Transmission and work vehicle

Abstract

To provide a transmission and a working vehicle capable of suppressing an increase in flow rate of oil discharged from a piston pump when a high load is applied at a high speed stage.SOLUTION: When a high load is applied to the motor-output-shaft HST33L of the hydraulic motor 42L in a state in which the high-speed stage is formed in the left 46L, the flow amount of the oil discharged from the hydraulic-pump 46L increases and the oil pressure of the closed-circuit increases in order to maintain the rotation speed of the motor-output-shaft 41L. When the oil pressure of the closed loop exceeds a predetermined pressure, the pressure of the oil supplied to the cylinder 71L of the auxiliary transmission servo piston 74L is reduced by the pressure-reducing valve 85 so that the force holding the rod 42L of the auxiliary transmission servo piston 71L at the high-speed position is overcome by the force received by the swash plate from the piston of the hydraulic motor 72L. Accordingly, when the oil pressure in the closed-circuit exceeds the predetermined pressure, the rod 74L is displaced from the high-speed position toward the low-speed position, and the inclination angle of the swash plate of the hydraulic motor 42L is changed.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a work vehicle such as a combine harvester and a transmission mounted on the work vehicle. [Background technology]

[0002] Combine harvesters are equipped with a power transmission mechanism that transmits power to a pair of left and right crawler travel devices. Some combine harvesters are equipped with an HST (Hydro Static Transmission) in the power transmission mechanism.

[0003] The HST is equipped with a swash plate-type piston pump and a piston motor. The piston pump has a suction / discharge port and a discharge / suction port. The piston motor has an inlet / outlet port and an outlet / inlet port. The hydraulic circuit of the HST has a closed circuit in which the outlet / suction port of the piston pump is connected to the inlet / outlet port of the piston motor by a first oil passage, and the suction / discharge port of the piston pump is connected to the outlet / inlet port of the piston motor by a second oil passage.

[0004] The hydraulic circuit of the HST also includes a charge pump and a charge oil line. The charge pump is driven by power from the engine mounted on the combine harvester. Oil discharged from the charge pump flows through the charge oil line. A check valve and a relief valve are interposed in parallel between the first oil line and the charge oil line. The check valve functions to maintain the oil pressure in the first oil line at or above the oil pressure in the charge oil line, and the relief valve operates to limit the oil pressure to a set pressure or lower. A check valve and a relief valve are interposed in parallel between the second oil line and the charge oil line. The check valve functions to maintain the oil pressure in the second oil line at or above the oil pressure in the charge oil line, and the relief valve operates to limit the oil pressure to a set pressure or lower.

[0005] Engine power is input to the input shaft of a piston pump. In a piston pump, the input shaft rotates due to engine power, and the cylinder block rotates together with the input shaft. As the cylinder block rotates, the pistons located inside the cylinder block reciprocate with a stroke that corresponds to the inclination angle of the swash plate. As the pistons reciprocate, oil is sucked from the second oil passage into the suction / discharge port, and oil is discharged from the discharge / suction port into the first oil passage.

[0006] When oil is discharged from the piston pump's discharge / suction port into the first oil passage, the oil flowing through the first oil passage flows into the cylinder block through the piston motor's inlet / outlet port. In a piston motor, the pressure of the oil flowing into the cylinder block presses the pistons located in the cylinder block against the swash plate. The reaction force that the pistons receive from the swash plate rotates the cylinder block, and the output shaft rotates integrally with the cylinder block. The rotation of the output shaft is transmitted to the crawler travelling device via a transmission gear or other device.

[0007] Some HSTs are equipped with a servo piston for changing the reduction ratio of the piston motor. The rod of the servo piston can be switched between a low-speed position and a high-speed position, for example, by hydraulic pressure. When the rod is in the low-speed position, the swash plate of the piston motor is in the low position with a relatively large inclination angle, and when the rod is in the high-speed position, the swash plate of the piston motor is in the high position with a relatively small inclination angle. By switching the position of the rod between the low-speed position and the high-speed position, it is possible to switch between two speeds: a low speed with a relatively large reduction ratio and a high speed with a relatively small reduction ratio, without changing the flow rate of oil supplied to the piston motor.

[0008] In the high-speed gear ratio, the rotation speed of the output shaft is relatively high, so when a high load is applied to the output shaft, the hydraulic pressure in the closed circuit tends to rise. Specifically, when a high load is applied to the output shaft, the flow rate of oil discharged from the piston pump increases to maintain the rotation speed of the output shaft, causing the hydraulic pressure in the closed circuit to rise. If the load on the engine driving the piston pump increases in order to increase the flow rate of oil discharged from the piston pump, this could result in an engine stall. Furthermore, if the hydraulic pressure in the closed circuit rises and the relief valve operates, the flow rate of oil supplied to the piston motor decreases, causing a significant drop in the rotation speed of the output shaft, potentially preventing the combine from traveling. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-149496 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a transmission and a work vehicle that can suppress an increase in the flow rate of oil discharged from a piston pump when a high load is applied at a high speed gear.

[0011] Another object of the present invention is to provide a transmission and a work vehicle that can suppress a decrease in the output shaft (vehicle speed) when a high load is applied at a high speed gear. [Means for solving the problem]

[0012] In order to achieve the above-mentioned object, a transmission according to one aspect of the present invention is a transmission that is mounted on a work vehicle, changes the speed of power input to an input shaft, outputs the power to an output shaft, and transmits the power from the output shaft to a travel device of the work vehicle, and includes a piston pump that converts rotational energy of the input shaft into fluid energy, a piston motor that converts the fluid energy into rotational energy of the output shaft, a closed circuit that circulates oil between the piston pump and the piston motor, and an auxiliary transmission servo piston that has a rod that is hydraulically displaceable between a low-speed position and a high-speed position, and the displacement of the rod changes the inclination angle of a swash plate of the piston motor, wherein a low-speed gear with a relatively large gear ratio is configured by positioning the rod of the auxiliary transmission servo piston in the low-speed position, and a high-speed gear with a relatively small gear ratio is configured by positioning the rod of the auxiliary transmission servo piston in the high-speed position, and the hydraulic pressure supplied to the auxiliary transmission servo piston is set so that when the hydraulic pressure in the closed circuit exceeds a predetermined pressure while the high-speed gear is configured, the force holding the rod of the auxiliary transmission servo piston in the high-speed position is greater than the force acting on the swash plate from the piston of the piston motor.

[0013] According to this configuration, when a high load is applied to the output shaft while the high-speed gear is engaged, the flow rate of oil discharged from the piston pump increases to maintain the rotation speed of the output shaft, causing the hydraulic pressure in the closed circuit to rise. When the hydraulic pressure in the closed circuit exceeds a predetermined pressure due to the setting of the hydraulic pressure supplied to the auxiliary transmission servo piston, the force holding the rod of the auxiliary transmission servo piston in the high-speed position overcomes the force acting on the swash plate from the piston of the piston motor. This causes the rod to move from the high-speed position toward the low-speed position, changing the inclination angle of the swash plate of the piston motor. As a result, the gear ratio of the transmission increases, increasing the torque of the output shaft, thereby suppressing the increase in the flow rate of oil discharged from the piston pump.

