Work vehicle

Abstract

The present invention reduces shift shock in a work vehicle comprising a continuously variable transmission and a stepped transmission. This work vehicle comprises: a continuously variable transmission that continuously changes a gear ratio by a variable displacement hydraulic pump and a variable displacement hydraulic motor connected through a hydraulic circuit; a stepped transmission that is connected in series to the continuously variable transmission and changes a gear ratio stepwise by switching a power transmission path; and a control device. The control device executes a preparation operation for reducing a volume of the variable displacement hydraulic pump and a volume of the variable displacement hydraulic motor in a state in which a volume ratio between the variable displacement hydraulic pump and the variable displacement hydraulic motor is maintained, a shifting operation for changing the gear ratio of the continuously variable transmission to offset a change in the gear ratio of the stepped transmission by changing either the volume of the variable displacement hydraulic pump or the volume of the variable displacement hydraulic motor while changing the volume ratio, and a termination operation for increasing the volume of the variable displacement hydraulic pump and the volume of the variable displacement hydraulic motor in a state in which the volume ratio is maintained.

Description

Work vehicles

[0001] This invention relates to a work vehicle equipped with a transmission for driving.

[0002] A wheel loader is a work vehicle that runs on wheels and uses a movable bucket at the front of the vehicle to excavate, transport, and load soil and sand. Because it runs on wheels, it has inferior off-road capability compared to vehicles that run on tracks, but it can travel at relatively high speeds on smooth surfaces.

[0003] Because wheel loaders operate at a wider range of vehicle speeds compared to tracked vehicles, when using an engine (internal combustion engine) as a power source, which has a more limited range of operating speeds than an electric motor, it is necessary to install a drive transmission with a wide gear range and high transmission efficiency.

[0004] Considering cost, the most common transmission system for medium-sized and larger wheel loaders, which meets the above requirements, is a combination of a torque converter and a stepped transmission mechanism using gears. However, since torque converters transmit power via fluid, they are less efficient than transmission using gears alone. On the other hand, while the gear range of a stepped transmission mechanism using gears can be designed arbitrarily, because it is a stepped transmission, it is not always possible to operate the engine at a highly efficient operating point.

[0005] As a type of continuously variable transmission (CVT) that can operate the engine at the most fuel-efficient operating point according to load conditions, there are HST (Hydrostatic Transmission) and HMT (Hydraulic Mechanical Transmission). An HST is a transmission mechanism that combines a variable displacement hydraulic pump and a hydraulic motor. Since the discharge volume of the hydraulic pump or hydraulic motor can be continuously changed, it becomes a continuously variable transmission, and in principle, the transmission range is unlimited. In contrast to an HST, where all power is transmitted via fluid, an HMT is a transmission mechanism that transmits a portion of the power via fluid using an HST, and the remaining power is transmitted mechanically rather than via fluid. In HMT, while having the functionality of continuously variable transmission, the use of mechanical transmission in conjunction with it achieves higher transmission efficiency than HST alone.

[0006] Because HSTs have lower transmission efficiency than HMTs, especially at higher speeds in their gear range, the gear range in which they can be used efficiently is narrower. Similarly, HMTs that include HSTs as a component inherently have a narrower gear range than HSTs alone. Therefore, both may require the addition of a stepped transmission mechanism to expand the gear range.

[0007] In continuously variable transmissions (CVTs), including HSTs and HMTs, a common method for expanding the gear range is to connect a stepped gear mechanism in series with the CVT. In HSTs, another method involves connecting two hydraulic motors in parallel to a single hydraulic pump on the hydraulic circuit, and switching between an operation mode using both the first and second motors and an operation mode using only the second motor with the first motor disconnected, thereby adding a stepped gear mechanism to the HST itself. However, in either method, a gear shift shock occurs when the gear is shifted discontinuously by the stepped gear mechanism, which may impair the inherent advantage of a CVT: smooth gear shifting.

[0008] For example, Patent Document 1 discloses a method for suppressing shift shock by changing the flow rate in the hydraulic circuit to cancel out changes in rotational speed by changing the hydraulic pump volume during discontinuous speed changes caused by switching the number of hydraulic motors.

[0009] Patent No. 6924159

[0010] Incidentally, the method of connecting a continuously variable transmission mechanism in series with a stepped transmission mechanism and the method described in Patent Document 1 above are methods that yield ideal results when the volume of the hydraulic pump or the volume of the hydraulic motor can be changed instantaneously, thereby changing the gear ratio of the continuously variable transmission mechanism instantaneously. However, if the responsiveness of the volume changes of the hydraulic pump and hydraulic motor is insufficient, it may not necessarily be possible to suppress the shift shock.

[0011] The present invention has been made in view of the above-mentioned problems, and its purpose is to suppress shifting shock in a work vehicle equipped with a continuously variable transmission mechanism and a stepped transmission mechanism connected in series.

[0012] The present invention relates to a work vehicle comprising a continuously variable speed transmission mechanism that continuously changes the gear ratio using a variable displacement hydraulic pump and a variable displacement hydraulic motor connected via a hydraulic circuit, a stepped gear transmission mechanism connected in series with the continuously variable speed transmission mechanism that changes the gear ratio in steps by switching the power transmission path, and a control device that controls the operation of the continuously variable speed transmission mechanism and the stepped gear transmission mechanism, wherein the control device performs a preparatory operation to decrease the volume of the variable displacement hydraulic pump and the volume of the variable displacement hydraulic motor while maintaining the volume ratio of the volume of the variable displacement hydraulic pump and the volume of the variable displacement hydraulic motor, a gear shift operation to change the gear ratio of the continuously variable speed transmission mechanism in such a way as to counteract the change in the gear ratio of the stepped gear transmission mechanism by changing either the volume of the variable displacement hydraulic pump or the volume of the variable displacement hydraulic motor while changing the volume ratio, and a final operation to increase the volume of the variable displacement hydraulic pump and the volume of the variable displacement hydraulic motor while maintaining the volume ratio.

[0013] According to the present invention, in a work vehicle equipped with a continuously variable transmission mechanism and a stepped transmission mechanism connected in series, shift shock can be suppressed. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

[0014] A side view showing the main configuration of the work vehicle according to Example 1. A plan view showing the power transmission system of the work vehicle in Figure 1. A schematic diagram showing the structure of the HMT continuously variable transmission mechanism. A diagram showing the power transmission path when the gear ratio of the HMT continuously variable transmission mechanism is at its lowest speed. A diagram showing the power transmission path when the gear ratio of the HMT continuously variable transmission mechanism is at its highest speed. A diagram showing the power transmission path when the HMT continuously variable transmission mechanism is locked up. A schematic diagram showing the structure of the HMT stepped transmission mechanism. A diagram showing the division of gear ratios between the continuously variable transmission mechanism and the stepped transmission mechanism. A diagram showing an example of the operation of the continuously variable transmission mechanism and the stepped transmission mechanism and the change in vehicle speed in a comparative example. A diagram showing an example of the operation of the continuously variable transmission mechanism and the stepped transmission mechanism and the change in vehicle speed when the clutch slip of the stepped transmission mechanism is excessive in a comparative example. A diagram showing an example of the operation of the continuously variable transmission mechanism and the stepped transmission mechanism and the change in vehicle speed in Example 1. A flowchart illustrating the operation of the continuously variable transmission mechanism in Example 1. A flowchart illustrating the operation of the continuously variable transmission mechanism in Example 2. A flowchart illustrating the operation of the continuously variable transmission mechanism in Example 3.

