JPH094603A - Backlash preventive device - Google Patents

Abstract

PURPOSE: To provide a backlash preventive device capable of surely preventing backlash as rotating in the direction as operated without a hydraulic motor making a sudden start and capable of securing its stopping position. CONSTITUTION: A backlash preventive device has the backlash of a hydraulic motor 3 prevented by a mechanical type brake 3c which is controlled by a mechanical brake control means 15 actuating by the pressure from a pilot pressure source which utilizes the pressure of an independent brake release pressure source and the pressure on the low pressure side of a relief valve composing the brake valve of an oil pressure circuit of the hydraulic motor 3. An opening and closing valve, which closes when the pressure in the circuit to the brake exceeds a set pressure and opens when the hydraulic motors 3 stops, is provided on a connection circuit 34 which is provided for connecting hydraulic motor 3 around trip lines 5, 6 together. Thus confined pressure does not occur in the round trip lines 5, 6 when the hydraulic motor 3 is stopped by the mechanical type brake, a sudden start and rotation of the hydraulic motor 3 in the opposite direction than prearranged can be prevented, backlash can be prevented and the stopping position car be secured.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anti-swing device for preventing a state in which a hydraulic motor stops rotating while rotating forward and backward when the hydraulic motor stops.

2. Description of the Related Art Conventionally, a hydraulic circuit for driving a revolving structure such as a hydraulic excavator is generally a revolving structure connected to a hydraulic pump by connecting a hydraulic pump and a tank to a reciprocating pipeline of a hydraulic motor by a switching valve. And a brake valve composed of a relief valve for discharging the fluid in the pipeline to the tank and a check valve for allowing the fluid to flow into the pipeline in the reciprocating pipeline. In order to prevent an impact generated on the revolving structure when the brake valve is provided and is stopped by operating the switching valve, it is generally performed while absorbing the inertial energy while relieving the relief of the brake valve.
However, in such a brake valve, even if the revolving structure once stops, the pressure in the two ports of the hydraulic motor becomes alternately high due to compression in the pipe, expansion of the rubber hose and inertia of the revolving structure. The hydraulic motor rotates back and forth several times, and then comes back to a stop phenomenon. In order to prevent such swing-back, the applicant has proposed a swing-back prevention device described in Japanese Utility Model Laid-Open No. 6-58205, and as a further improvement, a swing-back prevention device as shown in FIG. -4344. This actual Kaihei 6-58
In the device described in Japanese Patent Publication No. 205, a short-circuit valve having a throttle position provided in a short-circuit circuit connecting a reciprocating pipe to a hydraulic motor,
The high pressure oil generated in one of the reciprocating pipelines is operated by the pressure on the upstream side of the spring-loaded check valve provided in the tank return circuit of the relief valve in the pipeline, and the high-pressure oil generated in the throttle position of the short-circuit valve is used. It is intended to prevent swingback by circulating it to the other pipeline via the hydraulic motor, but the swingback phenomenon does not stop while the high-pressure oil passes through the throttle position of the short-circuit valve, and the hydraulic motor There was a defect that I couldn't keep still. In order to improve this, the applicant, as shown in FIG.
A hydraulic motor a is provided with a mechanical brake c having a hydraulic piston b, and reciprocating pipelines f and g connecting the hydraulic pump a and a tank e to the hydraulic motor a are provided with fluids in the pipelines f and g. A brake valve j composed of a relief valve h that discharges the fluid to the e and a check valve i that allows the fluid to flow into the pipelines f and g is provided to the hydraulic motor a to release the fluid in the pipeline. So that you can apply the brakes by
A brake release pressure source k that is independent of the hydraulic system of the hydraulic pump a for releasing the mechanical brake c and a pilot pressure source 1 that utilizes the pressure on the low pressure side of the relief valve h are provided, and the mechanical type When the pressure of the pilot pressure source 1 that has exceeded a certain value acts on the circuit m to the hydraulic piston b of the brake c, or when an electric signal or pressure signal for releasing the mechanical brake is input, the brake releasing pressure source The pressure of k is introduced to release the mechanical brake c, the pressure of the pilot pressure source 1 is below a certain value, and the electrical signal or the pressure signal of the release is not input, the mechanical brake c is released. By providing a mechanical brake control means n for removing the pressure acting on the mechanical brake and operating the mechanical brake, it is possible to reliably prevent swinging back and secure a stop position of the hydraulic motor. It proposed.

