JP2554649Y2 - Hydraulic motor drive circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive circuit for a hydraulic motor, and more particularly to a drive circuit for a hydraulic motor for reducing an impact caused by braking of a hydraulic traveling vehicle at a low speed.

[0002]

2. Description of the Related Art FIG. 9 shows a drive circuit of a hydraulic motor of a hydraulic traveling vehicle capable of switching a speed mode between a low speed mode and a high speed mode. A pilot-operated directional control valve 5 and a counterbalance valve 6 are provided in main pipelines 3 and 4 that connect the traveling hydraulic motor 1 and the hydraulic pump 2. The directional control valve 5 is controlled by a pilot operation unit 7, the directional control valve 5 is switched in conjunction with the movement of the operation lever 8, and the discharge oil of the hydraulic pump 2 is supplied to the main pipe via the directional control valve 5 and the counterbalance valve 6. The hydraulic motor 1 is led out from the road 3 or the main pipe 4. The hydraulic motor 1 can change the tilt angle by the operation of the tilt angle adjusting cylinder 9, and the state shown is the low-speed mode, and the rod 9 is biased by the spring 9 b built in the tilt angle adjusting cylinder 9. 9a
Is contracted into the cylinder, and the tilt angle of the hydraulic motor 1 is large. That is, the flow rate per rotation of the hydraulic motor 1 is large and the torque is high.

Further, crossover relief valves 10 and 11 acting in respective directions are provided between the two main pipelines 3 and 4,
A shuttle valve 14 is provided between the bypass lines 12 and 13. Then, a first opening / closing valve 15 that operates in the opening direction by the pilot pressure is provided, and a pipe line to the back pressure chamber 9c of the tilt angle adjusting cylinder 9 is connected to a secondary side of the first opening / closing valve 15. The shuttle valve 14 is provided on the primary side of the first on-off valve 15.
And the return tank 16 are connected. Further, a second on-off valve 17 which operates in the opening direction by energizing the solenoid is provided, and a pilot oil passage 18 is provided on the secondary side of the second on-off valve 17 so that a pilot port of the first on-off valve 15 is provided. And the pilot pump 19 and the return tank 16 are connected to the primary side of the second on-off valve 17.

When a high-speed switching signal is supplied to the solenoid of the second on-off valve from the state shown in the figure, the spool is switched to the position 17a, the second on-off valve is opened, and the spool is discharged from the pilot pump 19. The pilot oil passes through the pilot oil passage 18 and passes through the first on-off valve 15.
To the pilot port. For this reason, the spool of the first on-off valve 15 switches to the position of 15a and the first
The opening / closing valve 15 is opened, and the hydraulic oil from the outlet pipe of the shuttle valve 14 flows into the back pressure chamber 9c of the tilt angle adjusting cylinder 9 and resists the spring 9b built in the tilt angle adjusting cylinder 9. The rod 9a extends out of the cylinder and the hydraulic motor 1
Becomes small. That is, the flow rate per rotation of the hydraulic motor 1 is small and the torque is low, and the high-speed mode is set.

When the body is braked in the high-speed mode, the torque is low because the tilt angle of the hydraulic motor 1 is small, and the crossover relief valves 10 and 11 provided between the main pipelines 3 and 4 are used. Reduces the increase in hydraulic pressure in one of the main pipelines, thereby reducing shock during braking.

[0006]

As described above, the conventional hydraulic traveling vehicle reduces the shock at the time of the vehicle braking in the high-speed mode. However, in the low speed mode, the torque is high because the tilt angle of the hydraulic motor 1 is large, and a large flow of hydraulic oil is discharged in a short time during the braking operation of the body, so that the braking pressure rises sharply. As shown in FIG. 10, the braking pressure is rapidly increased to P _1, the initial reduction in the rate V ₁ of the body is steep and short time Delta] t ₁ until the aircraft stops. Therefore, the shock at the time of the braking of the machine body becomes large, and the fatigue of the operator increases, which causes a decrease in work efficiency.