[0015] (Example 1) Example 1 describes a method for temporarily limiting the maximum volume of the variable displacement hydraulic pump and variable displacement hydraulic motor of the continuously variable transmission mechanism to a pre-set virtual maximum volume when the stepped transmission mechanism located downstream of the continuously variable transmission mechanism is shifting speed.

[0016] Figure 1 shows the main configuration of the work vehicle 1 to which Embodiment 1 is applied, and Figure 2 shows the power transmission system 10 for driving the work vehicle 1 in Figure 1. As shown in Figures 1 and 2, the work vehicle 1, which is a wheel loader, is equipped with a bucket 2 at the front of the vehicle that can move up and down and tilt, and is driven by front wheels 3 and rear wheels 4. The bucket 2 is operated by the hydraulic pressure of the load handling hydraulic pump 7. The power of the engine 5 is input to the HMT 15 via the continuously variable transmission input shaft 28, while a portion of it drives the load handling hydraulic pump 7. The power, which has been shifted to the desired torque and rotational speed in the HMT 15, is transmitted to the front wheel differential 11 and the rear wheel differential 12 via a propeller shaft 8 equipped with a propeller shaft coupling 9, and the front wheels 3 and the rear wheels 4 are driven by the front wheel drive shaft 13 and the rear wheel drive shaft 14, respectively, resulting in a four-wheel drive configuration. The engine 5 generates a driving force 31 that counteracts the driving load 32 acting on the front wheels 3 and rear wheels 4, allowing the vehicle to move.

[0017] Figure 3 schematically shows the configuration of an HMT 15 for a wheel loader such as work vehicle 1. An HMT (Hydraulic Mechanical Transmission) is a general term for a transmission that transmits a portion of the power via fluid using an HST (Hydrostatic Transmission), and the remaining power is transmitted mechanically instead of fluidly. The HMT 15 is configured such that the ratio of gear transmission to transmission by HST 22 is continuously switched by power division using a planetary gear mechanism 21, and also has a path that bypasses the planetary gear mechanism 21 and HST 22 to transmit power via a lock-up clutch 25.

[0018] The HMT 15 of the work vehicle 1 is equipped with a continuously variable transmission mechanism 6, a stepped transmission mechanism 61, and a control device 27. The continuously variable transmission mechanism 6 changes the gear ratio steplessly using the variable displacement hydraulic pump 23 and variable displacement hydraulic motor 24 of the HST 22. The stepped transmission mechanism 61 is connected in series with the continuously variable transmission mechanism 6 and changes the gear ratio in steps by switching the power transmission path. The control device 27 controls the operation of the continuously variable transmission mechanism 6 and the stepped transmission mechanism 61.

[0019] The power of the engine 5, controlled by the engine control device 26, is divided by the planetary gear mechanism 21 into a path toward the HST 22 and a path toward the continuously variable transmission output shaft 29 without passing through the HST 22. The planetary gear mechanism 21 comprises a planetary carrier 41, a first sun gear 42, a second sun gear 43, a first planetary gear 44, and a second planetary gear 45. The planetary carrier 41 receives power from the engine 5 transmitted via the continuously variable transmission input shaft 28. The first sun gear 42 outputs the power transmitted from the planetary carrier 41 via the first planetary gear 44 to the variable displacement hydraulic pump 23 of the HST 22. The second sun gear 43 outputs the power transmitted from the planetary carrier 41 via the second planetary gear 45 to the continuously variable transmission output shaft 29.

[0020] The HST22 is configured by connecting a variable displacement hydraulic pump 23 and a variable displacement hydraulic motor 24 with a hydraulic circuit 50, which is a closed hydraulic circuit equipped with a charge circuit and a relief circuit. The relief circuit consists of a main relief valve 51 and a check valve 52, and the charge circuit consists of a charge pump 53 and a charge relief valve 54. In addition, the HMT15 of the work vehicle 1 is equipped with a pressure sensor 55 that detects the pressure in the hydraulic circuit 50 between the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24.

[0021] The relief circuit protects the hydraulic circuit 50 by maintaining the pressure of the hydraulic circuit 50 below the relief pressure set by the main relief valve 51. The charge circuit replenishes the hydraulic circuit 50 with the necessary hydraulic fluid so that the pressure of the hydraulic circuit 50 does not fall below a predetermined pressure. The continuously variable transmission mechanism 6 is composed of the planetary gear mechanism 21, HST 22, lock-up clutch 25, continuously variable transmission output shaft 29, and control device 27 shown in Figure 3.

[0022] Both the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 are of the variable displacement type, and the gear ratio of the HST 22 is determined by the ratio of the discharge volumes of the pump side and the motor side. The variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 of the HST 22 can continuously change the volume of pressurized oil discharged per rotational speed from zero to the maximum volume in accordance with commands from the control device 27. If the volume of the variable displacement hydraulic pump 23 is larger than the volume of the variable displacement hydraulic motor 24, the gear ratio of the HST 22 will be on the speed-increasing side, and if the volume of the variable displacement hydraulic pump 23 is smaller than the volume of the variable displacement hydraulic motor 24, the gear ratio of the HST 22 will be on the speed-decreasing side. In other words, the gear ratio of the HST 22 is the value obtained by taking the volume of the variable displacement hydraulic motor 24 as the denominator and the volume of the variable displacement hydraulic pump 23 as the numerator.

[0023] Note that the speed-increasing side refers to the side where the value obtained by taking the volume of the variable displacement hydraulic motor 24 as the denominator and the volume of the variable displacement hydraulic pump 23 as the numerator increases. The speed-decreasing side refers to the side where the value obtained by taking the volume of the variable displacement hydraulic motor 24 as the denominator and the volume of the variable displacement hydraulic pump 23 as the numerator decreases. An increase in the speed-increasing side of the gear ratio means that the value obtained by taking the volume of the variable displacement hydraulic motor 24 as the denominator and the volume of the variable displacement hydraulic pump 23 as the numerator increases. An increase in the speed-increasing side of the gear ratio means that the value obtained by taking the volume of the variable displacement hydraulic motor 24 as the denominator and the volume of the variable displacement hydraulic pump 23 as the numerator decreases. Furthermore, an increase in the speed-decreasing side of the gear ratio means that the reciprocal of the value obtained by taking the volume of the variable displacement hydraulic motor 24 as the denominator and the volume of the variable displacement hydraulic pump 23 as the numerator increases.

