputs rotational power of a second HST speed with the rotation direction set to the other side of the forward / reverse direction (for example, the forward direction) from the motor shaft 16. 【0056】 In this case, when the output adjustment member 20 is positioned in the neutral position between the first and second operating positions, the rotational speed of the HST output becomes the neutral speed (zero speed). 【0057】 In this embodiment, the HST10 has a movable swash plate as the output adjustment member 20, which is a typical axial piston pump that changes the volume of the HST pump 14 by swinging around a pivot axis, and has a movable swash plate that can swing to one side and the other side around the pivot axis, with a neutral position in which the discharge amount discharged from the HST pump 14 is zero (see Figure 10 below). 【0058】 When the movable swash plate is positioned in the neutral position, the discharge of pressurized oil from the HST pump 14 ceases, and the HST 10 enters a neutral state where the output of the HST motor 18 is zero. Then, when the movable swash plate is swung from the neutral position to the forward rotation side on one side of the pivot axis, pressurized oil is supplied from the HST pump 14 to one of the pair of HST lines 15 (for example, the first HST line 15a), and the first HST line 15a becomes the high-pressure side and the other second HST line 15b becomes the low-pressure side. As a result, the HST motor 18 is driven to rotate in the forward direction, and the HST 10 enters a forward rotation output state. 【0059】 Conversely, when the movable swash plate is swung from the neutral position to the other side of the pivot axis (the reverse side), pressurized oil is supplied from the HST pump 14 to the other of the pair of HST lines 15 (for example, the second HST line 15b), making the second HST line 15b the high-pressure side and the first HST line 15a the low-pressure side. As a result, the HST motor 18 is driven to rotate in the reverse direction, and the HST 10 enters a reverse output state. In the HST10, the volume of the HST motor 18 is fixed by a fixed swash plate. 【0060】 As shown in Figure 4, the HST10 is equipped with a charge line 130 that supplies pressurized oil to the pair of first and second HST lines 15a and 15b. 【0061】 More specifically, as shown in Figure 2, the transmission structure 1 includes a first hydraulic pump 110 that is operatively driven by the drive source 210, and a hydraulic oil line 120 to which discharge oil from the first hydraulic pump 110 is supplied. 【0062】 In this embodiment, the first hydraulic pump 110 is actuated to the main drive shaft 212 via a pump drive gear train 205 (see Figure 1). The hydraulic pressure in the aforementioned hydraulic fluid line 120 is set by a relief valve 122 (see Figure 4). 【0063】 As shown in Figure 4, the charge line 130 includes a common portion 132 whose base end is fluidly connected to the hydraulic fluid line 120, a first branch portion 134a whose base end is fluidly connected to the common portion 132 and whose tip end is fluidly connected to the first HST line 15a, and a second branch portion 134b whose base end is fluidly connected to the common portion 132 and whose tip end is fluidly connected to the second HST line 15b. 【0064】 Check valves 136 are interposed in the first and second branching sections 134a and 134b to allow the flow of pressurized oil from the common section 132 to the corresponding HST lines 15a and 15b while preventing reverse flow. 【0065】 As shown in Figure 4, the HST10 further includes a communication line 140 that connects the pair of first and second HST lines 15a and 15b, and a bidirectional relief valve 142 inserted in the communication line 140. The communication line 140 and the bidirectional relief valve 142 cause the pressurized oil from one of the pair of HST lines 15a and 15b to flow out to the other HST line when one of the HST lines becomes abnormally high pressure. 【0066】 As shown in Figure 1, the output adjustment member 20 is operated and controlled by the control device 100 in response to the operation of the gear shift operating member 90. 【0067】 In other words, the transmission structure 1 according to this embodiment has an HST shift actuator 150 that operates the output adjustment member 20, and the control device 100 operates the output adjustment member 20 via the HST shift actuator 150 in response to the operation of the shift operation member 90. 【0068】 The HST transmission actuator 150 can take various configurations, such as an electric motor or a hydraulic mechanism, as long as its operation can be controlled by the control device 100. 【0069】 As shown in Figure 4, the transmission structure 1 according to this embodiment has a hydraulic servo mechanism 152 as the HST shift actuator 150. 【0070】 The hydraulic servo mechanism 152 includes a servo piston 155 that is capable of reciprocating in the axial direction with first and second oil chambers separated on one and the other axial side, respectively; a servo pressure oil line 157 whose base end is fluidly connected to the hydraulic oil line 120; a drain line 159; first and second servo supply and discharge lines 160a and 160b, respectively, which are fluidly connected to the first and second oil chambers; a servo switching valve 162 that switches the connection state of the servo pressure oil line 157, the drain line 159, the first servo supply and discharge line 160a and the second servo supply and discharge line 160b; and an operating piston 164 which is operably connected to the servo switching valve 162. 【0071】 The servo piston 155 is actuarily connected to the movable swash plate, which acts as the output adjustment member 20, so as to move the movable swash plate around the pivot axis in accordance with the axial movement. 【0072】 The servo switching valve 162 is capable of selectively taking on three positions: a closed position in which the first and second servo supply and discharge lines 160a and 160b are closed; a first operating position in which the first servo supply and discharge line 160a is fluidly connected to the servo pressure oil line 157 and the second servo supply and discharge line 160b is fluidly connected to the drain line 159; and a second operating position in which the first servo supply and discharge line 160a is fluidly connected to the drain line 159 and the second servo supply and discharge line 160b is fluidly connected to the servo pressure oil line 157. 【0073】 The operating piston 164 is configured to take a first operating position, a holding position, and a second operating position, which position the servo switching valve 162 in a first operating position, a closed position, and a second operating position, respectively. 【0074】 In this embodiment, the operating piston 164 is capable of reciprocating in the axial direction with an oil chamber and a spring chamber separated on one and the other axial sides, respectively, and is pressed in a direction that reduces the size of the oil chamber by a biasing spring disposed in the spring chamber. 【0075】 The hydraulic servo mechanism 152 further includes a servo operating line 167 whose base end is fluidly connected to the hydraulic fluid line 120 and whose tip end is fluidly connected to the oil chamber, and an output control valve 165 that can adjust the amount of pressurized oil in the servo operating line 167. 【0076】 The output control valve 165 is operated under control by the control device 100. In other words, the control device 100 operates the output adjustment valve 165 so that the output adjustment member 20 is positioned in an operating position corresponding to the operating position of the gear shift operating member 90. The operating position of the gear shift operating member 90 is detected, for example, by an operating position sensor 92 such as a potentiometer. 【0077】 As shown in Figures 1 and 3, the planetary gear mechanism 30 includes a sun gear 32, a planetary gear 34 that meshes with the sun gear 32, an internal gear 36 that meshes with the planetary gear 34, and a carrier 38 that supports the planetary gear 34 so as to be rotatable around its axis and rotates around the axis of the sun gear 32 in conjunction with the revolution of the planetary gear 34 around the sun gear 32. The sun gear 32, the carrier 38, and the internal gear 36 form a planetary element. 【0078】 One of the three planetary elements, the third element, is actuarially connected to the motor shaft 16, and the third element acts as a variable power input unit that receives the HST output. As shown in Figures 1 and 3, in this embodiment, the sun gear 32 is the third element. In this embodiment, the sun gear 32 is actuated to the motor shaft 16 via the HST output gear train 216. 【0079】 The transmission structure 1 according to this embodiment is switchable between a first HMT transmission state in which the first element acts as a reference power input unit that receives reference rotational power from the drive source 210 and the second element acts as a combined power output unit that outputs combined rotational power, and a second HMT transmission state in which the first element acts as the combined power output unit and the second element acts as the reference power input unit. 【0080】 Specifically, as shown in Figures 1 to 3, the transmission structure 1 further includes an input-side first transmission mechanism 50a and an input-side second transmission mechanism 50b capable of operatingly transmitting the rotational power of the drive source 210 to the first element and the second element, respectively; an input-side clutch mechanism pair including an input-side first clutch mechanism 60a and an input-side second clutch mechanism 60b for engaging and disengaging the power transmission of the input-side first transmission mechanism 50a and the input-side second transmission mechanism 50b, respectively; an output-side first transmission mechanism 70a and an output-side second transmission mechanism 70b capable of operatingly transmitting the rotational power of the second element and the first element to the speed-shifting output shaft 45, respectively; and an output-side clutch mechanism pair including an output-side first clutch mechanism 80a and an output-side second clutch mechanism 80b for engaging and disengaging the power transmission of the output-side first transmission mechanism 70a and the output-side second transmission mechanism 70b, respectively. 【0081】 In this embodiment, the internal gear 36 and the carrier 38 act as the first and second elements, respectively. 【0082】 The input-side first transmission mechanism 50a is configured to transmit the rotational power of the drive source 210 to the first element (in this embodiment, the internal gear 36). 【0083】 More specifically, as shown in Figures 1 and 3, the input-side first transmission mechanism 50a includes an input-side first drive gear 52a that is rotatably connected to the main drive shaft 212, and an input-side first driven gear 54a that meshes with the input-side first drive gear 52a and is operably connected to the first element. 【0084】 As shown in Figures 1 and 3, the transmission structure 1 according to this embodiment has a variable speed intermediate shaft 43 arranged coaxially with the planetary gear mechanism 30 and connected to the second element so as not to rotate relative to the axis, and the input side first driven gear 54a is supported so as to be rotatable relative to the variable speed intermediate shaft 43 and is actuatedly connected to the input side first drive gear 52a and the first element (the internal gear 36 in this embodiment). 【0085】 The input-side second transmission mechanism 50b is configured to transmit the rotational power of the drive source 210 to the second element (in this embodiment, the carrier 38). 【0086】 More specifically, as shown in Figures 1 and 3, the input-side second transmission mechanism 50b includes an input-side second drive gear 52b supported so as to be rotatable relative to the main drive shaft 212, and an input-side second driven gear 54b that meshes with the input-side second drive gear 52b and is operably connected to the second element. 【0087】 In this embodiment, the input-side second driven gear 54b is meshed with the input-side second drive gear 52b while being supported in a manner that prevents relative rotation by the variable speed intermediate shaft 43, which is connected to the second element in a manner that prevents relative rotation. 【0088】 In this embodiment, the input-side first and second clutch mechanisms 60a and 60b are friction plate type clutch mechanisms. 【0089】 The input-side first and second clutch mechanisms 60a and 60b are supported on the main drive shaft 212 so as to engage and disengage the input-side first and second drive gears 52a and 52b with the main drive shaft 212, respectively. 【0090】 More specifically, the input-side first clutch mechanism 60a includes an input-side clutch housing 62 supported by the main drive shaft 212 so as not to rotate relative to it, an input-side first friction plate group 64a including a first drive-side friction plate supported by the input-side clutch housing 62 so as not to rotate relative to it and a first driven-side friction plate supported by the input-side first drive gear 52a so as not to rotate relative to it in a position opposite to the first drive-side friction plate, and an input-side first piston (not shown) that frictionally engages the input-side first friction plate group 64a. 【0091】 The input-side second clutch mechanism 60b includes the input-side clutch housing 62, an input-side second friction plate group 64b including a second drive-side friction plate supported by the input-side clutch housing 62 so as not to rotate relative to it, and a second driven-side friction plate supported by the input-side second drive gear 52b so as not to rotate relative to it, facing the second drive-side friction plate, and an input-side second piston (not shown) that frictionally engages the input-side second friction plate group 64b. 【0092】 The output-side first transmission mechanism 70a is configured to transmit the rotational power of the second element to the variable speed output shaft 45. 【0093】 In this embodiment, the output-side first transmission mechanism 70a is configured to transmit the rotational power of the second element to the variable speed output shaft 45 by utilizing the input-side second driven gear 54b of the input-side second transmission mechanism 50b. 【0094】 More specifically, as shown in Figures 1 and 3, the output-side first transmission mechanism 70a includes the input-side second driven gear 54b and the output-side first driven gear 74a which is rotatably supported relative to the speed-shifting output shaft 45 and is operatively coupled to the input-side second driven gear 54b. 【0095】 The output-side second transmission mechanism 70b is configured to transmit the rotational power of the first element to the variable speed output shaft 45. 【0096】 In this embodiment, the output-side second transmission mechanism 70b is configured to transmit the rotational power of the first element to the variable speed output shaft 45 by utilizing the input-side first driven gear 54a of the input-side first transmission mechanism 50a. 【0097】 More specifically, as shown in Figures 1 and 3, the output-side second transmission mechanism 70b includes the input-side first driven gear 54a and the output-side second driven gear 74b which is rotatably supported relative to the speed-shifting output shaft 45 and is operatively connected to the input-side first driven gear 54a. 【0098】 The output-side first and second clutch mechanisms 80a and 80b are friction plate type clutch mechanisms. 【0099】 In this embodiment, the output-side first and second clutch mechanisms 80a and 80b are supported on the gear shift output shaft 45 so as to engage and disengage the output-side first and second driven gears 74a and 74b with the gear shift output shaft 45, respectively. 【0100】 More specifically, the output-side first clutch mechanism 80a includes an output-side clutch housing 82 supported on the speed shift output shaft 45 so as not to rotate relative to it, an output-side first friction plate group 84a including a first drive-side friction plate supported on the output-side first driven gear 74a so as not to rotate relative to it and a first driven-side friction plate supported on the output-side clutch housing 82 so as not to rotate relative to it in a position opposite to the first drive-side friction plate, and an output-side first piston (not shown) that frictionally engages the output-side first friction plate group. 【0101】 The output-side second clutch mechanism 80b includes the output-side clutch housing 82, an output-side second friction plate group 84b including a second drive-side friction plate supported by the output-side second driven gear 74b so as not to rotate relative to it, and a second driven-side friction plate supported by the output-side clutch housing 82 so as not to rotate relative to it, facing the second drive-side friction plate, and an output-side second piston (not shown) that frictionally engages the output-side second friction plate group. 【0102】 The transmission structure 1 further includes a transmission state switching actuator 300 that switches the engagement and disengagement of the input-side first clutch mechanism 60a, the input-side second clutch mechanism 60b, the output-side first clutch mechanism 80a, and the output-side second clutch mechanism 80b. 【0103】 The transmission state switching actuator 300 can take various configurations, such as an electric motor or a hydraulic mechanism, as long as its operation can be controlled by the control device. 【0104】 As shown in Figure 2, the transmission structure 1 according to this embodiment has a transmission state switching hydraulic mechanism 302 as the transmission state switching actuator 300. 【0105】 The transmission state switching hydraulic mechanism 302 is configured to utilize a common oil source (the first hydraulic pump 110) shared with the charge line 130 of the HST 10 and the hydraulic servo mechanism 152. 