Power transmission device for work machine

Abstract

The present invention realizes a reduction in the size of a power transmission device for a work machine. A transmission (23) comprises: a low-speed clutch (114) that is brought into an engaged state when the travel speed of a forward-traveling work machine is greater than 0 and less than or equal to a first threshold value; a mid-speed clutch (126) that is brought into an engaged state when the travel speed of the forward-traveling work machine is greater than the first threshold value and less than or equal to a second threshold value; a high-speed clutch (134) that is brought into an engaged state when the travel speed of the forward-traveling work machine is greater than the second threshold value; and a first shaft (80). The low-speed clutch (114) and the high-speed clutch (134) are connected to the first shaft (80).

Description

Power Transmission Device for Work Machinery

[0001] The present disclosure relates to a power transmission device for work machinery that is mounted on a work machinery.

[0002] Japanese Unexamined Patent Application Publication No. 2020-159413 (Patent Document 1) discloses a wheel loader using an HMT (Hydraulic Mechanical Transmission) type transmission.

[0003] Japanese Unexamined Patent Application Publication No. 2020-159413

[0004] Further miniaturization is demanded in power transmission devices for work machinery. The present disclosure proposes a power transmission device for work machinery that enables further miniaturization.

[0005] The power transmission device for work machinery according to the present disclosure is mounted on a work machinery including a drive source and a travel device, and transmits driving force from the drive source to the travel device. The power transmission device for work machinery includes a low-speed clutch, a medium-speed clutch, a high-speed clutch, and a first shaft. The low-speed clutch is brought into an engaged state when the travel speed of the forward-traveling work machinery is greater than 0 and equal to or less than a first threshold. The medium-speed clutch is brought into an engaged state when the travel speed of the forward-traveling work machinery is greater than the first threshold and equal to or less than a second threshold. The high-speed clutch is brought into an engaged state when the travel speed of the forward-traveling work machinery is greater than the second threshold. The low-speed clutch is connected to the first shaft, and the high-speed clutch is connected to the first shaft.

[0006] According to the present disclosure, miniaturization of the power transmission device for work machinery can be achieved.

[0007] It is a side view of the work machinery according to the embodiment. It is a skeleton diagram of the power transmission device for work machinery according to the embodiment. It is an operating characteristic diagram of the power transmission device for work machinery.

[0008] The embodiments will be described below with reference to the drawings. In the following description, the same parts and components are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions of them will not be repeated. In the drawings, some configurations may be omitted or simplified for the sake of explanation. It is also intended from the outset that any configuration may be extracted from the embodiments and combined in any way.

[0009] In this embodiment, a wheel loader 1 will be described as an example of a work machine. Figure 1 is a side view of a wheel loader 1 as an example of a work machine according to this embodiment.

[0010] The wheel loader 1 mainly consists of a body frame 2, a work implement 3, a running gear 4, and a cab 5. The body of the wheel loader 1 is made up of the body frame 2, cab 5, etc. The running gear 4 is attached to the body of the wheel loader 1.

[0011] The traveling device 4 is used to move the body of the wheel loader 1 and includes traveling wheels 4a and 4b. The wheel loader 1 is self-propelled by the rotational drive of the traveling wheels 4a and 4b and can perform desired work using the work implement 3.

[0012] In this specification, the direction in which the wheel loader 1 travels in a straight line is referred to as the longitudinal direction of the wheel loader 1. In the longitudinal direction of the wheel loader 1, the side on which the work equipment 3 is positioned relative to the vehicle frame 2 is defined as the forward direction, and the side opposite to the forward direction is defined as the rear direction. The left-right direction of the wheel loader 1 is the direction perpendicular to the longitudinal direction when the wheel loader 1 is viewed from above on a flat ground surface.

[0013] The vehicle frame 2 includes a front frame 2a and a rear frame 2b. The wheel loader 1 is an articulated work machine in which the front frame 2a and the rear frame 2b are connected in a flexible manner. The direction of travel of the wheel loader 1 is changed left and right by the extension and retraction of the steering cylinder 11 by hydraulic fluid from a steering pump (not shown).

[0014] The work machine 3 includes a boom 14 rotatably mounted on the front frame 2a and a bucket 6 rotatably mounted on the tip of the boom 14. The work machine 3 further includes a bell crank 18 rotatably supported on the boom 14 and a link 15 connecting the bell crank 18 and the bucket 6.

