JP2000046151A - Hydrostatic transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrostatic transmission device that avoids any degradation in transmission efficiency by maintaining a continuously variable transmission gear ratio control characteristic at low vehicle speed and transmitting the driving force of a power source directly to its hydraulic motor at high vehicle speed. SOLUTION: When the swash plate of a variable displacement pump 1 is inclined over a preset angle in a vehicle forward running range, a clutch mechanism 6 is switched to couple a hydraulic motor 3 to a power source E. In response to the interlock of the hydraulic motor 3 with the power source E by the clutch mechanism 6, a communication valve 13 is switched to communicate a communication line 12.

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 displacement pump and a hydraulic motor. That is, when the variable displacement pump is rotated by the driving force of the engine, the variable displacement pump discharges hydraulic oil according to the inclination of the swash plate. Then, the hydraulic motor is rotated by the discharged oil to drive the vehicle,
The hydraulic oil discharged from the hydraulic motor is returned to the variable displacement pump.

[0003] In such a hydrostatic transmission device, if the engine speed is kept constant, the variable displacement pump also rotates at a constant speed. And
In this state, if the swash plate of the variable displacement 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. If the inclination of the swash plate of the variable displacement pump is reversed, the discharge direction can be reversed. Therefore, the vehicle can be made to travel forward or backward by rotating the hydraulic motor in both the forward and reverse directions.

[0004]

In such a hydrostatic transmission device as described above, a characteristic such as a continuously variable transmission becomes very effective especially at a low vehicle speed where the transmission is frequently 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 poor. When the vehicle normally travels at a high vehicle speed, there is almost no need for shifting, and the disadvantage that transmission efficiency is worse than the characteristic of stepless shifting is a serious problem. The purpose of this invention is
At low vehicle speeds, while maintaining characteristics such as stepless shifting, at high vehicle speeds, the driving force of the power source is transmitted directly to the hydraulic motor side, thereby avoiding deterioration in transmission efficiency. It is to provide a hydrostatic transmission device.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a variable displacement pump that is linked to a power source that rotates in one direction and that changes the discharge amount and discharge direction in accordance with the inclination of a swash plate. Hydraulic motor, a forward running line connecting one port of the variable displacement pump to one port of the hydraulic motor, and a reverse running connecting the other port of the variable displacement pump to the other port of the hydraulic motor. It assumes a hydrostatic transmission device with a running line. According to the first aspect of the present invention, the clutch mechanism for linking or disconnecting the fluid pressure motor to the power source side and the swash plate of the variable displacement pump are moved beyond the set angle in the forward traveling range of the vehicle. When tilted, the clutch mechanism is switched to connect the fluid pressure motor to the power source side, a communication line that connects the forward traveling line and the reverse traveling line, and a communication line that disconnects or disconnects the communication line. And a valve switching mechanism for switching the communication valve when the fluid pressure motor is connected to the power source by the clutch mechanism, thereby connecting the communication line.

According to a second aspect of the present invention, in the first aspect, the clutch switching mechanism rotates together with a cylinder linked to the clutch mechanism and a variable displacement pump to make both the discharge port and the suction port communicate with the tank. A second control pump, which permits flow in both directions while rotating with the fluid pressure motor and has both ports communicating with the tank, and a second control pump provided on the discharge port side of the first control pump. A first orifice, and a second orifice provided on a port side of the port of the second control pump that discharges when the fluid pressure motor is rotating in the forward direction, and the swash plate of the variable displacement pump is moved forward of the vehicle. The cylinder switches the clutch mechanism due to the pressure difference generated upstream of the first and second orifices when the cylinder is tilted beyond the set angle in the traveling range, and the hydraulic motor is moved to the power source side. It has a feature in that a configuration to coordinate.

[0007] A third invention is the second invention, wherein the second invention is the second invention.
Of the ports of the control pump, a port that discharges when the fluid pressure motor is rotating in the forward direction communicates with the tank through a passage separate from the passage provided with the second orifice. (2) It is characterized in that a check valve that allows only the flow to the control pump side is provided.