[0014] A work vehicle according to another aspect of the present invention includes an engine, a traveling device, and a transmission that changes the speed of engine power input to an input shaft and outputs it to an output shaft, and transmits power from the output shaft to the traveling device. The transmission includes a piston pump that converts rotational energy of the input shaft into fluid energy, a piston motor that converts the fluid energy into rotational energy of the output shaft, a closed circuit that circulates oil between the piston pump and the piston motor, and an auxiliary transmission servo piston that has a rod that is hydraulically displaceable between a low-speed position and a high-speed position, and the displacement of the rod changes the inclination angle of the swash plate of the piston motor. When the rod of the auxiliary transmission servo piston is positioned in the low-speed position, a low-speed gear with a relatively large gear ratio is configured, and when the rod of the auxiliary transmission servo piston is positioned in the high-speed position, a high-speed gear with a relatively small gear ratio is configured. When the hydraulic pressure in the closed circuit exceeds a predetermined pressure while the high-speed gear is configured, the hydraulic pressure supplied to the auxiliary transmission servo piston is set so that the force holding the rod of the auxiliary transmission servo piston in the high-speed position is overcome by the force acting on the swash plate from the piston of the piston motor.

[0015] This configuration can provide the same effects as those provided by the transmission configuration described above.

[0016] The transmission preferably further includes a relief valve that limits the hydraulic pressure in the closed circuit to a set pressure or less, the set pressure being set to a value lower than the set pressure.

[0017] With this configuration, when the hydraulic pressure in the closed circuit exceeds a predetermined pressure that is lower than the set pressure at which the relief valve operates, the gear ratio of the transmission increases, and the torque of the output shaft increases, thereby suppressing an increase in the flow rate of oil discharged from the piston pump. As a result, operation of the relief valve can be suppressed, and a decrease in the rotation speed of the output shaft due to operation of the relief valve can be suppressed, preventing the work vehicle from becoming unable to travel.

[0018] The transmission may further include a main speed-change servo piston having a rod that is displaced by hydraulic pressure and that changes the inclination angle of the swash plate of the piston pump by displacing the rod, and a control device that controls the hydraulic pressure supplied to the main speed-change servo piston so that a decrease in the vehicle speed of the work vehicle is suppressed in response to a change in the inclination angle of the swash plate of the piston motor when the force holding the rod of the sub-speed-change servo piston in a high-speed position is overcome by the force that the swash plate receives from the piston of the piston motor.

[0019] The transmission may also be configured to have a rod that is displaced by hydraulic pressure and further include a main speed-change servo piston that changes the inclination angle of the swash plate of the piston pump by displacing the rod, and the work vehicle may further include a control device that controls the hydraulic pressure supplied to the main speed-change servo piston so that a decrease in the vehicle speed of the work vehicle is suppressed in response to a change in the inclination angle of the swash plate of the piston motor when the force holding the rod of the sub-speed-change servo piston in a high-speed position is overcome by the force that the swash plate receives from the piston of the piston motor.

[0020] By changing the inclination angle of the piston pump's swash plate, the flow rate of oil discharged from the piston pump can be increased, thereby increasing the rotation speed of the output shaft (reducing the transmission ratio). This causes the rod of the sub-transmission servo piston to displace from the high-speed position toward the low-speed position, compensating for the decrease in rotation speed of the output shaft caused by the increased transmission ratio. Therefore, when a high load is applied to the output shaft with the high-speed gear configured, it is possible to suppress a decrease in rotation speed of the output shaft, and ultimately a decrease in the vehicle speed of the work vehicle. As a result, the operator of the work vehicle can be given a good driving feel without feeling a decrease in vehicle speed.

[0021] A transmission according to yet another aspect of the present invention is a transmission that is mounted on a work vehicle, changes the speed of power input to an input shaft, outputs the power to an output shaft, and transmits the power from the output shaft to a traveling device of the work vehicle, and includes a piston pump that converts rotational energy of the input shaft into fluid energy, a piston motor that converts the fluid energy into rotational energy of the output shaft, a closed circuit that circulates oil between the piston pump and the piston motor, a main speed-change servo piston that has a rod that is displaced by hydraulic pressure and changes the inclination angle of a swash plate of the piston pump by displacement of the rod, and a rod that is displaced hydraulically between a low-speed position and a high-speed position. The work vehicle is provided with a sub-speed change servo piston that changes the inclination angle of the swash plate of the piston motor by displacing the rod, and a control device, wherein a low speed stage with a relatively large gear ratio is formed when the rod of the sub-speed change servo piston is positioned at a low speed position, and a high speed stage with a relatively small gear ratio is formed when the rod of the sub-speed change servo piston is positioned at a high speed position, and the control device controls the oil pressure supplied to the main speed change servo piston so as to suppress a decrease in the vehicle speed of the work vehicle in response to the displacement of the rod of the sub-speed change servo piston from the high speed position to the low speed position and the change in the inclination angle of the swash plate of the piston motor.

[0022] According to this configuration, when a high load is applied to the output shaft while the high-speed gear is engaged, the rod of the auxiliary transmission servo piston displaces from the high-speed position to the low-speed position, changing the inclination angle of the swash plate of the piston motor. This increases the gear ratio of the transmission and reduces the rotation speed of the output shaft. At this time, the hydraulic pressure supplied to the main transmission servo piston is controlled to change the inclination angle of the swash plate of the piston pump, thereby increasing the flow rate of oil discharged from the piston pump, thereby compensating for the reduction in rotation speed of the output shaft due to the change in the inclination angle of the swash plate of the piston motor. Therefore, when a high load is applied to the output shaft while the high-speed gear is engaged, a reduction in rotation speed of the output shaft and, ultimately, a reduction in the vehicle speed of the work vehicle can be suppressed. As a result, the operator of the work vehicle can enjoy a good driving experience without feeling a reduction in vehicle speed.

[0023] In such a configuration, when the high-speed stage is configured and the oil pressure in the closed circuit exceeds a predetermined pressure, the oil pressure supplied to the sub-transmission servo piston may be set so that the force holding the rod of the sub-transmission servo piston in the high-speed position is overcome by the force that the swash plate receives from the piston of the piston motor. Alternatively, for example, a hydraulic pressure detector may be provided that detects the oil pressure in the closed circuit, and when the oil pressure detected by the hydraulic pressure detector exceeds the predetermined pressure, the control device may control the oil pressure supplied to the sub-transmission servo piston to displace the rod of the sub-transmission servo piston from the high-speed position toward the low-speed position.