【0106】 More specifically, the transmission state switching hydraulic mechanism 302 has a clutch line 310 whose base end is fluidly connected to the hydraulic fluid line 120, and input side first supply / discharge line 320a, input side second supply / discharge line 320b, output side first supply / discharge line 330a and output side second clutch mechanism 80b, respectively, whose tip ends are fluidly connected to the input side first clutch mechanism 60a, the input side second clutch mechanism 60b, the output side first clutch mechanism 80a and the output side second clutch mechanism 80b. The system includes a second supply / discharge line 330b, a drain line 340, and input-side first solenoid valve 325a, input-side second solenoid valve 325b, output-side first solenoid valve 335a, and output-side second solenoid valve 335b, which are interposed between the clutch line 310 and the drain line 340 and the input-side first supply / discharge line 320a, the input-side second supply / discharge line 320b, the output-side first supply / discharge line 330a, and the output-side second supply / discharge line 330b, respectively. 【0107】 The input-side first solenoid valve 325a, the input-side second solenoid valve 325b, the output-side first solenoid valve 335a, and the output-side second solenoid valve 335b are operated under control by the control device 100 and can take on a supply position in which the corresponding supply and discharge lines 320a, 320b, 330a, and 330b are fluidly connected to the clutch line 310, and a discharge position in which the corresponding supply and discharge lines 320a, 320b, 330a, and 330b are fluidly connected to the drain line 340. 【0108】 As shown in Figure 2, in this embodiment, the input-side first solenoid valve 325a and the input-side second solenoid valve 325b are electromagnetic switching valves that instantaneously increase or decrease the hydraulic pressure of the corresponding input-side first supply / discharge line 320a and the input-side second supply / discharge line 320b. 【0109】 On the other hand, the output-side first solenoid valve 335a and the output-side second solenoid valve 335b are solenoid proportional valves capable of adjusting the hydraulic pressure increase / decrease speed of the corresponding output-side first supply / discharge line 330a and the output-side second supply / discharge line 330b. 【0110】 As shown in Figures 1 and 3, the transmission structure 1 according to this embodiment further includes a travel output shaft 47 that outputs drive rotational power toward the drive wheels 220, a forward-side transmission mechanism 400F and a reverse-side transmission mechanism 400R that operate and transmit the rotational power of the shift output shaft 45 to the travel output shaft 47 as drive rotational power in the forward and reverse directions, respectively, and a friction plate type forward-side clutch mechanism 410F and a reverse-side clutch mechanism 410R that engage and disengage the power transmission of the forward-side transmission mechanism 400F and the reverse-side transmission mechanism 400R, respectively. 【0111】 As shown in Figures 1 and 3, the forward transmission mechanism 400F has a forward gear train including a forward drive gear 402F supported on the speed shift output shaft 45 and a forward driven gear 404F that is supported on the travel output shaft 47 and meshes with the forward drive gear 402F. 【0112】 In this embodiment, the forward drive gear 402F is supported on the speed shift output shaft 45 so as not to rotate relative to it, and the forward driven gear 404F is supported on the travel output shaft 47 so as to rotate relative to it. 【0113】 The reverse side transmission mechanism 400R has a reverse side gear train including a reverse side drive gear 402R supported on the transmission output shaft 45 and a reverse side driven gear 404R that is supported on the travel transmission shaft 47 and meshes with the reverse side drive gear 402R via an idler gear 403 (see Figure 1). 【0114】 In this embodiment, the reverse drive gear 402R is supported on the speed shift output shaft 45 so as not to rotate relative to it, and the reverse driven gear 404R is supported on the travel output shaft 47 so as to rotate relative to it. 【0115】 In this embodiment, the forward clutch mechanism 410F and the reverse clutch mechanism 410R are supported on the travel output shaft 47 such that they engage and disengage the forward driven gear 404F and the reverse driven gear 404R with the travel output shaft 47, respectively. 【0116】 More specifically, the forward clutch mechanism 410F includes a forward / reverse clutch housing 412 supported on the travel output shaft 47 so as not to rotate relative to it, a group of forward friction plates 414F including a forward driven friction plate supported on the forward / reverse clutch housing 412 so as not to rotate relative to it, and a forward drive friction plate supported on the forward driven gear 404F so as not to rotate relative to it, facing the forward driven friction plate, and a forward piston (not shown) that frictionally engages the group of forward friction plates 414F. 【0117】 The reverse clutch mechanism 410R includes the forward / reverse clutch housing 412, a group of reverse friction plates 414R including a reverse driven friction plate supported by the forward / reverse clutch housing 412 so as not to rotate relative to it, and a reverse drive friction plate supported by the reverse driven gear 404R so as not to rotate relative to it, facing the reverse driven friction plate, and a reverse piston (not shown) that frictionally engages the group of reverse friction plates 414R. 【0118】 The forward clutch mechanism 410F and the reverse clutch mechanism 410R are switched between engaged and disengaged by the transmission state switching actuator 300 (in this embodiment, the transmission state switching hydraulic mechanism 302). 【0119】 Specifically, when the control device 100 recognizes that the gear shift operating member 90 has been operated to the forward side, it operates the transmission state switching actuator 300 so that the forward clutch mechanism 410F becomes engaged and the reverse clutch mechanism 410R becomes disengaged. When the control device 100 recognizes that the gear shift operating member 90 has been operated to the reverse side, it operates the transmission state switching actuator 300 so that the forward clutch mechanism 410F becomes disengaged and the reverse clutch mechanism 410R becomes engaged. 【0120】 As described above, the transmission structure 1 has the transmission state switching hydraulic mechanism 302 as the transmission state switching actuator 300. 【0121】 As shown in Figure 2, the transmission state switching hydraulic mechanism 302 further includes forward supply and discharge lines 350F and reverse supply and discharge lines 350R, whose tip ends are fluidly connected to the forward clutch mechanism 410F and the reverse clutch mechanism 410R, respectively, and forward solenoid valves 355F and reverse solenoid valves 355R, which are interposed between the clutch line 310 and the drain line 340 and the forward supply and discharge lines 350F and the reverse supply and discharge lines 350R, respectively. 【0122】 The forward-side solenoid valve 355F and the reverse-side solenoid valve 355R are operated under control by the control device 100, and can take on a supply position in which the corresponding supply and discharge lines 350F and 350R are fluidly connected to the clutch line 310, and a discharge position in which the corresponding supply and discharge lines 350F and 350R are fluidly connected to the drain line 340. 【0123】 As shown in Figure 2, in this embodiment, the forward-side solenoid valve 355F and the reverse-side solenoid valve 355R are electromagnetic switching valves that instantaneously increase or decrease the hydraulic pressure of the corresponding forward-side supply and discharge line 350F and the reverse-side supply and discharge line 350R. 【0124】 As shown in Figures 1 to 3, the transmission structure 1 according to this embodiment further includes an output-side third transmission mechanism 70c that can transmit the rotational power of the first element to the travel output shaft 47 as driving rotational power in the forward direction, and an output-side third clutch mechanism 80c that engages and disengages the power transmission of the output-side third transmission mechanism 70c. 【0125】 The output-side third transmission mechanism 70c is configured such that the rotational speed of the travel output shaft 47 when the rotational power of the first element is transmitted to the travel output shaft 47 via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F is higher than the rotational speed of the travel output shaft 47 when the rotational power of the first element is transmitted to the travel output shaft 47 via the output-side third transmission mechanism 70c. 【0126】 In this embodiment, the output-side third transmission mechanism 70c is configured to transmit the rotational power of the first element to the travel output shaft 47 by utilizing the output-side second driven gear 74b of the output-side second transmission mechanism 70b. 【0127】 More specifically, as shown in Figures 1 and 3, the output-side third transmission mechanism 70c includes the output-side second driven gear 74b and the output-side third driven gear 74c which is rotatably supported relative to the travel output shaft 47 and is operatively connected to the output-side second driven gear 74b. 【0128】 The output-side third clutch mechanism 80c is supported on the travel transmission shaft 47 so as to engage and disengage the output-side third driven gear 74c from the travel output shaft 47. 【0129】 More specifically, the output-side third clutch mechanism 80c includes an output-side clutch housing 83 supported on the travel output shaft 47 so as not to rotate relative to it, an output-side third friction plate group 84c including a third drive-side friction plate supported on the output-side third driven gear 74c so as not to rotate relative to it, and a third driven-side friction plate supported on the output-side clutch housing 83 so as not to rotate relative to it while facing the third drive-side friction plate, and an output-side third piston (not shown) that frictionally engages the output-side third friction plate group 84c. 【0130】 The output-side third clutch mechanism 80c is configured to be engaged and disengaged by the transmission state switching actuator 300. 【0131】 As described above, the transmission structure 1 has the transmission state switching hydraulic mechanism 302 as the transmission state switching actuator 300. 【0132】 As shown in Figure 2, the transmission state switching hydraulic mechanism 302 further includes an output side third supply / discharge line 330c whose tip is fluidly connected to the output side third clutch mechanism 80c, and an output side third solenoid valve 335c interposed between the clutch line 310 and the drain line 340 and the output side third supply / discharge line 330c. 【0133】 The output-side third solenoid valve 335c is operated under control by the control device 100 and can take on a supply position in which the corresponding supply / discharge line 330c is fluidly connected to the clutch line 310, and a discharge position in which the corresponding supply / discharge line 330c is fluidly connected to the drain line 340. 【0134】 As shown in Figure 2, in this embodiment, the output-side third solenoid valve 335c is a solenoid proportional valve capable of adjusting the hydraulic pressure increase / decrease speed of the corresponding output-side third supply / discharge line 330c. 【0135】 The work vehicle 200 has a pair of left and right main drive wheels as the drive wheels 220. Accordingly, as shown in Figure 1, the work vehicle 200 further includes a pair of main drive axles 250 that drive the pair of main drive wheels, and a differential mechanism 260 that differentially transmits the rotational power of the travel output shaft to the pair of main drive axles 250. 【0136】 As shown in Figure 1, the work vehicle 200 further includes a travel brake mechanism 255 that selectively applies braking force to the main drive axle 250, a differential lock mechanism 265 that forcibly drives the pair of main drive axles 250 synchronously with rotational power from the travel output shaft, and a drive force extraction mechanism 270 for auxiliary drive wheels that can selectively output the rotational power taken from the travel output shaft toward the auxiliary drive wheels. 【0137】 Furthermore, the work vehicle 200 has a PTO shaft 280 that outputs rotational power to the outside, and a PTO clutch mechanism 285 and a PTO multi-stage transmission mechanism 290 that are interposed in the PTO transmission path from the drive source 210 to the PTO shaft 280. 【0138】 Now, let's explain the gear shift control by the control device 100. First, we will describe the gear shift control of the control device 100 when the gear shift operating member 90 is operated to increase the vehicle speed from zero speed to the forward speed side. 【0139】 Figure 5 shows the relationship between the passage of time, the rotational speed of the drive rotational power of the transmission structure 1, the output rotational speed of the HST 10, and the hydraulic pressure of the clutch mechanisms 60a-60b, 80a-80c, and 410F when the gear shifting operating member 90 is operated to increase the vehicle speed from zero to forward. 【0140】 The gear shift operating member 90 is operable in the following states: zero vehicle speed, forward first gear region F1 which is a forward low-speed region, forward second gear region F2 which is a forward high-speed region further forward than forward first gear region F1, and forward third gear region F3 which is a forward high-speed region further forward than forward second gear region F2. 【0141】 Furthermore, the gear shifting operating member 90 can be operated not only on the forward side, but also on the reverse side low-speed region, which is the first reverse gear region, and on the reverse side second gear region, which is a higher speed region further backward than the first reverse gear region. 【0142】 When the gear shift operating member 90 is positioned at zero vehicle speed and in the first gear range (forward first gear range F1 and reverse first gear range), the control device 90 operates the transmission state switching actuator 300 so that the input side first and second clutch mechanisms 60a and 60b are engaged and disengaged, respectively, thereby creating a first HMT transmission state in which the first element acts as the reference power input unit and the second element acts as the combined power output unit that outputs the combined rotational power of the planetary gear mechanism 30 toward the gear shift output shaft 45. 【0143】 Specifically, as shown in Figure 5, when the gear shift operating member 90 is positioned at zero vehicle speed and in the first gear range (forward first gear range F1 and reverse first gear range), the control device 100 sets the input-side first solenoid valve 325a to the supply position to set the hydraulic pressure of the input-side first clutch mechanism 60a to above the engagement hydraulic pressure (in this embodiment, the set hydraulic pressure by the relief valve (clutch hydraulic pressure ON)), and sets the input-side second solenoid valve 325b to the discharge position to set the hydraulic pressure of the input-side second clutch mechanism 60b to below the engagement hydraulic pressure (in this embodiment, the drain hydraulic pressure (clutch hydraulic pressure OFF)), thereby creating the first HMT transmission state. 【0144】 Then, the control device 100 operates the transmission state switching actuator 300 so that the output-side first and second clutch mechanisms 70a and 70b are engaged and disengaged, respectively, thereby creating a second element output state in which the rotational power of the second element is transmitted to the speed-shifting output shaft 45. 【0145】 Specifically, as shown in Figure 5, when the gear shift operating member 90 is positioned at zero vehicle speed and in the first gear range (forward first gear range F1 and reverse first gear range), the control device 100 sets the output-side first solenoid valve 335a to the supply position, thereby setting the hydraulic pressure of the output-side first clutch mechanism 80a to be equal to or greater than the engagement hydraulic pressure (in this embodiment, the set hydraulic pressure (clutch hydraulic pressure ON)), and sets the output-side second solenoid valve 335b to the discharge position, thereby setting the hydraulic pressure of the output-side second clutch mechanism 80b to less than the engagement hydraulic pressure (in this embodiment, the drain hydraulic pressure (clutch hydraulic pressure OFF)), thereby creating a second element output state. 【0146】 On the other hand, when the gear shift operating member 90 is positioned in the second gear range (the forward second gear range F2 and the reverse second gear range), the control device 100 operates the transmission state switching actuator 300 so that the input-side first and second clutch mechanisms 60a and 60b are in the disengaged and engaged states, respectively, thereby creating a second HMT transmission state in which the second element acts as the reference power input and the first element acts as the combined power output. 【0147】 Specifically, as shown in Figure 5, when the gear shift operating member 90 is located in the second gear range (forward second gear range F2 and reverse second gear range), the control device 100 sets the input-side first solenoid valve 325a to the discharge position, thereby setting the hydraulic pressure of the input-side first clutch mechanism 60a to less than the engagement hydraulic pressure (in this embodiment, drain hydraulic pressure (clutch hydraulic pressure OFF)), and sets the input-side second solenoid valve 325b to the supply position, thereby setting the hydraulic pressure of the input-side second clutch mechanism 60b to more than or equal to the engagement hydraulic pressure (in this embodiment, set hydraulic pressure (clutch hydraulic pressure ON)), thereby creating a second HMT transmission state. 【0148】 Then, the control device 100 operates the transmission state switching actuator 300 so that the output-side first and second clutch mechanisms 80a and 80b are in a disengaged state and an engaged state, respectively, thereby creating a first element output state in which the rotational power of the first element is transmitted to the speed-shifting output shaft. 