Therefore, there is a technical problem to be solved in order to improve the operability by reducing the shock at the time of the body braking even in the low-speed mode and to improve the work efficiency by reducing the physical burden on the operator. The present invention aims to solve this problem.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been proposed to achieve the above-mentioned object, and the tilt angle of a hydraulic motor can be freely changed by a tilt angle adjusting cylinder, so that a high-speed mode or a low-speed mode can be set. In a hydraulic traveling vehicle capable of switching speeds, a pipe line to a back pressure chamber of the tilt angle adjusting cylinder is provided on a secondary side of a first opening / closing valve that operates in an opening direction by pilot pressure, A first shuttle valve is disposed between two main lines of the motor, and a line from an outlet of the first shuttle valve and a return tank are connected to a primary side of the first on-off valve; The secondary side of a second on-off valve which is operated by energizing the first on-off valve is connected to a pilot port of the first on-off valve, and between two pilot oil passages from a pilot operating section for controlling a directional control valve of a hydraulic motor. Second
The shuttle valve is arranged and connected to the pressure detecting means.
A first exciting means for exciting the solenoid of the second on-off valve by the operation of the pressure detecting means, and a second exciting means for exciting the solenoid of the second on-off valve by the speed switching operation.
When the braking operation of the hydraulic motor is performed by the pilot operation unit during low-speed running of the body, the first opening / closing valve is opened to return to the back pressure chamber of the tilt angle adjusting cylinder. It is an object of the present invention to provide a hydraulic motor drive circuit characterized in that hydraulic oil is derived and the tilt angle of the hydraulic motor is changed to a small value.

[0009]

[Function] When the pilot operating unit is returned to the neutral position to brake the aircraft traveling in the low speed mode, the pilot oil from the pilot pump is shut off and the directional control valve of the hydraulic motor returns to the neutral position. I do. When that happens,
The derivation of the pilot pressure from the second shuttle valve to the pressure detecting means is stopped, the first exciting means excites the solenoid of the second on-off valve, and the second on-off valve is opened. Then, the pilot oil passes through the second on-off valve and is led out to the pilot port of the first on-off valve. Accordingly, the spool of the first on-off valve moves in the opening direction, and the hydraulic oil is led out to the back pressure chamber of the tilt angle adjusting cylinder via the first shuttle valve provided between the two main pipelines. , The tilt angle of the hydraulic motor changes to a small value.

Therefore, at the time of body braking in the low speed mode,
Since the flow rate per rotation of the hydraulic motor becomes small and the torque becomes low, the braking torque decreases, the time until the braking pressure rises becomes slightly longer, and the shock at the time of body braking is reduced.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below in detail with reference to FIGS. For convenience of explanation, the same components as those of the related art will be described at the same time. FIG. 1 shows a drive circuit of a hydraulic motor of a hydraulic traveling vehicle in which a speed mode can be manually switched between a low speed mode and a high speed mode by a switch, and a main line connecting a hydraulic motor 21 for traveling and a hydraulic pump 22. A pilot-operated directional control valve 25 and a counterbalance valve 26 are provided at 23 and 24. The directional control valve 25 is controlled by a pilot operation unit 27, and interlocks with the operation of the operation lever 28 to control the directional control valve 2
5 is switched, and the discharge oil of the hydraulic pump 22 is supplied to the main pipeline 2 via the directional control valve 25 and the counterbalance valve 26.
3 or from the main line 24 to the hydraulic motor 21.
The hydraulic motor 21 can change the tilt angle by the operation of the tilt angle adjusting cylinder 29, and the state shown is the low speed mode, and the rod 29 is urged by the spring 29b built in the tilt angle adjusting cylinder 29. 29a contracts into the cylinder, and the tilt angle of the hydraulic motor 21 becomes large. That is, the flow rate per rotation of the hydraulic motor 21 is large and the torque is high.

Further, crossover relief valves 30 and 31 acting in the respective directions are provided between the two main lines 23 and 24, and a first shuttle valve 34 is provided between the bypass lines 32 and 33.
Is arranged. Then, a first opening / closing valve 35 which operates in the opening direction by the pilot pressure is provided.
5, the back pressure chamber 2 of the tilt angle adjusting cylinder 29
The pipe to 9c is connected, and the outlet pipe of the first shuttle valve 34 and the return tank 36 are connected to the primary side of the first on-off valve 35.