[0024] A work vehicle according to yet another aspect of the present invention includes an engine, a traveling device, a transmission that changes the speed of engine power input to an input shaft and outputs it to an output shaft, and transmits the power from the output shaft to the traveling device, and a control device that controls the speed change by the transmission, the transmission including a piston pump that converts rotational energy of the input shaft into fluid energy, a piston motor that converts the fluid energy into rotational energy of the output shaft, a closed circuit that circulates oil between the piston pump and the piston motor, a main speed change servo piston having a rod that is displaced by hydraulic pressure and that changes the inclination angle of a swash plate of the piston pump by displacement of the rod, and a control device that controls the speed change by the transmission. and a sub-speed change servo piston having a rod that can be displaced between a low speed position and a high speed position, and the displacement of the rod changes the inclination angle of the swash plate of the piston motor. When the rod of the sub-speed change servo piston is positioned at the low speed position, a low speed stage with a relatively large gear ratio is formed, and when the rod of the sub-speed change servo piston is positioned at the high speed position, a high speed stage with a relatively small gear ratio is formed. The control device controls the oil pressure supplied to the main speed change servo piston so that a decrease in the vehicle speed of the work vehicle is suppressed in response to the displacement of the rod of the sub-speed change servo piston from the high speed position to the low speed position and the change in the inclination angle of the swash plate of the piston motor.

[0025] According to this configuration, it is possible to achieve the same effects as those of the transmission according to the other aspects.

[0026] The control device may use the vehicle speed of the work vehicle detected by a vehicle speed detector to control the hydraulic pressure supplied to the main transmission servo piston. Control using the vehicle speed detector can minimize a decrease in vehicle speed. [Effects of the Invention]

[0027] According to the present invention, when a high load is applied in the high-speed gear ratio, an increase in the flow rate of oil discharged from the piston pump can be suppressed, and when a high load is applied in the high-speed gear ratio, a decrease in the output shaft (vehicle speed) can be suppressed. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a right side view of a combine harvester according to an embodiment of the present invention. [Figure 2] FIG. 2 is a perspective view of the power transmission unit as seen from the left front. [Figure 3] FIG. 2 is a perspective view of the power transmission unit as seen from the front right. [Figure 4] FIG. 2 is a hydraulic circuit diagram of the power transmission unit. [Figure 5] FIG. 2 is a block diagram showing the main parts of the electrical configuration of the combine harvester. [Figure 6] 4 is a flowchart showing a flow of vehicle speed decrease suppression control. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0030] <Combine> FIG. 1 is a right side view of a combine harvester 1 according to one embodiment of the present invention.

[0031] The combine harvester 1 is a harvester that harvests and threshes grain stalks while traveling in a field, and is an example of a work vehicle.

[0032] The combine harvester 1 employs a pair of left and right crawler traveling devices 11 as traveling devices capable of traveling over uneven terrain such as farm fields. The crawler traveling device 11 includes a drive roller (drive sprocket) 12, a tension roller 13, multiple track rollers 14, a carrier roller 15, and crawlers (crawler belts) 16. The rotation axes of the drive roller 12, tension roller 13, multiple track rollers 14, and carrier roller 15 extend in the left-right direction. The drive roller 12 and tension roller 13 are spaced apart from each other in the front-to-rear direction. The multiple track rollers 14 are arranged parallel to each other and spaced apart in the front-to-rear direction between the drive roller 12 and tension roller 13, at a position lower than the drive roller 12. The carrier roller 15 is arranged between the drive roller 12 and tension roller 13 so that the highest point of its circumferential surface is at the same height as the highest point of the circumferential surface of the drive roller 12. The crawler 16 is wound around a drive roller 12, a tension roller 13, a plurality of track rollers 14, and a carrier roller 15. The tension roller 13 applies tension to the crawler 16, and the track rollers 14 keep the outer circumferential surface of the crawler 16 in contact with the ground.

[0033] The combine harvester 1 also includes a machine frame 21 supported by the left and right crawler traveling devices 11. A cabin 22 and a grain tank 23 are provided on the machine frame 21.

[0034] The cabin 22 is disposed on the front end of the machine frame 21. The cabin 22 provides a space for the driver to board, and within this space, for example, a driver's seat where the operator sits and operating members operated by the operator are disposed. An openable door 24 is provided on the right side of the cabin 22, and the operator can open the door 24 to enter the cabin 22.

[0035] The grain tank 23 is located behind the cabin 22.

[0036] Also provided on the machine frame 21 are a reaping device 25 and a thresher (not shown). The reaping device 25 is disposed in front of the crawler traveling device 11 and reaps stalks planted in the field as the combine harvester 1 moves forward. The thresher is disposed to the left of the grain tank 23 and transports the base side of the stalks reaped by the reaping device 25 to the rear side via a threshing feed chain, and supplies the ear side of the stalk to a threshing chamber for threshing. Grain grains detached from the stalks are then transported from the thresher to the grain tank 23, where they are stored. An unloader 26 is connected to the grain tank 23, and the grains stored in the grain tank 23 can be removed by the unloader 26 and discharged outside the machine.

[0037] <Power transmission mechanism> Fig. 2 is a perspective view of the power transmission unit 27 as seen from the front left. Fig. 3 is a perspective view of the power transmission unit 27 as seen from the front right.

[0038] The combine harvester 1 is equipped with a power transmission unit 27 that transmits power to the left and right crawler traveling devices 11. The power transmission unit 27 includes an input shaft 31, a left drive shaft 32L, a right drive shaft 32R, a left HST (Hydro Static Transmission) 33L, and a right HST 33R.

[0039] The input shaft 31 receives power from an engine 101 (see FIG. 5).

[0040] The left drive shaft 32L and the right drive shaft 32R are connected to the drive rollers 12 (see FIG. 1) of the left and right crawler traveling devices 11, respectively.

[0041] Power is transmitted (input) from the input shaft 31 to the left HST 33L and the right HST 33R via a gear train. The left HST 33L and the right HST 33R individually reduce the speed of the power before outputting it. The power output from the left HST 33L is transmitted to the left drive shaft 32L via a gear train. This drives the drive roller 12 of the left crawler traveling device 11, causing the crawler 16 to rotate. The power output from the right HST 33R is transmitted to the right drive shaft 32R via a gear train. This drives the drive roller 12 of the right crawler traveling device 11, causing the crawler 16 to rotate.

[0042] FIG. 4 is a hydraulic circuit diagram of the power transmission unit 27.

[0043] Power transmission unit 27 includes a charge pump 34 and a charge oil passage 35. Charge pump 34 is a fixed displacement gear pump or vane pump. An input shaft 31 is coupled to a rotor of charge pump 34. When power from engine 101 is input to input shaft 31, the power causes the rotor of charge pump 34 to rotate integrally with input shaft 31, and oil is delivered from charge pump 34 to charge oil passage 35.

[0044] A filter 36 is installed in the charge oil passage 35. Oil delivered from the charge pump 34 to the charge oil passage 35 passes through the filter 36 to be filtered, and then is supplied to each part of the power transmission unit 27. A relief valve 37 is also connected to the charge oil passage 35. The relief valve 37 functions to limit the oil pressure in the charge oil passage 35 to a certain pressure (the upper limit of the original pressure) or less.