[0008]

FIG. 1 shows a first embodiment of a hydrostatic transmission device according to the present invention.
The variable displacement pump 1 is linked to an engine E as its power source. Then, one port 1 a of the variable displacement 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 displacement pump 1 is connected to the other port 3b of the hydraulic motor 3 via the reverse traveling line 4. Therefore, the variable displacement pump 1 and the hydraulic motor 3 form a closed circuit.

The variable displacement pump 1 always rotates in the direction of rotation of the engine E. Then, as shown in FIG. 1, when the swash plate of the variable displacement pump 1 is tilted in the forward traveling range, the variable displacement pump 1 discharges hydraulic oil from the port 1a to the forward traveling line 2 side. Therefore, the hydraulic motor 3 rotates forward according to the discharge amount, and the vehicle travels forward. If the swash plate of the variable displacement pump 1 is tilted in the reverse traveling range, the variable displacement pump 1
Is discharged in the opposite direction, and hydraulic oil is discharged from the port 1b to the reverse traveling line 4 side. Therefore, the hydraulic motor 3 rotates in the reverse direction according to the discharge amount, and the vehicle travels backward.

Further, in the first embodiment, unlike the conventional hydrostatic transmission device,
The hydraulic motor 3 is connected to a variable displacement pump 1 via a shaft 5.
It is linked to the side. However, a clutch mechanism 6 is provided in the middle of the shaft 5. Although not specifically shown, the clutch mechanism 6 employs a friction clutch system, and is normally in a disconnected state in which the hydraulic motor 3 is disconnected from the variable displacement pump 1 side. When the clutch mechanism 6 is switched to the linked state, the hydraulic motor 3 is linked to the variable displacement pump 1 to directly transmit the driving force of the engine E to the hydraulic motor 3 via the shaft 5. Like that.

The clutch mechanism 6 is switched by the cylinder 7. That is, until the pressure in the rod-side chamber 7a of the cylinder 7 becomes higher than the pressure in the bottom-side chamber 7b by a predetermined pressure, the clutch mechanism 6 is in the disconnected state, and the hydraulic motor 3 is disconnected from the variable displacement pump 1 side. When the pressure of the rod side chamber 7a of the cylinder 7 becomes higher than the pressure of the bottom side chamber 7b by a predetermined pressure, the clutch mechanism 6 is switched to the linked state by the thrust of the cylinder 7, and the hydraulic motor 3 is moved to the variable displacement pump 1 side. It will be linked.

On the other hand, the variable displacement pump 1 has a first
The control pump 8 is provided continuously. Therefore, the first control pump 8 rotates together with the variable displacement pump 1 by the driving force of the engine E. The suction port 8a of the first control pump 8 is connected to the tank T, and the discharge port 8b is connected to the tank T via the first orifice 9. The pressure on the upstream side of the first orifice 9 is guided to the bottom chamber 7b of the cylinder 7.

A second control pump 10 is connected to the hydraulic motor 3. Therefore, the second control pump 10 rotates together with the hydraulic motor 3. Since the hydraulic motor 3 rotates in both forward and reverse directions, the second control pump 10 is also of a type that can rotate in both forward and reverse directions. Both ports 10a and 10b of the second control pump 10 are connected to the tank T. However, when the hydraulic motor 3 is rotating forward and the vehicle is traveling forward, the port 1 for discharging hydraulic oil is provided.
The second orifice 11 is interposed on the 0b side. The pressure on the upstream side of the second orifice 11 is guided to the rod-side chamber 7a of the cylinder 7.

The first and second control pumps 8, 10
Have the same capacity as each other. And the first
The opening of the orifice 9 is set slightly larger than the opening of the second orifice 11.

Further, there is provided a communication line 12 for connecting the two running lines 2, 4. And this communication line 1
2, a communication valve 13 is provided. Communication valve 13
Is in the normal state held by the spring 18,
As shown in FIG. 1, the communication line 12 is in a blocking position for blocking the communication line 12. Then, when the solenoid 14 is excited, the position is switched to the communication position where the communication line 12 is communicated.

