JP2000074184A - Hydrostatic transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrostatic transmission device which can avoid degradation of the transmitting efficiency by introducing such a structure that the characteristics including continuous variable speed change are maintained in the low speed running and the drive force of a power source is transmitted directly to a hydraulic motor in the high speed running. SOLUTION: A hydraulic motor 3 is set to the first or second speed position automatically in compliance with the pressure of a forward running line 2. When the swash plate inclining angle of a variable discharge pump 1 is made over the set angle in the condition that the hydraulic motor 3 is set to the second speed position, a clutch 13 is engaged to put the hydraulic motor 3 in direct coupling with the engine E, and the motor 3 is driven by the engine E, while in any other cases, the arrangement is allowed to work as a usual hydrostatic transmission device whereby the hydraulic motor 3 is driven by the discharged fluid of the pump 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrostatic transmission device used as a drive mechanism for a working vehicle such as a road roller.

[0002]

2. Description of the Related Art Although not specifically shown, a hydrostatic transmission device has a closed circuit constituted by a variable discharge pump and a hydraulic motor. That is, when the variable discharge pump is rotated by the driving force of the engine, the variable discharge pump discharges the hydraulic oil according to the inclination of the swash plate. Then, the hydraulic oil is rotated by the discharged oil to drive the vehicle, and the hydraulic oil discharged from the hydraulic motor is returned to the variable discharge pump.

[0003] In such a hydrostatic transmission device, if the engine speed is kept constant, the variable discharge pump also rotates at a constant speed. Then, in this state, if the swash plate of the variable discharge pump is tilted to control the discharge amount from the minimum to the maximum, the rotation speed of the hydraulic motor can be continuously controlled. Therefore, stepless speed change is possible, and for example, smooth start, acceleration, and deceleration can be realized. Further, if the inclination of the swash plate of the variable discharge pump is reversed, the discharge direction can be reversed. Therefore, the hydraulic motor can be rotated in both the forward and reverse directions to cause the vehicle to travel forward or to travel backward.

[0004]

In such a hydrostatic transmission device as described above, a characteristic such as a stepless speed change becomes very effective especially at a low speed running where the speed is often changed. However, since the driving force of the engine is transmitted to the hydraulic motor side indirectly via the hydraulic oil instead of directly, the transmission efficiency is poor, and the fuel efficiency is inferior. When the vehicle normally travels at high speed, there is almost no need for shifting. for that reason,
At the time of high-speed running, a drawback that transmission efficiency is worse than a characteristic such as a stepless shift is conspicuous, and this drawback becomes a serious problem. An object of the present invention is to maintain the characteristics of stepless shifting during low-speed running, but to transmit the driving force of the power source directly to the hydraulic motor side during high-speed running, thereby reducing the transmission efficiency. It is to provide a hydrostatic transmission device that can be avoided.

[0005]

According to a first aspect of the present invention, there is provided a variable discharge pump which is connected to a power source which rotates in one direction and changes a discharge amount and a discharge direction in accordance with the inclination of a swash plate. A fluid motor driven by the discharge fluid of the variable discharge pump and having two large and small capacities set according to the inclination of the swash plate; and one port of the variable discharge pump A forward traveling line connected to one port of the motor, a pressure sensor for detecting the pressure of the forward traveling line, and a reverse traveling line connecting the other port of the variable discharge pump to the other port of the fluid pressure motor; A bypass line that communicates with the forward traveling line and the reverse traveling line and bypasses the fluid pressure motor;
A communication valve that communicates or shuts off this bypass line, and a fluid pressure motor is linked to the power source side,
A clutch for interrupting the linkage, a clutch link mechanism that operates on the hydraulic pressure to act on the clutch to link the fluid pressure motor to the power source side, a clutch control valve, and the clutch control valve are switched. A control pump for supplying fluid to the clutch linkage mechanism, a displacement switching mechanism for switching the inclination of the swash plate of the fluid pressure motor, and a tilt angle sensor for detecting the tilt angle of the swash plate of the variable discharge pump. A controller that controls the capacity switching mechanism, the clutch control valve, and the communication valve in accordance with detection signals from the pressure sensor and the tilt angle sensor.