[0015] The boom cylinder 16 is a hydraulic actuator that moves the boom 14 up and down relative to the front frame 2a. As the boom 14 moves up and down, the bucket 6 attached to the tip of the boom 14 also moves up and down. The bucket cylinder 19 is a hydraulic actuator that rotates the bucket 6 up and down relative to the boom 14.

[0016] The box-shaped cab 5 is located behind the boom 14. The cab 5 is mounted on the rear frame 2b. Inside the cab 5 are a seat where the wheel loader 1 operator sits, and operating devices that the operator uses to operate the wheel loader 1.

[0017] The rear frame 2b houses the engine 21 and the transmission 23. The engine 21 is the power source for the wheel loader 1 and is an internal combustion engine, such as a diesel engine. The engine 21 generates the driving force for the operation of the wheel loader 1. The transmission 23 is connected to the engine 21. The running wheels 4a and 4b of the running gear 4 are connected to the transmission 23 via axles (not shown). The transmission 23 corresponds to an example of a power transmission device for a work machine.

[0018] The wheel loader 1 further includes a controller 27. The controller 27 includes, for example, a processor and memory. The controller 27 controls the engine 21 and the transmission 23.

[0019] Figure 2 is a skeleton diagram of a transmission 23 as an example of a power transmission device for a work machine according to the embodiment.

[0020] The input shaft 30 is connected to the engine 21. Driving force is transmitted from the engine 21 to the input shaft 30. The output shaft 150 is connected to the running gear 4. The output shaft 150 outputs the driving force that is transmitted to the running gear 4. The skeleton diagram shown in Figure 2 is a simplified representation of the power transmission path from the engine 21 to the running gear 4. The driving force generated by the engine 21 is transmitted to the running gear 4 via the transmission 23, and further via a propeller shaft and axle (not shown).

[0021] An input gear 32 is connected to the input shaft 30 so as to rotate integrally with the input shaft 30.

[0022] The transmission 23 includes a planetary shaft 70. The planetary shaft 70 is positioned eccentrically from the input shaft 30 and the output shaft 150 such that its axis of rotation is offset from the axis of rotation of the input shaft 30 and the axis of rotation of the output shaft 150. The transmission 23 includes a first planetary gear mechanism 40, a second planetary gear mechanism 50, and a third planetary gear mechanism 60. The first planetary gear mechanism 40, the second planetary gear mechanism 50, and the third planetary gear mechanism 60 are rotatably supported about the axis of rotation of the planetary shaft 70. The first planetary gear mechanism 40, the second planetary gear mechanism 50, and the third planetary gear mechanism 60 are arranged coaxially.

[0023] The first planetary gear mechanism 40 includes a first sun gear 41, a plurality of first planetary gears 42, a first carrier 43, and a first ring gear 44. The first sun gear 41 is connected to the planetary shaft 70 so as to rotate integrally with the planetary shaft 70. Each first planetary gear 42 meshes with the first sun gear 41 and is connected to the first sun gear 41.

[0024] The first carrier 43 supports each first planetary gear 42. Each first planetary gear 42 can revolve together with the first carrier 43 around the rotation axis of the planetary axis 70. The outer surface of the first carrier 43 includes an external gear. The gear of the first carrier 43 meshes with the input gear 32. Through this gear meshing, the first carrier 43 is connected to the input shaft 30. The first ring gear 44 meshes with each first planetary gear 42 and is connected to the first planetary gear 42.

[0025] The second planetary gear mechanism 50 includes a second sun gear 51, a plurality of second planetary gears 52, a second carrier 53, and a second ring gear 54. The second sun gear 51 is annular, and the planetary shaft 70 passes through the second sun gear 51. The second sun gear 51 and the planetary shaft 70 are rotatable relative to each other.

[0026] The second sun gear 51 is connected to the first carrier 43. The second sun gear 51, together with the first carrier 43, can rotate integrally around the rotation axis of the planetary shaft 70. Each second planetary gear 52 meshes with the second sun gear 51 and is connected to the second sun gear 51.

[0027] The second carrier 53 supports each second planetary gear 52. Each second planetary gear 52, together with the second carrier 53, is capable of orbiting about the rotation axis of the planetary axis 70. The second ring gear 54 meshes with and is connected to each second planetary gear 52.

[0028] The second ring gear 54 is connected to the first ring gear 44. The first ring gear 44 and the second ring gear 54 are integrally formed. More specifically, the transmission 23 includes a ring member 46. The ring member 46 is a single component including the integrated first ring gear 44 and the second ring gear 54. The outer circumferential surface of the ring member 46 includes an external gear. The gear of the ring member 46 meshes with a PM input gear 47. The PM input gear 47 is connected to a PM input shaft 48 which constitutes the rotation axis of the second variable device 49.