The solenoid 14 is connected to a switch 15 associated with a swash plate of the variable displacement pump 1. When the swash plate of the variable displacement pump 1 is tilted at a set angle in the forward traveling range of the vehicle, the switch 15 detects this and excites the solenoid 14. In addition,
As this set angle, an angle near the maximum angle in the forward traveling range of the vehicle is set. In other words, the solenoid 14 is excited when the discharge amount of the variable displacement pump 1 becomes close to the maximum amount and the vehicle moves forward at a high vehicle speed. Moreover, when the swash plate of the variable displacement pump 1 is tilted at a set angle in the forward traveling range of the vehicle, the discharge amount of the variable displacement pump 1 is made to exactly match the displacement of the hydraulic motor 3.

Next, the operation of the hydrostatic transmission device of the first 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 displacement pump 1. Hereinafter, a case where the swash plate of the variable displacement pump 1 is inclined in the forward traveling range of the vehicle will be described. In the forward traveling range of the vehicle, as described above, the variable displacement pump 1 discharges hydraulic oil to the forward traveling line 2 side, and causes the hydraulic motor 3 to rotate forward. Then, when the hydraulic motor 3 rotates forward, the second control pump 10 also rotates forward, and the hydraulic oil is discharged from the port 10b to the second orifice 11 side.

First, the operation when the swash plate of the variable displacement pump 1 is tilted in the range from the minimum angle to the set angle will be described. When tilting the swash plate of the variable displacement pump 1 in a range from the minimum angle to the set angle, the variable displacement pump 1
Is smaller than the capacity of the hydraulic motor 3. That is, in order to make the hydraulic motor 3 make one rotation, the variable displacement pump 1 must rotate more than one rotation. If there is a difference between the rotational speeds of the hydraulic motor 3 and the variable displacement pump 1, a difference also occurs between the rotational speeds of the first and second control pumps 8, 10. That is, while the second control pump 10 makes one rotation, the first control pump 8 rotates more than one rotation. Therefore, the first control pump 8 discharges more hydraulic oil than the second control pump 10 by the difference in the number of rotations.

When the discharge amount of the first control pump 8 is larger than the discharge amount of the second control pump 10, conversely, when the discharge amount of the second control pump 10 is smaller than the discharge amount of the first control pump 8, The pressure generated on the upstream side of the second orifice 11 does not exceed the pressure generated on the upstream side of the first orifice 9, or even if it does, the pressure does not increase by a predetermined pressure. Therefore, cylinder 7
The pressure in the rod-side chamber 7a does not become higher than the pressure in the bottom-side chamber 7b by a predetermined pressure, and the clutch mechanism 6 is maintained in the disconnected state, and the hydraulic motor 3 is disconnected from the variable displacement pump 1 side. it can. Also, since the swash plate of the variable displacement pump 1 is not tilted to the set angle, the solenoid 1
4, the communication valve 13 is in the shut-off position, as shown in FIG.

As described above, when the swash plate of the variable displacement pump 1 is tilted in the range from the minimum angle to the set angle,
That is, during low vehicle speed traveling, the clutch mechanism 6 is in the disconnected state, and the communication valve 13 is in the disconnected position. Therefore, it functions as a normal hydrostatic transmission device, and continuously controls the rotation speed of the hydraulic motor 3 to enable stepless speed change.

Next, the operation when the swash plate of the variable displacement pump 1 is inclined beyond the set angle will be described. When the swash plate of the variable displacement pump 1 is tilted at the set angle, the discharge amount of the variable displacement pump 1 exactly matches the displacement of the hydraulic motor 3 as described above. That is, the hydraulic motor 3
In order to make one rotation, the variable displacement pump 1 only needs to make one rotation. When the rotation speeds of the hydraulic motor 3 and the variable displacement pump 1 become the same, the rotation speeds of the first and second control pumps 8 and 10 also become the same. Therefore, the same amount of hydraulic oil is discharged from the first and second control pumps 8 and 10 having the same capacity.