On the other hand, the pilot operating section 27 rotates the operating lever 28 in either the forward side or the reverse side of the body, so that the pilot pump 39a is connected to the pilot oil passage 40 through the pilot valve 37 or 38. Or 41
It is configured to derive pilot oil to the engine. The pilot oil passage 40 is connected to one pilot port 42 of the directional control valve 25, and the pilot oil passage 41 is connected to the other pilot port 43 of the directional control valve 25. A bypass oil passage 44 is provided between 41 and a second shuttle valve 45 is provided in the middle of the bypass oil passage 44.

Here, a second opening / closing valve 46 which operates in the opening direction by energizing the solenoid is provided. A pilot pump 39b and a return tank 36 are connected to the primary side of the second opening / closing valve 46, The pilot oil passage on the secondary side of the on-off valve 46 is connected to the pilot port of the first on-off valve 35. Then, the second shuttle valve 45
The pressure detecting means 47 is provided at the outlet of the
7 is connected to the first excitation means 50 through signal lines 48 and 49. The first excitation means 50 is provided with a coil 52 between a COM1 terminal connected to a signal line 48 from a power supply 51 and a COIL terminal connected to another signal line 49, and receives a signal from a power supply 53 for solenoid excitation. COM2 to which the signal line 54 is connected
Terminal, an NC terminal connected to a signal line 55 to the solenoid of the second on-off valve 46, and a NO provided between both terminals.
When the coil 52 is not excited, the movable contact piece 56 is in contact with the NC terminal.

A second exciting means 57 is provided in parallel with the first exciting means 50, a signal line from a power supply 53 is connected to a COM3 terminal of the second exciting means 57, and a speed switching operation is performed. The switch 59 is connected to the solenoid of the second on-off valve 46 via the signal line 55. Next, the operation of the hydraulic motor drive circuit according to the present invention will be described. As shown in FIG. 1, when the body is stopped on a flat ground and the switch 59 of the second excitation means 57 is switched to the low-speed mode, the second excitation means 57 is off. Further, since the operating lever 28 of the pilot operating section 27 is at the neutral position, the pilot pressure from the second shuttle valve 45 is not led out to the pressure detecting means 47, and the pressure detecting means 47 does not operate. Therefore, the coil 52 is not excited, so that the movable contact piece 56 comes into contact with the NC terminal, the first exciting means 50 is turned on, and the second exciting means 50 is turned on via the signal line 55.
The solenoid of the on-off valve 46 is excited. Therefore, the second
The spool of the on-off valve 46 is located at the position 46a, the second on-off valve 46 is opened, and the pilot oil from the pilot pump 39b is led out to the pilot port of the first on-off valve 35 through the second on-off valve 46. , First on-off valve 35
Is located at the position 35a, and the first on-off valve 35 is open. When the airframe is stopped on a flat ground, no hydraulic pressure is generated in the two main pipelines 23 and 24 of the hydraulic motor 21, so the first shuttle valve 34 switches the first on-off valve 35.
Hydraulic oil is not drawn out to the back pressure chamber 29c of the tilt angle adjusting cylinder 29 through the cylinder, and the rod 29a contracts into the cylinder by the bias of the spring 29b built in the tilt angle adjusting cylinder 29, and the hydraulic motor 21 has a large tilt angle. That is, the flow rate per rotation of the hydraulic motor 21 is large and the torque is high, and the hydraulic motor 21 is in a state suitable for starting the aircraft.

Here, as shown in FIG. 2, if the operating lever 28 of the pilot operating section 27 is rotated in either the forward side or the backward side of the fuselage to start the fuselage, the pilot valve 37 or 38 is started. The pilot oil is led out from the pilot pump 39a to the pilot oil passage 40 or 41 via the pilot oil. Then, a pilot pressure is applied to one of the pilot ports 42 or 43 of the direction control valve 25, the spool of the direction control valve 25 is switched, hydraulic fluid is drawn out to the main pipeline 23 or 24, and the hydraulic motor 21 rotates. In that case, the pilot oil from the second shuttle valve 45 is led out to the pressure detecting means 47, the pressure detecting means 47 is operated to excite the coil 52, and the movable contact piece 56 is moved from the NC terminal side to the NO terminal side. And the first excitation means 50 is turned off, and the solenoid of the second on-off valve 46 is not excited via the signal line 55. As a result, the spool of the second on-off valve 46 moves to the position 46b, the second on-off valve 46 is closed, and the pilot oil is not discharged to the pilot port of the first on-off valve 35. The spool of the valve 35 switches to the position 35b and is closed. Therefore, the hydraulic oil derived from the bypass line 32 or 33 via the first shuttle valve 34 is closed by the first on-off valve 35,
Hydraulic oil is not led out to the back pressure chamber 29c of the tilt angle adjusting cylinder 29, the tilt angle of the hydraulic motor 21 is kept large, and the body can be easily started in the low speed mode.