[0024] Furthermore, offsetting a change in gear ratio means, for example, that when the gear ratio of the stepped transmission mechanism 61 is changed to the speed-increasing side, the gear ratio of the continuously variable transmission mechanism 6 is changed to the speed-decreasing side. Furthermore, offsetting a change in gear ratio means, for example, that when the gear ratio of the stepped transmission mechanism 61 is changed to the speed-decreasing side, the gear ratio of the continuously variable transmission mechanism 6 is changed to the speed-increasing side. In addition to the gear ratio of the continuously variable transmission mechanism 6 and the gear ratio of the stepped transmission mechanism 61 remaining completely constant, offsetting also includes temporary fluctuations in the gear ratio of the continuously variable transmission mechanism 6 and the gear ratio of the stepped transmission mechanism 61. Furthermore, the condition that the combined gear ratio of the continuously variable transmission mechanism 6 and the gear ratio of the stepped transmission mechanism 61 remains constant includes not only ensuring that the combined gear ratio of the continuously variable transmission mechanism 6 and the gear ratio of the stepped transmission mechanism 61 remains constant without any fluctuation, but also, as a result, causing fluctuations in the combined gear ratio of the continuously variable transmission mechanism 6 and the gear ratio of the stepped transmission mechanism 61.

[0025] The output from the variable displacement hydraulic motor 24 of the HST 22 and the output of the planetary gear mechanism 21 that does not go to the HST 22 are both connected to the continuously variable transmission output shaft 29 via gears. The planetary gear mechanism 21 has the function of passively distributing one input rotational speed to two output rotational speeds, so when the lock-up clutch 25 is released, the gear ratio of the continuously variable transmission mechanism 6 is determined steplessly by the gear ratio of the HST 22, which can be changed steplessly.

[0026] If the gear ratio of the HST22 is set to zero, then regardless of the input rotational speed of the HST22, i.e., the rotational speed of the variable displacement hydraulic pump 23, the output rotational speed of the HST22, i.e., the rotational speed of the variable displacement hydraulic motor 24, must be zero. Therefore, the gear ratio of the continuously variable transmission mechanism 6 becomes the lowest speed, i.e., the stopped state. In a completely stopped state, no power is transmitted, but if the gear ratio of the HST22 is a small non-zero value, and the gear ratio of the continuously variable transmission mechanism 6 is an extremely large finite value, then almost all power passes through the HST22, resulting in the power transmission path shown in Figure 4.

[0027] Conversely, if the gear ratio of the HST22 is set to infinity, the power transmission path will be as shown in Figure 5. This is because, regardless of the output rotational speed of the HST22, that is, the rotational speed of the variable displacement hydraulic motor 24, the input rotational speed of the HST22, that is, the rotational speed of the variable displacement hydraulic pump 23, must be zero, so the HST22 will not receive any power. Therefore, all the rotational speed input from the engine 5 to the planetary gear mechanism 21 is distributed toward the continuously variable transmission output shaft 29, and the gear ratio of the continuously variable transmission mechanism 6 becomes the maximum speed.

[0028] Therefore, when the volume of the variable displacement hydraulic motor 24 is set to maximum and the volume of the variable displacement hydraulic pump 23 is set to zero, the gear ratio of the continuously variable transmission mechanism 6 becomes the lowest speed state, and when the volume of the variable displacement hydraulic pump 23 is set to maximum and the volume of the variable displacement hydraulic motor 24 is set to zero, the gear ratio of the continuously variable transmission mechanism 6 becomes the highest speed state.

[0029] Furthermore, the pressing pressure of the lock-up clutch 25 is controlled by a command from the control device 27. By engaging the lock-up clutch 25 and setting the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 to zero, the power transmission via the HST 22 is interrupted, resulting in the power transmission path shown in Figure 6, where the power of the engine 5 is transmitted to the continuously variable transmission output shaft 29 without going through either the planetary gear mechanism 21 or the HST 22.

[0030] Figure 7 schematically shows the internal configuration of the stepped transmission mechanism 61. The stepped transmission mechanism 61 has three forward gears and one reverse gear, and is configured to transmit power mechanically through shafts, gears, and clutches.

[0031] The input shaft 66 of the stepped transmission is connected to the output shaft 29 of the continuously variable transmission (CVT) 6. The output shaft 67 of the stepped transmission is connected to the vehicle's propeller shaft 8, and the rotation of the continuously variable transmission output shaft 29 is transmitted to the front wheels 3 and rear wheels 4 by the front differential 11, rear differential 12, front drive shaft 13, and rear drive shaft 14.

[0032] By engaging one of the clutches, namely the reverse clutch 62, the first-speed clutch 63, the second-speed clutch 64, and the third-speed clutch 65, and releasing the other clutches, the power transmission path switches to the one corresponding to the engaged clutch. Since the ratio of the number of teeth of the gears constituting each power transmission path is different, different gear ratios can be obtained by selecting the engaged clutch.

[0033] Figure 8 is a diagram showing the sharing of the gear ratio between the continuously variable transmission mechanism 6 and the stepped transmission mechanism 61 during forward movement. The horizontal axis represents the overall gear ratio R of HMT15 ＣＶＴ , and the vertical axis represents the gear ratio R of the continuously variable transmission mechanism 6 ＣＶＴ . For a desired overall gear ratio R ＨＭＴ , by shifting the gear ratio R of the continuously variable transmission mechanism 6 along the thick line according to the used gear stage of the stepped transmission mechanism 61, the continuously variable transmission function can be maintained for the entire HMT15.

[0034] The relationships among the first-speed gear ratio R of the stepped transmission mechanism 61 Ｓｕｂ１ , the second-speed gear ratio R Ｓｕｂ２ , the third-speed gear ratio R Ｓｕｂ３ , the ratio R between the first and second speeds Ｓｕｂ１－２ and the ratio R between the second and third speeds Ｓｕｂ２－３ are as follows: R Ｓｕｂ１－２ = R Ｓｕｂ２ / R Ｓｕｂ１ R Ｓｕｂ２－３ = R Ｓｕｂ３ / R Ｓｕｂ２ . Figure 8 shows the values of each endpoint of the diagram using the above gear ratios, step ratios, and the upper limit R of the gear ratio of the continuously variable transmission mechanism 6 ＣＶＴｍａｘ .

[0035] ​​However, Fig. 8 shows an ideal for maintaining the continuously variable transmission function. For realization, it is necessary to instantaneously transition between region A and region B and between region B and region C. The stepped transmission mechanism 61 shifts gears discontinuously, but at that time, the release of the clutch in the previous stage and the engagement of the clutch in the next stage cannot be performed instantaneously. Further, the continuously variable transmission mechanism 6 is a mechanism premised on continuous shifting. To increase or decrease the volumes of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 and cause a change in the gear ratio corresponding to between region A and region B and between region B and region C, more shifting time is required than for the stepped transmission mechanism 61.

[0036] Fig. 9 is an example showing the transition of the volumes of the variable displacement hydraulic pump and the variable displacement hydraulic motor 24 of the continuously variable transmission mechanism 6, the gear ratio of the continuously variable transmission mechanism 6, the gear ratio of the stepped transmission mechanism 61, and the vehicle speed of the work vehicle 1 when transitioning from region A to region B and from region B to region C in Fig. 8 for a comparative example.