The controller switches the capacity switching mechanism to the small capacity side when a detection signal of a set pressure or less is input from the pressure sensor. In this state, the tilt angle sensor moves the vehicle forward. When detecting a tilt angle equal to or larger than the set angle within the traveling range, the controller switches between the clutch control valve and the communication valve to supply fluid from the control pump to the clutch linkage mechanism to engage the clutch. In addition, it is characterized in that the above-mentioned bypass line is connected.

In a second aspect of the present invention, there is provided a communication line connecting a forward traveling line and a reverse traveling line, a connection portion connecting a discharge port of a control pump to the communication line, and the connection line on the communication line. A check valve provided on the forward traveling line side from the portion and allowing only the flow from the connection portion to the forward traveling line, and a check valve provided on the communication line and on the reverse traveling line side from the connection portion, from the connection portion A check valve that allows only the flow to the reverse traveling line is provided, and the capacity switching mechanism is operated by a fluid supplied from a control pump.

[0008]

FIG. 1 shows an embodiment of a hydrostatic transmission device according to the present invention. In this device, the variable discharge pump 1 is linked to an engine E which is a power source thereof. And this variable discharge pump 1
Is connected to one port 3 a of the hydraulic motor 3 via the forward traveling line 2. Further, the other port 1b of the variable discharge pump 1 is connected to the other port 3b of the hydraulic motor 3 via the reverse traveling line 4. Therefore, the variable discharge pump 1 and the hydraulic motor 3 form a closed circuit.

The variable discharge pump 1 always rotates in the direction of rotation of the engine E. If the swash plate of the variable discharge pump 1 is tilted in the forward traveling range, the variable discharge pump 1 discharges hydraulic oil from the port 1a to the forward traveling line 2 side. Therefore, the hydraulic motor 3 rotates forward at a speed corresponding to the discharge amount, and the vehicle travels forward. The maximum discharge amount is obtained when the tilt angle of the swash plate of the variable discharge pump 1 is maximum. If the swash plate of the variable discharge pump 1 is tilted in the reverse travel range, the discharge direction of the variable discharge pump 1 is reversed, and the reverse travel line 4
Discharge hydraulic oil to the side. Therefore, the hydraulic motor 3 rotates in reverse at a speed corresponding to the discharge amount, and the vehicle travels backward.

The swash plate of the variable discharge pump 1 is associated with a tilt angle sensor 5 for detecting the tilt of the swash plate. The tilt angle sensor 5 is an electrical sensor, is connected to the controller C, and inputs a tilt angle detection signal to the controller C. Further, a control pump 6 is connected to the variable discharge pump 1. The control pump 6 rotates together with the variable discharge pump 1 by the driving force of the engine E, and discharges oil from the discharge port 6a.

Also, between the two traveling lines 2 and 4,
A bypass line 7 that bypasses the hydraulic motor 3 is provided, and a communication valve 8 is provided in the bypass line 7.
The communication valve 8 is maintained at a closed position, which is a normal position, by the action of a spring 8a, and is switched to an open position for communicating the ports 8c and 8d by exciting the solenoid 8b. The solenoid 8b is controlled by an output signal of the controller C. In the bypass line 7, a pressure sensor 9 is provided on the forward traveling line 2 side from the communication valve 8. This pressure sensor 9
The pressure of the forward traveling line 2 is detected, and the detection signal is input to the controller C.

On the other hand, the hydraulic motor 3 is a two-speed motor capable of changing the inclination of the swash plate in two stages and setting its capacity to two types, large and small. In the apparatus of this embodiment, the cylinder 10 is used as a capacity switching mechanism for switching the capacity according to the inclination of the swash plate of the hydraulic motor 3. That is, when the cylinder 10 is in the contracted state shown in the figure, the tilt angle of the hydraulic motor 3 is kept at the maximum. When the hydraulic motor 3 extends, the swash plate is pushed by the piston rod 10a to keep the tilt angle to a minimum.

When the tilt angle of the hydraulic motor 3 is large, the torque increases, and when the tilt angle is small, the torque decreases. If the discharge rate of the variable discharge pump 1 is constant, the hydraulic motor 3 rotates at a low speed when the tilt angle is large, and at a high speed when the tilt angle is small. Therefore, when the tilt angle of the hydraulic motor 3 is large and the capacity is large, the hydraulic motor 3 has a high torque and a low rotation, and is in a so-called first speed state. On the other hand, when the tilt angle is small and the displacement is small, the hydraulic motor 3 has a low torque and a high rotation, and is in a so-called second speed state. However, in this hydrostatic transmission device, the rotation speed of the hydraulic motor 3 is controlled by the variable discharge pump 1
Because it depends on the discharge amount of
High torque and high rotation are possible, and low torque and low rotation are also possible. Therefore, this embodiment is characterized in that the range of speed control in first speed or second speed is increased.