Next, when the switch 59 is switched to the high-speed mode while the vehicle is traveling in the low-speed mode, the signal line 58 from the power supply 53 and the signal line 55 to the solenoid are connected as shown in FIG. The second excitation means 57 is turned on, and the solenoid of the second on-off valve 46 is excited. Therefore, the spool of the second on-off valve 46 moves to the position 46a to open the second on-off valve 46, and the pilot oil from the pilot pump 39b is supplied to the first on-off valve 35 through the second on-off valve 46. To the pilot port of
The spool of the on-off valve 35 moves to the position 35a and the first
Is opened. Therefore, the bypass line 32
Alternatively, the hydraulic oil derived from 33 via the first shuttle valve 34 is guided to the back pressure chamber 29c of the tilt angle adjusting cylinder 29 and resists the bias of the spring 29b of the tilt angle adjusting cylinder 29. As a result, the rod 29a projects outside the cylinder, and the tilt angle of the hydraulic motor 21 changes to a small value. In other words, the flow rate per rotation of the hydraulic motor 21 becomes small and the torque becomes low, so that the aircraft is in a state suitable for high-speed traveling.

On the other hand, when the fuselage is stopped on a flat ground and the switch 59 is switched to the high-speed mode, the operating lever 28 of the pilot operating section 27 is turned in either the forward or backward direction of the fuselage. If it moves, as shown in FIG. 4, pilot oil is drawn out from pilot pump 39a to pilot oil passage 40 or 41 via pilot valve 37 or 38. Then, the pilot pressure is applied to one of the pilot ports 42 or 43 of the direction control valve 25, and the spool of the direction control valve 25 is switched, so that the main pipeline 23
Alternatively, the hydraulic oil is led out to 24 and the hydraulic motor 21 rotates. In that case, pilot pressure is applied to the pressure detecting means 47 from the second shuttle valve 45, and the pressure detecting means 47
Is activated, the coil 52 is excited, and the movable contact 56
The first excitation means 50 is turned off by moving from the terminal side to the NO terminal side. However, since the switch 59 has been switched to the high-speed mode, the signal line 58 from the power supply 53 and the signal line 55 to the solenoid are connected, the second exciting means 57 is turned on, and the second on-off valve 46 The solenoid is excited. Therefore, the spool of the second on-off valve 46 moves to the position 46a, the second on-off valve 46 is opened, and the pilot oil from the pilot pump 39b is supplied to the second on-off valve 46a.
Through the pilot port of the first on-off valve 35, the spool of the first on-off valve 35 moves to the position 35a, and the first on-off valve 35 is opened.

At this time, since the tilt angle of the hydraulic motor 21 is still large, the torque is high, and the vehicle can be easily started even if the switch 57 is switched to the high speed mode. Then, as the flow rate of the hydraulic oil led to the main line 23 or 24 increases, the hydraulic oil led from the bypass line 32 or 33 via the first shuttle valve 34 adjusts the tilt angle. Pressure chamber 29 of cylinder 29
c of the cylinder 29 for adjusting the tilt angle.
The rod 29a protrudes out of the cylinder against the urging of the hydraulic motor 21, and the tilt angle of the hydraulic motor 21 changes to a small value. In other words, the flow rate per rotation of the hydraulic motor 21 becomes small and the torque becomes low, so that the aircraft is in a state suitable for high-speed traveling.