【0149】 Specifically, as shown in Figure 5, when the gear shift operating member 90 is located in the second gear range (forward second gear range F2 and reverse second gear range), the control device 100 sets the output-side first solenoid valve 335a to the discharge position, making the hydraulic pressure of the output-side first clutch mechanism 80a less than the engagement hydraulic pressure (in this embodiment, drain hydraulic pressure (clutch hydraulic pressure OFF)), and sets the output-side second solenoid valve 335b to the supply position, making the hydraulic pressure of the output-side second clutch mechanism 80b greater than or equal to the engagement hydraulic pressure (in this embodiment, set hydraulic pressure (clutch hydraulic pressure ON)), thereby creating the first element output state. 【0150】 As shown in Figure 5, the planetary gear mechanism 30 is configured such that, under the first HMT transmission state, when the HST output is set to the first HST speed, the output rotational power of the second element becomes zero, and as the HST output changes from the first HST speed to the second HST speed, the combined rotational power output from the second element increases. On the other hand, under the second HMT transmission state, as the HST output changes from the second HST speed to the first HST speed, the combined rotational power output from the first element increases. 【0151】 Furthermore, the gear ratio of the input-side first transmission mechanism 50a (input-side first gear ratio) and the gear ratio of the input-side second transmission mechanism 50b (input-side second gear ratio) are set such that the rotational speed of the second element when the HST output is set to the second HST speed under the first HMT transmission state is the same as the rotational speed of the second element due to the rotational power transmitted via the input-side second transmission mechanism 50b under the second HMT transmission state, and the rotational speed of the first element when the HST output is set to the second HST speed under the second HMT transmission state is the same as the rotational speed of the first element due to the rotational power transmitted via the input-side first transmission mechanism 50a under the first HMT transmission state. 【0152】 The control device 100 operates the HST shift actuator 150 (in this embodiment, the hydraulic servo mechanism 152) so that the HST output becomes the first HST speed in response to the operation of the shift operation member 90 to the zero vehicle speed position, thereby generating zero speed for the drive rotational power. Furthermore, the control device 100 operates the HST shift actuator 150 (in this embodiment, the hydraulic servo mechanism 152) so that the HST output shifts from the first HST speed to the second HST speed in response to the acceleration operation of the shift operation member 90 within the first gear stage region, thereby increasing the drive rotational power output from the second element in response to the acceleration operation of the shift operation member 90 within the first gear stage region. 【0153】 When the gear shift operating member 90 is shifted up from the forward first gear range F1 to the forward second gear range F2, the control device 100 instantly shifts the clutch mechanism (i.e., the second clutch mechanism) in one of the clutch mechanism pairs (the input clutch mechanism pair and the output clutch mechanism pair) that is in the released state during the first HMT transmission state from the released state to the engaged state, at the start of the first / second gear shift up when the drive rotational power reaches a predetermined speed (the first / second gear shift up start speed), and at the end of the first / second gear shift up after a predetermined time has elapsed from the start of the first / second gear shift up, it instantly shifts the clutch mechanism (i.e., the first clutch mechanism) in one of the clutch mechanism pairs that is in the engaged state during the first HMT transmission state from the engaged state to the released state, thereby shifting the gear shift up to the first / second gear From the start of a gear shift up to the end of the first / second gear shift up, a double-shift-up transmission state is created in which both the first and second clutch mechanisms of one of the clutch mechanism pairs are engaged. During this double-shift-up transmission state, the first clutch mechanism of the other clutch mechanism pair (the input clutch mechanism pair and the output clutch mechanism pair), which is engaged during the first HMT transmission state, is moved from the engaged state to the disengaged state by sliding the friction plates, and the second clutch mechanism of the other clutch mechanism pair, which is disengaged during the first HMT transmission state, is moved from the disengaged state to the engaged state by sliding the friction plates, thereby switching the other clutch mechanism pair from the engaged state of the first clutch mechanism to the engaged state of the second clutch mechanism. 【0154】 With this configuration, the shift up from the first HMT transmission state to the second HMT transmission state can be performed smoothly without causing a power transmission interruption. 【0155】 Furthermore, in this embodiment, the gear ratio of the output-side first transmission mechanism 70a (output-side first gear ratio) and the gear ratio of the output-side second transmission mechanism 70b (output-side first gear ratio) are set such that the rotational speed that appears on the gear shift output shaft 45 when the HST output is set to the second HST speed in the first HMT transmission state is substantially the same as the rotational speed that appears on the gear shift output shaft 45 when the HST output is set to the second HST speed in the second HMT transmission state. 【0156】 In this embodiment, as shown in Figure 5, the starting speed for the first / second gear shift up is the speed of the drive rotational power that is generated when the HST output is set to the second HST speed in the first HMT transmission state. With this configuration, changes in vehicle speed during switching between the first and second HMT transmission states can be effectively prevented or reduced. 【0157】 As mentioned above, in this embodiment, the output-side first and second clutch mechanisms 80a and 80b are of the friction plate type. Therefore, even if a slight speed difference occurs in the speed-shifting output shaft 45 when switching the transmission state, this speed difference can be effectively absorbed by the sliding of the friction plates. 【0158】 The control device 100 can recognize when the drive rotational power reaches the first / second gear shift-up starting speed, for example, by a signal from an output sensor 95 that detects the rotational speed of the travel output shaft 47, the gear shift output shaft 45, or the motor shaft 16. In this embodiment, as shown in Figure 1, the output sensor 95 is arranged to detect the rotational speed of the travel output shaft 47. 【0159】 In this embodiment, the input-side first and second solenoid valves 325a and 325b, which switch the supply and discharge of pressurized oil to the input-side first and second clutch mechanisms 60a and 60b that form the input-side clutch mechanism pair, are solenoid switching valves, and the increase or decrease in hydraulic pressure of the input-side first and second clutch mechanisms 60a and 60b is performed instantaneously. 【0160】 On the other hand, the output-side first and second solenoid valves 335a and 335b, which switch the supply and discharge of pressurized oil to the output-side first and second clutch mechanisms 80a and 80b that form the output-side clutch mechanism pair, are solenoid proportional valves, and the increase or decrease in hydraulic pressure of the output-side first and second clutch mechanisms 80a and 80b is speed-adjustable. 【0161】 In this case, the input-side clutch mechanism pair is the one clutch mechanism pair, and the output-side clutch mechanism pair is the other clutch mechanism pair. 【0162】 Specifically, as shown in Figure 5, the control device 100 moves the input-side second solenoid valve 325b from the discharge position to the supply position at the start of the first / second gear shift up, instantly transitioning the input-side second clutch mechanism 60b, which is in the released state in the first HMT transmission state, from the released state to the engaged state, and at the end of the first / second gear shift up, moves the input-side first solenoid valve 325a from the supply position to the discharge position, instantly transitioning the input-side first clutch mechanism 60a, which is in the engaged state in the first HMT transmission state, from the engaged state to the released state, thereby creating a double shift-up transmission state in which both the input-side first and second clutch mechanisms 60a and 60b of the input-side clutch mechanism pair are engaged from the start of the first / second gear shift up to the end of the first / second gear shift up. 【0163】 Furthermore, the control device 100 moves the output-side first solenoid valve 335a, which is a solenoid proportional valve, from the supply position to the discharge position, gradually decreasing the hydraulic pressure of the output-side first clutch mechanism 80a from the set hydraulic pressure (clutch hydraulic pressure ON) to the drain hydraulic pressure (clutch hydraulic pressure OFF) via the engagement hydraulic pressure, thereby moving the output-side first clutch mechanism 80a from the engaged state to the released state while sliding the friction plates. Additionally, the control device 100 moves the output-side second solenoid valve 335b, which is a solenoid proportional valve, from the discharge position to the supply position, gradually increasing the hydraulic pressure of the output-side second clutch mechanism 80b from the drain hydraulic pressure (clutch hydraulic pressure OFF) to the set hydraulic pressure (clutch hydraulic pressure ON) via the engagement hydraulic pressure, thereby moving the output-side second clutch mechanism 80b from the released state to the engaged state while sliding the friction plates. This switching from the engaged state of the output-side first clutch mechanism 80a to the engaged state of the output-side second clutch mechanism 80b is performed during the shift-up double transmission state. 【0164】 In this embodiment, as shown in Figure 5, the movement of the output-side first solenoid valve 335a from the supply position to the discharge position, and the movement of the output-side second solenoid valve 335b from the discharge position to the supply position are performed before the start of the first / second gear shift up. 【0165】 More specifically, when the rotational speed of the drive rotational power reaches a first / second gear shift-up preparation speed which is a predetermined speed lower than the first / second gear shift-up start speed, the control device 100 moves the output-side first solenoid valve 335a from the supply position to the discharge position, and moves the output-side second solenoid valve 335b from the discharge position to the supply position. 【0166】 In this case, the output-side first and second solenoid valves 335a and 335b, which are electromagnetic proportional valves, have their hydraulic pressure increase / decrease speed set such that, during the period from the start of the first / second gear shift up to the end of the first / second gear shift up, the hydraulic pressure of the output-side first clutch mechanism 80a falls below the set hydraulic pressure (clutch hydraulic pressure ON) and the hydraulic pressure of the output-side second clutch mechanism 80b rises above the engagement hydraulic pressure from the drain hydraulic pressure (clutch hydraulic pressure OFF), and then the hydraulic pressure of the output-side first clutch mechanism 80a reaches the drain hydraulic pressure (clutch hydraulic pressure OFF) and the hydraulic pressure of the output-side second clutch mechanism 80b reaches the set hydraulic pressure (clutch hydraulic pressure ON). 【0167】 As shown in Figure 5, when the gear shift operating member 90 is located within the second speed range (the forward second speed range F2 and the reverse second speed range), the control device 100 operates the HST gear shift actuator 150 to increase the drive rotational power in accordance with the speed increase operation of the gear shift operating member 90, so that the HST output shifts from the second HST speed to the first HST speed. 【0168】 When the gear shifting operating member 90 is shifted up from the forward second gear range F2 to the forward third gear range F3, the control device 100 instantly switches the output side second clutch mechanism 80b and the forward side clutch mechanism 410F from the engaged state to the disengaged state at the start of the second / third gear shift up when the drive rotational power reaches a predetermined second / third gear shift up start speed, and then switches the output side third clutch mechanism 80c from the disengaged state to the engaged state at the end of the second / third gear shift up after a predetermined time has elapsed from the start of the second / third gear shift up. As a result, a shift-up coasting state is created in which power transmission to the travel output shaft 47 is interrupted from the start of the second / third gear shift up to the end of the second / third gear shift up, and during the shift-up coasting state, the HST gear shift actuator 150 is operated so that the HST output becomes a predetermined second / third gear switching speed. 【0169】 Here, the predetermined second / third gear shifting speed is defined as the speed at which the rotational speed of the travel output shaft 47, which is generated by the rotational power transmitted from the second element via the output-side third transmission mechanism 70c, matches or approaches the rotational speed of the travel output shaft 47, which is generated by the rotational power transmitted from the second element via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F at the start of the second / third gear shift up. 【0170】 In this embodiment, the second / third gear shift-up start speed, which marks the start of the second / third gear shift-up, is defined as the rotational speed of the travel output shaft 47, which is generated by the rotational power transmitted from the second element via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F when the HST output is set to the first HST speed. 【0171】 In this embodiment, as described above, the output-side third solenoid valve 335c that switches the supply and discharge of pressurized oil to the output-side third clutch mechanism 80c is a solenoid proportional valve. 【0172】 The output-side third solenoid valve 335c is configured such that, over a predetermined time from the start of the second / third gear shift up to the end of the second / third gear shift up, the hydraulic pressure of the output-side third clutch mechanism 80c is gradually increased from drain hydraulic pressure (clutch hydraulic pressure OFF) to engagement hydraulic pressure. 【0173】 In this case, the control device 100 can reliably create a shift-up coasting state between the start of the second / third gear shift and the end of the second / third gear shift by moving the output-side third solenoid valve 335c from the discharge position to the supply position at the start of the second / third gear shift. 【0174】 When the gear shift operating member 90 is positioned in the forward third gear region F3, the control device 100, while exhibiting the second HMT transmission state, disengages the output-side first and second clutch mechanisms 80a and 80b, and engages the output-side third clutch mechanism 80c, and controls the operation of the HST gear shift actuator 150 so that the rotational speed of the drive rotational power becomes the rotational speed corresponding to the operating position of the gear shift operating member 90. 【0175】 More specifically, as shown in Figure 5, when the gear shift operating member 90 is positioned in the forward third gear region F3, the control device 100 operates the HST gear shift actuator 150 so that the HST output shifts from the second / third gear switching speed side to the first HST speed side in response to the gear shift operating member 90's speed increase operation. 【0176】 Furthermore, when the gear shift operating member 90 is positioned in the forward third gear range F3, there is no need to transmit power from the gear shift output shaft 45 to the travel output shaft 47. Therefore, the control device 100 disengages the forward clutch mechanism 410F, as shown in Figure 5. 【0177】 The following describes the gear shift control of the control device 100 when the gear shift operating member 90 is decelerated. 【0178】 Figure 6 shows the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure 1, the output rotational speed of the HST 10, and the hydraulic pressure of the clutch mechanisms 60a-60b, 80a-80c, and 410F when the gear shift operating member 90 is decelerated from the forward third gear region F3 through the forward second gear region F2 to the forward first gear region F1. 【0179】 As shown in Figure 6, when the gear shift operating member 90 is positioned in the forward third gear region F3, the control device 100, while exhibiting the second HMT transmission state, disengages the output-side first and second clutch mechanisms 80a and 80b, engages the output-side third clutch mechanism 80c, and disengages the forward clutch mechanism 410F, and then operates the HST gear shift actuator 150 so that the HST output changes from the first HST speed side to the second HST speed side in response to the deceleration operation of the gear shift operating member 90. This reduces the rotational speed of the drive rotational power transmitted from the first element to the travel output shaft 47 via the output-side third transmission mechanism 70c and the output-side third clutch mechanism 80c in response to the deceleration operation of the gear shift operating member 90. 【0180】 When the gear shift operating member 90 performs a downshift operation from the forward third gear range F3 to the forward second gear range F2, the control device 100 is configured to move the output side third clutch mechanism 80c from the engaged state to the disengaged state at the start of the third / second gear downshift when the drive rotational power reaches a predetermined speed (third / second gear downshift start speed), and to move the output side second clutch mechanism 80b and the forward side clutch mechanism 410F from the disengaged state to the engaged state at the end of the third / second gear downshift after a predetermined time has elapsed from the start of the third / second gear downshift, thereby creating a downshift coasting state in which the power transmission path to the travel output shaft 47 is interrupted from the start of the third / second gear downshift to the end of the third / second gear downshift. 