[0045] The left HST 33L is equipped with a hydraulic pump 41L and a hydraulic motor 42L.

[0046] The hydraulic pump 41L is a variable displacement swash plate piston pump. It includes a pump input shaft 43L, a cylinder block, multiple pistons, a swash plate, piston shoes, and a valve plate. Power from the input shaft 31 is input to the pump input shaft 43L via a gear train. The cylinder block is arranged to rotate integrally with the pump input shaft 43L. Multiple pistons are arranged in the cylinder block at equal angular intervals centered on the axis of the cylinder block. The swash plate is arranged so that its inclination with respect to the axis of the pump input shaft can be changed. The piston shoes are arranged between the pistons and the swash plate so that they rotate integrally with the cylinder block. The valve plate is located on the opposite side of the cylinder block from the swash plate and has ports 44L and 45L for discharging and suctioning oil.

[0047] The hydraulic motor 42L is a variable displacement swash plate piston motor. It includes a cylinder block, multiple pistons, a valve plate, a swash plate, piston shoes, and a motor output shaft 46L. The multiple pistons are arranged in the cylinder block at equal angular intervals around the axis of the cylinder block. The valve plate is located on the opposite side of the cylinder block from the swash plate and has ports 47L and 48L through which oil flows in and out. The swash plate is located on the opposite side of the cylinder block from the valve plate, and its inclination relative to the axis of the cylinder block can be changed. The piston shoes are located between the pistons and the swash plate so as to rotate integrally with the cylinder block. The motor output shaft 46L is located so as to rotate integrally with the cylinder block.

[0048] The port 44L of the hydraulic pump 41L and the port 47L of the hydraulic motor 42L are connected by an oil passage 51L, and the port 45L of the hydraulic pump 41L and the port 48L of the hydraulic motor 42L are connected by an oil passage 52L. As a result, the left HST 33L has a closed circuit that circulates oil between the hydraulic pump 41L and the hydraulic motor 42L through the oil passages 51L and 52L.

[0049] A check valve 53L and a relief valve 54L are interposed in parallel between oil passage 51L and charge oil passage 35. Check valve 53L functions to maintain the oil pressure in oil passage 51L at or above the original pressure, which is the oil pressure in charge oil passage 35. Relief valve 54L also functions to limit the oil pressure in oil passage 51L to a set pressure or lower. This maintains the oil pressure in oil passage 51L within a range above the original pressure and below the set pressure.

[0050] A check valve 55L and a relief valve 56L are interposed in parallel between the oil passage 52L and the charge oil passage 35. The check valve 55L functions to maintain the oil pressure in the oil passage 52L at or above the source pressure. The relief valve 56L also functions to limit the oil pressure in the oil passage 52L to a set pressure or less. This maintains the oil pressure in the oil passage 52L within a range above the source pressure and below the set pressure. The set pressures of the oil passages 51L and 52L are the same.

[0051] The hydraulic pump 41L converts the rotational energy of the pump input shaft 43L into fluid energy (kinetic energy of oil). Specifically, when the pump input shaft 43L of the hydraulic pump 41L rotates due to power from the input shaft 31, the cylinder block rotates integrally with the pump input shaft 43L. As the cylinder block rotates, the pistons reciprocate with a stroke that corresponds to the inclination angle of the swash plate. As the pistons reciprocate, oil is sucked from one of the oil passages 51L, 52L to one of the ports 44L, 45L, and oil is discharged from the other of the ports 44L, 45L to the other of the oil passages 51L, 52L.

[0052] A main speed-change servo piston 61L is provided to change the inclination angle of the swash plate of the hydraulic pump 41L (the acute angle that the swash plate forms with respect to a plane perpendicular to the axis of the cylinder block). The main speed-change servo piston 61L is, for example, a double-acting hydraulic cylinder, and includes a cylinder 62L, a piston 63L, and a rod 64L.

[0053] Oil passages 65L and 66L are connected to the cylinder 62L, and allow oil to flow in and out of spaces on one side and the other side of the piston 63L in the axial direction. Proportional control valves 67L and 68L are provided in the oil passages 65L and 66L, respectively.

[0054] The proportional control valves 67L, 68L control the supply of oil from the oil passages 65L, 66L into the cylinder 62L, and in response to this control, the piston 63L reciprocates axially within the cylinder 62L. The rod 64L is connected to the piston 63L and moves linearly in accordance with the movement of the piston 63L. The linear movement of the rod 64L changes the inclination angle of the swash plate of the hydraulic pump 41L.

[0055] In hydraulic pump 41L, when the tilt angle of the swash plate is zero, the piston does not move, and the oil discharge and suction flow rates are zero. Furthermore, the greater the tilt angle of the swash plate, the greater the piston stroke (volume) and the greater the oil discharge and suction flow rates. Reversing the tilt of the swash plate reverses the oil discharge direction. That is, when the swash plate is tilted in one direction, oil is discharged from one port 44L to oil passage 51L, and when the swash plate is tilted in the opposite direction, oil is discharged from the other port 45L to oil passage 52L.

[0056] The hydraulic motor 42L converts fluid energy supplied from the hydraulic pump 41L into rotational energy for the motor output shaft 46L. Specifically, when oil is discharged from port 44L of the hydraulic pump 41L to the oil passage 51L, the oil flowing through the oil passage 51L flows into the cylinder block through port 47L of the hydraulic motor 42L. In the hydraulic motor 42L, the pistons are pressed against the swash plate by the pressure of the oil flowing into the cylinder block. The pistons receive a reaction force from the swash plate, causing the cylinder block to rotate, and the motor output shaft 46L rotates integrally with the cylinder block. Additionally, when oil is discharged from port 45L of the hydraulic pump 41L to the oil passage 52L, the oil flowing through the oil passage 52L flows into the cylinder block through port 48L of the hydraulic motor 42L. The pressure of the oil flowing into the cylinder block presses the pistons against the swash plate. The pistons receive a reaction force from the swash plate, causing the cylinder block to rotate in the opposite direction to when oil flows in from port 47L, and the motor output shaft 46L rotates integrally with the cylinder block. The rotation of the motor output shaft 46L is transmitted to the left drive shaft 32L (see Figures 2 and 3) via a gear train.

[0057] An auxiliary speed-change servo piston 71L is provided to change the inclination angle of the swash plate of the hydraulic motor 42L (the acute angle that the swash plate forms with respect to a plane perpendicular to the axis of the cylinder block). The auxiliary speed-change servo piston 71L is, for example, a double-acting hydraulic cylinder, and includes a cylinder 72L, a piston 73L, and a rod 74L.