[0037] Region a means before the operation of changing the gear ratio, region c means during the operation of changing the gear ratio (transition region), and region e means after the operation of changing the gear ratio. The time required for region c is the shifting time of HMT15 and the shifting time of the continuously variable transmission mechanism 6.

[0038] Although the rotational speed of the engine 5 is not shown, it is premised on maintaining a highly efficient constant value in order to utilize the advantages of continuously variable transmission. Therefore, since the rotational speed of the continuously variable transmission input shaft 28 is constant throughout, the gear ratio of the continuously variable transmission mechanism 6 is synonymous with the rotational speeds of the continuously variable transmission output shaft 29 and the stepped transmission input shaft 66, and the vehicle speed determined by the rotational speed of the stepped transmission output shaft 67 is synonymous with the substantial gear ratio of HMT15. The vehicle speeds in regions a and e before and after the transition are the same because the gear ratio of the continuously variable transmission mechanism 6 is decreased so as to cancel out the increase in the gear ratio of the stepped transmission mechanism 61.

[0039] Also, not only is the vehicle speed the same before and after the operation of changing the gear ratio, but the vehicle speed is constant in regions a and e. This is to clearly show the states before and after shifting, including the volumes of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24, the gear ratio of the continuously variable transmission mechanism 6, and the gear ratio of the stepped transmission mechanism 61, when explaining the shifting operation. In reality, there is no need to provide a period during which the vehicle speed is constant in either region a or region e, and it is also possible to transition between regions A, B, and C in Fig. 8 while the vehicle speed, that is, the gear ratio of the HMT15, is gradually increasing or decreasing.

[0040] The volume of the variable displacement hydraulic pump 23 remains at the maximum value from region a to region e. This is because, by setting either the variable displacement hydraulic pump 23 or the variable displacement hydraulic motor 24 to the maximum volume with good efficiency for a single unit and changing the volume of the other from 0 to the maximum to create the required gear ratio, it is the principle of the operation to suppress the decrease in efficiency as the HST22 and the continuously variable transmission mechanism 6.

[0041] Fig. 9 shows an example in which the volume of the variable displacement hydraulic motor 24 is increased while the volume of the variable displacement hydraulic pump 23 is at the maximum to decrease the gear ratio of the continuously variable transmission mechanism 6. However, even if the volume of the variable displacement hydraulic pump 23 is decreased while the volume of the variable displacement hydraulic motor 24 is at the maximum, the manner of decreasing the gear ratio of the continuously variable transmission mechanism 6 is the same. Also, Fig. 9 shows an example in which the gear ratio of the continuously variable transmission mechanism 6 is decreased and the gear ratio of the stepped transmission mechanism 61 is increased. When increasing the gear ratio of the continuously variable transmission mechanism 6 and decreasing the gear ratio of the stepped transmission mechanism 61, that is, when transitioning from region B to region A or from region C to region B in Fig. 8, the operation is the same except that the directions of increase and decrease are reversed.

[0042] As mentioned above, the continuously variable transmission mechanism 6 is a mechanism that assumes continuous speed changes, and there are constraints on the rate at which the volume of either the variable displacement hydraulic pump 23 or the variable displacement hydraulic motor 24 is increased or decreased. Therefore, time is required to change the volume enough to offset the increase in the gear ratio of the stepped transmission mechanism 61, and this time becomes the shift time of the continuously variable transmission mechanism 6. Figure 9 shows the case where the rate of change of volume of the variable displacement hydraulic motor 24 is set to the maximum speed. In other words, the absolute value of the slope of the volume graph of the variable displacement hydraulic motor 24 cannot be greater than the absolute value of the slope in region c.

[0043] Furthermore, as mentioned above, the stepped transmission mechanism 61 also requires time between the release of the clutch of the previous stage and the engagement of the clutch of the next stage during the operation of changing the gear ratio, and this time constitutes the gear shift time of the stepped transmission mechanism 61.

[0044] Even if the rate of change in the volume of the variable displacement hydraulic pump 23 or the variable displacement hydraulic motor 24 is set to the maximum speed, the shifting time of the continuously variable transmission mechanism 6 is longer than the shifting time of the stepped transmission mechanism 61 due to the disengagement of the clutch. Therefore, in the transient region during the operation of changing the gear ratio, the decrease in the gear ratio of the continuously variable transmission mechanism 6 and the increase in the gear ratio of the stepped transmission mechanism 61 do not cancel each other out. As a result, even if the vehicle speed in region a and region e before and after the transition is the same, the vehicle speed fluctuates during the shift in region c. Furthermore, since the shifting of the continuously variable transmission mechanism 6 must be completed in order for the shifting of the HMT 15 to be completed, the shifting time of the continuously variable transmission mechanism 6 becomes the shifting time of the HMT 15.

[0045] In Figure 9, the gear ratio of the continuously variable transmission mechanism 6 is shown as a solid line even during gear changes, whereas the gear ratio of the stepped transmission mechanism 61 is shown as an apparent gear ratio with a dotted line during gear changes. This means that the gear ratio of the continuously variable transmission mechanism 6 can be directly controlled, whereas the gear ratio that can be specified in the stepped transmission mechanism 61 is a discontinuous value corresponding to each gear, and the gear ratio during the transition from the previous gear to the next gear depends on the degree to which the clutch slips.

[0046] The degree to which a clutch slips cannot be directly controlled; it is passively determined by the relationship between the load, including rolling resistance and inertia-induced components, and the clutch pressure. While the clutch pressures of the preceding and succeeding clutches can be controlled independently, if there is an extreme difference in their pressures, only the clutch with the lower pressure will slip, while the one with the higher pressure will not. In this state, the gear ratio is the gear ratio of the power transmission path where the clutch with the higher pressure is located, and therefore not a transient state in terms of gear ratio change. For this reason, subsequent explanations of clutch pressure will assume that both the preceding and succeeding clutches are increased or decreased by roughly the same amount.

[0047] Figure 9 shows that the upper limit torque determined by the clutch pressure is sufficient for the load, and the apparent gear ratio of the stepped transmission mechanism 61 increases linearly during gear changes. In this case, the gear changes of the stepped transmission mechanism 61 are completed earlier than those of the continuously variable transmission mechanism 6, and the vehicle speed temporarily increases due to the influence of the gear ratio of the continuously variable transmission mechanism 6, which has not yet decreased at that point.

[0048] In contrast, Figure 10 shows that in the comparative example, when the clutch pressing pressure is too weak relative to the load, the clutch slippage during gear shifting becomes excessive, causing the vehicle to decelerate once before accelerating. Compared to the case in Figure 9, the clutch slippage increases the gear shifting time of the stepped transmission mechanism 61, resulting in a case where it becomes equivalent to the gear shifting time of the continuously variable transmission mechanism 6.