【0181】 In this embodiment, as shown in Figure 6, the output-side second solenoid valve 335b is configured such that, over a predetermined period of time from the start of the third / second gear downshift to the end of the third / second gear downshift, the hydraulic pressure increase rate of the output-side second clutch mechanism 80b is set to gradually increase from drain hydraulic pressure (clutch hydraulic pressure OFF) to engagement hydraulic pressure. 【0182】 On the other hand, as shown in Figure 6, when the output-side third solenoid valve 335c is repositioned from the supply position to the discharge position, it is configured to instantly lower the hydraulic pressure of the corresponding output-side third clutch mechanism 80c from the set hydraulic pressure (clutch hydraulic pressure ON) to the drain hydraulic pressure (clutch hydraulic pressure OFF). 【0183】 Furthermore, the forward-side solenoid valve 355F, by changing its position from the discharge position to the supply position, instantly increases the hydraulic pressure of the corresponding forward-side clutch mechanism 410F from drain hydraulic pressure (clutch hydraulic pressure OFF) to engagement hydraulic pressure (in this embodiment, set hydraulic pressure (clutch hydraulic pressure ON)). 【0184】 As shown in Figure 6, the control device 100 is configured to move the output-side third solenoid valve 335c from the supply position to the discharge position and the output-side second solenoid valve 335b from the discharge position to the supply position at the start of the third / second gear downshift, and to move the forward-side solenoid valve 355F from the discharge position to the supply position at the end of the third / second gear downshift, thereby ensuring that a downshift coasting state is reliably created for a predetermined time from the start of the third / second gear downshift to the end of the third / second gear downshift. 【0185】 The control device 100 is further configured to change the HST 10 via the HST shift actuator 150 during the downshift coasting state so that the rotational speed of the travel output shaft 47, which is rotationally driven via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F at the end of the downshift to the third / second gear, matches or approaches the rotational speed of the travel output shaft 47, which is rotationally driven via the output-side third transmission mechanism 70c at the start of the downshift to the third / second gear. 【0186】 Here, the predetermined third / second gear switching speed is defined as a speed such that, when the HST output is set to the third / second gear switching speed under conditions in which rotational power is transmitted from the first element to the travel output shaft 47 via the output-side third transmission mechanism 70c, the rotational speed that appears on the travel output shaft 47 matches or approaches the rotational speed that can appear on the travel output shaft 47 under conditions in which rotational power is transmitted from the first element to the travel output shaft 47 via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F. 【0187】 In this embodiment, as shown in Figure 6, the third / second gear switching speed is set such that the rotational speed displayed on the travel output shaft 47 when the HST output is set to the third / second gear switching speed, under conditions in which rotational power is transmitted from the first element to the travel output shaft 47 via the output-side third transmission mechanism 70c, matches or approaches the rotational speed displayed on the travel output shaft 47 when the HST output is set to the first HST speed, under conditions in which rotational power is transmitted from the first element to the travel output shaft 47 via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F. 【0188】 Preferably, the third / second gear shift speed is approximately the same as the second / third gear shift speed. This configuration allows for a simplification of the control structure of the control device 100. 【0189】 As shown in Figure 6, when the gear shift operating member 90 is positioned in the forward second gear range F2, the control device 100 activates the second HMT transmission state, engages the output second clutch mechanism 80b and the forward clutch mechanism 410F, and disengages the output first and third clutch mechanisms 80a and 80c, and then operates the HST gear shift actuator 150 so that the HST output shifts from the first HST speed to the second HST speed in response to the deceleration operation of the gear shift operating member 90. As a result, the drive rotational power is reduced in response to the deceleration operation of the gear shift operating member 90. 【0190】 When the gear shift operating member 90 is performing a downshift operation from the forward second gear range F2 to the forward first gear range F1, the control device 100 instantly moves the first clutch mechanism (input side first clutch mechanism 60a) in one of the clutch mechanism pairs (for example, the input side clutch mechanism pair) from the released state to the engaged state at the start of the second / first gear downshift when the drive rotational power reaches a predetermined speed (second / first gear downshift start speed), and instantly moves the second clutch mechanism (input side second clutch mechanism 60b) in the other clutch mechanism pair (input side clutch mechanism pair) from the engaged state to the released state at the end of the second / first gear downshift after a predetermined time has elapsed from the start of the second / first gear downshift, Up until the end of the downshift, a downshift double transmission state is created in which both the first and second clutch mechanisms of one of the clutch mechanism pairs (input clutch mechanism pair) are engaged. During this downshift double transmission state, the first clutch mechanism (output first clutch mechanism 80a) of the other clutch mechanism pair (output clutch mechanism pair) is moved from a released state to an engaged state by sliding the friction plates, and the second clutch mechanism (output second clutch mechanism 80b) of the other clutch mechanism pair is moved from an engaged state to a released state by sliding the friction plates, thereby switching the second clutch mechanism of the other clutch mechanism pair (output clutch mechanism pair) from an engaged state to an engaged state. 【0191】 With this configuration, a downshift from the second HMT transmission state to the first HMT transmission state can be performed smoothly without causing a power transmission interruption. 【0192】 In this embodiment, as shown in Figure 6, the starting speed for the second / first gear downshift is the speed of the drive rotational power that is generated when the HST output is set to the second HST speed in the second HMT transmission state. 【0193】 As described above, in this embodiment, the output-side first and second solenoid valves 335a and 335b, which switch the supply and discharge of pressurized oil to the output-side first and second clutch mechanisms 80a and 80b that form the output-side clutch mechanism pair, are solenoid proportional valves, and the rate at which the hydraulic pressure of the output-side first and second clutch mechanisms 80a and 80b is increased or decreased is adjustable. 【0194】 In this case, the input-side clutch mechanism pair is the one clutch mechanism pair, and the output-side clutch mechanism pair is the other clutch mechanism pair. 【0195】 In this embodiment, as shown in Figure 6, the control device 100 moves the input-side first solenoid valve 325a from the discharge position to the supply position at the start of the second / first gear downshift, instantly transitioning the input-side first clutch mechanism 60a, which is in the released state during the second HMT transmission state, from the released state to the engaged state. Furthermore, at the end of the second / first gear downshift, the control device 100 moves the input-side second solenoid valve 325b from the supply position to the discharge position, instantly transitioning the input-side second clutch mechanism 60b, which is in the engaged state during the second HMT transmission state, from the engaged state to the released state. Thus, from the start of the first / second gear downshift to the end of the downshift, a double downshift transmission state is created in which both the input-side first and second clutch mechanisms 60a and 60b of the input-side clutch mechanism pair are engaged. 【0196】 Furthermore, the control device 100 moves the output-side second solenoid valve 335b, which is a solenoid proportional valve, from the supply position to the discharge position, gradually decreasing the hydraulic pressure of the output-side second clutch mechanism 80b from the set hydraulic pressure (clutch hydraulic pressure ON) to less than the engagement hydraulic pressure (specifically, the drain hydraulic pressure (clutch hydraulic pressure OFF)), thereby moving the output-side second clutch mechanism 335b from the engaged state to the disengaged state while sliding the friction plates. At the same time, the control device 100 moves the output-side first solenoid valve 335a, which is a solenoid proportional valve, from the discharge position to the supply position, gradually increasing the hydraulic pressure of the output-side first clutch mechanism 80a from the drain hydraulic pressure (clutch hydraulic pressure OFF) to an hydraulic pressure equal to or greater than the engagement hydraulic pressure (specifically, the set hydraulic pressure (clutch hydraulic pressure ON)), thereby moving the output-side first clutch mechanism 80a from the disengaged state to the engaged state while sliding the friction plates. This switching from the engaged state of the output-side second clutch mechanism 80b to the engaged state of the output-side first clutch mechanism 80a is performed during the downshift double transmission state. 【0197】 In this embodiment, as shown in Figure 6, the movement of the output-side second solenoid valve 335b from the supply position to the discharge position, and the movement of the output-side first solenoid valve 335a from the discharge position to the supply position are performed before the start of the second / first gear downshift. 【0198】 More specifically, the control device 100 is configured to move the output-side second solenoid valve 335b from the supply position to the discharge position, and the output-side first solenoid valve 335a from the discharge position to the supply position, when the rotational speed of the drive rotational power reaches a second / first gear down shift preparation speed which is a predetermined speed faster than the second / first gear down shift start speed. 【0199】 In this case, the output-side first and second solenoid valves 335a and 335b, which are electromagnetic proportional valves, are configured such that during the double transmission downshift state from the start of the second / first gear downshift to the end of the second / first gear downshift, the hydraulic pressure of the output-side first clutch mechanism 60a rises from the drain hydraulic pressure (clutch hydraulic pressure OFF) to the engagement hydraulic pressure and the hydraulic pressure of the output-side second clutch mechanism 60b falls from the set hydraulic pressure (clutch hydraulic pressure ON) to the engagement hydraulic pressure. After a predetermined time has elapsed from the end of the second / first gear downshift, the hydraulic pressure of the output-side first clutch mechanism 60a reaches the set hydraulic pressure (clutch hydraulic pressure ON) and the hydraulic pressure of the output-side second clutch mechanism 60b reaches the drain hydraulic pressure (clutch hydraulic pressure OFF). 【0200】 In this embodiment, as described above, the output-side first and second solenoid valves 335a and 335b are configured as electromagnetic proportional valves, gradually increasing and decreasing the hydraulic pressure of the output-side first and second clutch mechanisms 80a and 80b, while the input-side first and second solenoid valves 325a and 325b are configured as electromagnetic switching valves, allowing for instantaneous increases and decreases in the hydraulic pressure of the input-side first and second clutch mechanisms 60a and 60b. However, the present invention is not limited to this configuration. 【0201】 For example, it is also possible to modify the system so that the output-side first and second solenoid valves 335a and 335b are electromagnetic switching valves, and the input-side first and second solenoid valves 325a and 325b are electromagnetic proportional valves. Figures 7 and 8 show graphs corresponding to Figures 5 and 6 in the first modified example in which such deformation was performed. 【0202】 Here, we will describe a modified control structure for shifting gears between the forward second gear range F2 and the forward third gear range F3. 【0203】 Figure 9 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is operated to increase the vehicle speed from zero speed to the forward direction in a transmission structure according to a second modified example of this embodiment. 【0204】 The second modification described above differs from this embodiment in its control method when shifting up from the forward second gear range F2 to the forward third gear range F3. 【0205】 That is, as shown in Figure 9, in the second modified example, when the gear shift operating member 90 is shifted up from the forward second gear range to the forward third gear range, the control device 100, - At the time of the 2nd / 3rd gear shift-up preparation, when the drive rotational power reaches a predetermined 2nd / 3rd gear shift-up preparation speed, one of the output-side 2nd clutch mechanism 80b and the forward-side clutch mechanism 410F (in Figure 9, the output-side 2nd clutch mechanism 80b) is initiated to transition from the engaged state to the disengaged state while sliding, and at the time of the 2nd / 3rd gear shift-up start, after a predetermined time has elapsed from the time of the 2nd / 3rd gear shift-up preparation, one of the output-side 2nd clutch mechanism 80b and the forward-side clutch mechanism 410F is released. · At the point midway through the second / third gear shift, after a predetermined time has elapsed from the start of the second / third gear shift, the output-side second clutch mechanism 80b and the forward-side clutch mechanism 410F (the forward-side clutch mechanism 410F in Figure 9) are transitioned from the engaged state to the disengaged state. - At the point of completion of the second / third gear shift, after a predetermined time has elapsed from the midpoint of the second / third gear shift up, the output-side third clutch mechanism 80c is transitioned from the disengaged state to the engaged state. From the start of the second / third gear shift up to the end of the second / third gear shift up, a state of coasting during the shift up is created in which the power transmission path to the aforementioned drive output shaft is interrupted. 【0206】 Furthermore, the control device 100 shifts the HST 10 via the HST shift actuator 150 during the shift-up coasting state so that the rotational speed of the travel output shaft 47, which is rotationally driven via the output-side third transmission mechanism 70c at the end of the second / third gear shift-up, matches or approaches the rotational speed of the travel output shaft 47 just before the output-side third clutch mechanism 70c transitions to the engaged state. 【0207】 In the second modified example shown in Figure 9, the HST output is shifted from the second / third gear shift switching speed on the second gear side (the first HST speed in this example) to the second / third gear shift switching speed on the third gear side during the shift-up coasting state from the start of the second / third gear shift up to the end of the second / third gear shift up. 【0208】 Figure 10 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is operated to increase the vehicle speed from zero speed to the forward direction in a transmission structure according to a third modified example of this embodiment. 【0209】 The third modified example described above also differs from this embodiment in its control method when shifting up from the forward second gear range F2 to the forward third gear range F3. 【0210】 That is, as shown in Figure 10, in the third modified example, when the gear shift operating member 90 is shifted up from the forward second gear range to the forward third gear range, the control device 100, - At the time of preparation for the second / third gear shift up, when the drive rotational power reaches a predetermined second / third gear shift up preparation speed, the output side second clutch mechanism 80b is made to transition from the engaged state to the disengaged state and the output side third clutch mechanism 80c is made to transition from the disengaged state to the engaged state, and at the time of completion of the second / third gear shift up, after a predetermined time has elapsed from the time of preparation for the second / third gear shift up, the output side second clutch mechanism 80b is made to the disengaged state and the output side third clutch mechanism 80c is made to the engaged state. By transitioning the forward clutch mechanism 410F from the engaged state to the disengaged state at a point before the completion of the second / third gear shift (i.e., at the start of the second / third gear shift), From the start of the second / third gear shift up to the end of the second / third gear shift up, a state of coasting during the shift up is created in which the power transmission path to the aforementioned drive output shaft 47 is interrupted. 【0211】 Furthermore, the control device 100 shifts the HST 10 via the HST shift actuator 150 during the shift-up coasting state so that the rotational speed of the travel output shaft 47, which is rotationally driven via the output-side third transmission mechanism 70c at the end of the second / third gear shift-up, matches or approaches the rotational speed of the travel output shaft 47 just before the output-side third clutch mechanism 70c transitions to the engaged state. 