[0058] One ends of oil passages 75L and 76L, which allow oil to flow in and out of spaces on one and the other sides of the piston 73L in the axial direction, are connected to the cylinder 72L. The other ends of the oil passages 75L and 76L are connected to one ends of branch oil passages 81 and 82, respectively. The other ends of the branch oil passages 81 and 82 are connected to one end of a connecting oil passage 83, and the other end of the connecting oil passage 83 is connected to the charge oil passage 35. A solenoid valve 84 is provided in the branch oil passage 81. A pressure reducing valve 85 and a solenoid valve 86 are provided in the branch oil passage 82, in this order from the connecting oil passage 83 side.

[0059] When the solenoid valve 84 is turned on (open) and the solenoid valve 86 is turned off (closed), oil is supplied from the charge oil passage 35 to the cylinder 72L of the sub transmission servo piston 71L through the connecting oil passage 83 and the branch oil passage 81, and as a result, the piston 73L and the rod 74L of the sub transmission servo piston 71L are positioned at the low speed position. On the other hand, when the solenoid valve 84 is turned off and the solenoid valve 86 is turned on, oil is supplied from the charge oil passage 35 to the cylinder 72L of the sub transmission servo piston 71L through the connecting oil passage 83 and the branch oil passage 82, and as a result, the piston 73L and the rod 74L of the sub transmission servo piston 71L are positioned at the high speed position.

[0060] When the rod 74L of the auxiliary transmission servo piston 71L is in the low-speed position, the swash plate of the hydraulic motor 42L is in the low position, where it is tilted at a relatively large angle. When the rod 74L is in the high-speed position, the swash plate of the hydraulic motor 42L is in the high position, where it is tilted at a relatively small angle. When the swash plate of the hydraulic motor 42L is in the low position, the displacement of the hydraulic motor 42L is relatively large, and the rotation speed of the motor output shaft 46L is relatively small. On the other hand, when the swash plate of the hydraulic motor 42L is in the high position, the displacement of the hydraulic motor 42L is relatively small, and the rotation speed of the motor output shaft 46L is relatively large. Therefore, by switching the solenoid valves 84 and 86 on and off to switch the position of the swash plate of the hydraulic motor 42L between the low position and the high position, the left HST 33L can be switched between two speeds: a high speed with a relatively small reduction ratio and a low speed with a relatively large reduction ratio.

[0061] In the high-speed gear ratio, the rotation speed of the motor output shaft 46L is relatively high. Therefore, when a high load is applied to the motor output shaft 46L, the hydraulic pressure in the closed circuit that circulates oil between the hydraulic pump 41L and the hydraulic motor 42L tends to increase. That is, when a high load is applied to the motor output shaft 46L, the flow rate of oil discharged from the hydraulic pump 41L increases to maintain the rotation speed of the motor output shaft 46L, and the hydraulic pressure in the oil passages 51L and 52L (hereinafter referred to as the "closed circuit hydraulic pressure") increases. If the load on the engine 101 that drives the hydraulic pump 41L increases in order to increase the flow rate of oil discharged from the hydraulic pump 41L, engine stall may occur. Furthermore, if the relief valves 54L and 56L are activated due to an increase in the hydraulic pressure in the closed circuit, the flow rate of oil supplied to the hydraulic motor 42L decreases, causing a significant decrease in the rotation speed of the motor output shaft 46L, which may prevent the combine harvester 1 from traveling.

[0062] Therefore, a pressure reducing valve 85 is provided in the branch oil passage 82 closer to the charge oil passage 35 than the solenoid valve 86. The pressure reducing valve 85 reduces the pressure of the oil supplied from the branch oil passage 82 to the cylinder 72L of the sub transmission servo piston 71L from the oil pressure in the charge oil passage 35. The pressure reducing ratio of the pressure reducing valve 85 (the pressure difference between the primary and secondary sides of the pressure reducing valve 85) is set so that when the oil pressure in the closed circuit exceeds a predetermined pressure with the rod 74L of the sub transmission servo piston 71L positioned at the high speed position, the force of the oil pressure supplied from the branch oil passage 82 to the cylinder 72L of the sub transmission servo piston 71L to hold the rod 74L at the high speed position is overcome by the force acting on the swash plate from the piston of the hydraulic motor 42L. The predetermined pressure is set to a value lower than the set pressure, which is the upper limit of the oil pressure in the closed circuit, i.e., the oil pressure at which the relief valves 54L and 56L operate.

[0063] The right HST 33R has the same configuration as the left HST 33L, and therefore, for the right HST 33R, parts corresponding to those of the left HST 33L are given reference numerals with the suffix "L" changed to "R" for those parts, and the description thereof will be omitted. Note that the configuration for supplying oil to the sub transmission servo pistons 71L, 71R, i.e., the configuration consisting of branch oil passages 81, 82, connecting oil passage 83, solenoid valve 84, pressure reducing valve 85, and solenoid valve 86, is common to both the left HST 33L and the right HST 33R.

[0064] <Electrical configuration> FIG. 5 is a block diagram showing the main parts of the electrical configuration of the combine harvester 1. As shown in FIG.

[0065] The combine harvester 1 is equipped with a control device 91. The control device 91 includes a microcontroller unit (MCU), which includes, for example, a CPU and a memory. The memory includes a nonvolatile memory such as a flash memory and a volatile memory such as a dynamic random access memory (DRAM).

[0066] The cabin 22 of the combine harvester 1 is provided with a main speed change lever 92, a steering lever 93, and an auxiliary speed change lever 94 as operating members. The main speed change lever 92 can be tilted in the front-rear direction, and by tilting it, it is possible to instruct the combine harvester 1 to switch between forward and reverse travel and to change the forward or reverse speed. The steering lever 93 can be tilted left and right and front and rear, and by tilting it left and right, it is possible to instruct the combine harvester 1 to travel straight, turn left, or turn right, and by tilting it front and rear, it is possible to instruct the harvester 25 to be raised or lowered. The auxiliary speed change lever 94 can be tilted in the front and rear, and it is possible to instruct the combine harvester 1 to switch between a high speed and a low speed.

[0067] The combine harvester 1 is also provided with a main speed change lever sensor 95 that outputs a detection signal corresponding to the position of the main speed change lever 92, a steering lever sensor 96 that outputs a detection signal corresponding to the position of the steering lever 93, an auxiliary speed change lever sensor 97 that outputs a detection signal corresponding to the position of the auxiliary speed change lever 94, a left vehicle speed sensor 98L that outputs a detection signal in pulse synchronization with the rotation of the left drive shaft 32L, and a right vehicle speed sensor 98R that outputs a detection signal in pulse synchronization with the rotation of the right drive shaft 32R. Because there is a correlation between the speed of the left drive shaft 32L and the right drive shaft 32R and the speed of the combine harvester 1, the left vehicle speed sensor 98L and the right vehicle speed sensor 98R can be said to be an example of a vehicle speed detector that detects the vehicle speed of the combine harvester 1.

[0068] Detection signals from a main shift lever sensor 95, a steering lever sensor 96, an auxiliary shift lever sensor 97, a left vehicle speed sensor 98L, and a right vehicle speed sensor 98R are input to the control device 91. Based on the input detection signals, the control device 91 controls the operation of the engine 101, and also controls the operation of the proportional control valves 67L, 68L, 67R, 68R and the solenoid valves 84, 86 of the power transmission unit 27.