However, in the apparatus shown in FIG. 1, when the rotation of the hydraulic motor a stops and the brake is applied by the action of the mechanical brake c, the hydraulic motor a is stopped.
Since the mechanical brake c is applied at the same timing as the stop of the rotation, the high pressure oil is trapped in either one of the reciprocating pipelines f and g. This closing pressure decreases with the lapse of time, but if the hydraulic motor a is started in a state where the pressure does not decrease sufficiently, the following problems occur. That is, when the switching valve is operated to activate the hydraulic motor a, the mechanical brake c is released first.
At the moment of releasing the brake c, the hydraulic motor a is suddenly activated by the high-pressure oil trapped in the pipeline f or g, and depending on the rotation direction of the hydraulic motor a, the direction in which the operator operates There was a problem of moving once in the opposite direction. The present invention has no sudden start of the hydraulic motor,
It is an object of the present invention to provide a rocking-back preventing device that can rotate in a direction according to an operation, can surely rock-back, and can secure a stop position.

According to the present invention, a switching valve for switching and connecting a hydraulic pump and a tank to a reciprocating conduit of a hydraulic motor having a mechanical brake having a hydraulic piston is provided, and the reciprocating conduit is provided. In the above, by providing a brake valve composed of a relief valve for discharging the fluid in the pipeline to the tank and a check valve for allowing the fluid to flow into the pipeline, the hydraulic motor is provided with the fluid in the reciprocating pipeline. Is a hydraulic circuit device for applying a brake by relieving the brake valve by a relief valve, the brake releasing pressure source having a pressure equal to or higher than a reference pressure for releasing the mechanical brake and independent of a hydraulic system of the hydraulic pump. And a pilot pressure source that utilizes the pressure on the low pressure side of the relief valve, and the pressure of the pilot pressure source that has exceeded a certain value acts on the circuit to the hydraulic piston of the mechanical brake. When or when an electrical or pressure signal for mechanical brake release is input, the brake release pressure source is introduced to release the mechanical brake, and the pressure of the pilot pressure source is below a certain value and In a swing-back prevention device provided with a mechanical brake control means for excluding the pressure acting on the mechanical brake to operate the mechanical brake when a release electric signal or a pressure signal is not inputted, A connection circuit for connecting the reciprocating pipelines of the hydraulic motor to each other is provided, and the connection circuit has a closed position and a closed position for closing the connection circuit when the pressure of the circuit to the hydraulic piston of the mechanical brake is equal to or higher than a set pressure. The above object is achieved by providing an opening / closing valve having an open position for opening the connection circuit when the hydraulic motor is stopped by the brake operation of the mechanical brake. The on-off valve has an open position that connects the connection circuit to the tank, and closes the connection circuit when the pressure of the circuit to the hydraulic piston of the mechanical brake falls below a certain range below the set pressure. It may have a second closed position, and the mechanical brake control means controls the pressure of the brake release pressure source to release the mechanical brake when an electric signal or a pressure signal for releasing the mechanical brake is input. The pressure acting on the mechanical brake when the pressure of the pilot pressure source is equal to or lower than a certain value and the electric signal is not input, and the mechanical brake is braked.

When the hydraulic motor in the stopped state is started,
An electric signal or a pressure signal is input to the mechanical brake control means to introduce the pressure of the brake release pressure source into the opening / closing valve of the circuit and connection circuit to the hydraulic piston of the mechanical brake to release and open / close the mechanical brake. When the connection circuit by the valve is closed and then the switching valve provided in the reciprocating pipe of the hydraulic motor is switched to the switching position, the hydraulic motor operates normally with the fluid from the hydraulic pump. When stopping the hydraulic motor, when the input of the electric signal or the pressure signal to the mechanical brake control means is cut off and the switching valve is returned to the neutral position for stopping the hydraulic motor, the reciprocating pipe line is cut off. Due to the inertial rotation of the hydraulic motor, pressure is generated in one of the reciprocating pipelines to operate the relief valve of the brake valve, and due to the operation of the relief valve, the pilot pressure source is also provided on the low pressure side. Pressure is generated. In this case, even if a signal such as an electric signal is turned off, the pressure of the pilot pressure source generated on the low pressure side of the relief valve acts on the mechanical brake control means. It can guide the brake and continue to maintain its release. When the relief valve is actuated, the pressure in one of the reciprocating pipelines escapes to the other pipeline and drops, and when the operation of the relief valve is stopped, the pressure on the low pressure side, that is, the pressure of the pilot pressure source, reaches a certain value. It drops below.
As a result, the control means operates so as to eliminate the pressure acting on the brake, the hydraulic motor is braked by the mechanical brake, and at the same time, the opening / closing valve of the connection circuit is opened. Even if a high pressure is generated, rotation of the hydraulic motor is blocked, swingback of the hydraulic motor is prevented, and high pressure oil is not trapped in the pipeline.
Since this closing is eliminated, the hydraulic motor can be prevented from being suddenly activated at the time of activation, and can be activated in the direction of the operation.