【0212】 In the second modified example shown in Figure 10, the HST output is also changed from the second / third gear shifting speed on the second gear side (the first HST speed in this example) to the second / third gear shifting speed on the third gear side during the shift-up coasting state from the start of the second / third gear shift-up to the end of the second / third gear shift-up. 【0213】 Figure 11 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to a fourth modified example of this embodiment. 【0214】 The fourth modification described above differs from this embodiment in its control method when shifting down from the forward third gear region F3 to the forward second gear region F2. 【0215】 That is, as shown in Figure 11, in the fourth modified example, when the gear shift operating member 90 is shifted down from the forward third gear range to the forward second gear range, the control device 100, - At the start of the third / second gear downshift, when the drive rotational power reaches a predetermined third / second gear downshift start speed, the output-side third clutch mechanism 80c is switched from the engaged state to the disengaged state. - At the midpoint of the third / second gear downshift, after a predetermined time has elapsed from the start of the third / second gear downshift, the forward clutch mechanism 410F is transitioned from the disengaged state to the engaged state. - By transitioning the output-side second clutch mechanism 80b from the disengaged state to the engaged state at the end of the third / second gear downshift, after a predetermined time has elapsed from the midpoint of the third / second gear downshift, From the start of the third / second gear downshift to the end of the third / second gear downshift, a downshift coasting state is created in which the power transmission path to the aforementioned drive output shaft 47 is interrupted. 【0216】 Furthermore, the control device 100 shifts the HST 10 via the HST shift actuator 150 during the downshift coasting state so that the rotational speed of the travel output shaft 47, which is rotationally driven via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F, matches or approaches the rotational speed of the travel output shaft 47 immediately before the output-side second transmission mechanism 70b transitions to the engaged state, at the time the downshift to the third / second gear is completed. 【0217】 In the fourth modified example shown in Figure 11, the HST output is shifted from the third / second gear shift switching speed on the third gear side to the third / second gear shift switching speed on the second gear side (the first HST speed in this example) during the downshift coasting state from the start of the third / second gear downshift to the end of the third / second gear downshift. 【0218】 According to the fourth modified example, the transition of the output-side second clutch mechanism 80b from the disengaged state to the engaged state during a downshift from the forward-side third gear to the forward-side second gear can be performed with a reduced speed difference between the drive side and the driven side of the output-side second clutch mechanism 80b. 【0219】 Therefore, when shifting down from the third forward gear to the second forward gear, a large reverse torque is prevented from being transmitted from the drive output shaft 47 to the main drive shaft 212, thereby effectively preventing engine stall of the drive source 210 due to overload. 【0220】 Figure 12 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to a fifth modified example of this embodiment. 【0221】 As shown in Figure 12, the fifth modified example differs from the fourth modified example only in that the transition of the output-side second clutch mechanism 80b from the disengaged state to the engaged state is performed gradually while sliding. 【0222】 Figure 13 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the sixth modified example of this embodiment. 【0223】 The sixth modification has a different control method from the control methods in the fourth and fifth modifications when shifting down from the forward third gear region F3 to the forward second gear region F2. 【0224】 That is, as shown in Figure 13, in the sixth modified example, when the gear shift operating member 90 is shifted down from the forward third gear range to the forward second gear range, the control device 100, - At the start of the third / second gear downshift, when the drive rotational power reaches a predetermined third / second gear downshift start speed, the output-side third clutch mechanism 80c is switched from the engaged state to the disengaged state. - At the midpoint of the third / second gear downshift, after a predetermined time has elapsed from the start of the third / second gear downshift, the output-side second clutch mechanism 80b is transitioned from the disengaged state to the engaged state. - By transitioning the forward clutch mechanism 410F from the disengaged state to the engaged state at the end of the third / second gear downshift, after a predetermined time has elapsed from the midpoint of the third / second gear downshift, From the start of the third / second gear downshift to the end of the third / second gear downshift, a downshift coasting state is created in which the power transmission path to the aforementioned drive output shaft 47 is interrupted. 【0225】 The control device 100 shifts the HST 10 via the HST shift actuator 150 between the time the output side third clutch mechanism 80c transitions from the engaged state to the engaged state (i.e., the start of the third / second gear downshift) and the time the output side second clutch mechanism 80b transitions from the engaged state to the engaged state (i.e., the intermediate point in the third / second gear downshift), so that the rotational speed of the gear shift output shaft 45 due to the rotational power transmitted via the output side second transmission mechanism 70b at the time the output side second clutch mechanism 80c transitions from the engaged state to the engaged state (i.e., the intermediate point in the third / second gear downshift). 【0226】 Furthermore, the control device 100 shifts the HST 10 via the HST shift actuator 150 between the time the output-side second clutch mechanism 80b transitions from the disengaged state to the engaged state and the time the forward-side clutch mechanism 410F transitions from the disengaged state to the engaged state (i.e., the time when the third / second gear downshift is completed), so that the rotational speed of the travel output shaft 47, which is rotationally driven via the output-side second transmission mechanism 70b and the forward-side transmission mechanism 400F, matches or approaches the rotational speed of the travel output shaft 47 immediately before the forward-side clutch mechanism 400F transitions to the engaged state. 【0227】 In more detail, since the output-side second clutch mechanism 80b is in a disengaged state when the forward-side third gear is engaged, theoretically, rotational power from the drive source 210 is not transmitted to the gear shift output shaft 45. 【0228】 However, for lubrication and cooling purposes, oil is stored in the transmission case 500 that houses the transmission structure, and even when the output-side second clutch mechanism 80b is disengaged, rotational power is actually transmitted from the drive side to the driven side of the output-side second clutch mechanism 80b via the stored oil, causing the gear shift output shaft 45 to rotate. 【0229】 Based on this, in the sixth modified example, the HST10 is shifted such that, at the point when the forward clutch mechanism 410F remains in the disengaged state and only the output-side second clutch mechanism 80b transitions from the disengaged state to the engaged state (midway through the third / second gear downshift), the rotational speed of the gear shift output shaft driven via the output-side second transmission mechanism 70b matches or approaches the rotational speed of the gear shift output shaft 45 driven via the reservoir oil at that point (hereinafter referred to as the first gear shift). Subsequently, at the point when the forward clutch mechanism 410F transitions from the disengaged state to the engaged state (end of the third / second gear downshift), the rotational speed of the travel output shaft 47 driven via the output-side second transmission mechanism 70b and the forward transmission mechanism 400F matches or approaches the rotational speed of the travel output shaft 47 immediately before the forward clutch mechanism 400F transitions to the engaged state (hereinafter referred to as the second gear shift). 【0230】 Therefore, when shifting down from the third forward gear to the second forward gear, a large reverse torque is prevented from being transmitted from the drive output shaft 47 to the main drive shaft 212, thereby effectively preventing engine stall of the drive source due to overload. 【0231】 As shown in Figure 13, in the sixth modified example, the first gear shift is configured to shift the HST output from the third / second gear switching speed on the third gear side to the third / second gear switching intermediate speed, and the second gear shift is configured to shift the HST output from the second third / second gear switching intermediate speed to the second gear side to the third / second gear switching speed. 【0232】 Figure 14 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the seventh modified example of this embodiment. 【0233】 As shown in Figure 14, the seventh modified example differs from the sixth modified example only in that the transition from the disengaged state to the engaged state of the output-side second clutch mechanism 80b occurs instantaneously. 【0234】 Figure 15 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is operated to increase the vehicle speed from zero speed to the forward direction in the transmission structure according to the eighth modified example of this embodiment. 【0235】 The eighth modification modifies the HST shift control when shifting up from the second forward gear to the third forward gear, compared to the second modification. 【0236】 In the second modified example (see Figure 9), the control device 100 performs HST shift control during the coasting period from the start of the second / third gear shift up to the end of the second / third gear shift up, using the rotational speed that the HST 10 should output at the end of the second / third gear shift up (i.e., the second / third gear shifting speed on the third gear side) as the target rotational speed. 【0237】 In contrast, in the eighth modified example, as shown in Figure 15, the control device 100 controls the HST shift when shifting up from the forward second gear to the forward third gear, and during the start period from the start of the second / third gear shift up until a predetermined time has elapsed, the actual HST at the start of the second / third gear shift up is determined from the changed shift gear switching speed that the HST 10 should output at the end of the second / third gear shift up (i.e., the second / third gear switching speed on the third gear side). The initial period target speed, which is increased in the opposite direction to the output rotational speed, is used as the target rotational speed, and after the end of the initial period, the HST output rotational speed is used as the target rotational speed for HST shift control, such that the rotational speed of the travel output shaft 47, which is rotationally driven via the output side third transmission mechanism 70c at the time the output side third clutch mechanism 80c is engaged, matches the rotational speed of the travel output shaft 47 immediately before the output side third clutch mechanism 80c is engaged. 【0238】 According to this eighth modification, the HST10 can quickly shift gears when shifting up from the second forward gear to the third forward gear. 【0239】 In the eighth modified example described above, the target speed for the start period is set to the HST speed (in this example, the second HST speed) that is on the opposite side of the actual HST output rotational speed at the start of the shift stage change, based on the shift stage switching speed after the change, among the first and second HST speeds. 【0240】 Figure 16 is a graph showing the relationship between the passage of time and the rotational speed of the drive rotational power of the transmission structure, the rotational speed of the HST output, and the hydraulic pressure of the clutch mechanism when the gear shifting operating member 90 is decelerated from the forward third gear region to the forward second gear region and then to the forward first gear region in a transmission structure according to the ninth modified example of this embodiment. 【0241】 The ninth modification modifies the HST shift control when shifting up from the third forward gear to the second forward gear, compared to the fifth modification. 【0242】 In the fifth modified example (see Figure 12), the control device 100 performs HST shift control during the coasting period from the start of the third / second gear downshift to the end of the third / second gear downshift, using the rotational speed that the HST 10 should output at the end of the third / second gear upshift (i.e., the third / second gear switching speed on the second gear side, or the first HST speed in Figure 12) as the target rotational speed. 【0243】 In contrast, in the ninth modified example described above, as shown in Figure 16, the control device 100 controls the HST shift when shifting down from the forward third gear to the forward second gear, and during the start period from the start of the third / second gear shift up until a predetermined time has elapsed, it controls the actual HST output rotation at the start of the third / second gear shift down from the changed shift gear switching speed that the HST 10 should output at the end of the third / second gear shift down (i.e., the third / second gear switching speed on the second gear side). The target rotational speed is set to the initial target speed which is increased in the opposite direction to the speed, and after the end of the initial period, the HST output rotational speed is set to match the rotational speed of the travel output shaft 47, which is rotationally driven via the output second transmission mechanism 70b and the forward transmission mechanism 400F at the time the output second clutch mechanism 80b is engaged, to the rotational speed of the travel output shaft 47 immediately before the output second clutch mechanism 80b is engaged, and the HST output rotational speed is set to the target rotational speed. According to this ninth modification, the HST10 can quickly shift gears when downshifting from the forward third gear to the forward second gear. 【0244】 In the ninth modified example described above, the target speed for the start period is set to the HST speed (in this example, the first HST speed) that is on the opposite side of the actual HST output rotational speed at the start of the shift stage change, based on the shift stage switching speed after the change, among the first and second HST speeds. 【0245】 Naturally, the HST shift control according to the eighth and ninth modified examples can also be applied to this embodiment and other modified examples. 【0246】 The installation structure of the transmission structure 1 will be described below. In this embodiment, the HST10, the planetary gear mechanism 30, the input-side first and second transmission mechanisms 50a and 50b, the input-side first and second clutch mechanisms 60a and 60b, the output-side first and second transmission mechanisms 70a and 70b, the output-side first and second clutch mechanisms 80a and 80b, the transmission intermediate shaft 43, the transmission output shaft 45, and the travel output shaft 47 are housed in the transmission case 500 of the work vehicle 1. 【0247】 More specifically, as shown in Figure 3, the mission case 500 has a hollow case body 510. In this embodiment, the case body 510 has first and second cases 512 and 515 arranged in series along the longitudinal direction. 【0248】 The transmission case 500 includes a first bearing plate 520 that divides the internal space of the case body 510 into front and rear sections, and a second bearing plate 530 that divides the internal space of the case body 510 into front and rear sections, located rearward from the first bearing plate 520 in the vehicle's longitudinal direction. 【0249】 In other words, the internal space of the case body 510 is divided by the first and second bearing plates 520 and 530 into a front chamber 510F located in front of the first bearing plate 520, a middle chamber 510M sandwiched between the first and second bearing plates 520 and 530 in the front-rear direction, and a rear chamber 510R located behind the second bearing plate 530. 【0250】 As shown in Figure 3, in this embodiment, the first bearing plate 520 is detachably connected to the first case 512 via fastening members such as bolts near the rear opening of the first case 512, while the second bearing plate 530 is detachably connected to the second case 515 via fastening members such as bolts near the front opening of the second case 515. 【0251】 As shown in Figures 1 and 3, the main drive shaft 212, the intermediate gear shift shaft 43, the gear shift output shaft 45, and the travel output shaft 47 are supported by the first and second bearing plates 520 and 530 so as to be rotatable around their axes in a manner aligned with the longitudinal direction of the vehicle. 【0252】 As shown in Figure 3, the input-side first and second clutch mechanisms 60a and 60b are supported by the main drive shaft 212 while being located in the middle chamber 510M. 【0253】 The input-side first and second transmission mechanisms 50a and 50b are supported by the main drive shaft 212 and the speed-shifting intermediate shaft 43 while positioned in the middle chamber 510M. 【0254】 As shown in Figure 3, the transmission case 500 has a third bearing plate 540 provided on the case body 510 behind the second bearing plate 530. In this embodiment, the third bearing plate 540 is integrally formed with the case body 510 (the second case 515). 【0255】 The output-side first and second clutch mechanisms 80a and 80b are supported by the transmission output shaft 45 while being positioned in the middle chamber 510M. 【0256】 The output-side first and second transmission mechanisms 70a and 70b are supported by the transmission intermediate shaft 43 and the transmission output shaft 45 while being positioned in the middle chamber 510M. 