[0049] Even if the shifting time is the same, for the speed to remain constant during shifting, the decrease in the gear ratio of the continuously variable transmission (CVT) 6 and the increase in the gear ratio of the stepped transmission 61 must always cancel each other out at any point during that shifting time. Theoretically, if the clutch pressing pressure can be precisely controlled, it is possible to keep the shifting time of the stepped transmission 61 and the shifting time of the CVT 6 the same, and ensure that the apparent increase in the gear ratio of the stepped transmission 61 during shifting is always canceled out by the decrease in the gear ratio of the CVT 6.

[0050] However, since it is greatly affected by load conditions, at least the input and output rotational speeds of the stepped transmission mechanism 61 must be detected at a sufficiently high sampling frequency, and the desired apparent gear ratio must be created by the clutch pressure. In practice, it is thought that it will be necessary to consider the responsiveness of the clutch pressure and to detect and precisely control the torque, which complicates the system. In the comparative example shown in Figure 10, there is no such control, and because the clutch slip is excessive, the increase in the apparent gear ratio of the stepped transmission mechanism 61 is delayed, resulting in deceleration during gear changes.

[0051] However, if the shifting times of the stepped transmission mechanism 61 and the continuously variable transmission mechanism 6 are equivalent, and the shifting time of the continuously variable transmission mechanism 6 is sufficiently short, then even if the control is not precise, the shift will be completed before the difference in the gear ratio changes between the two mechanisms widens, and speed fluctuations during shifting will be suppressed.

[0052] Therefore, in this embodiment, the shift time of the continuously variable transmission mechanism 6 is shortened by the following method.

[0053] Figure 11 is an example showing the changes in the volume of the variable displacement hydraulic pump 23 and variable displacement hydraulic motor 24 of the continuously variable transmission mechanism 6, the gear ratio of the continuously variable transmission mechanism 6, the gear ratio of the stepped transmission mechanism 61, and the vehicle speed of the work vehicle 1 during the transition from region A to region B and from region B to region C in Figure 8. Note that even in the case of the reverse transition in Figure 8, that is, the transition from region B to region A or from region C to region B, the operation is the same except that the direction of increase and decrease of the gear ratio of the continuously variable transmission mechanism 6 and the gear ratio of the stepped transmission mechanism 61 are reversed.

[0054] In order to take advantage of the benefits of continuously variable transmission, the premise that the rotational speed of engine 5 is kept at a highly efficient constant value is the same as in the comparative example above. The gear ratio of the continuously variable transmission mechanism 6 is synonymous with the rotational speed of the continuously variable transmission output shaft 29 and the stepped transmission input shaft 66, the vehicle speed determined by the rotational speed of the stepped transmission output shaft 67 is synonymous with the effective gear ratio of the HMT 15, the gear ratio of the continuously variable transmission mechanism 6 is reduced so as to cancel out the increase in the gear ratio of the stepped transmission mechanism 61 before and after the transition, and the section in which the vehicle speed is constant in regions a and e before and after the transition is also the same as in Figure 9 of the comparative example.

[0055] On the other hand, unlike in the comparative example Figure 9, in addition to regions a, c, and e, regions b and d also exist. Region b is the preparatory operation region before the gear ratio of the continuously variable transmission mechanism 6 is changed, and region d is the final operation region after the gear ratio of the continuously variable transmission mechanism 6 has been changed. Therefore, as in the case of Figure 9, only region c is the transition region where the gear ratio changes, and regions b and d are not included in the gear change time.

[0056] In the comparative example shown in Figure 9, the volume of the variable displacement hydraulic pump 23 is kept at its maximum value throughout, whereas in the example shown in Figure 11, the volume of the variable displacement hydraulic pump 23 is gradually reduced from its maximum value to a preset volume in region b, and then returned to its maximum value in region d.

[0057] Regarding the volume of the variable displacement hydraulic motor 24, in the comparative example shown in Figure 9, it was increased in region c for speed shifting, whereas in this embodiment shown in Figure 11, it is temporarily decreased in region b before speed shifting and then increased in region c.

[0058] Region b, where the volumes of both the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 are reduced, and region d, where the volumes of both the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 are increased or decreased while maintaining the volume ratio between the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24. Therefore, the gear ratio of the continuously variable transmission mechanism 6 does not change between region b and region d, and the gear transition region remains within region c. This series of operations means that the volumes of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 are limited to a virtual maximum volume.

[0059] In region c, the volumes of both the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 are reduced. Therefore, in region c, when the volume of the variable displacement hydraulic motor 24 is increased at the same maximum speed as in the comparative example Figure 9, the rate at which the gear ratio of the continuously variable transmission mechanism 6 decreases becomes faster than in the case of Figure 9.

[0060] Here, the slope of the dashed line indicating the motor volume in region c in Figure 9 of the comparative example is the same as the slope of the dashed line indicating the motor volume in region c in Figure 11 of this embodiment. In other words, in this embodiment, the rate at which the volume of the variable displacement hydraulic motor 24 is increased is the same as in the case of Figure 9 of the comparative example.

[0061] However, in this embodiment, the volumes of both the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 are reduced in region c. Also, as described above, the gear ratio is expressed as a value obtained by using the volume of the variable displacement hydraulic pump 23 as the numerator and the volume of the variable displacement hydraulic motor 24 as the denominator.

[0062] In this embodiment, the volume of the variable displacement hydraulic pump 23 in the numerator is small. Since the volume of the variable displacement hydraulic motor 24 in the denominator is also small, even with the same volume increase, the rate at which the volume of the variable displacement hydraulic motor 24 in the denominator increases is larger. Therefore, in Figure 11 of this embodiment, the rate at which the gear ratio decreases, as shown by the value when the volume of the variable displacement hydraulic pump 23 is the numerator and the volume of the variable displacement hydraulic motor 24 is the denominator, is faster than in Figure 9 of the comparative example. This is shown by the slope of the line showing the gear ratio of the continuously variable transmission mechanism in region c of Figure 9 of the comparative example and the slope of the line showing the gear ratio of the continuously variable transmission mechanism in region c of Figure 11 of this embodiment.

[0063] The fact that the rate of decrease in the gear ratio of the continuously variable transmission mechanism 6 in region c is faster than in the case of Figure 9 means that the time spent in region c is shorter than in the case of Figure 9, that is, the gear shifting time is shorter. In region d, the gear ratio of the continuously variable transmission mechanism 6 does not change, so the volume ratio of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 at the stage from region c to region d is already the same as the volume ratio of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 in the final region e.

[0064] Therefore, although the preparation operation in region b and the termination operation in region d are necessary in this invention, the shift time of the continuously variable transmission mechanism 6 can be shortened and made equivalent to the shift time of the stepped transmission mechanism 61. By shortening and equalizing the shift time, the decrease in the gear ratio of the continuously variable transmission mechanism 6 and the increase in the gear ratio of the stepped transmission mechanism 61 are largely offset, thereby suppressing speed fluctuations during shifting, i.e., shift shock.