[0069] <Straight-line control> In the combine harvester 1, the control device 91 executes straight-line control to control the straight-line running of the combine harvester 1. In the straight-line control, the position of the main speed change lever 92 is detected from the detection signal of the main speed change lever sensor 95, and the position of the steering lever 93 is detected from the detection signal of the steering lever sensor 96.

[0070] When the steering lever 93 is in the straight-ahead position (position instructing straight-ahead travel) in the center of the left-right direction within its operable range and the main speed change lever 92 is in the neutral position (position instructing stop) in the center of the front-rear direction within its operable range, the current supplied to the proportional control valves 67L, 68L, 67R, 68R is controlled to set the tilt angles of the swash plates of the hydraulic pump 41L of the left HST 33L and the hydraulic pump 41R of the right HST 33R to zero. As a result, oil is not discharged from the hydraulic pumps 41L, 41R, so the motor output shaft 46L of the hydraulic motor 42L and the motor output shaft 46R of the hydraulic motor 42R do not rotate, and power is not transmitted to the left drive shaft 32L and the right drive shaft 32R. As a result, the left and right crawler traveling devices 11 do not operate, and the combine 1 stops.

[0071] When the main speed change lever 92 is tilted forward from the neutral position, a target vehicle speed in the forward direction is set according to the tilting operation. Then, the actual rotational speed of the left drive shaft 32L is obtained from the detection signal of the left vehicle speed sensor 98L, and the actual rotational speed of the right drive shaft 32R is obtained from the detection signal of the right vehicle speed sensor 98R. The currents supplied to the proportional control valves 67L, 68L, 67R, and 68R are controlled to change the inclination angles of the swash plates of the hydraulic pumps 41L and 41R so that the left drive shaft 32L and the right drive shaft 32R rotate at rotational speeds corresponding to the target vehicle speeds. As a result, the combine harvester 1 travels forward at a vehicle speed according to the tilting operation of the main speed change lever 92 forward.

[0072] When the main speed change lever 92 is tilted rearward from the neutral position, a target vehicle speed in the reverse direction is set according to the tilting operation. Then, the actual rotational speed of the left drive shaft 32L is obtained from the detection signal of the left vehicle speed sensor 98L, and the actual rotational speed of the right drive shaft 32R is obtained from the detection signal of the right vehicle speed sensor 98R. The currents supplied to the proportional control valves 67L, 68L, 67R, and 68R are controlled to change the inclination angles of the swash plates of the hydraulic pumps 41L and 41R so that the left drive shaft 32L and the right drive shaft 32R rotate at rotational speeds corresponding to the target vehicle speeds. As a result, the combine harvester 1 travels in reverse at a vehicle speed according to the tilting operation of the main speed change lever 92 rearward.

[0073] <Rotation control> In the combine harvester 1, a control device 91 executes a swing control for controlling the swing of the combine harvester 1. In the swing control, the position of a main speed change lever 92 is detected from a detection signal of a main speed change lever sensor 95, and the position of a steering lever 93 is detected from a detection signal of a steering lever sensor 96.

[0074] When the main speed change lever 92 is tilted and the combine harvester 1 is traveling at a speed corresponding to the tilt operation of the main speed change lever 92, if the steering lever 93 is tilted to the left or right, a target turning ratio is set according to the tilt operation of the steering lever 93. The turning ratio is the ratio of the rotational speed of the left drive shaft 32L or right drive shaft 32R on the inside of the turn (the side closer to the turning center) to the rotational speed of the left drive shaft 32L or right drive shaft 32R on the outside of the turn (the side farther from the turning center). Then, using the left drive shaft 32L or the right drive shaft 32R on the outside of the turn as a reference, the current supplied to the proportional control valves 67L, 68L, 67R, and 68R is controlled to change the tilt angle of the swash plates of the hydraulic pumps 41L and 41R so that the reference left drive shaft 32L or the right drive shaft 32R rotates at a rotational speed corresponding to the target vehicle speed, and the left drive shaft 32L or the right drive shaft 32R on the inside of the turn rotates at a rotational speed obtained by multiplying the rotational speed of the reference left drive shaft 32L or the right drive shaft 32R by the target turning ratio. As a result, the combine harvester 1 turns at a speed corresponding to the tilting operation of the main speed change lever 92, and in a direction and with a turning radius corresponding to the tilting operation of the steering lever 93.

[0075] <Vehicle speed reduction suppression control> FIG. 6 is a flowchart showing the flow of the vehicle speed decrease suppression control.

[0076] When the left HST 33L and the right HST 33R are in the high-speed gear position, a high load is applied to the left drive shaft 32L and the right drive shaft 32R, which may result in a high load being applied to the motor output shafts 46L, 46R. For example, when the combine harvester 1 starts to turn from a straight traveling state, a high load is applied to the left drive shaft 32L and the right drive shaft 32R. Other examples of when a high load is applied to the left drive shaft 32L and the right drive shaft 32R include when the crawler traveling device 11 (crawler 16) enters a muddy area in the field or when the harvesting device 25 starts harvesting straw.

[0077] When a high load is applied to the left drive shaft 32L, a high load is applied to the motor output shaft 46L. This causes the hydraulic pressure in the closed circuit to exceed a predetermined pressure, and the force holding the rod 74L of the sub-transmission servo piston 71L at the high-speed position is overwhelmed by the force acting on the swash plate from the piston of the hydraulic motor 42L. This causes the swash plate of the hydraulic motor 42L to move from the high-speed position to the low-speed position, increasing the reduction ratio of the left HST 33L. When a high load is applied to the right drive shaft 32R, a high load is applied to the motor output shaft 46R. This causes the hydraulic pressure in the closed circuit to exceed a predetermined pressure, and the force holding the rod 74R of the sub-transmission servo piston 71R at the high-speed position is overwhelmed by the force acting on the swash plate from the piston of the hydraulic motor 42R. This causes the swash plate of the hydraulic motor 42R to move from the high-speed position to the low-speed position, increasing the reduction ratio of the right HST 33R. As the reduction ratios of the left HST 33L and the right HST 33R increase, the vehicle speed of the combine 1 decreases.

[0078] In order to suppress such a decrease in vehicle speed, the control device 91 executes vehicle speed decrease suppression control.

[0079] In the vehicle speed reduction suppression control, it is determined whether the vehicle speed of the combine harvester 1 is a vehicle speed (target vehicle speed) corresponding to the position (tilting operation) of the main speed change lever 92 (step S1). Specifically, the actual rotational speed of the left drive shaft 32L is obtained from the left vehicle speed sensor 98L, and the actual rotational speed of the right drive shaft 32R is obtained from the detection signal of the right vehicle speed sensor 98R. When the combine harvester 1 is traveling straight, the actual rotational speed of the left drive shaft 32L and the actual rotational speed of the right drive shaft 32R become the same due to the straight traveling control, so it is determined whether the vehicle speed corresponding to one of the actual rotational speeds is a vehicle speed corresponding to the position (tilting operation) of the main speed change lever 92. When the combine harvester 1 is turning, it is determined whether the vehicle speed corresponding to the actual rotational speed of the left drive shaft 32L or the right drive shaft 32R, which is used as the reference on the outside of the turn, is a vehicle speed corresponding to the position (tilting operation) of the main speed change lever 92.