As described above, the cylinder 10 for controlling the tilt angle of the hydraulic motor 3 comprises a hydraulic chamber 10b and a rod-side chamber 10
c, and a spring 10d is provided in the rod side chamber 10c. Therefore, when the hydraulic pressure is not acting on the hydraulic chamber 10b, the cylinder 10 maintains the contracted state shown in the drawing by the action of the spring 10d. In other words, when the hydraulic pressure is not acting on the hydraulic chamber 10b,
The hydraulic motor 3 is set to the first speed state. Also,
When a hydraulic pressure acts on the hydraulic chamber 10b, the cylinder 10 expands against the spring 10d, the tilt angle of the hydraulic motor 3 is reduced, and the hydraulic motor 3 is set to the second speed state.

The hydraulic chamber 1 of the cylinder 10 as described above
0b is connected to the port 11a of the capacity control valve 11. Another port 1 of the capacity control valve 11
1b is connected to the discharge port 6a of the control pump 6,
The hydraulic chamber 10b is connected to the control pump 6 via a capacity control valve 11. The capacity control valve 11
Is a valve which is switched by exciting the solenoid 11c, and the normal position shown is maintained by a spring 11d. And this solenoid 11c
Is controlled by the controller C. In the illustrated state, the hydraulic chamber 10b is connected to the tank T via the port 11a of the displacement control valve 11. Therefore,
The cylinder 10 is contracted, and the hydraulic motor 3 is set to the first speed.

On the other hand, when the solenoid 11c is excited and the displacement control valve 11 is switched from the normal position to the offset position, the ports 11a and 11b communicate with each other, and pressure oil is supplied from the control pump 6 to the hydraulic chamber 10b. . Therefore, the cylinder 10 is extended and the hydraulic motor 3
Set to the fast side. As is clear from the above,
Depending on the switching position of the displacement control valve 11, the hydraulic motor 3 is set to first speed or second speed. The capacity control valve 11 is controlled by a controller C. When a pressure detection signal equal to or less than a set pressure is input from the pressure sensor 9, the controller C excites the solenoid 11 c, and controls the capacity control valve 11. Is switched. That is, the pressure of the forward traveling line 2 is
When the pressure falls below the set pressure, the hydraulic motor 3 is set to the second speed.

The pressure of the forward traveling line 2 varies depending on the traveling state of the vehicle. For example, when the vehicle is traveling on level ground and no load is applied, the pressure is low. Conversely, when the vehicle is starting or climbing a hill, the pressure of the forward traveling line 2 increases. That is, when the pressure sensor 9 detects high pressure, it is when high torque is required, and when it detects low pressure, it is when low torque is sufficient. Therefore, the controller C has the pressure sensor 9
When a detection signal below the set pressure is input,
The displacement control valve 11 is switched to set the hydraulic motor 3 to the second speed with a low torque.
On the other hand, when the pressure sensor 9 detects a pressure exceeding the set pressure, the displacement control valve 11 is kept at the normal position shown in the figure. That is, the first speed and the second speed are automatically set to the hydraulic motor 3 in accordance with the detection signal from the pressure sensor 9.

Further, in this embodiment, unlike the conventional hydrostatic transmission device, the hydraulic motor 3 is linked to the variable discharge pump 1 via a shaft 12. However, a clutch 13 is provided in the middle of the shaft 12. Although not specifically shown, the clutch 13 employs a friction clutch system, and is normally in a disconnected state in which the hydraulic motor 3 is disconnected from the variable discharge pump 1. When the clutch 13 is switched to the linked state, the hydraulic motor 3 is mechanically linked to the variable discharge pump 1 so that the driving force of the engine E is directly transmitted to the hydraulic motor 3 via the shaft 12. I have to.