Embodiments of the present invention will now be described with reference to the attached drawings. In FIG. 2, reference numeral 1 is a hydraulic pump, 2 is a tank, and 3 is a hydraulic pump.
Is a hydraulic motor that rotationally drives a revolving structure of a hydraulic excavator, and these hydraulic pump 1 and tank 2 are connected to reciprocating pipelines 5 and 6 of the hydraulic motor 3 by a manual, electromagnetic or hydraulic switching valve 4. It The hydraulic motor 3 is of a known type including a mechanical brake 3c that presses a brake plate 3b with a hydraulic piston 3a, as illustrated in FIG.
In the illustrated example, the brake plate 3b is configured as a multi-plate type in which the cylinder block 3d side and the casing 3e side of the hydraulic motor 3 are alternately engaged, and when the hydraulic piston 3a is pushed by the disc spring 3f, the brake plate 3d is When the drive shaft 3g is brought into pressure contact with each other to brake and the brake release pressure is introduced from the circuit 25 into the chamber 3h in front of the hydraulic piston 3a, the hydraulic piston 3a retreats against the disc spring 3f,
The brake is released. The reciprocating pipelines 5, 6
The brake valve 11 includes relief valves 7 and 8 that discharge the fluid in each pipeline to the tank 2 and check valves 9 and 10 that allow the fluid to flow into each pipeline. In order to prevent the hydraulic motor 3 from swinging back when stopped, a brake release pressure source 12 having a pressure equal to or higher than a reference pressure for releasing the mechanical brake 3c and independent of the hydraulic system of the hydraulic pump 1 is provided. And a pilot pressure source 14 that utilizes the pressure of the tank return circuit 13 on the low pressure side of the relief valves 7 and 8, and these pressure sources 12 and
The pressure of 14 is input to the mechanical brake control means 15 as a signal to control the mechanical brake 3c of the hydraulic motor 3. As the pilot pressure source 14, the pressure in front of the throttle check valve 17 provided for clarifying the pressure generated in the tank return circuit 13 is used, and the pressure behind the throttle check valve 17 acts on the other end of the spool 15b. I was allowed to. Further, 19 is a shockless valve provided in the vent circuit of the relief valves 7 and 8, and removes the shock generated in the pipelines 5 and 6 when the relief valves 7 and 8 are closed.
The throttle check valve 17 includes a spring-loaded check valve 17a and an orifice 1 provided in a circuit in parallel with the check valve 17a.
7b and. In the embodiment of FIG. 2, the mechanical brake control means 15 is composed of a first spool valve 15e and a second spool valve 15f, and the mechanical type is controlled by switching the first spool valve 15e by an electric signal or a pressure signal. Brake 3
2 position 2 connecting circuit 25 of hydraulic piston 3a of c to first pilot pressure source 12 or second spool valve 15f
7 and 28, the second spool valve 15f has a spool on which the pressure before and after the throttle check valve 17 acts in opposition, and a spring force is applied to the side behind the throttle check valve 17 on which the pressure acts. It is configured to have two positions 29 and 30 that operate to connect the connection path 26 between the first spool valve 15e and the second spool valve to the brake release pressure source 12 or the tank 2. The electric signal or pressure signal is
It is preferable to input to the mechanical brake control means 15 immediately before the switching valve 4 enters its neutral position and to interlock with the switching valve 4 so that it is turned off immediately before switching from the neutral position to the switching position. . The reciprocating pipelines 5 and 6 have an open position 32.
Are connected to each other by a connection circuit 34 provided with a spool-type on-off valve 31 having a closed position 33, and the opening and closing operation of the on-off valve 31 is guided by the circuit 25 of the hydraulic piston 3a of the mechanical brake 3c. It was carried out in proportion to 35. In this embodiment, when the hydraulic motor 3 is started, an electric signal or a pressure signal is input to the first spool valve 15e to switch to the position 27, and the pressure of the brake release pressure source 12 is set to the mechanical pressure of the hydraulic motor 3. -Type brake 3c to release the brake and open / close valve 3