When the discharge rates of the first and second control pumps 8 and 10 become equal, the pressure generated upstream of the second orifice 11 having a small opening increases the pressure upstream of the first orifice 9 having a large opening. It becomes higher by a predetermined pressure than the generated pressure. Therefore, the pressure in the rod side chamber 7a of the cylinder 7 becomes higher than the pressure in the bottom side chamber 7b by a predetermined pressure, and the clutch mechanism 6 is switched to the linked state, so that the hydraulic motor 3 can be linked to the variable displacement pump 1 side. . When the swash plate of the variable displacement pump 1 is tilted to the set angle, the switch 15 detects this. Therefore, the solenoid 14 is excited,
The communication valve 13 is switched to the communication position.

As described above, the clutch mechanism 6 can be switched to the linked state when the swash plate of the variable displacement pump 1 is tilted beyond the set angle, that is, at high vehicle speed. Therefore, the driving force of the engine E is directly transmitted to the hydraulic motor 3 via the shaft 5,
It is possible to transmit the driving force of the engine E to the output side most efficiently. Since the communication valve 13 is switched to the communication position, the two traveling lines 2 and 4 communicate with each other via the communication line 12. Accordingly, the two traveling lines 2 and 4 have the same pressure, and the hydraulic motor 3 is not rotated by the discharge oil of the variable displacement pump 1, and there is no possibility that the transmission efficiency is impaired there.

When the vehicle travels backward, it is unlikely that the vehicle will run at a high speed. Therefore, there is almost no need to directly rotate the hydraulic motor 3 with the driving force of the engine E. Therefore, when the swash plate of the variable displacement pump 1 is inclined in the reverse traveling range of the vehicle, the swash plate always functions as a hydrostatic transmission device.

That is, when the vehicle travels in the reverse direction, the variable displacement pump 1 discharges the hydraulic oil to the reverse traveling line 4 to rotate the hydraulic motor 3 in the reverse direction. Therefore, the second control pump 10 also rotates in the reverse direction, and does not discharge hydraulic oil to the second orifice 11 side. If hydraulic oil is not discharged to the second orifice 11 side, even if the rotation speed of the second control pump 10 increases, the second orifice 1
No pressure is generated upstream of 1. Therefore, the pressure in the rod-side chamber 7a of the cylinder 7 is substantially maintained at the tank pressure,
The clutch mechanism 6 can be kept in the disengaged state and can always function as a hydrostatic transmission device.

Here, the port 10 of the second control pump 10
b is a passage 1 different from the passage provided with the second orifice 11.
6 is connected to the tank T. And this passage 16
Further, a check valve 17 that allows only the flow from the tank T to the second control pump 10 is provided. Therefore, when the second control pump 10 rotates in the reverse direction, the check valve 17 is opened, and the operating oil can be sucked from the tank T via the port 10b.

In the first embodiment, the first and second control pumps 8 and 10 have the same capacity and the first and second orifices 9 and 11 have different degrees of opening. Is by no means limiting. For example, contrary to the first embodiment, the openings of the first and second orifices 9 and 11 may be the same, and the capacities of the first and second control pumps 8 and 10 may be different. In short, it is only necessary that a predetermined differential pressure can be generated upstream of the first and second orifices 9 and 11 when the swash plate of the variable displacement pump 1 is tilted beyond the set angle in the forward traveling range of the vehicle.

In the first embodiment, the switch 1
5. The solenoid 14 constitutes a valve switching mechanism according to the present invention. However, the valve switching mechanism may have any configuration as long as it switches the communication valve 13. For example, the cylinder 7 is mechanically linked to the communication valve 13 so that the clutch mechanism 6
May be switched to the linked state, and at the same time, the communication valve 13 may be switched to the communicating position. The second to the following
The fourth embodiment differs from the first embodiment in the configuration of the valve switching mechanism. Therefore, in the following, the differences will be mainly described, and the same components as those in the first embodiment will be denoted by the same reference numerals, and detailed description thereof will be omitted.