[0029] The second variable device 49 is a hydraulic pump / motor. The capacity of the second variable device 49 is controlled by the controller 27. The controller 27 controls the rotational speed of the second variable device 49. As a result, the second variable device 49 continuously changes the speed ratio of the output shaft 150 to the input shaft 30.

[0030] The third planetary gear mechanism 60 includes a third sun gear 61, a plurality of third planetary gears 62, a third carrier 63, and a third ring gear 64. The third carrier 63 is connected to the second carrier 53. The third carrier 63, together with the second carrier 53, can rotate integrally around the rotation axis of the planetary shaft 70. Each third planetary gear 62 is supported by the third carrier 63. Each third planetary gear 62, together with the third carrier 63, can revolve around the rotation axis of the planetary shaft 70.

[0031] The third sun gear 61 meshes with and is connected to the third planetary gear 62. The third sun gear 61 is annular, and the planetary shaft 70 passes through the third sun gear 61. The third sun gear 61 and the planetary shaft 70 are rotatable relative to each other.

[0032] The third ring gear 64 meshes with and is connected to each of the third planetary gears 62. The third ring gear 64 is non-rotatable. The third ring gear 64 is fixed to, for example, the housing of the transmission 23. The third planetary gear mechanism 60 increases the speed of the rotation input from the second planetary gear mechanism 50 and outputs it.

[0033] The planetary shaft 70 is connected to the first variable gear 79. The planetary shaft 70 constitutes the rotation axis of the first variable gear 79. The planetary shaft 70 directly connects the first sun gear 41 of the first planetary gear mechanism 40 to the first variable gear 79. The first variable gear 79 is arranged coaxially with the first planetary gear mechanism 40. In the direction in which the planetary shaft 70 extends, the second planetary gear mechanism 50 and the third planetary gear mechanism 60 are arranged between the first planetary gear mechanism 40 and the first variable gear 79.

[0034] The first variable device 79 is a hydraulic pump / motor. The capacity of the first variable device 79 is controlled by the controller 27. The controller 27 controls the rotational speed of the first variable device 79. As a result, the first variable device 79 continuously changes the speed ratio of the output shaft 150 to the input shaft 30.

[0035] The first variable device 79 is connected to the second variable device 49 by a hydraulic circuit (not shown). When the second variable device 49 functions as a pump and discharges hydraulic fluid, the first variable device 79 functions as a motor and is driven by the hydraulic fluid from the second variable device 49.

[0036] The transmission 23 comprises a low-speed gear train 110, a medium-speed gear train 120, and a high-speed gear train 130. The low-speed gear train 110 includes a low-speed gear 111. The medium-speed gear train 120 includes a medium-speed gear 121. The high-speed gear train 130 includes a high-speed gear 131. The low-speed gear 111, the medium-speed gear 121, and the high-speed gear 131 are rotatably supported about the rotation axis of the planetary shaft 70. The low-speed gear 111, the medium-speed gear 121, and the high-speed gear 131 are arranged coaxially with the first planetary gear mechanism 40, the second planetary gear mechanism 50, and the third planetary gear mechanism 60. The high-speed gear 131 corresponds to an example of the first gear.

[0037] The low-speed gear 111 is connected to the planetary shaft 70 so as to rotate integrally with the planetary shaft 70. The medium-speed gear 121 is connected to the third sun gear 61. The medium-speed gear 121, together with the third sun gear 61, is rotatable integrally with the planetary shaft 70 around its axis of rotation. The medium-speed gear 121 is annular, and the planetary shaft 70 passes through it. The medium-speed gear 121 and the planetary shaft 70 are rotatable relative to each other. The high-speed gear 131 is connected to the planetary shaft 70 so as to rotate integrally with the planetary shaft 70. The low-speed gear 111, the medium-speed gear 121, and the high-speed gear 131 are positioned between the third planetary gear mechanism 60 and the first variable device 79 in the direction in which the planetary shaft 70 extends.

[0038] The transmission 23 further includes a first shaft 80 and a second shaft 90. The first shaft 80 and the second shaft 90 are arranged eccentrically from the planetary shaft 70 and extend parallel to the planetary shaft 70.

[0039] The transmission 23 further includes a low-speed clutch 114, a medium-speed clutch 126, and a high-speed clutch 134. The low-speed clutch 114, the medium-speed clutch 126, and the high-speed clutch 134 are, for example, hydraulic clutches controlled by a controller 27.