It also leads to 1 and sets it as the closed position 33. After that, when the switching valve 4 is switched to the switching position, the hydraulic motor 3 is normally rotated by the pressure oil from the pump 1. When the rotation of the hydraulic motor 3 is stopped, the switching valve 4 is returned to the neutral position, the input of the electric signal or the pressure signal to the spool valve 15e is stopped, and the spool valve 15e is returned to the position 28. As a result, the pressure acting on the mechanical brake 3c is removed to the tank 2 and the brake can be operated by the disc spring 3f, but as described above, the inside of the conduit 5 or 6 blocked by the switching valve 4 Since a high pressure is generated by the inertia of the hydraulic motor 3 and the relief valve 7 or 8 operates, a pilot pressure source 14 is generated on the low pressure side of the relief valve 7 or 8, and the pressure causes the second spool valve 15f to position. Two
9, the mechanical brake 3c is maintained in the released state by the pressure of the brake release pressure source 12, and is decelerated only by the fluid pressure brake by the brake valve 11. After this, when the pressure in the pipeline 5 or 6 is reduced to the pressure at which the relief valve 7 or 8 that was operating is closed, the pressure in the pilot pressure source 12 also becomes a certain value or less, so the second pilot valve 1
5f is switched to the position 30, and the brake release pressure acting on the mechanical brake 3c is released to the tank 2, so that the mechanical brake 3c is braked by the disc spring 3f and the hydraulic motor 3 swings back. Stop without
The stop position can be reliably retained. The on-off valve 31 is switched to the open position 32 at the same time when the pressure of the circuit 25 becomes a set pressure, that is, the pressure applied by the mechanical brake 3c, and the high pressure oil generated by closing one of the reciprocating pipes 5 or 6 is released to the other. It is possible to prevent the inconvenience that the hydraulic motor 3 is suddenly started when it is started, or that the hydraulic motor 3 is temporarily rotated in a direction opposite to the desired rotation direction. By adjusting the spring 35 of the opening / closing valve 31, the timing at which the mechanical brake 3c is applied and the opening timing of the opening / closing valve 31 can be matched. As shown in FIG. 4, the on-off valve 31 may have an open position 32 for connecting the connection circuit 34 to the tank 2. In this case, the reciprocating pipe lines 5 and 6 have It becomes the tank pressure. Further, the on-off valve 31 is shown in FIG. 5 provided with a second closed position 36 which is closed when the pressure of the circuit 25 to the hydraulic piston 3a of the mechanical brake 3c becomes a pressure within a certain range or lower than the set pressure. The configuration may be adopted, and in this case, after the high pressure oil inside the pipes 5 and 6 is removed, the communication with the tank 2 is closed, and the outflow of the hydraulic oil inside the pipes can be prevented. Further, as shown in FIG. 6, the open position 3 for connecting to the tank 2
Alternatively, the on-off valve 31 may be provided with 2 and the second closed position 36. In the case of FIGS. 5 and 6, the circuits for introducing the switching pressure from the circuit 25 to the opening / closing valve 31 and the circuit for discharging the pressure to the tank for switching are provided with the throttles 37, 38 for adjusting the switching speed. did. As shown in FIG. 7, the mechanical brake control means 15 has a spool having a position 20 for removing the pressure of the chamber 3h of the hydraulic piston 3a to the tank 2 and a position 21 for guiding the pressure of the brake release pressure source 12 to the chamber 3h. Valve 15
The auxiliary switching valve 1 configured with a and interlocking with the switching valve 4
6 may be provided to control the input of the pressure signal from the brake release pressure source 12 to the control means 15.
In this case, a two-stage piston is provided at one end of the spool 15b of the spool valve 15a, and one piston chamber 15c
The pressure of the brake release pressure source 12 is introduced as a pressure signal to the other piston chamber 15 via the auxiliary switching valve 16.
The pressure of the pilot pressure source 14 was introduced into d. Reference numeral 18 is a return spring that acts on the other end. The auxiliary switching valve 16 has three positions like the switching valve 4, and its switching positions 22, 2
In 3, the brake release pressure source 12 is connected to the brake control means 1
No. 5 spool 15b and its piston chamber 15c
The brake release pressure source 1 at its neutral position 24.
2 is sent only to the spool 15b of the brake control means 15, and the switching valve 4 is used to rotationally drive the hydraulic motor 3.