[0065] Figure 12 is a flowchart showing the operation of the continuously variable transmission mechanism 6 in Embodiment 1. As shown in Figure 12, the control device 27 starts a preparatory operation (S101). Before changing the gear ratio of the stepped transmission mechanism 61, the control device 27 performs a preparatory operation to reduce the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 while maintaining the volume ratio of the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 (S102).

[0066] The control device 27 performs a gear change operation to change the gear ratio of the continuously variable transmission 6 in such a way that it cancels out the change in the gear ratio of the stepped transmission 61 by changing either the volume of the variable displacement hydraulic pump 23 or the volume of the variable displacement hydraulic motor 24 while changing the volume ratio during the gear ratio change operation of the stepped transmission 61 (S103). In the example in Figure 11, the control device 27 increases the volume of the variable displacement hydraulic motor 24 until the target gear ratio is achieved. The control device 27 performs a gear change operation so that the combined gear ratio of the continuously variable transmission 6 and the gear ratio of the stepped transmission 61 remains constant before and after the gear ratio change operation of the stepped transmission 61.

[0067] After the gear ratio change operation of the stepped transmission mechanism 61, the control device 27 performs a termination operation to increase the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 while maintaining the volume ratio (S104). The control device 27 performs the termination operation if the volume of the variable displacement hydraulic pump 23 has not reached its maximum (S105) (S104). The control device 27 completes the termination operation if the volume of the variable displacement hydraulic pump 23 has reached its maximum (S105) (S106).

[0068] In this embodiment, even if the shifting time of the continuously variable transmission mechanism 6 cannot be shortened to the same level as the shifting time of the stepped transmission mechanism 61, the effect of suppressing speed fluctuations during shifting, i.e., shifting shock, can be obtained by completing the shift before the difference in the shift ratio changes between the two expands.

[0069] Furthermore, in this embodiment, the time required for the shifting operation in region c is shorter compared to the comparative example, but a preparatory operation in region b is required before the shifting operation in region c, and a termination operation in region d is required after the shifting operation in region c. Therefore, in this embodiment, the time from commanding the HMT 15 to shift gears until the shifting operation begins is longer than in the comparative example, and time is also required for the volume ratio of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 after shifting to a state where the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 is more efficient for power transmission than immediately after shifting.

[0070] However, since the shift time during which the HMT15 changes the gear ratio is extremely short, the decrease in power transmission efficiency during the shift time does not affect the behavior of the work vehicle 1. Also, since the HMT15 is an automatic transmission, if the timing when a shift is needed can be predicted in advance, the shift can be prevented from being delayed by starting the preparation operation earlier by the amount of time required for the preparation operation in area b.

[0071] Even if the preparatory operation is initiated but the vehicle does not actually shift gears, the virtual maximum volume limit can be released at the point when it is determined that the vehicle will not shift gears. In this sequence of operations, although there may be a decrease in power transmission efficiency, the behavior of the work vehicle 1 will not be affected.

[0072] Conversely, if an immediate gear change is required due to a sudden increase in load or the like, the gear change can be performed in the same way as in Figure 9, where the present invention is not applied, without waiting for the virtual maximum volume limit to be reached. In this case, the gear change time of the continuously variable transmission mechanism 6 cannot be shortened, and the effect of suppressing gear change shock cannot be obtained, but the more important gear change operation itself is not hindered.

[0073] Regarding the termination operation in region d, it is possible to change speed during the termination operation, and the time required for the termination operation after the speed change does not hinder the next speed change operation.

[0074] According to this embodiment, before changing the gear ratio of the stepped transmission mechanism 61, the control device 27 performs a preparatory operation to reduce the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 while maintaining the volume ratio of the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24. Therefore, the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 can be reduced without changing the gear ratio.

[0075] The control device 27 performs a gear change operation to change the gear ratio of the continuously variable transmission 6 in such a way that it cancels out the change in the gear ratio of the stepped transmission 61 by changing either the volume of the variable displacement hydraulic pump 23 or the volume of the variable displacement hydraulic motor 24 while changing the volume ratio during the operation to change the gear ratio of the stepped transmission 61. Because the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 are reduced by the preparatory operation, the speed at which the gear ratio, which is the ratio of the volume of the variable displacement hydraulic pump 23 to the volume of the variable displacement hydraulic motor 24, is changed can be increased even at the same speed of volume change.

[0076] After the gear ratio change operation of the stepped transmission mechanism 61, the control device 27 performs a final operation to increase the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 while maintaining the volume ratio. Therefore, without changing the gear ratio, the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 can be increased, returning the system to a state of high power transmission efficiency.

[0077] Furthermore, in this embodiment, the control device 27 performs the gear shift operation so that the combined gear ratio of the continuously variable transmission 6 and the gear ratio of the stepped transmission 61 remains constant before and after the operation to change the gear ratio of the stepped transmission 61. As a result, the gear ratio, and thus the vehicle speed, remains constant before and after the operation to change the gear ratio, and gear shift shock can be further suppressed.

[0078] (Example 2) In Example 2, a method is described in which, when the stepped transmission mechanism 61 located downstream of the continuously variable transmission mechanism 6 changes speed, the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 of the continuously variable transmission mechanism 6 are temporarily limited to a virtual maximum volume corresponding to the pressure of the hydraulic circuit 50.

[0079] In Example 1, the degree of temporary limitation on the virtual maximum volume was predetermined, but in that case, the degree of limitation may be excessive or insufficient. When the virtual maximum volume is limited, power is transmitted at a smaller volume state, i.e., a lower flow rate, than when there is no limitation, while the limit is in place. Since the power transmitted by hydraulics is the product of flow rate and pressure, when transmitting the same amount of power, a lower flow rate results in a higher pressure.

[0080] The HST22 has an allowable operating pressure range, with the relief pressure set by the main relief valve 51 being the upper limit. When the pressure in the hydraulic circuit 50 reaches the relief pressure, hydraulic fluid is discharged from the main relief valve 51 into the relief circuit, causing the discharge flow rate of the variable displacement hydraulic pump 23 and the suction flow rate of the variable displacement hydraulic motor 24 to no longer match. As a result, the gear ratio of the continuously variable transmission mechanism 6 is no longer determined by the volume ratio of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24. In addition, pressure energy is lost as the hydraulic fluid passes through the main relief valve 51, which also reduces power transmission efficiency.

[0081] Therefore, in this embodiment, the pressure in the hydraulic circuit 50 of the HST 22 is constantly measured by the pressure sensor 55, and relief from the main relief valve 51 is prevented by preventing the pressure from becoming too high. The qualitative volume change between the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 is the same as in Figure 11 in Embodiment 1, but the degree to which the virtual maximum volume is limited from region b to region d is changed according to the pressure of the hydraulic circuit 50.

[0082] To prevent relief by the relief circuit, it is necessary not to excessively restrict the virtual maximum volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24. However, since the responsiveness of the volume changes between the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 is not necessarily sufficient to keep pace with the rate of pressure rise, it may be too late to relax the restriction on the virtual maximum volume after the pressure has reached near the relief pressure.