The clutch 13 is operated by a clutch linkage 14 which is operated by hydraulic pressure. A clutch control valve 15 is connected to the clutch linkage mechanism 14. This clutch control valve 1
5 normally maintains the illustrated normal position by the action of the spring 15a. In this normal position, the clutch linking mechanism 14 is linked to the tank T via the port 15b, and keeps the operating force released. If the clutch linking mechanism 14 is in the operating force released state, the clutch 13 remains disconnected. When the solenoid 15c of the clutch control valve 15 is excited, the clutch control valve 15 switches to the offset position on the right side in the drawing against the spring 15a. In this switching position, the port 15b
And 15d communicate with each other, so that the pressure oil from the control pump 6 is guided to the clutch linkage mechanism 14. When pressure oil is introduced into the clutch linking mechanism 14, it acts as an operating force to link the clutch 13.

Since the solenoid 15c is connected to the controller C, it is controlled by the controller C to keep the clutch control valve 15 at the normal position or at the switching position. Become. Then, when the following two conditions are satisfied, the controller C excites the solenoid 15c,
The clutch 13 is linked. That is, the first condition is that the displacement control valve 11 is switched and the hydraulic motor 3 is
It is set to speed. The second condition is
This is when the variable discharge pump 1 is tilted at a set angle or more in the forward traveling range. When these two conditions are simultaneously satisfied, the solenoid 15c is excited. The set angle is a maximum tilt angle of the swash plate of the variable discharge pump 1 or a tilt angle close thereto. That is,
When the vehicle reaches the maximum speed, the solenoid 15
c is excited, and the hydraulic motor 3 is directly driven by the engine E.

When the above two conditions are satisfied,
The controller C also excites the solenoid 8b of the communication valve 8 provided in the bypass line 7. That is, the controller C sets the hydraulic motor 3 to the second speed, and excites the solenoid 8b when the tilt angle sensor 5 detects a tilt angle equal to or larger than the set angle in the forward traveling range. That is, the bypass line 7 is communicated under the same conditions as when the clutch control valve 15 described above is switched.

Further, between the two traveling lines 2 and 4,
A communication line 16 is provided. This communication line 16
It is connected to the discharge port 6a of the control pump 6 via the connection section 17. A check valve 18 is provided on the communication line 16 and on the side of the forward traveling line 2 from the connection portion 17.
Is provided. This check valve 18 is
From the vehicle to the forward traveling line 2 only. on the other hand,
A check valve 19 is provided on the communication line 16 and also on the reverse traveling line 4 side from the connection portion 17. The check valve 19 allows only the flow from the connection portion 17 to the reverse traveling line 4 side. Such a communication line 1
6, the variable discharge pump 1 → the forward traveling line 2
The oil can be replenished from the control pump 6 when the oil leaks from the closed circuit consisting of the hydraulic motor 3 and the reverse traveling line 4 and the amount of oil is reduced.

Next, the operation of the hydrostatic transmission of this embodiment will be described. In a work vehicle such as a load roller, the rotation speed of the engine E is kept constant, and an operation lever (not shown) is moved to operate the swash plate of the variable discharge pump 1. First, the case where the swash plate of the variable discharge pump 1 is tilted in the forward traveling range of the vehicle will be described. In the forward traveling range of the vehicle, as described above, the variable discharge pump 1 discharges the operating oil to the forward traveling line 2 to rotate the hydraulic motor 3 forward.

At this time, if the pressure sensor 9 detects a pressure exceeding the set pressure, the displacement control valve 11 is held at the illustrated normal position. Therefore, the cylinder 10
The hydraulic chamber 10b communicates with the tank T. If the hydraulic chamber 10b communicates with the tank T, the cylinder 10
The contraction state is maintained by the action of d, and the hydraulic motor 3 is maintained in the first speed state. If a pressure detection signal exceeding the set pressure is input from the pressure sensor 9 to the controller C, the hydraulic motor 3 is in the first speed state. At this time, the clutch control valve 15 also holds the illustrated normal position. . Therefore, the clutch linkage mechanism 14 maintains the operating force released state, and keeps the clutch 13 disconnected. At this time, the communication valve 8 also maintains the closed position. Since the communication valve 8 keeps the closed position in this manner, the discharge oil of the variable discharge pump 1 is supplied to the hydraulic motor 3, and the hydraulic motor 3 operates at a high torque as described above, causing the vehicle to travel at the first speed. . And the running speed of the vehicle at this time is
It is controlled by the tilt angle of the variable discharge pump 1.