In the second embodiment shown in FIG.
3 is provided with the second orifice 11
The upstream pressure is leading. The pressure on the upstream side of the first orifice 9 is guided to the other pilot chamber 13 b provided with the spring 18. In the second embodiment as described above, as described above, when the swash plate of the variable displacement pump 1 is tilted beyond a set angle in the forward traveling range of the vehicle, the swash plate occurs upstream of the second orifice 11. The pressure is higher than the pressure generated upstream of the first orifice 9. Therefore, the pressure in the rod-side chamber 7a of the cylinder 7 becomes higher than the pressure in the bottom-side chamber 7b by a predetermined pressure, and the clutch mechanism 6 is switched so that the hydraulic motor 3 is linked to the variable displacement pump 1 side. At this time, since the differential pressure acts on both pilot chambers 13a and 13b of the communication valve 13, the communication valve 13 can be switched to the communication position against the spring 18.

In the third embodiment shown in FIG. 3, a tilt angle sensor 19 for detecting the tilt angle of the variable displacement pump 1 and a rotation speed sensor 20 for detecting the rotation speed of the variable displacement pump 1 are provided. The results detected by these sensors 19 and 20 are input to the controller C. A rotation speed sensor 21 for detecting the rotation speed of the hydraulic motor 3 is provided.
Are also input to the controller C. Then, the controller C excites the solenoid 14 when the swash plate of the variable displacement pump 1 is tilted beyond the set angle and the rotation speed of the variable displacement pump 1 matches the rotation speed of the hydraulic motor 3. Then, the communication valve 13 is switched to the communication position.

In the third embodiment, as described above, when the swash plate of the variable displacement pump 1 is tilted beyond a set angle in the forward traveling range of the vehicle, the upstream side of the second orifice 11 can be used. Is higher than the pressure generated on the upstream side of the first orifice 9. Therefore,
The pressure in the rod-side chamber 7a of the cylinder 7 is
, The clutch mechanism 6 is switched, and the hydraulic motor 3 is linked to the variable displacement pump 1 side. At this time, the controller C detects that the swash plate of the variable displacement pump 1 has tilted beyond the set angle and that the rotation speed of the variable displacement pump 1 matches the rotation speed of the hydraulic motor 3. By energizing the solenoid 14, the communication valve 13 can be switched to the communication position.

In the third embodiment, the passage 16
An on / off valve 22 is provided instead of the check valve 17. The on / off valve 22 is in an off position for shutting off the passage 16 in a normal state. When the controller C detects, for example, that the rotation direction of the variable displacement pump 1 and the rotation direction of the hydraulic motor 3 are opposite, the controller C determines that the vehicle is traveling backward and the on / off valve 22 Is switched to the ON position. Therefore, when the second control pump 10 rotates in the reverse direction, the on / off valve 2
2, the hydraulic oil can be sucked from the tank T via the port 10b.

In the fourth embodiment shown in FIG. 4, not the switch 15 but the switching valve 2
3 are linked. The pilot chamber 24 of the communication valve 13 is connected to the upstream side of the first orifice 9 via the switching valve 23. The switching valve 23 is
In a normal state, the pilot chamber 24 of the communication valve 13 communicates with the tank T and is shut off from the upstream side of the first orifice 9. Therefore, the communication valve 13
It is in the blocking position by the spring 18.

On the other hand, when the swash plate of the variable displacement pump 1 is inclined beyond the set angle in the forward traveling range of the vehicle,
The switching valve 23 is switched to connect the pilot chamber 24 of the communication valve 13 to the upstream side of the first orifice 9. As described above, since pressure is generated upstream of the first orifice 9, the pressure is guided to the pilot chamber 24, and the communication valve 13 is switched to the communication position.

In the fourth embodiment, as described above, when the swash plate of the variable displacement pump 1 is tilted beyond a set angle in the forward traveling range of the vehicle, the upstream side of the second orifice 11 is moved. Is higher than the pressure generated on the upstream side of the first orifice 9. Therefore,
The pressure in the rod-side chamber 7a of the cylinder 7 is
, The clutch mechanism 6 is switched, and the hydraulic motor 3 is linked to the variable displacement pump 1 side. At this time, as described above, the switching valve 23
And the pressure on the upstream side of the first orifice 9 is guided to the pilot chamber 24 of the communication valve 13, so that the communication valve 13 can be switched to the communication position.