[0040] The low-speed clutch 114 is connected to the first shaft 80. The low-speed clutch 114 switches between connecting and disconnecting the planetary shaft 70 and the first shaft 80. When the low-speed clutch 114 is in an engaged state, rotation of the planetary shaft 70 is transmitted to the first shaft 80 via the low-speed gear 111.

[0041] The medium-speed clutch 126 is connected to the second shaft 90. The medium-speed clutch 126 switches between connecting and disconnecting the third sun gear 61 and the second shaft 90. When the medium-speed clutch 126 is in an engaged state, rotation of the third sun gear 61 is transmitted to the second shaft 90 via the medium-speed gear 121.

[0042] The high-speed clutch 134 is connected to the first shaft 80. The high-speed clutch 134 switches between connecting and disconnecting the planetary shaft 70 and the first shaft 80. When the high-speed clutch 134 is in an engaged state, rotation of the planetary shaft 70 is transmitted to the first shaft 80 via the high-speed gear 131.

[0043] The low-speed clutch 114 and the high-speed clutch 134 are connected to the first shaft 80. The medium-speed clutch 126 is connected to the second shaft 90, which is a shaft different from the first shaft 80. No clutch is connected to the planetary shaft 70, which is a shaft different from the first shaft 80 and the second shaft 90.

[0044] A first output gear 87 is connected to the first shaft 80 so as to rotate integrally with the first shaft 80. A second output gear 97 is connected to the second shaft 90 so as to rotate integrally with the second shaft 90.

[0045] The transmission 23 further includes an intermediate shaft 140. An intermediate input gear 142 and an intermediate output gear 147 are coupled to the intermediate shaft 140 so as to rotate integrally with the intermediate shaft 140. The intermediate input gear 142 meshes with the first output gear 87 and also meshes with the second output gear 97. Driving force is transmitted from the first shaft 80 to the intermediate shaft 140 via the first output gear 87 and the intermediate input gear 142 that mesh with each other. Driving force is transmitted from the second shaft 90 to the intermediate shaft 140 via the second output gear 97 and the intermediate input gear 142 that mesh with each other.

[0046] The output gear 152 is coupled to an output shaft 150 so as to rotate integrally with the output shaft 150. The intermediate output gear 147 meshes with the output gear 152. Driving force is transmitted from the intermediate shaft 140 to the output shaft 150 via the intermediate output gear 147 and the output gear 152 that mesh with each other.

[0047] FIG. 3 is an operation characteristic diagram of a power transmission device for a working machine. FIG. 3 illustrates the operation characteristics of the transmission 23 in accordance with the traveling speed when the wheel loader 1 travels forward. The traveling speed V1 shown in FIG. 3 corresponds to an example of a first threshold value related to the traveling speed of the wheel loader 1 traveling forward. The traveling speed V2 corresponds to an example of a second threshold value related to the traveling speed of the wheel loader 1 traveling forward.

[0048] The traveling speed of the wheel loader 1 is classified into a low speed range, a medium speed range, and a high speed range. A range where the traveling speed of the wheel loader 1 traveling forward is greater than 0 and equal to or less than V1 is defined as the low speed range. A range where the traveling speed of the wheel loader 1 traveling forward is greater than V1 and equal to or less than V2 is defined as the medium speed range. A range where the traveling speed of the wheel loader 1 traveling forward is greater than V2 is defined as the high speed range.

[0049] The controller 27 controls the state of each clutch in accordance with the traveling speed of the wheel loader 1. FIG. 3 shows clutches that are brought into an engaged state in accordance with the traveling speed of the wheel loader 1.

[0050] The controller 27 also controls the operation of the first variable device 79 and the second variable device 49 according to the travel speed of the wheel loader 1. Figure 3 shows whether the first variable device 79 and the second variable device 49 function as a pump or a motor according to the travel speed of the wheel loader 1.

[0051] When the wheel loader 1 is traveling at a high speed, the controller 27 engages the high-speed clutch 134 and disengages the other clutches. The controller 27 also causes the second variable device 49 to function as a pump and the first variable device 79 to function as a motor.

[0052] When the wheel loader 1 is traveling at a medium speed, the controller 27 engages the medium-speed clutch 126 and disengages the other clutches. The controller 27 also causes the first variable device 79 to function as a pump and the second variable device 49 to function as a motor.