【0257】 The output-side third clutch mechanism 80c is supported by the travel output shaft 47 while being located in the middle chamber 510M. 【0258】 The output-side third transmission mechanism 70c is located in the middle chamber 510M and is supported by the gear shift output shaft 45 and the travel output shaft 47. 【0259】 As shown in Fig. 3, the transmission output shaft 45 and the traveling output shaft 47 extend rearward beyond the second bearing plate 530 at the rear end side and are supported by the third bearing plate 540 so as to be rotatable about the axis. 【0260】 That is, the transmission output shaft 45 and the traveling output shaft 47 are supported by the first to third bearing plates 520, 530, and 540 in a state of extending across the middle chamber 510M and the rear chamber 510R. 【0261】 The forward clutch mechanism 410F and the reverse clutch mechanism 410R are supported by a portion of the traveling output shaft 47 located within the rear chamber 510R. 【0262】 The forward transmission mechanism 400F and the reverse transmission mechanism 400R are supported by a portion of the transmission output shaft 45 and the traveling output shaft 47 located within the rear chamber 510R. 【0263】 Fig. 17 shows a cross-sectional view taken along line XVII-XVII in Fig. 3. Fig. 18 shows a transverse developed cross-sectional view of the transmission structure 1. 【0264】 As shown in Figs. 3, 17, and 18, the planetary gear mechanism 30 is disposed within the front chamber 510F coaxially with the transmission intermediate shaft 43. 【0265】 Specifically, in addition to the sun gear 32, the planetary gear 34, the carrier 38, and the internal gear 36, the planetary gear mechanism 30 has a carrier pin 35 that supports the planetary gear 34 so as to be rotatable about the axis. 【0266】 As shown in Figs. 17 and 18, the front end of the transmission intermediate shaft 43 penetrates the first bearing plate 520 and extends into the front chamber 510F, and the carrier 38 is supported by the transmission intermediate shaft 43. 【0267】 In other words, the carrier 38 has a flange portion 38a that supports the carrier pin 35 and a shaft portion 38b that is externally fitted to the front end of the gear shift intermediate shaft 43 so as not to rotate relative to the axis. 【0268】 As shown in Figure 17, the internal gear 36 has a ring gear portion 36a that meshes with the planetary gear 34, and a cylindrical portion 36b that is externally fitted to the shaft portion of the input-side first driven gear 54a, which is supported to rotate relative to the speed-shifting intermediate shaft 43, so as not to rotate relative to the axis. 【0269】 In this embodiment, as shown in Figure 18, the HST10 has the pump shaft 12 and the motor shaft 16 arranged coaxially. 【0270】 More specifically, the HST10 includes, in addition to the HST pump 14, the pump shaft 12, the HST motor 18, the motor shaft 16, and the output adjustment member 20, a center section 450 on which the pair of HST lines 15a and 15b are formed, a pump case 460 connected to the center section 450 to form a pump space for housing the HST pump 14, and a motor case 470 connected to the center section 450 to form a motor space for housing the HST motor 18. 【0271】 As shown in Figure 18, the center section 450 has a first surface 451 that faces a first direction (towards the front of the vehicle in the illustrated configuration) and includes a pump surface to which the HST pump 14 is slidably contacted, and a second surface 452 that faces a second direction opposite to the first direction (towards the rear of the vehicle in the illustrated configuration) and includes a motor surface to which the HST motor 18 is slidably contacted. 【0272】 The pump case 460 has a hollow peripheral wall portion 462 surrounding the HST pump 14 and an end wall portion 464 that closes the opening at the tip of the peripheral wall portion 462, and is connected to the center section 450 with the base end face of the peripheral wall portion 462 in contact with the first surface 451. 【0273】 The pump shaft 12 is supported at its base end by a bearing in a bearing hole 455 formed in the center section 450, and its tip is supported directly or indirectly by a bearing in the end wall 464 of the pump case 460, extending outward from the end wall 464. 【0274】 In this embodiment, the HST input gear 215 that forms the HST input gear train 214 is supported so as not to rotate relative to the tip of the pump shaft 12 in the portion extending outward from the end wall portion 464, and the shaft portion of the HST input gear 215 is supported by the end wall portion 464 via a bearing. 【0275】 The motor case 470 has a hollow peripheral wall portion 472 surrounding the HST motor 18 and an end wall portion 474 that closes the opening at the tip of the peripheral wall portion 472, and is connected to the center section 450 with the base end face of the peripheral wall portion 472 in contact with the second surface 452. 【0276】 The motor shaft 16 has its base end supported by a bearing in the bearing hole 455, and its tip is supported by a bearing directly or indirectly by the end wall 474 of the motor case 470, extending outward from the end wall 474. 【0277】 In this embodiment, the HST output first gear 217a, which forms the HST output gear train 216, is supported in a manner that prevents relative rotation in the portion of the motor shaft 16 that extends outward from the end wall portion 474, and the shaft portion of the HST output first gear 217a is supported by the end wall portion 474 via a bearing. 【0278】 The HST output gear train 216 includes the HST output first gear 217a, the HST output second gear 217b which meshes with the HST output first gear 217a, and the HST output shaft 218 which supports the HST output second gear 217b in a manner that prevents relative rotation. 【0279】 The HST output shaft 218 supports the sun gear 32 at its tip in such a way that it cannot rotate relative to its axis. 【0280】 The motor case 470 is provided with an extended portion 476 that extends outward from the peripheral wall portion 472 or the end wall portion 474 and supports the base end of the HST output shaft 218 so as to be rotatable around its axis via a bearing member. 【0281】 In this embodiment, as shown in Figure 18, the case body 510 of the transmission case 500 is provided with a mounting opening 511 through which an HST assembly, which comprises the center section 450, the pump shaft 12, the HST pump 14, the pump case 460, the motor shaft 16, the HST motor 18, the motor case 470, the HST input gear 215, and the HST output gear train 216, can be inserted. 【0282】 In this embodiment, the mounting opening 511 is formed in the first case 512 so as to open the front chamber 510F to the outside. 【0283】 The HST assembly is connected to a mounting plate 550 which is attached to the outer surface of the case body 510 while closing the mounting opening 511. 【0284】 In other words, the HST assembly is fixed within the front chamber 510F by attaching the mounting plate 550, which is connected to the HST assembly, to the outer surface of the case body 510. 【0285】 As shown in Figure 18, in this embodiment, the pump case 460 and the center section 450 are connected to the mounting plate 550. 【0286】 In other words, the pump case 460 is provided with a connecting portion 466 that extends outward from the peripheral wall portion 462 and the end wall 464 and abuts against the mounting plate 550. Further, the center section 450 is configured such that a third surface 453 orthogonal to both the first surface 451 and the second surface 452 abuts against the mounting plate 550. 【0287】 As shown in FIG. 18, the servo piston 155 is accommodated in a connecting portion 466 of the pump case 460. In the present embodiment, the servo switching valve 162 is accommodated in an axial hole formed in the servo piston 155, and the mounting plate 550 accommodates the operation piston 164 operatively connected to the servo switching valve 162 and an output adjustment valve 165 for adjusting pressure oil supplied to the operation piston 164. 【0288】 The transmission structure 1 according to the present embodiment further has a lubricating oil supply mechanism 600 that supplies lubricating oil to various components. 【0289】 The lubricating oil supply mechanism 600 is configured to use oil from a hydraulic source different from the hydraulic source (the first hydraulic pump 110) of the hydraulic oil line 120 that supplies pressure oil to the HST 10 (and the hydraulic servo mechanism 152) and the clutch mechanisms 60a - 60b, 80a - 80c, 410F, 410R. 【0290】 Thus, by separating the hydraulic oil line 120 of the HST 10 and the clutch mechanisms 60a - 60b, 80a - 80c, 410F, 410R from the lubricating oil line 620, the hydraulic pressure fluctuations in the hydraulic oil line 120 can be effectively suppressed. 【0291】 More specifically, as shown in Figure 2, the lubrication oil supply mechanism 600 includes a second hydraulic pump 610 operatively driven by the drive source 210, a lubrication oil line 620 to which discharge oil from the second hydraulic pump 610 is supplied, a transmission rotation shaft lubrication line 630 whose base end is fluidly connected to the lubrication oil line 620 and which guides lubrication oil toward the transmission rotation shaft and the planetary gear mechanism 30 to which power is transmitted by the plurality of transmission mechanisms 50a-50b, 70a-70c, 400F, and 400R, and an HST lubrication line 660 (see Figure 4) whose base end is fluidly connected to the lubrication oil line 620 and which guides lubrication oil toward the HST 10. 【0292】 In this embodiment, the main drive shaft 212, the intermediate gear shift shaft 43, the gear shift output shaft 45, and the travel output shaft 47 are included in the transmission rotation shaft. 【0293】 In this case, as shown in Figures 2, 3 and 17, the lubrication oil supply mechanism 600 further includes a main drive shaft hole 642 formed along the axial direction of the main drive shaft 212 to receive oil from the transmission rotation shaft lubrication line 630, a transmission intermediate shaft hole 644 formed along the axial direction of the transmission intermediate shaft 43 to receive oil from the transmission rotation shaft lubrication line 630, a transmission output shaft hole 646 formed along the axial direction of the transmission output shaft 45 to receive oil from the transmission rotation shaft lubrication line 630, and a travel output shaft hole 648 formed along the axial direction of the travel output shaft 47 to receive oil from the transmission rotation shaft lubrication line 630. 【0294】 The main drive shaft hole 642, the intermediate gear shift shaft hole 644, the gear shift output shaft hole 646, and the travel output shaft hole 648 are each opened on the outer surface at positions corresponding to predetermined lubrication points. 【0295】 As shown in Figure 17, in this embodiment, the intermediate gear shift shaft 43 and the HST output shaft 218 are arranged opposite each other on the same axis, and the HST output shaft 218 has an HST output shaft hole 645 that takes over a portion of the oil flowing through the intermediate gear shift shaft hole 644. 【0296】 More specifically, the intermediate shaft hole 644 for the gear shift is opened on the end face facing the HST output shaft 218. The HST output shaft hole 645 is opened on the end face facing the transmission intermediate shaft 43 so that it can receive oil from the transmission intermediate shaft hole 644, and is further opened on the outer surface at a position corresponding to a predetermined lubrication part so that the oil received from the transmission intermediate shaft hole 644 can be guided to a predetermined lubrication part. 【0297】 As shown in Figures 3 and 17, one of the first and second bearing plates 520 and 530 (the second bearing plate 530 in this embodiment) has a transmission rotating shaft lubrication oil passage 632 that forms the transmission rotating shaft lubrication line 630, and oil transfer portions 642a, 644a, 646a, and 648a that transfer oil from the transmission rotating shaft lubrication oil passage 632 to the main drive shaft hole 642, the variable speed intermediate shaft hole 644, the variable speed output shaft hole 646, and the travel output shaft hole 648, respectively. 【0298】 Furthermore, in this embodiment, the transfer of oil in the supply and discharge lines 320a-320b, 330a-330c, 350F, and 350R is also performed on one of the bearing plates (the second bearing plate 530 in this embodiment). 【0299】 More specifically, the input-side first supply and discharge line 320a includes an input-side first supply and discharge oil passage (not shown) formed in one of the bearing plates (the second bearing plate 530 in this embodiment) and an input-side first axial oil passage 321a (see Figure 3) formed along the axial direction of the main drive shaft 212. 【0300】 The input-side first axial oil passage 321a has its base end opening to the outer surface of the portion of the main drive shaft 212 that is supported by one of the bearing plates (the second bearing plate 530 in this embodiment), and is fluidly connected to the input-side first oil supply and discharge passage via the oil transfer portion 322a, while its tip end opens to the oil chamber of the corresponding clutch mechanism 60a. 【0301】 The input-side second supply and discharge line 320b includes an input-side second supply and discharge oil passage (not shown) formed in one of the bearing plates (the second bearing plate 530 in this embodiment) and an input-side second axial oil passage 321b (see Figure 3) formed along the axial direction of the main drive shaft 212. 【0302】 The input-side second axial oil passage 321b has its base end opening to the outer surface of the portion of the main drive shaft 212 that is supported by one of the bearing plates (the second bearing plate 530 in this embodiment), and is fluidly connected to the input-side second oil supply and discharge passage via the oil transfer portion 322b, while its tip end opens to the oil chamber of the corresponding clutch mechanism 60b. 【0303】 The output-side first supply and discharge line 330a includes an output-side first supply and discharge oil passage (not shown) formed in one of the bearing plates (the second bearing plate 530 in this embodiment) and an output-side first axial oil passage 331a (see Figure 3) formed along the axial direction of the speed-shifting output shaft 45. 【0304】 The output-side first axial oil passage 331a has its base end opening to the outer surface of the portion of the speed-shifting output shaft 45 that is supported by one of the bearing plates (the second bearing plate 530 in this embodiment), and is fluidly connected to the output-side first oil supply and discharge passage via an oil transfer portion 322a, while its tip end opens to the oil chamber of the corresponding clutch mechanism 80a. 【0305】 The output-side second supply and discharge line 330b includes an output-side second supply and discharge oil passage (not shown) formed in one of the bearing plates (the second bearing plate 530 in this embodiment) and an output-side second axial oil passage 331b (see Figure 3) formed along the axial direction of the speed-shifting output shaft 45. 【0306】 The output-side second shaft oil passage 331b has its base end open to the outer surface of the portion of the speed-shifting output shaft 45 that is supported by one of the bearing plates (the second bearing plate 530 in this embodiment), and is fluidly connected to the output-side second oil supply and discharge passage via the oil transfer portion 332b, and its tip end opens to the oil chamber of the corresponding clutch mechanism 80b. 【0307】 The forward-side supply and discharge line 350F includes a forward-side supply and discharge oil passage (not shown) formed in one of the bearing plates (the second bearing plate 530 in this embodiment) and a forward-side axial oil passage 351F (see Figure 3) formed along the axial direction of the travel output shaft 47. 【0308】 The forward-side shaft oil passage 351F has its base end open to the outer surface of the portion of the travel output shaft 47 that is supported by one of the bearing plates (the second bearing plate 530 in this embodiment), and is fluidly connected to the forward-side oil supply and discharge passage via the oil transfer portion 352F, and its tip end opens to the oil chamber of the corresponding clutch mechanism 410F. 【0309】 The reverse side supply and discharge line 350R has a reverse side supply and discharge oil passage (not shown) formed in one of the bearing plates (the second bearing plate 530 in this embodiment) and a reverse side axial oil passage 351R (see Figure 3) formed along the axial direction of the travel output shaft 47. 【0310】 The reverse-side shaft oil passage 351R has its base end open to the outer surface of the portion of the travel output shaft 47 that is supported by one of the bearing plates (the second bearing plate 530 in this embodiment), and is fluidly connected to the reverse-side oil supply and discharge passage via the oil transfer portion 352R, and its tip end opens to the oil chamber of the corresponding clutch mechanism 410R. 【0311】 The output-side third supply and discharge line 330c has an output-side third supply and discharge oil passage (not shown) formed in one of the bearing plates (the second bearing plate 530 in this embodiment) and an output-side third axial oil passage 331c (see Figure 3) formed along the axial direction of the traveling output shaft 47. 【0312】 The output-side third axis oil passage 331c has its base end opening to the outer surface of the portion of the traveling output shaft 47 that is supported by one of the bearing plates (the second bearing plate 530 in this embodiment), and is fluidly connected to the output-side third oil supply and discharge passage via an oil transfer portion 332c, while its tip end opens to the oil chamber of the corresponding clutch mechanism 80c. 【0313】 Figure 19 shows a cross-sectional view along the line XIX-XIX in Figure 18. As shown in Figures 18 and 19, the HST lubrication line 660 is an HST lubrication oil passage 662 formed in the mounting plate 550 and the center section 450, with one end of the HST lubrication oil passage 662 opening to the outer surface of the mounting plate 550 to form an HST lubrication port IN2. The end of the lubrication oil line 620 shown in Figure 2 is fluidly connected to the HST lubrication port IN2. 【0314】 As shown in Figure 18, the base ends (opposing ends) of the pump shaft 12 and the motor shaft 16 are supported in the axial hole 455 with a gap between them, and the HST lubrication oil passage 662 is formed to guide a portion of the oil into the gap. 【0315】 The HST lubrication line 660 further includes pump shaft holes 663 and motor shaft holes 664 formed axially in the pump shaft 12 and motor shaft 16, respectively, to guide the oil in the gap to a predetermined lubrication area, and oil passages 665 formed in the center section 450, the pump case 460 and the motor case 470 to guide a portion of the oil in the HST lubrication oil passage 662 to a predetermined lubrication area. 【0316】 The lubrication oil supply mechanism 600 has a lubrication oil pressure setting relief valve 625 that sets the oil pressure of the lubrication oil line 620. In this embodiment, the lubrication hydraulic pressure setting relief valve 625 is provided on the mounting plate 550 so as to act on the HST lubrication oil passage 662. [Explanation of symbols] 【0317】 1. Transmission structure 10 HST 30 Planetary gear mechanism 45. Transmission output shaft 47. Drive output shaft 50a, 50b Input side first and second transmission mechanisms 60a, 60b Input-side first and second clutch mechanisms 70a~70c Output side 1st~3rd transmission mechanism 80a~80c Output side 1st to 3rd clutch mechanism 90 Gear shifting operating member 100 Control device 152 Hydraulic servo mechanism (HST transmission actuator) 210 Power source 212 Main drive shaft 325a, 325b Input side first and second solenoid valves 335a~335c Output side 1st to 3rd solenoid valves 355F, 355R Forward and Reverse Solenoid Valves 400F, 400R Forward and Reverse Transmission Mechanisms 410F, 410R Forward and Reverse Clutch Mechanisms