[0080] If the speed of the combine 1 is not the speed corresponding to the position of the main speed change lever 92 (NO in step S1), the current supplied to the proportional control valves 67L, 68L, 67R, 68R is controlled to control the hydraulic pressure supplied to the main speed change servo pistons 61L, 61R, and the inclination angle of each swash plate of the hydraulic pumps 41L, 41R is adjusted (step S2) so that the speed of the combine 1 corresponds to the position of the main speed change lever 92.

[0081] Specifically, if the speed of the combine harvester 1 is not the speed corresponding to the position of the main speed change lever 92 (NO in step S1), the current supplied to the proportional control valves 67L, 68L, 67R, 68R is controlled based on the deviation between the speed of the combine harvester 1 and the speed corresponding to the position of the main speed change lever 92, and the hydraulic pressure supplied to the main speed change servo pistons 61L, 61R is controlled. This control increases the flow rate of oil discharged from the hydraulic pumps 41L, 41R, and adjusts the inclination angle of the swash plates of the hydraulic pumps 41L, 41R so that the rotation speed of the motor output shafts 46L, 46R increases (the reduction ratio decreases) (step S2).

[0082] By such vehicle speed reduction suppression control, the reduction in the vehicle speed of the combine 1 can be suppressed.

[0083] <Action and effect> As described above, when a high load is applied to the motor output shaft 46L of the hydraulic motor 42L while the left HST 33L is in the high-speed gear position, the flow rate of oil discharged from the hydraulic pump 41L increases to maintain the rotational speed of the motor output shaft 46L, thereby increasing the hydraulic pressure in the closed circuit. The HST 33L includes relief valves 54L and 56L that limit the hydraulic pressure in the closed circuit to a set pressure or less. When the hydraulic pressure in the closed circuit exceeds a predetermined pressure, which is set lower than the set pressure by setting the hydraulic pressure supplied to the auxiliary transmission servo piston 71L, the force of the auxiliary transmission servo piston 71L that holds the rod 74L in the high-speed position is overcome by the force acting on the swash plate from the piston of the hydraulic motor 42L. This displaces the rod 74L from the high-speed position toward the low-speed position, changing the inclination angle of the swash plate of the hydraulic motor 42L. As a result, the speed ratio (reduction ratio) of the HST 33L increases, increasing the torque of the motor output shaft 46L, thereby suppressing the increase in the flow rate of oil discharged from the hydraulic pump 41L. As a result, the operation of the relief valves 54L, 56L can be suppressed, and a decrease in the rotation speed of the motor output shaft 46L due to the operation of the relief valves 54L, 56L can be suppressed. Similarly, for the right HST 33R, a decrease in the rotation speed of the motor output shaft 46R of the hydraulic motor 42R due to the operation of the relief valves 54R, 56R can be suppressed. Therefore, it is possible to prevent the combine 1 from becoming unable to travel.

[0084] Furthermore, the vehicle speed reduction suppression control can suppress a reduction in the vehicle speed of the combine harvester 1, so that the operator of the combine harvester 1 can be given a good driving feeling without feeling a reduction in vehicle speed.

[0085] In addition, when the vehicle speed decrease suppression control is executed, the operator may be notified of this.

[0086] Furthermore, when the vehicle speed decrease suppression control is continuously executed for a certain period of time or more, the operator may be notified that a high load is being applied to the left HST 33L and the right HST 33R.

[0087] <Modification> Although one embodiment of the present invention has been described above, the present invention can be embodied in other forms.

[0088] In the above-described embodiment, an example was given in which a high load is applied to the left drive shaft 32L and the right drive shaft 32R when the left HST 33L and the right HST 33R are configured in a high-speed gear. However, if a high load is applied only to the left drive shaft 32L, vehicle speed reduction suppression control is performed on the left HST 33L, and if a high load is applied only to the right drive shaft 32R, vehicle speed reduction suppression control is performed on the right HST 33R.

[0089] Furthermore, the actual rotational speed of the left drive shaft 32L is obtained from the left vehicle speed sensor 98L, and the actual rotational speed of the right drive shaft 32R is obtained from the detection signal of the right vehicle speed sensor 98R, and the inclination angles of the swash plates of the hydraulic pumps 41L, 41R are adjusted so that the vehicle speed corresponding to one of the actual rotational speeds coincides with the vehicle speed corresponding to the position of the main shift lever 92. Alternatively, a left swash plate position sensor that detects the position (inclination angle) of the swash plate of the hydraulic pump 41L and a right swash plate position sensor that detects the position of the swash plate of the hydraulic pump 41R may be provided, and the vehicle speed of the combine 1 may be obtained from the positions of the swash plates of the hydraulic pumps 41L, 41R and the rotation speeds of the pump input shafts 43L, 43R, and the inclination angles of the swash plates of the hydraulic pumps 41L, 41R may be adjusted so that the vehicle speed coincides with the vehicle speed corresponding to the position of the main shift lever 92.

[0090] Although a combine harvester 1 has been taken up as an example of a work vehicle, the present invention can be applied to various work vehicles, such as a harvester equipped with a harvesting and transporting device as a working device for harvesting and transporting vegetables such as carrots, radishes, edamame beans, and cabbage, a rice transplanter equipped with a seedling planting device as a working device, a direct seeder equipped with a sowing device as a working device, a tractor equipped with a plow as a working device, and a construction work vehicle equipped with a bucket as a working device.

[0091] In addition, various design modifications can be made to the above-described configuration within the scope of the claims. [Explanation of symbols]

[0092] 1: Combine (work vehicle) 11: Crawler running device (running device) 27: Power transmission unit (transmission) 41L, 41R: Hydraulic pump (piston pump) 42L, 42R: Hydraulic motor (piston motor) 43L, 43R: Pump input shaft (input shaft) 46L, 46R: Motor output shaft (output shaft) 51L, 52L, 51R, 52R: Oil path (closed circuit) 54L, 54R, 56L, 56R: Relief valve 61L, 61R: Main transmission servo piston 64L: Rod 71L, 71R: Sub-transmission servo piston 74L, 74R: Rod 91: Control device 98L: Left vehicle speed sensor 98R: Right speed sensor 101: Engine

Claims

1. A transmission that is mounted on a work vehicle, changes the speed of power input to an input shaft, outputs the power to an output shaft, and transmits the power from the output shaft to a traveling device of the work vehicle, a piston pump that converts rotational energy of the input shaft into fluid energy; a piston motor that converts fluid energy into rotational energy for the output shaft; a closed circuit for circulating oil between the piston pump and the piston motor; a sub-speed-change servo piston having a rod that is hydraulically displaced between a low-speed position and a high-speed position, and that changes the inclination angle of a swash plate of the piston motor by displacement of the rod; When the rod of the auxiliary transmission servo piston is positioned at the low speed position, a low speed stage with a relatively large transmission ratio is configured, and when the rod of the auxiliary transmission servo piston is positioned at the high speed position, a high speed stage with a relatively small transmission ratio is configured, a transmission in which, when the high-speed stage is configured and the hydraulic pressure in the closed circuit exceeds a predetermined pressure, the hydraulic pressure supplied to the auxiliary transmission servo piston is set so that the force holding the rod of the auxiliary transmission servo piston at the high-speed position is overcome by the force that the swash plate receives from the piston of the piston motor.