[0083] The reason for the sudden increase in pressure within the HST22's hydraulic circuit is the torque fluctuation when the stepped transmission mechanism 61 shifts gears, and the magnitude of the pressure increase at that time can be predicted by theoretical calculation or experiment. Therefore, a first threshold pressure is set at a pressure that approaches the relief pressure when the pressure rises by the predicted amount, and the virtual maximum volume limit is set until the pressure measured by the pressure sensor 55 reaches that first threshold.

[0084] Figure 13 is a flowchart illustrating the above operation. As shown in Figure 13, the control device 27 starts a preparatory operation when it moves from region a to region b in Figure 11 (S201), and reduces the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 while maintaining the volume ratio between the two (S202). The control device 27 determines whether the pressure of the hydraulic circuit 50 of the HST 22, measured by the pressure sensor 55, has exceeded the aforementioned first threshold such that the pressure rises by the predicted amount and approaches the relief pressure (S203). If it has not exceeded the first threshold, it continues to reduce the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 (S202). S202 and S203 correspond to the preparatory operation in region b in Figure 11.

[0085] On the other hand, if the pressure exceeds the first threshold (S203), the control device 27 controls the volume of the variable displacement hydraulic motor 24 to control the gear ratio R of the continuously variable transmission mechanism 6. ＣＶＴThe pressure is increased until it reaches the target value based on Figure 8 (S204). S204 is a gear shift operation and corresponds to region c in Figure 11. In other words, in this embodiment, if the pressure detected by the pressure sensor 55 exceeds the first threshold while the control device 27 is performing the preparation operation, the control device 27 stops the preparation operation. Furthermore, after stopping the preparation operation, the control device 27 performs a gear shift operation. The first threshold may be changed according to the state of the driving load 32 shown in Figure 2.

[0086] Next, the control device 27 increases the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 while maintaining the volume ratio between the two (S205). The control device 27 determines whether the pump volume has reached its maximum (S206), and if not, continues to increase the volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24 (S205). S205 and S206 correspond to the termination operation of region d in Figure 11. If the pump volume has reached its maximum, the termination operation is completed (S207).

[0087] In this embodiment, the control device 27 stops the preparation operation if the pressure detected by the pressure sensor 55 exceeds a first threshold while the preparation operation is being performed. This prevents relief by the relief circuit and suppresses a decrease in power transmission efficiency.

[0088] Furthermore, in this embodiment, the control device 27 executes the gear shift operation after stopping the preparation operation. Therefore, while preventing relief by the relief circuit, the speed at which the gear ratio of the continuously variable transmission mechanism 6 is changed can be increased in proportion to the extent to which the volume of the variable displacement hydraulic pump 23 and the volume of the variable displacement hydraulic motor 24 have been reduced by the preparation operation up to that point.

[0089] (Example 3) In Example 3, in addition to the operations of Examples 1 and 2, a method will be described in which securing driving force is prioritized over smooth shifting in cooperation with the continuously variable transmission mechanism 6 when the stepped transmission mechanism 61 located downstream of the continuously variable transmission mechanism 6 shifts speed.

[0090] In Embodiment 2, the objective is to limit the degree of restriction so that the pressure does not reach the relief pressure when virtually limiting the maximum volume of the variable displacement hydraulic pump 23 and the variable displacement hydraulic motor 24, whereas in this embodiment, the objective is to quickly obtain a driving force 31 to counteract the load when the running load 32 shown in Figure 2 suddenly increases.

[0091] The operation of Examples 1 and 2 has the effect of increasing the pressure in the hydraulic circuit 50 of the HST 22, making it easier for the relief circuit to relieve pressure. Example 2 is operated to prevent the pressure from rising too high, but since it only anticipates the pressure rise due to torque fluctuations when the stepped transmission mechanism 61 changes gears, if the driving load 32 itself rises suddenly, the relief pressure will be reached.

[0092] When the continuously variable transmission mechanism 6 and the stepped transmission mechanism 61 are shifted according to Figure 8, if an attempt is made to decrease the gear ratio of the HMT 15 when the driving load 32 suddenly increases, the gear ratio of the stepped transmission mechanism 61 will decrease while the gear ratio of the continuously variable transmission mechanism 6 will increase, resulting in an operation that either decreases the volume of the variable displacement hydraulic motor 24 or increases the volume of the variable displacement hydraulic pump 23.

[0093] However, the driving force 31 can be maximized by reducing the gear ratios of both the stepped transmission mechanism 61 and the continuously variable transmission mechanism 6. In this case, the fluctuation in driving speed due to gear changes will be large, but in situations where the driving load 32 is rapidly increasing, if the driving force 31 is not secured, it will result in a sudden deceleration and ultimately the engine 5 will stall. Therefore, it is better to prioritize securing the driving force 31 over smooth gear changes.

[0094] Therefore, in this embodiment, when the driving load 32 increases rapidly, the gear ratio of the continuously variable transmission mechanism 6 is reduced as the highest priority. The rapid increase in the driving load 32 is manifested as a pressure increase in the hydraulic circuit 50 of the HST 22, and is detected by the pressure sensor 55, as in Embodiment 2. If the pressure in the hydraulic circuit 50 of the HST 22 exceeds a second threshold, which is higher than the first threshold in Embodiment 2, the volume of the variable displacement hydraulic motor 24 is increased and the volume of the variable displacement hydraulic pump 23 is decreased, with priority given to controlling the gear ratio according to Figure 8. If the pressure in the hydraulic circuit of the HST 22 falls below a third threshold, which is lower than the second threshold, the gear ratio control returns to that according to Figure 8.

[0095] Figure 14 is a flowchart showing the operation of the continuously variable transmission mechanism 6 in Embodiment 3. As shown in Figure 14, the same processes as those in S201 to S207 in Figure 13 of Embodiment 2 are performed in S301, S302, S304, S305, S307, S309, and S310. In this embodiment, when the control device 27 is performing a preparation operation (S302), if the pressure detected by the pressure sensor 55 exceeds a second threshold that is higher than the first threshold (S303), it stops the preparation operation that was being performed and then performs a deceleration operation to change the gear ratio of the continuously variable transmission mechanism 6 (S311).

[0096] The deceleration operation is an operation to change the gear ratio of the continuously variable transmission mechanism 6 so that the gear ratio of the continuously variable transmission mechanism 6 increases toward the deceleration side by changing either the volume of the variable displacement hydraulic pump 23 or the volume of the variable displacement hydraulic motor 24 while changing the volume ratio.

[0097] If the pressure detected by the pressure sensor 55 exceeds a second threshold (S306) while the control device 27 is performing a gear shift operation (S305), it will stop the gear shift operation and then perform a deceleration operation (S311).

[0098] If the pressure detected by the pressure sensor 55 exceeds the second threshold (S308) while the control device 27 is performing a termination operation (S307), it stops the termination operation that was being performed and then performs a deceleration operation (S311).