Claims

[Claim 1] A transmission structure that takes rotational power input operationally from a drive source, continuously variable speed control, and outputs it as driving rotational power to the drive wheels, An HST that outputs rotational power, which is operatively input from the drive source, by steplessly changing the speed to at least between the first HST speed and the second HST speed according to the operating position of the output adjustment member, The HST transmission actuator that operates the output adjustment member, A planetary gear mechanism having first to third elements, wherein the third element acts as an input to the HST output, An input-side first transmission mechanism capable of transmitting the rotational power of the drive source to the first element at an input-side first gear ratio, An input-side second transmission mechanism capable of transmitting the rotational power of the drive source to the second element at an input-side second gear ratio, A pair of input-side clutch mechanisms, including friction plate type input-side first and second clutch mechanisms for engaging and disengaging power transmission between the input-side first and second transmission mechanisms, The gear shift output shaft, A first output-side transmission mechanism capable of transmitting the rotational power of the second element to the gear shift output shaft at the output-side first gear ratio, A second output transmission mechanism capable of transmitting the rotational power of the first element to the variable speed output shaft at a second output gear ratio, A pair of output-side clutch mechanisms, including friction plate type output-side first and second clutch mechanisms for engaging and disengaging power transmission between the output-side first and second transmission mechanisms, A gear shift operating member that can be operated at zero vehicle speed, in the first gear range, and in the second gear range which is faster than the first gear range, The system includes a control device that receives commands from the gear shift operating member and controls the operation of the HST gear shift actuator, the input clutch mechanism, and the output clutch mechanism. The control device, when the gear shift operating member is positioned at zero vehicle speed and in the first gear range, sets the input-side first and second clutch mechanisms to engaged and disengaged, respectively, thereby creating a first HMT transmission state in which the first element acts as a reference power input unit that receives reference rotational power from the drive source and the second element acts as a combined power output unit that outputs combined rotational power toward the gear shift output shaft, while simultaneously setting the output-side first and second clutch mechanisms to engaged and disengaged, respectively, thereby transmitting the rotational power of the second element to the gear shift output shaft. On the other hand, when the gear shift operating member is positioned in the second gear range, the control device, when the input-side first and second clutch mechanisms are set to disengaged and engaged, respectively, thereby creating a second HMT transmission state in which the second element acts as the reference power input unit and the first element acts as the combined power output unit, while simultaneously setting the output-side first and second clutch mechanisms to disengaged and engaged, respectively, thereby transmitting the rotational power of the first element to the gear shift output shaft. The planetary gear mechanism is configured such that when the HST output is set to the first HST speed in the first HMT transmission state, the output rotational power of the second element becomes zero speed. The input-side first and second gear ratios are set such that the rotational speed of the second element when the HST output is set to the second HST speed in the first HMT transmission state is the same as the rotational speed of the second element due to the rotational power transmitted via the input-side second transmission mechanism in the second HMT transmission state, and the rotational speed of the first element when the HST output is set to the second HST speed in the second HMT transmission state is the same as the rotational speed of the first element due to the rotational power transmitted via the input-side first transmission mechanism in the first HMT transmission state. The control device further, The operation of the HST shift actuator is controlled such that the HST output becomes the first HST speed in response to the operation of the shift operating member to the zero vehicle speed position, and the rotational speed of the drive rotational power increases in response to the speed increase operation of the shift operating member from the zero vehicle speed position. When the gear shifting operating member is operated to shift up from the first gear range to the second gear range, at the start of the first / second gear shift when the drive rotational power reaches a predetermined first / second gear shift-up start speed, the second clutch mechanism in one of the clutch mechanism pairs of the input-side clutch mechanism pair and the output-side clutch mechanism pair is instantaneously switched from the disengaged state to the engaged state, and at the end of the first / second gear shift after a predetermined time has elapsed from the start of the first / second gear shift-up, the first clutch mechanism in the one clutch mechanism pair is instantaneously switched from the engaged state to the disengaged state, from the start of the first / second gear shift-up to the end of the shift-up. A transmission structure characterized in that, in between, a shift-up double transmission state is created in which both the first and second clutch mechanisms of one of the clutch mechanism pairs are engaged, and during the shift-up double transmission state, the first clutch mechanism of the other clutch mechanism pair of the input clutch mechanism pair and the output clutch mechanism pair is moved from an engaged state to a disengaged state by sliding the friction plates, and the second clutch mechanism of the other clutch mechanism pair is moved from a disengaged state to an engaged state by sliding the friction plates, thereby switching from the engaged state of the first clutch mechanism of the other clutch mechanism pair to the engaged state of the second clutch mechanism. [Claim 2] The transmission structure according to claim 1, characterized in that the starting speed for shifting up to the first / second gear is the speed of the drive rotational power that is manifested on the gear shift output shaft via the output-side first transmission mechanism when the HST output is set to the second HST speed in the first HMT transmission state. [Claim 3] The input-side first and second clutch mechanisms and the output-side first and second clutch mechanisms are hydraulic, and their engaged and disengaged states are switched by supplying and discharging hydraulic fluid. The transmission structure includes an input-side solenoid valve pair, which includes input-side first and second solenoid valves that switch the supply and discharge of hydraulic fluid to the input-side first and second clutch mechanisms, respectively, in accordance with the operation control by the control device, and an output-side solenoid valve pair, which includes output-side first and second solenoid valves that switch the supply and discharge of hydraulic fluid to the output-side first and second clutch mechanisms, respectively, in accordance with the operation control by the control device. The transmission structure according to claim 1 or 2, characterized in that one of the input-side solenoid valve pair and the output-side solenoid valve pair is an electromagnetic proportional valve capable of gradually increasing and decreasing the hydraulic pressure of the corresponding clutch mechanism, and the other of the input-side solenoid valve pair and the output-side solenoid valve pair is an electromagnetic switching valve that instantly increases or decreases the hydraulic pressure of the corresponding clutch mechanism. [Claim 4] The transmission structure according to claim 1 or 2, characterized in that the output-side first and second gear ratios are set such that the rotational speed that appears on the gear shift output shaft when the HST output is set to the second HST speed in the first HMT transmission state is substantially the same as the rotational speed that appears on the gear shift output shaft when the HST output is set to the second HST speed in the second HMT transmission state. [Claim 5] The transmission structure according to claim 1 or 2, characterized in that the control device starts the switching operation of the first clutch mechanism in the other clutch mechanism pair, which moves the first clutch mechanism from an engaged state to an engaged state while sliding the friction plates, and the switching operation of the second clutch mechanism in the other clutch mechanism pair, which moves the second clutch mechanism from an engaged state to an engaged state while sliding the friction plates, before the start of the first / second gear shift up and after the end of the first / second gear shift up. [Claim 6] When the gear shift operating member is shifted down from the second gear range to the first gear range, the control device instantly switches the first clutch mechanism of one of the clutch mechanism pairs (the input clutch mechanism pair and the output clutch mechanism pair) from the disengaged state to the engaged state at the start of the second / first gear downshift when the drive rotational power reaches a predetermined second / first gear downshift start speed, and instantly switches the second clutch mechanism of the one clutch mechanism pair from the engaged state to the disengaged state at the end of the second / first gear downshift after a predetermined time has elapsed from the start of the second / first gear downshift, between the start of the second / first gear downshift and the end of the downshift. The transmission structure according to claim 1 or 2, wherein a downshift double transmission state is created in which both the first and second clutch mechanisms of one of the clutch mechanism pairs are engaged, and during the downshift double transmission state, the first clutch mechanism of the other clutch mechanism pair of the input clutch mechanism pair and the output clutch mechanism pair is moved from a disengaged state to an engaged state by sliding the friction plates, and the second clutch mechanism of the other clutch mechanism pair is moved from an engaged state to a disengaged state by sliding the friction plates, thereby switching from the engaged state of the second clutch mechanism of the other clutch mechanism pair to the engaged state of the first clutch mechanism. [Claim 7] The transmission structure according to claim 6, characterized in that the starting speed for the second / first gear downshift is the speed of the drive rotational power that is manifested on the gear shift output shaft via the output-side second transmission mechanism when the HST output is set to the second HST speed in the second HMT transmission state. [Claim 8] The transmission structure according to claim 6, characterized in that the control device starts the switching operation of the second clutch mechanism in the other clutch mechanism pair, which moves the second clutch mechanism from an engaged state to a disengaged state while sliding the friction plates, and the switching operation of the first clutch mechanism in the other clutch mechanism pair, which moves the first clutch mechanism from a disengaged state to an engaged state while sliding the friction plates, before the start of the second / first gear downshift and after the end of the second / first gear downshift. [Claim 9] The transmission structure according to claim 6, characterized in that the rotational speed of the driving power that marks the start of the second / first gear downshift and the rotational speed of the driving power that marks the start of the first / second gear upshift are approximately the same. [Claim 10] A drive output shaft that outputs drive rotational power toward the aforementioned drive wheels, A forward-side transmission mechanism and a reverse-side transmission mechanism that transmit the rotational power of the aforementioned speed-shifting output shaft as driving rotational power in the forward and reverse directions to the aforementioned travel output shaft, respectively. It includes a friction plate type forward clutch mechanism and a reverse clutch mechanism for engaging and disengaging power transmission between the forward-side transmission mechanism and the reverse-side transmission mechanism, respectively. The HST is configured to output rotational power in both forward and reverse directions, such that the output of the first HST speed is rotational power in one direction (forward or reverse) and the output of the second HST speed is rotational power in the other direction (forward or reverse). In the planetary gear mechanism, in the first HMT transmission state, the combined rotational power output from the second element increases as the HST output changes from the first HST speed to the second HST speed, and in the second HMT transmission state, the combined rotational power output from the first element increases as the HST output changes from the second HST speed to the first HST speed. The first gear region includes a forward first gear region and a reverse first gear region, and the second gear region includes a forward second gear region that is faster than the forward first gear region and a reverse second gear region that is faster than the reverse first gear region. The transmission structure according to claim 1 or 2, characterized in that the control device causes the forward and reverse clutch mechanisms to be engaged and disengaged, respectively, in response to the forward operation of the gear shift operating member, and causes the forward and reverse clutch mechanisms to be disengaged and engaged, respectively, in response to the reverse operation of the gear shift operating member. [Claim 11] The forward and reverse clutch mechanisms are hydraulic, with the engagement and disengagement states being switched by the supply and discharge of hydraulic fluid. The transmission structure according to claim 10, characterized in that the transmission structure is provided with a pair of forward and reverse switching solenoid valves, including forward and reverse solenoid valves, which switch the supply and discharge of hydraulic fluid to the forward and reverse clutch mechanisms, respectively, in accordance with the operation control by the control device. [Claim 12] A third output-side transmission mechanism capable of transmitting the rotational power of the first element to the travel output shaft as forward-direction driving rotational power, wherein the gear ratio of the third output-side transmission mechanism is set such that the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the third output-side transmission mechanism is faster than the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the second output-side transmission mechanism and the forward-side transmission mechanism, It comprises an output-side third clutch mechanism for engaging and disengaging power transmission of the output-side third transmission mechanism, The gear shift operating member is also operable in the forward third gear range, which is higher in speed than the forward second gear range. The control device is When the gear shift operating member is positioned in the forward third gear range, the second HMT transmission state is activated, the output side first and second clutch mechanisms are released, and the output side third clutch mechanism is engaged, while the operation of the HST gear shift actuator is controlled so that the rotational speed of the drive rotational power becomes the rotational speed corresponding to the operating position of the gear shift operating member. When the gear shifting operating member is operated to shift up from the forward second gear range to the forward third gear range, the output side second clutch mechanism and the forward side clutch mechanism are instantaneously switched from the engaged state to the disengaged state at the start of the second / third gear shift up when the drive rotational power reaches a predetermined second / third gear shift up start speed, and the output side third clutch mechanism is switched from the disengaged state to the engaged state at the end of the second / third gear shift up after a predetermined time has elapsed from the start of the second / third gear shift up, thereby enabling the shifting operation from the start of the second / third gear shift up to the