[0053] When the wheel loader 1 is traveling at a low speed, the controller 27 engages the low-speed clutch 114 and disengages the other clutches. The controller 27 also causes the second variable device 49 to function as a pump and the first variable device 79 to function as a motor.

[0054] As shown in Figure 2, the transmission 23 described above includes a first shaft 80 to which a low-speed clutch 114 and a high-speed clutch 134 are connected. In conventional transmissions, the high-speed clutch 134 was connected to the planetary shaft 70, but in this transmission, it has been moved from the planetary shaft 70 to the first shaft 80. Since the clutch is not connected to the planetary shaft 70, the axial length of the planetary shaft 70 can be shortened, making the planetary shaft 70 more compact. The first variable device 79, which is connected to the end of the planetary shaft 70, can be positioned further rearward on the body of the wheel loader 1. As a result, the transmission 23 as a whole can be made smaller, improving the ease of mounting the transmission 23 on the vehicle body.

[0055] As shown in Figure 2, the transmission 23 further includes a planetary shaft 70, which is a different shaft from the first shaft 80. By configuring the transmission 23 to include the first shaft 80 and the planetary shaft 70, and having the high-speed clutch 134 connected to the first shaft 80, it is possible to realize a configuration in which the planetary shaft 70 is not connected to a clutch.

[0056] As shown in Figure 2, the transmission 23 further includes an intermediate shaft 140 that transmits the driving force transmitted from the first shaft 80 or the second shaft 90 to the output shaft 150.

[0057] The output shaft 150 is positioned in the center of the wheel loader 1's body in the left-right direction. In a conventional configuration where the drive force is transmitted directly from the first shaft 80 or the second shaft 90 to the output shaft 150 without an intermediate shaft 140, the first output gear 87 of the first shaft 80 and the second output gear 97 of the second shaft 90 mesh with an output gear 152 that rotates integrally with the output shaft 150. The first shaft 80 and the second shaft 90 also have gear pairs that mesh with each other. Therefore, once the arrangement of the planetary shafts 70 is determined, the arrangement of the first shaft 80 and the second shaft 90 is automatically determined, resulting in a low degree of freedom in arrangement.

[0058] By interposing an intermediate shaft 140 in the power transmission path between the first shaft 80 or the second shaft 90 and the output shaft 150, the first shaft 80 and the second shaft 90 can be moved in the circumferential direction around the rotation axis of the planetary shaft 70 and positioned arbitrarily. The degree of freedom in positioning the first shaft 80 and the second shaft 90 relative to the output shaft 150 is improved. By positioning the first shaft 80 and the second shaft 90 in the optimal position, it is possible to reduce the dimensions of the transmission 23 in the left-right direction of the vehicle body, improving the mountability of the transmission 23 to the vehicle body. By changing the position of the first shaft 80 and the second shaft 90, the second variable device 49 can also be optimally positioned, thus improving the mountability of the second variable device 49.

[0059] By adding an intermediate shaft 140, it becomes possible to move the output shaft 150 in the longitudinal direction of the vehicle body. By offsetting the output shaft 150 in the forward direction, the length of the propeller shaft connected to the output shaft 150 and extending in the longitudinal direction of the vehicle body can be secured. When the propeller shaft is spline-fitted with the output shaft 150, increasing the length of the propeller shaft increases the contact area of ​​the spline fitting and reduces the surface pressure. This increases the torque transmitted by the propeller shaft and ensures the strength of the propeller shaft.

[0060] In the above embodiment, a wheel loader 1 was described as an example of a work machine, but the ideas of this disclosure may be applied to other types of work machines. The power source of the work machine is not limited to the engine 21 and may include an electric motor. The work machine may be remotely controlled from a location away from the machine.

[0061] <Note> The above description includes the following features.

[0062] (Note 1) A power transmission device for a work machine, which is mounted on a work machine having a drive source and a travel device, and transmits driving force from the drive source to the travel device, comprising: a low-speed clutch that is engaged when the travel speed of the work machine traveling forward is greater than 0 and less than or equal to a first threshold; a medium-speed clutch that is engaged when the travel speed of the work machine traveling forward is greater than the first threshold and less than or equal to a second threshold; a high-speed clutch that is engaged when the travel speed of the work machine traveling forward is greater than the second threshold; and a first shaft to which the low-speed clutch and the high-speed clutch are connected.

[0063] (Note 2) The power transmission device for a work machine according to claim 1, further comprising a second shaft which is a different shaft from the first shaft, and the medium-speed clutch being connected to the second shaft.