Here, if the operating lever 28 of the pilot operating section 27 is turned to the neutral position in order to brake the body,
As shown in the figure, the pilot oil is no longer led from the pilot pump 39a to the pilot oil passage 40 or 41,
The direction control valve 25 returns to the neutral position. In that case, the pilot pressure is not led out from the second shuttle valve 45 to the pressure detecting means 47, and the pressure detecting means 47 does not operate. Therefore, since the coil 52 is not excited, the movable contact piece 56 moves from the NO terminal side to the NC terminal side, and the first exciting means 50 is turned on. That is, the first excitation means 50
And both the second excitation means 57 are turned on, and the signal line 5
5, the solenoid of the second on-off valve 46 is excited,
The spool of the second on-off valve 46 switches to the position 46a and is opened. Since the pilot oil from the pilot pump 39b is always drawn to the pilot port of the first on-off valve 35, the spool of the first on-off valve 35 is at the position 35a and the first on-off valve 35 is opened. The rod 29a of the tilt angle adjusting cylinder 29 projects outside the cylinder, and the tilt angle of the hydraulic motor 21 is kept small. That is, the flow rate per rotation of the hydraulic motor 21 is small and the torque is low at the time of the body braking in the high-speed mode.
Naturally, even if a braking pressure equivalent to that at low speed is generated, the braking torque is minimized. Therefore, compared to high braking torque at low speed,
Shock during aircraft braking is reduced.

On the other hand, if the vehicle continues traveling in the low-speed mode shown in FIG. 2 and the operation lever 28 of the pilot operation section 27 is turned to the neutral position in order to brake the aircraft in the low-speed mode, as shown in FIG. Then, the pilot oil is no longer led from the pilot pump 39a to the pilot oil passage 40 or 41, and the direction control valve 25 returns to the neutral position. In that case, the pilot pressure is not led out from the second shuttle valve 45 to the pressure detecting means 47, and the pressure detecting means 47 does not operate. Therefore, since the coil 52 is not excited, the movable contact piece 56 moves from the NO terminal side to the NC terminal side, and
Is turned on, and the second exciting means 50
The solenoid of the on-off valve 46 is excited. And the second
The spool of the on-off valve 46 moves to the position 46a and the second
Is opened and the pilot oil from the pilot pump 39b is led out to the pilot port of the first on-off valve 35, so that the spool of the first on-off valve 35
The first opening / closing valve 35 is opened by moving to the position a. Therefore, the hydraulic oil derived from the bypass line 32 or 33 via the first shuttle valve 34 is guided to the back pressure chamber 29c of the tilt angle adjusting cylinder 29, and the spring oil of the tilt angle adjusting cylinder 29 The rod 29a protrudes out of the cylinder against the urging of the cylinder 29b, and the tilt angle of the hydraulic motor 21 becomes small.

[0022] That is, the flow rate per rotation of the hydraulic motor 21 has a low torque at small during aircraft braking, as shown in FIG. 7, it takes a little time for the brake pressure rises to P _2, the body initial drop velocity V ₁ is a slow and prolonged time Delta] t ₂ until the aircraft stops. Therefore, even when the vehicle is traveling in the low speed mode, the shock at the time of the vehicle braking is reduced.

When the body is stopped on a slope such as a hill, the rod 29a of the tilt angle adjusting cylinder 29 projects out of the cylinder by its own weight, and as shown in FIG. Although the tilt angle is small and the mode is in the high-speed mode, if the operation lever 28 of the pilot operation unit 27 is rotated in either the forward side or the reverse side of the body at the time of starting the body, FIG. Returning to the state shown in (1), the aircraft can be started.

FIG. 8 shows a second embodiment, which shows a drive circuit of a hydraulic motor of a hydraulic traveling vehicle that switches between high speed and low speed automatically, and replaces the 4-port counterbalance valve 26 shown in FIG. , A six-port counterbalance valve 60, and one port of a mechanical brake 62 is connected to a newly provided oil passage 61. Then, one pilot port of the load sensing on-off valve 63 and the other port of the mechanical brake 62 are connected by an oil passage 64, and the other pilot port of the load sensing on-off valve 63 is connected to the second on-off valve. Connect to the secondary side of 46. Further, similarly to the hydraulic circuit shown in FIG. 1, a back pressure chamber 29c of the tilt angle adjusting cylinder 29 is provided on the secondary side of the load sensing on-off valve 63.
The outlet line of the first shuttle valve 34 and the return tank 36 are connected to the primary side of the load sensing on-off valve 63.

The drive circuit of the hydraulic motor having the structure shown in FIG. 8 operates in the same manner as the hydraulic circuit having the structure shown in FIG. 1, and a detailed description thereof will be omitted. Note that the switching circuit according to the change in the tilt angle of the hydraulic motor at the time of stoppage in the above embodiment can be applied to other various speed switching circuits, and the present invention has the spirit of the present invention. Various modifications can be made without departing from the scope of the present invention, and the present invention naturally extends to the modified ones.