end of the second / third gear shift up. The transmission structure according to claim 10, characterized in that, while creating a shift-up coasting state in which the power transmission path to the drive output shaft is interrupted until the end of the shift-up, the HST is shifted via the HST shift actuator during the shift-up coasting state so that the rotational speed of the drive output shaft, which is rotationally driven via the output-side third transmission mechanism at the end of the second / third gear shift-up, matches or approaches the rotational speed of the drive output shaft, which is rotationally driven via the output-side second transmission mechanism and the forward-side transmission mechanism at the start of the second / third gear shift-up. [Claim 13] A third output-side transmission mechanism capable of transmitting the rotational power of the first element to the travel output shaft as forward-direction driving rotational power, wherein the gear ratio of the third output-side transmission mechanism is set such that the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the third output-side transmission mechanism is faster than the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the second output-side transmission mechanism and the forward-side transmission mechanism, It comprises an output-side third clutch mechanism for engaging and disengaging power transmission of the output-side third transmission mechanism, The gear shift operating member is also operable in the forward third gear range, which is higher in speed than the forward second gear range. The control device is When the gear shift operating member is positioned in the forward third gear range, the second HMT transmission state is activated, the output side first and second clutch mechanisms are released, and the output side third clutch mechanism is engaged, while the operation of the HST gear shift actuator is controlled so that the rotational speed of the drive rotational power becomes the rotational speed corresponding to the operating position of the gear shift operating member. When the gear shifting operating member is operated to shift up from the forward second gear range to the forward third gear range, one of the output second clutch mechanism and the forward clutch mechanism is moved from the engaged state to the disengaged state by sliding, and after a predetermined time has elapsed since one of the output second clutch mechanism and the forward clutch mechanism was disengaged, the other of the output second clutch mechanism and the forward clutch mechanism is moved from the engaged state to the disengaged state, and after a predetermined time has elapsed since the other of the output second clutch mechanism and the forward clutch mechanism was disengaged, the output third clutch mechanism is moved from the disengaged state to the engaged state, thereby disengaging the output second clutch. The transmission structure according to claim 10, characterized in that, while a shift-up coasting state is created in which the power transmission path to the travel output shaft is interrupted between the release state of one of the mechanism and the forward clutch mechanism and the engagement state of the output side third clutch mechanism, the HST is shifted via the HST shift actuator during the shift-up coasting state so that the rotational speed of the travel output shaft, which is rotationally driven via the output side third transmission mechanism at the time the output side third clutch mechanism transitions to the engagement state, matches or approaches the rotational speed of the travel output shaft immediately before the output side third clutch mechanism transitions to the engagement state. [Claim 14] A third output-side transmission mechanism capable of transmitting the rotational power of the first element to the travel output shaft as forward-direction driving rotational power, wherein the gear ratio of the third output-side transmission mechanism is set such that the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the third output-side transmission mechanism is faster than the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the second output-side transmission mechanism and the forward-side transmission mechanism, It comprises an output-side third clutch mechanism for engaging and disengaging power transmission of the output-side third transmission mechanism, The gear shift operating member is also operable in the forward third gear range, which is higher in speed than the forward second gear range. The control device is When the gear shift operating member is positioned in the forward third gear range, the second HMT transmission state is activated, the output side first and second clutch mechanisms are released, and the output side third clutch mechanism is engaged, while the operation of the HST gear shift actuator is controlled so that the rotational speed of the drive rotational power becomes the rotational speed corresponding to the operating position of the gear shift operating member. When the gear shifting operating member is shifted up from the forward second gear range to the forward third gear range, the output second clutch mechanism is moved from the engaged state to the disengaged state while sliding, and the output third clutch mechanism is moved from the disengaged state to the engaged state while sliding, and the forward clutch mechanism is moved from the engaged state to the disengaged state before the output third clutch mechanism is moved to the engaged state, thereby creating a shift-up coasting state in which the power transmission path to the travel output shaft is interrupted between the time the forward clutch mechanism is disengaged and the time the output third clutch mechanism is engaged, and the HST is shifted via the HST shift actuator during the shift-up coasting state so that the rotational speed of the travel output shaft, which is rotationally driven via the output third transmission mechanism at the time the output third clutch mechanism is moved to the engaged state, matches or approaches the rotational speed of the travel output shaft immediately before the output third clutch mechanism is moved to the engaged state. [Claim 15] The output-side third clutch mechanism is a hydraulic type, which switches between engaged and disengaged states by supplying and discharging hydraulic fluid. The transmission structure according to claim 12, characterized in that the transmission structure is provided with an output-side third solenoid valve that switches the supply and discharge of hydraulic fluid to the output-side third clutch mechanism in accordance with the operation control by the control device. [Claim 16] When the gear shift operating member is shifted down from the forward third gear range to the forward second gear range, the control device moves the output side third clutch mechanism from the engaged state to the disengaged state at the start of the third / second gear downshift when the drive rotational power reaches a predetermined third / second gear downshift start speed, and moves the output side second clutch mechanism and the forward side clutch mechanism from the disengaged state to the engaged state at the end of the third / second gear downshift after a predetermined time has elapsed from the start of the third / second gear downshift, thereby enabling the third / second gear downshift from the start of the third / second gear downshift to the third / second gear downshift. The transmission structure according to claim 12, characterized in that, while creating a downshift coasting state in which the power transmission path to the drive output shaft is interrupted until the end of the downshift, the HST is shifted via the HST shift actuator during the downshift coasting state so that the rotational speed of the drive output shaft, which is rotationally driven via the output-side second transmission mechanism and the forward-side transmission mechanism at the end of the third / second gear downshift, matches or approaches the rotational speed of the drive output shaft, which is rotationally driven via the output-side third transmission mechanism at the start of the third / second gear downshift. [Claim 17] The transmission structure according to claim 16, characterized in that the starting speed for shifting down to the third / second gear and the starting speed for shifting up to the second / third gear are approximately the same speed. [Claim 18] A third output-side transmission mechanism capable of transmitting the rotational power of the first element to the travel output shaft as forward-direction driving rotational power, wherein the gear ratio of the third output-side transmission mechanism is set such that the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the third output-side transmission mechanism is faster than the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the second output-side transmission mechanism and the forward-side transmission mechanism, It comprises an output-side third clutch mechanism for engaging and disengaging power transmission of the output-side third transmission mechanism, The gear shift operating member is also operable in the forward third gear range, which is higher in speed than the forward second gear range. The control device is When the gear shift operating member is positioned in the forward third gear range, the second HMT transmission state is activated, the output side first and second clutch mechanisms are released, and the output side third clutch mechanism is engaged, while the operation of the HST gear shift actuator is controlled so that the rotational speed of the drive rotational power becomes the rotational speed corresponding to the operating position of the gear shift operating member. When the gear shifting operating member is shifted down from the forward third gear range to the forward second gear range, the transmission structure according to 10 is characterized in that, by performing the transition of the output third clutch mechanism from the engaged state to the disengaged state, the transition of the forward clutch mechanism from the disengaged state to the engaged state, and the transition of the output second clutch mechanism from the disengaged state to the engaged state in this order, a downshift coasting state is created in which the power transmission path to the travel output shaft is interrupted between the transition of the output third clutch mechanism to the disengaged state and the transition of the output second clutch mechanism to the engaged state, and the HST is shifted via the HST shift actuator during the downshift coasting state so that at the time of the transition to the engaged state of the output second clutch mechanism, the rotational speed of the travel output shaft, which is rotationally driven via the output second transmission mechanism and the forward transmission mechanism, matches or approaches the rotational speed of the travel output shaft immediately before the output second clutch mechanism is engaged. [Claim 19] A third output-side transmission mechanism capable of transmitting the rotational power of the first element to the travel output shaft as forward-direction driving rotational power, wherein the gear ratio of the third output-side transmission mechanism is set such that the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the third output-side transmission mechanism is faster than the rotational speed of the travel output shaft when the rotational power of the first element is transmitted to the travel output shaft via the second output-side transmission mechanism and the forward-side transmission mechanism, It comprises an output-side third clutch mechanism for engaging and disengaging power transmission of the output-side third transmission mechanism, The gear shift operating member is also operable in the forward third gear range, which is higher in speed than the forward second gear range. The control device is When the gear shift operating member is positioned in the forward third gear range, the second HMT transmission state is activated, the output side first and second clutch mechanisms are released, and the output side third clutch mechanism is engaged, while the operation of the HST gear shift actuator is controlled so that the rotational speed of the drive rotational power becomes the rotational speed corresponding to the operating position of the gear shift operating member. When the gear shift operating member is shifted down from the forward third gear range to the forward second gear range, the transition of the output side third clutch mechanism from the engaged state to the disengaged state, the transition of the output side second clutch mechanism from the disengaged state to the engaged state, and the transition of the forward side clutch mechanism from the disengaged state to the engaged state are performed in this order, thereby creating a downshift coasting state in which the power transmission path to the travel output shaft is interrupted between the transition of the output side third clutch mechanism to the engaged state of the forward side clutch mechanism, while the rotational speed of the gear shift output shaft due to the rotational power transmitted via the output side second transmission mechanism at the time of the transition from the disengaged state to the engaged state of the output side second clutch mechanism matches the actual rotational speed of the gear shift output shaft immediately before the output side second clutch mechanism is engaged. The transmission structure according to 10, characterized in that the HST is shifted via the HST shift actuator between the time the output-side third clutch mechanism transitions from an engaged state to an engaged state and the time the output-side second clutch mechanism transitions from an engaged state to an engaged state, so that the rotational speed of the travel output shaft, which is rotationally driven via the output-side second transmission mechanism and the forward-side transmission mechanism, at the time the forward-side clutch mechanism transitions from an engaged state to an engaged state, matches or approaches the rotational speed of the travel output shaft immediately before the forward-side clutch mechanism transitions to an engaged state, and thereafter the HST is shifted via the HST shift actuator between the time the output-side second clutch mechanism transitions to an engaged state and the time the forward-side clutch mechanism transitions to an engaged state. [Claim 20] The transmission structure according to claim 18, characterized in that the transition of the output-side second clutch mechanism from the disengaged state to the engaged state during a downshift from the forward-side third gear range to the forward-side second gear range is performed gradually. [Claim 21] The control device is During the initial period from the start of the shift-up from the forward second gear to the forward third gear until a predetermined time has elapsed, the target rotational speed for HST shift control is set to an HST output rotational speed that is increased in the opposite direction to the actual HST output rotational speed at the start of the shift-up, rather than the second / third gear switching speed on the third gear side that matches the rotational speed of the travel output shaft, which is rotationally driven via the output third transmission mechanism when the output third clutch mechanism is engaged, to the rotational speed of the travel output shaft at the start of the shift-up when one of the output second clutch mechanism and the forward clutch mechanism is released, and The transmission structure according to claim 13, characterized in that, after the end of the start period, the HST output rotational speed is set as the target rotational speed for HST shift control such that the rotational speed of the travel output shaft, which is rotationally driven via the output side third transmission mechanism at the time the output side third clutch mechanism transitions to the engaged state, matches the rotational speed of the travel output shaft immediately before the output side third clutch mechanism transitions to the engaged state. [Claim 22] The transmission structure according to claim 21, characterized in that the target speed for HST shift control during the starting period is an HST speed among the first and second HST speeds that is located on the opposite side of the actual HST output rotational speed at the start of the shift up, from the second / third gear shifting speed on the third gear side. [Claim 23] The control device is During the initial period from the start of the downshift from the forward third gear to the forward second gear until a predetermined time has elapsed, the target rotational speed for HST shift control is set to an HST output rotational speed that is increased in the opposite direction to the actual HST output rotational speed at the start of the downshift, rather than the second gear side third / second gear switching speed that matches the rotational speed of the travel output shaft, which is rotationally driven via the output side second transmission mechanism and the forward side transmission mechanism, at the point when the output side second clutch mechanism is engaged, to the rotational speed of the travel output shaft, which is rotationally driven via the output side third transmission mechanism at the start of the downshift of the third / second gear, and The transmission structure according to claim 18, characterized in that, after the end of the start period, the HST output rotational speed is set as the target rotational speed for HST shift control such that the rotational speed of the travel output shaft, which is rotationally driven via the output side second transmission mechanism and the forward side transmission mechanism at the time the output side second clutch mechanism is engaged, matches the rotational speed of the travel output shaft immediately before the output side second clutch mechanism is engaged. [Claim 24] The transmission structure according to claim 23, characterized in that the target speed for HST shift control during the start period is an HST speed among the first and second HST speeds that is located on the opposite side of the actual HST output rotational speed at the start of the downshift, from the target speed for the start period of the third / second gear switching speed on the second gear side.

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