2. a relief valve that limits the hydraulic pressure in the closed circuit to a set pressure or less; 2. The transmission according to claim 1, wherein the predetermined pressure is set to a value lower than the set pressure.

3. a main speed-change servo piston having a rod that is displaced by hydraulic pressure, the main speed-change servo piston changing the tilt angle of a swash plate of the piston pump by displacing the rod; 3. The transmission according to claim 1, further comprising: a control device that controls hydraulic pressure supplied to the main shift servo piston so that a decrease in vehicle speed of the work vehicle is suppressed in response to a change in the inclination angle of the swash plate of the piston motor caused by a force that holds the rod of the auxiliary shift servo piston at the high speed position being overcome by a force that the swash plate of the piston motor receives from a piston.

4. a vehicle speed detector for detecting a vehicle speed of the work vehicle; 4. The transmission according to claim 3, wherein the control device controls the hydraulic pressure supplied to the main shift servo piston using the vehicle speed detected by the vehicle speed detector.

5. The engine and Running gear and a transmission that changes the speed of the power of the engine input to an input shaft and outputs the power to an output shaft, and transmits the power from the output shaft to the traveling device, The transmission is a piston pump that converts rotational energy of the input shaft into fluid energy; a piston motor that converts fluid energy into rotational energy for the output shaft; a closed circuit for circulating oil between the piston pump and the piston motor; a relief valve that limits the hydraulic pressure in the closed circuit to a set pressure or less; a sub-speed-change servo piston having a rod that is hydraulically displaced between a low-speed position and a high-speed position, and changing the inclination angle of a swash plate of the piston motor by displacement of the rod; When the rod of the auxiliary transmission servo piston is positioned at the low speed position, a low speed stage with a relatively large speed ratio is configured, and when the rod of the auxiliary transmission servo piston is positioned at the high speed position, a high speed stage with a relatively small speed ratio is configured, a hydraulic pressure supplied to the auxiliary transmission servo piston is set so that, when the high speed stage is configured and the hydraulic pressure in the closed circuit exceeds a predetermined pressure, the force holding the rod of the auxiliary transmission servo piston at the high speed position is overcome by a force that the swash plate receives from the piston of the piston motor.

6. The work vehicle according to claim 5 , wherein the predetermined pressure is set to a value lower than the set pressure.

7. the transmission further includes a main speed-change servo piston having a rod that is displaced by hydraulic pressure and that changes the tilt angle of the swash plate of the piston pump by displacement of the rod; 7. The work vehicle according to claim 5 or 6, further comprising a control device that controls hydraulic pressure supplied to the main transmission servo piston so that a decrease in vehicle speed of the work vehicle is suppressed in response to a change in the inclination angle of the swash plate of the piston motor caused by a force that holds the rod of the auxiliary transmission servo piston at the high speed position being overcome by a force that the swash plate of the piston motor receives from a piston.

8. a vehicle speed detector for detecting a vehicle speed of the work vehicle; The work vehicle according to claim 7, wherein the control device controls the hydraulic pressure supplied to the main transmission servo piston using the vehicle speed detected by the vehicle speed detector.

9. A transmission that is mounted on a work vehicle, changes the speed of power input to an input shaft, outputs the power to an output shaft, and transmits the power from the output shaft to a traveling device of the work vehicle, a piston pump that converts rotational energy of the input shaft into fluid energy; a piston motor that converts fluid energy into rotational energy for the output shaft; a closed circuit for circulating oil between the piston pump and the piston motor; a main speed-change servo piston having a rod that is displaced by hydraulic pressure, the main speed-change servo piston changing the tilt angle of a swash plate of the piston pump by displacing the rod; an auxiliary speed-change servo piston having a rod that is hydraulically displaced between a low-speed position and a high-speed position, and that changes the inclination angle of a swash plate of the piston motor by displacement of the rod; a control device; When the rod of the auxiliary transmission servo piston is positioned at the low speed position, a low speed stage with a relatively large speed ratio is configured, and when the rod of the auxiliary transmission servo piston is positioned at the high speed position, a high speed stage with a relatively small speed ratio is configured, The control device controls the hydraulic pressure supplied to the main transmission servo piston so as to suppress a decrease in the vehicle speed of the work vehicle in response to a change in the inclination angle of the swash plate of the piston motor as the rod of the sub transmission servo piston is displaced from the high speed position toward the low speed position.

10. a vehicle speed detector for detecting a vehicle speed of the work vehicle; 10. The transmission according to claim 9, wherein the control device controls the hydraulic pressure supplied to the main shift servo piston using the vehicle speed detected by the vehicle speed detector.

11. The engine and Running gear and a transmission that changes the speed of the engine power input to an input shaft and outputs the power to an output shaft, and transmits the power from the output shaft to the traveling device; a control device for controlling the speed change by the transmission, The transmission is a piston pump that converts rotational energy of the input shaft into fluid energy; a piston motor that converts fluid energy into rotational energy for the output shaft; a closed circuit for circulating oil between the piston pump and the piston motor; a main speed-change servo piston having a rod that is displaced by hydraulic pressure, the main speed-change servo piston changing the tilt angle of a swash plate of the piston pump by displacing the rod; a sub-speed-change servo piston having a rod that is hydraulically displaced between a low-speed position and a high-speed position, and changing the inclination angle of a swash plate of the piston motor by displacement of the rod; When the rod of the auxiliary transmission servo piston is positioned at the low speed position, a low speed stage with a relatively large speed ratio is configured, and when the rod of the auxiliary transmission servo piston is positioned at the high speed position, a high speed stage with a relatively small speed ratio is configured, The control device controls the hydraulic pressure supplied to the main transmission servo piston so as to suppress a decrease in vehicle speed of the work vehicle in response to a change in the inclination angle of the swash plate of the piston motor as the rod of the sub transmission servo piston is displaced from the high speed position toward the low speed position.

12. a vehicle speed detector for detecting a vehicle speed of the work vehicle; The work vehicle according to claim 11, wherein the control device controls the hydraulic pressure supplied to the main transmission servo piston using the vehicle speed detected by the vehicle speed detector.

Images