[0026]

According to the present invention, as described in detail in the above-described embodiment, the first opening / closing valve is opened when the airframe running in the low speed mode is braked, and is provided between the two main pipelines. From the first shuttle valve, the hydraulic oil passes through the first on-off valve and is led out to the back pressure chamber of the tilt angle adjusting cylinder. Therefore,
The tilt angle of the hydraulic motor is reduced, and the flow per rotation of the hydraulic motor is reduced, resulting in lower torque.

Therefore, various factors such as an increase in braking pressure and a decrease in shock at the time of body braking are reduced, the operability is improved, and the physical fatigue of the operator is reduced to improve the working efficiency. It is a device of practical value.

[Brief description of the drawings]

FIG. 1 is a drive circuit diagram of a hydraulic motor in a low-speed mode of a hydraulic traveling vehicle in which a high-speed low-speed manual switching is performed when an airframe is stopped according to an embodiment of the present invention;

FIG. 2 is a drive circuit diagram of a hydraulic motor in a state in which the vehicle starts moving in a low-speed mode.

FIG. 3 is a drive circuit diagram of a hydraulic motor in a state where a switch is switched from a low-speed mode to a high-speed mode.

FIG. 4 is a drive circuit diagram of a hydraulic motor in a state in which the aircraft starts in a high-speed mode.

FIG. 5 is a drive circuit diagram of the hydraulic motor in a state where the body is braked in a high-speed mode.

FIG. 6 is a drive circuit diagram of a hydraulic motor in a state where the body is braked in a low-speed mode.

FIG. 7 is a graph showing the relationship between braking pressure, speed, and time during body braking in a low-speed mode.

FIG. 8 shows a second embodiment of the present invention, and is a drive circuit diagram of a hydraulic motor in a low-speed mode of a hydraulic traveling vehicle with automatic switching between high-speed and low-speed when the body is stopped.

FIG. 9 shows a conventional example, and is a drive circuit diagram of a hydraulic motor in a state in which the body of the hydraulic traveling vehicle is switched to a low-speed mode with manual switching between high-speed and low-speed when the body is stopped.

FIG. 10 shows a conventional example, and is a graph showing the relationship between braking pressure, speed, and time during body braking in a low-speed mode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 21 Hydraulic motor 22 Hydraulic pump 23, 24 Main pipeline 25 Direction control valve 27 Pilot operation part 29 Tilting angle adjustment cylinder 29c Back pressure chamber 34 First shuttle valve 35 First open / close valve 36 Return tank 39a Pilot pump 39b Pilot Pumps 40, 41 Pilot oil passage 45 Second shuttle valve 46 Second on-off valve 47 Pressure detecting means 50 First exciting means 57 Second exciting means

Claims (1)

(57) [Scope of request for utility model registration] 1. A hydraulic traveling vehicle in which a tilt angle of a hydraulic motor can be freely changed by a tilt angle adjusting cylinder and a speed can be switched between a high speed mode and a low speed mode.
A pipe to the back pressure chamber of the tilt angle adjusting cylinder is provided on the secondary side of the first opening / closing valve that operates in the opening direction by pilot pressure, and a first shuttle is provided between the two main pipes of the hydraulic motor. A valve is disposed, a conduit from the outlet of the first shuttle valve and a return tank are connected to the primary side of the first on-off valve, and a second on-off valve is operated by energizing a solenoid. The second side is connected to a pilot port of the first opening / closing valve, and a second shuttle valve is disposed between two pilot oil passages from a pilot operating section for controlling a directional control valve of the hydraulic motor, thereby detecting pressure. Means for exciting the solenoid of the second on-off valve by actuation of the pressure detecting means, and second means for exciting the solenoid of the second on-off valve by speed switching operation. Excitation means are installed side by side, When the hydraulic operation of the hydraulic motor is performed by the pilot operation unit, the first opening / closing valve is opened to draw hydraulic oil into the back pressure chamber of the tilt angle adjusting cylinder, and the tilt angle of the hydraulic motor is reduced. A drive circuit for a hydraulic motor, wherein the drive circuit is configured to change to a small amount.