Next, a case where the traveling condition of the vehicle changes and the pressure of the forward traveling line 2 becomes lower than the set pressure will be described. When the pressure sensor 9 inputs a pressure detection signal lower than the set pressure to the controller C, the controller C excites the solenoid 11c of the displacement control valve 11. When the solenoid 11c is excited, the displacement control valve 11 is switched, and the hydraulic chamber 10b of the cylinder 10 communicates with the control pump 6 via the displacement control valve 11. Therefore,
The hydraulic pressure is guided to the hydraulic chamber 10b, and the cylinder 10 is extended by the pressure action. When the cylinder 10 is extended, the tilt angle of the hydraulic motor 3 is reduced, and the second speed is established.

At this time, if the tilt angle of the variable discharge pump 1 detected by the tilt angle sensor 5 is within the set angle, the controller C judges this and keeps the clutch control valve 15 at the illustrated normal position. , The clutch 13 is kept disconnected. At the same time, the communication valve 8 also maintains the closed position. Therefore, the hydraulic motor 3 is rotated by the discharge oil of the variable discharge pump 1, and the vehicle runs at the second speed. The running speed of the vehicle at this time is controlled by the tilt angle of the variable discharge pump 1, similarly to the case of the first speed.

As described above, when the speed is increased while increasing the tilt angle of the variable discharge pump 1 during the second speed running, the tilt angle becomes greater than or equal to the set angle in the process. When the tilt angle becomes equal to or larger than the set angle, the tilt angle sensor 5 detects this and outputs a detection signal to the controller C. As described above, when the tilt angle of the variable discharge pump 1 is equal to or larger than the set angle during the second speed running, in other words, when the running speed of the vehicle is equal to or higher than the set speed, the controller C excites the solenoid 15c. Then, the clutch control valve 15 is switched to the right position in the drawing.

When the clutch control valve 15 is switched to the right position, the ports 15b and 15d communicate with each other, and the oil discharged from the control pump 6 is supplied to the clutch linking mechanism 14. Therefore, the clutch linking mechanism 14 operates to link the clutch 13. At the same time, the controller C also excites the solenoid 8b and switches the communication valve 8 to the open position. Therefore, the hydraulic motor 3 is directly connected to the engine E via the clutch 13. At this time, the discharge oil of the variable discharge pump 1 is not supplied to the hydraulic motor 3 but is returned to the variable discharge pump 1 through the bypass line 7.

As described above, when the swash plate of the variable discharge pump 1 is tilted at a predetermined angle or more with the hydraulic motor 3 set to the second speed, that is, during high-speed running, the clutch 13 can be switched to the linked state. it can. Therefore, the driving force of the engine E is directly transmitted to the hydraulic motor 3 via the shaft 12, and the driving force of the engine E can be transmitted to the output side most efficiently. When the bypass line 7 communicates, the two traveling lines 2,
4 has the same pressure, and the hydraulic motor 3 is not rotated by the discharge oil of the variable discharge pump 1, so that there is no possibility that the transmission efficiency is impaired there.

When oil leakage occurs from the closed circuit as described above, the control pump 6 sends the oil to the communication line 1.
6 can be added to the forward travel line 2 or the reverse travel line 4. For example, when the pressure of the reverse traveling line 4 becomes lower than the pressure of the connection portion 17 of the communication line 16 due to oil leakage during traveling, the check valve 1
9, oil is supplied from the control pump 6 to the reverse traveling line 4. Conversely, when the pressure of the forward traveling line 2 becomes low, oil is similarly supplied via the check valve 18. If the oil in the closed circuit leaks and the pressure is kept low, the transmission of power from the variable discharge pump 1 to the hydraulic motor 3 may not be performed well, but as described above. When replenished from the control pump 6, a required amount of oil always flows during the closed circuit, and stable operation can be performed.

When the vehicle travels backward, it is unlikely that the vehicle will run at high speed. Therefore, there is almost no need to directly rotate the hydraulic motor 3 by the driving force of the engine E.
Therefore, when the swash plate of the variable discharge pump 1 is inclined in the reverse traveling range of the vehicle, the clutch 13 is always kept disconnected. Further, in this embodiment, the control pump 6 is connected to the variable discharge pump 1, but it is not always necessary to connect the control pump 6 to the variable discharge pump 1. However, if it is connected to the variable discharge pump 1,
The engine E can be a common power source for both pumps.