[0064] (Note 3) A power transmission device for a work machine according to Note 1 or Note 2, further comprising a planetary gear mechanism and a planetary shaft which is a different shaft from the first shaft to which the planetary gear mechanism is connected.

[0065] (Note 4) The power transmission device for a work machine according to Note 3, further comprising a first gear that rotates integrally with the planetary shaft, wherein when the travel speed of the work machine traveling forward is greater than the second threshold, driving force is transmitted from the planetary shaft to the first shaft via the first gear and the engaged high-speed clutch.

[0066] (Note 5) A power transmission device for a work machine as described in Note 2, or Note 3 or Note 4, which references Note 2, further comprising: an output shaft that outputs a driving force transmitted to the running device; and an intermediate shaft that transmits a driving force transmitted from the first shaft or the second shaft to the output shaft.

[0067] (Note 6) The power transmission device for a work machine as described in Note 5, further comprising: a first output gear that rotates integrally with the first shaft; a second output gear that rotates integrally with the second shaft; and an intermediate input gear that meshes with the first output gear and the second output gear and rotates integrally with the intermediate shaft.

[0068] (Note 7) The power transmission device for a work machine according to Note 5 or Note 6, further comprising: an output gear that rotates integrally with the output shaft; and an intermediate output gear that meshes with the output gear and rotates integrally with the intermediate shaft.

[0069] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the claims rather than the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included.

[0070] 1 Wheel loader, 2 Body frame, 4 Running gear, 4a, 4b Driving wheels, 21 Engine, 23 Transmission, 27 Controller, 30 Input shaft, 32 Input gear, 40 First planetary gear mechanism, 41 First sun gear, 42 First planetary gear, 43 First carrier, 44 First ring gear, 46 Ring member, 47 PM input gear, 48 PM input shaft, 49 Second variable device, 50 Second planetary gear mechanism, 51 Second sun gear, 52 Second planetary gear, 53 Second carrier, 54 Second ring gear, 60 Third planetary gear mechanism, 61 Third sun gear, 62 Third planetary gear, 63 Third carrier, 64 Third ring gear, 70 Planetary shaft, 79 First variable device, 80 First shaft, 87 First output gear, 90 Second shaft, 97 Second output gear, 110 Low-speed gear train, 111 Low-speed gear, 114 Low-speed clutch, 120 Medium-speed gear train, 121 Medium-speed gear, 126 Medium-speed clutch, 130 High-speed gear train, 131 High-speed gear, 134 High-speed clutch, 140 Intermediate shaft, 142 Intermediate input gear, 147 Intermediate output gear, 150 Output shaft, 152 Output gear.

Claims

1. A power transmission device for a work machine, which is mounted on a work machine comprising a drive source and a travel device, and transmits driving force from the drive source to the travel device, comprising: a low-speed clutch that is engaged when the travel speed of the work machine traveling forward is greater than 0 and less than or equal to a first threshold; a medium-speed clutch that is engaged when the travel speed of the work machine traveling forward is greater than the first threshold and less than or equal to a second threshold; a high-speed clutch that is engaged when the travel speed of the work machine traveling forward is greater than the second threshold; and a first shaft to which the low-speed clutch and the high-speed clutch are connected.

2. The power transmission device for a work machine according to claim 1, further comprising a second shaft which is a different shaft from the first shaft, wherein the medium-speed clutch is connected to the second shaft.

3. The power transmission device for a working machine according to claim 1, further comprising a planetary gear mechanism and a planetary shaft which is a different shaft from the first shaft to which the planetary gear mechanism is connected.

4. The power transmission device for a work machine according to claim 3, further comprising a first gear that rotates integrally with the planetary shaft, wherein when the travel speed of the work machine traveling forward is greater than the second threshold, a driving force is transmitted from the planetary shaft to the first shaft via the first gear and the engaged high-speed clutch.

5. The power transmission device for a work machine according to claim 2, further comprising: an output shaft that outputs a driving force transmitted to the traveling device; and an intermediate shaft that transmits a driving force transmitted from the first shaft or the second shaft to the output shaft.

6. The power transmission device for a work machine according to claim 5, further comprising: a first output gear that rotates integrally with the first shaft; a second output gear that rotates integrally with the second shaft; and an intermediate input gear that meshes with the first output gear and the second output gear and rotates integrally with the intermediate shaft.

7. The power transmission device for a work machine according to claim 6, further comprising: an output gear that rotates integrally with the output shaft; and an intermediate output gear that meshes with the output gear and rotates integrally with the intermediate shaft.

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