[0099] If the control device 27 is performing a deceleration operation (S311) and the pressure detected by the pressure sensor 55 falls below a third threshold that is lower than the second threshold (S312), then, if it is before the gear ratio change operation of the stepped transmission mechanism 61 (S313), it performs a preparatory operation (S302). If the control device 27 is performing a gear ratio change operation of the stepped transmission mechanism 61 or after the gear ratio change operation of the stepped transmission mechanism 61 (S313), then, after the gear shift operation (S305), it performs a termination operation (S307).

[0100] In steps S303, S306, and S308, if the pressure detected by the pressure sensor 55 does not exceed the second threshold, the same process as in Figure 13 of the above embodiment 2 is performed. Also, if the pressure detected by the pressure sensor 55 exceeds the second threshold but does not fall below the third threshold (S312), the deceleration operation continues (S311). The preparation operation, speed change operation, and termination operation performed after the deceleration operation are performed as described above.

[0101] In this embodiment, when the control device 27 is performing any of the preparation, shifting, or termination operations, if the pressure detected by the pressure sensor 55 exceeds a second threshold higher than the first threshold, it stops any of the preparation, shifting, or termination operations being performed, and then performs a deceleration operation to change the gear ratio of the continuously variable transmission 6 by changing either the volume of the variable displacement hydraulic pump 23 or the volume of the variable displacement hydraulic motor 24 while changing the volume ratio, so that the gear ratio of the continuously variable transmission 6 increases towards the deceleration side. Therefore, when the pressure detected by the pressure sensor 55 is high and the driving load 32 is large, the gear ratio of the HMT 15 can be increased towards the deceleration side to secure the driving force 31 and suppress sudden deceleration and engine stalling of the engine 5.

[0102] Furthermore, in this embodiment, when the control device 27 is performing a deceleration operation, if the pressure detected by the pressure sensor 55 falls below a third threshold, which is lower than the second threshold, it will perform a preparatory operation before the gear ratio change operation of the stepped transmission mechanism 61. Therefore, when the driving load 32 decreases, the preparatory operation is performed before the gear ratio change operation of the stepped transmission mechanism 61, and the gear shift time of the continuously variable transmission mechanism 6 can be shortened.

[0103] On the other hand, in this embodiment, when the deceleration operation is being performed, if the pressure detected by the pressure sensor 55 falls below the third threshold, the control unit U27 will perform a termination operation after the gear shift operation, either during or after the gear ratio change operation of the stepped transmission mechanism 61. Therefore, when the driving load 32 decreases, the gear shift operation will be performed to counteract the gear ratio change of the stepped transmission mechanism 61, either during or after the gear ratio change operation of the stepped transmission mechanism 61, and the termination operation after the gear shift operation will increase the power transmission efficiency of the continuously variable transmission mechanism 6.

[0104] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are included. For example, the embodiments described above are described in detail to make the present invention easier to understand, and are not necessarily limited to those having all the configurations described. Furthermore, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. In addition, it is possible to add, delete, and replace parts of the configuration of each embodiment with other configurations. For example, the upstream and downstream positional relationship between the continuously variable transmission mechanism 6 and the stepped transmission mechanism 61 may be reversed, and the part that uses pressure information for control may use information on the rate of change of pressure over time.

[0105] 1. Work vehicle 2. Bucket 3. Front wheels 4. Rear wheels 5. Engine 6. Continuously Variable Transmission 7. Lifting hydraulic pump 8. Propeller shaft 9. Propeller shaft coupling 10. Power transmission system for driving 11. Front differential 12. Rear differential 13. Front drive shaft 14. Rear drive shaft 15. HMT 21. Planetary gear mechanism 22. HST 23. Variable displacement hydraulic pump 24. Variable displacement hydraulic motor 25. Lock-up clutch 26. Engine control device 27. Control device 28. Continuously Variable Transmission input shaft 29. Continuously Variable Transmission output shaft 31. Driving force 32. Driving load 41. Planetary carrier 42. First sun gear 43. Second sun gear 44. First planetary gear 45. Second planetary gear 50. Hydraulic circuit 51. Main relief valve 52. Check valve 53 Charge pump 54 Charge relief valve 55 Pressure sensor 61 Stepped transmission mechanism 62 Reverse clutch 63 1st gear clutch 64 2nd gear clutch 65 3rd gear clutch 66 Stepped transmission input shaft 67 Stepped transmission output shaft

Claims

A continuously variable transmission mechanism that changes the gear ratio steplessly using a variable displacement hydraulic pump and a variable displacement hydraulic motor connected via a hydraulic circuit, A stepped transmission mechanism is connected in series with the aforementioned continuously variable transmission mechanism and changes the gear ratio in stages by switching the power transmission path. A control device for controlling the operation of the continuously variable transmission mechanism and the stepped transmission mechanism, A work vehicle equipped with, The control device is While maintaining the volume ratio between the volume of the variable displacement hydraulic pump and the volume of the variable displacement hydraulic motor, a preparatory operation is performed to reduce the volumes of the variable displacement hydraulic pump and the variable displacement hydraulic motor. A gear shift operation that changes the gear ratio of the continuously variable transmission mechanism in such a way that the change in gear ratio of the stepped transmission mechanism is offset by changing either the volume of the variable displacement hydraulic pump or the volume of the variable displacement hydraulic motor while changing the volume ratio, While maintaining the aforementioned volume ratio, a final operation is performed to increase the volume of the variable displacement hydraulic pump and the volume of the variable displacement hydraulic motor. A work vehicle characterized by the following features. 　 The hydraulic circuit is further equipped with a pressure sensor for detecting the pressure, The control device is If the pressure detected by the pressure sensor exceeds a first threshold while the aforementioned preparatory operation is being performed, the preparatory operation is terminated. The work vehicle according to feature 1. 　 The control device, when the preparation operation is canceled, executes the gear shift operation of the continuously variable transmission mechanism. The work vehicle according to feature 2. 　 The control device is If, while performing any of the preparatory operation, the gear shifting operation, or the termination operation, the pressure detected by the pressure sensor exceeds a second threshold higher than the first threshold, the control device cancels any of the preparatory operation, the gear shifting operation, or the termination operation being performed, and performs a deceleration operation to change the gear ratio of the continuously variable transmission mechanism by changing either the volume of the variable displacement hydraulic pump or the volume of the variable displacement hydraulic motor while changing the volume ratio, so that the gear ratio of the continuously variable transmission mechanism increases towards the deceleration side. The work vehicle according to feature 3. 　 The control device is If, while the deceleration operation is being performed, the pressure detected by the pressure sensor falls below a third threshold that is lower than the second threshold, If the gear ratio change operation of the aforementioned stepped transmission mechanism has not yet been performed, the preparatory operation shall be performed. During the operation to change the gear ratio of the stepped transmission mechanism, or after the operation to change the gear ratio of the stepped transmission mechanism, the termination operation is performed after the gear shift operation is performed. The work vehicle according to feature 4. 　 The control device is The work vehicle according to claim 5, characterized in that the gear shift operation is performed such that, before and after the operation to change the gear ratio of the stepped gear shift mechanism, the combined gear ratio of the continuously variable transmission mechanism and the gear ratio of the stepped gear shift mechanism remains constant.

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