[0032]

According to the first aspect of the present invention, when the hydraulic motor is set to a large capacity, or when the swash plate of the variable discharge pump is tilted within a range not reaching the set angle even with a small capacity, During low-speed running, the clutch is in the disengaged state. Therefore, when the vehicle runs at low speed, it functions as a normal hydrostatic transmission device, can continuously control the rotation speed of the hydraulic motor, and can perform stepless speed change. On the other hand, when the fluid pressure motor is set to a small capacity and the swash plate of the variable discharge pump is tilted at a set angle or more in the forward traveling range of the vehicle, that is, at high speed traveling, the clutch is switched to the linked state, The driving force of the power source can be transmitted directly to the hydraulic motor side.

When the vehicle travels at high speed, the communication valve is switched to the communication position, and the bypass line communicates. Therefore, both traveling lines have the same pressure, and the fluid motor is not rotated by the discharge fluid of the variable discharge pump. Therefore, there is no possibility that transmission efficiency is impaired. Further, by using a two-speed motor whose capacity can be set in two stages as the fluid pressure motor, it is possible to control the speed in high torque and low torque, respectively. Further, the capacity of the fluid pressure motor can be automatically set according to the pressure of the forward traveling line.

According to the second aspect, when a fluid leaks from both traveling lines, the fluid can be replenished by the control pump. In addition, the operation of the clutch linking mechanism and the capacity switching mechanism and the supply of the fluid to the two traveling lines can be performed economically by sharing one control pump.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a hydrostatic transmission device according to an embodiment of the present invention.

[Explanation of symbols]

 E engine 1 variable discharge pump 2 forward running line 3 hydraulic motor 4 reverse running line 5 tilt angle sensor 6 control pump 6a discharge port 7 bypass line 8 communication valve 9 pressure sensor 10 cylinder 11 capacity control valve 12 shaft 13 clutch 14 clutch linkage Mechanism 15 Clutch control valve 16 Communication line 17 Connection 18, 19 Check valve C Controller

Claims (2)

[Claims] 1. A power source that rotates in one direction, a variable discharge pump that is linked to the power source, and that changes a discharge amount and a discharge direction according to a tilt of a swash plate, and that is driven by a discharge fluid of the variable discharge pump. And a hydraulic motor in which two types of large and small displacements are set according to the inclination of the swash plate, and a forward traveling line connecting one port of the variable discharge pump to one port of the hydraulic motor. A pressure sensor for detecting the pressure of the forward travel line, a reverse travel line connecting the other port of the variable discharge pump to the other port of the fluid pressure motor, and communication between the forward travel line and the reverse travel line. A bypass line that bypasses the hydraulic motor, a communication valve that communicates or shuts off the bypass line, and a hydraulic motor that links the power source to the power source. A clutch for interrupting the connection of the clutch, a clutch link mechanism that operates on the clutch to actuate the fluid pressure motor to the power source side by acting on the clutch, a clutch control valve, and the clutch control valve. A control pump for supplying fluid to the clutch linkage mechanism, a displacement switching mechanism for switching the inclination of the swash plate of the fluid pressure motor, and a tilt angle sensor for detecting the tilt angle of the swash plate of the variable discharge pump. A controller that controls the capacity switching mechanism, a clutch control valve, and a communication valve in accordance with detection signals from the pressure sensor and the tilt angle sensor, wherein the controller detects a detection signal of a set pressure or less from the pressure sensor. Is input, the capacity switching mechanism is switched to the small capacity side, and in this state, the tilt angle sensor When a tilt angle equal to or greater than the set angle is detected within the line range, the controller switches between the clutch control valve and the communication valve to supply fluid from the control pump to the clutch linkage mechanism to engage the clutch. A hydrostatic transmission device configured to communicate the bypass line. 2. A communication line connecting the forward traveling line and the reverse traveling line, a connection portion connecting a discharge port of a control pump to the communication line, and traveling forward from the connection portion on the communication line. A check valve provided on the line side and allowing only the flow from the connection portion to the forward travel line; and a check valve provided on the communication line and on the reverse travel line side from the connection portion, from the connection portion to the reverse travel line. The hydrostatic transmission device according to claim 1, further comprising: a check valve that allows only flow to the hydraulic pump, and wherein the capacity switching mechanism is operated by a fluid supplied from a control pump.