[0036]

According to the first aspect of the present invention, when the swash plate of the variable displacement pump is tilted in the range from the minimum angle to the set angle in the forward traveling range of the vehicle, that is, at low vehicle speed, the clutch mechanism is disconnected. State and the communication valve is in the shut-off position. Therefore, it functions as a normal hydrostatic transmission device, and continuously controls the rotation speed of the fluid pressure motor, thereby enabling stepless speed change. On the other hand, when the swash plate of the variable displacement pump is tilted beyond the set angle in the forward traveling range of the vehicle, that is, during high vehicle speed traveling, the clutch mechanism is switched to the linked state, so the driving force of the power source is reduced. It can be transmitted directly to the fluid pressure motor side. Further, since the communication valve is switched to the communication position, the two traveling lines have the same pressure, and the fluid pressure motor does not rotate by the discharge fluid of the variable displacement pump, and there is no possibility that the transmission efficiency is impaired there.

According to the second aspect, since the fluid pressure is used as the clutch switching mechanism, there is almost no malfunction.
The clutch mechanism can be reliably switched. According to the third aspect, even when the fluid pressure motor is rotating in the reverse direction, the second control pump opens the check valve,
Hydraulic oil can be sucked from the tank.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a hydrostatic transmission device according to a first embodiment.

FIG. 2 is a circuit diagram showing a hydrostatic transmission device according to a second embodiment.

FIG. 3 is a circuit diagram showing a hydrostatic transmission device according to a third embodiment.

FIG. 4 is a circuit diagram showing a hydrostatic transmission device according to a fourth embodiment.

[Explanation of symbols]

 E engine 1 variable displacement pump 1a, 1b port 2 forward traveling line 3 hydraulic motor 3a, 3b port 4 reverse traveling line 5 shaft 6 clutch mechanism 7 cylinder 8 first control pump 8a suction port 8b discharge port 9 first orifice 10th 2 control pump 10a, 10b port 11 second orifice 13 communication valve 14 solenoid 15 switch 16 passage 17 check valve 19 tilt angle sensor 20 rotation speed sensor 21 rotation speed sensor 23 switching valve

Claims (3)

[Claims] 1. A variable displacement pump which is linked to a power source rotating in one direction and changes a discharge amount and a discharge direction in accordance with a tilt of a swash plate, a fluid pressure motor permitting flow in both directions, and a variable displacement pump. Hydrostatic having a forward traveling line connecting one port of a hydraulic pump to one port of a hydraulic motor, and a reverse traveling line connecting the other port of the variable displacement pump to the other port of the hydraulic motor In a transmission device, when a fluid pressure motor is linked to a power source side or a clutch mechanism for disconnecting the linkage and a swash plate of a variable displacement pump are tilted beyond a set angle in a forward traveling range of a vehicle, A clutch switching mechanism for switching the clutch mechanism to connect the fluid pressure motor to the power source side, and a communication line for communicating the forward traveling line and the reverse traveling line. And a communication valve for connecting or disconnecting the communication line, and a valve switching mechanism for switching the communication valve when the fluid pressure motor is connected to the power source by a clutch mechanism, thereby connecting the communication line. A hydrostatic transmission device. 2. A clutch switching mechanism comprising: a cylinder connected to a clutch mechanism; a first control pump rotating together with a variable displacement pump and having both a discharge port and a suction port connected to a tank; A second control pump that rotates with the fluid pressure motor and has both ports communicating with the tank, a first orifice provided on the discharge port side of the first control pump, and a port of the second control pump. A second orifice provided on the port side for discharging when the fluid pressure motor is rotating in the forward direction, wherein the swash plate of the variable displacement pump is tilted beyond a set angle in the forward travel range of the vehicle. In some cases, the cylinder switches the clutch mechanism by a pressure difference generated upstream of the first and second orifices to link the fluid pressure motor to the power source. The hydrostatic transmission device according to claim 1, wherein: 3. A port of the second control pump, which discharges when the fluid pressure motor is rotating in the forward direction, communicates with the tank through a passage different from the passage provided with the second orifice. 3. The hydrostatic transmission device according to claim 2, wherein a check valve that allows only the flow from the tank to the second control pump is provided in the passage.