To the switching position, the auxiliary switching valve 16 is also switched, and the pressure of the brake release pressure source 12 acts as a signal on the pilot chamber 15c of the brake control means 15, so that the brake control valve 15 is operated by the mechanical brake 3c. The hydraulic brake 3c operates so as to send the pressure of the brake release pressure source 12, and the mechanical brake 3c is released. At this time, the on-off valve 31 is closed by the pressure of the circuit 25,
The hydraulic motor 3 can be driven to rotate normally. When the rotation of the hydraulic motor 3 is stopped, the switching valve 4 is returned to the neutral position. At this time, the auxiliary switching valve 16 introduces the pressure of the brake release pressure source 12 into the pilot chamber 15c of the brake control means 15. So that the brake control means 1
The spool 15b of No. 5 tries to return to the position 20 by the force of the return spring 18, but at this time, a high pressure is generated due to the inertia of the hydraulic motor 3 in one of the pipelines 5 and 6, and with this, one of the relief lines is released. The valves 7 and 8 are actuated to generate a pressure serving as the pilot pressure source 14 on the low pressure side. This pressure is used as a signal in the pilot chamber 15 of the brake control means 15.
acting on d, the spool 15b of the brake control means 15 remains in position 21 and the brake release pressure source 12
The pressure of the hydraulic motor 3 in order to guide the pressure to the mechanical brake 3c.
Is decelerated only by the fluid pressure brake by the brake valve 11 that short-circuits between the pipelines 5 and 6. Then, when the pressure in the pipeline 5 or 6 is reduced to the pressure at which the relief valve 7 or 8 that was operating is closed, the pressure in the pilot pressure source 12 is also a certain value or less, specifically, the pressure conversion value of the return spring 18. And the pressure value behind the throttle check valve 17 drops below the total value, the brake control means 15 moves to the position 20, and the pressure acting on the mechanical brake 3c is increased.
Therefore, the rotation of the hydraulic motor 3 can be stopped by the brake of the mechanical brake 3c without swinging back. That is, when the pressure signal from the brake release pressure source 12 does not act and the pressure signal from the pilot pressure source 14 is below a certain value, the mechanical brake is applied. In other words, even if the relief valve 7 or 8 is performing the relief operation, the brake control means 15 does not perform the operation of releasing the brake unless the pressure on the low pressure side becomes a certain value or more. Since a relatively large amount of fluid rarely flows through the relief valve 7 or 8 when the switching valve 4 is set to the neutral position,
In most cases, the brake control means 15 does not operate to release the mechanical brake 3c, and the rotation stop position of the hydraulic motor 3 can be held. Also in this case, the open / close valve 31 is in the open position 32 when the mechanical brake 3c is applied, and prevents the high pressure oil from being closed in the pipeline 5 or 6. The mechanical brake control means 15 controls the mechanical brake 3 as shown in FIG.
A main spool valve 15g having two positions, one for connecting the hydraulic circuit 25 to the brake release pressure source 12 and one for connecting the circuit 25 to the tank, and a pilot valve that switches between two positions by an electric signal or a pressure signal. It is also possible to form a combination of 15h.

As described above, according to the present invention, in the swing-back preventing device provided with the mechanical brake control means for controlling the mechanical brake of the hydraulic motor, the reciprocating pipeline of the hydraulic motor is provided. The connected connection circuit has a closed position that is closed when the pressure of the circuit to the hydraulic piston of the mechanical brake is equal to or higher than a set pressure, and an opening that opens the connection circuit when the hydraulic motor is stopped by the brake operation of the mechanical brake. Since the on-off valve having the position is provided, when the hydraulic motor is stopped by the mechanical brake, no closing pressure is generated in the reciprocating pipeline to the hydraulic motor, and when the hydraulic motor is started. It is possible to prevent inconveniences caused by sudden activation or temporary movement in a direction opposite to the intended rotation direction, and there is an effect that the function of preventing swinging back and the function of securing a stop position are not impaired.

[Brief description of drawings]

FIG. 1 is a diagram of a conventional example.

FIG. 2 is a diagram of an embodiment of the present invention.

FIG. 3 is a sectional view of a hydraulic motor used in the present invention.

FIG. 4 is a diagram of a second embodiment of the present invention.

FIG. 5 is a diagram of a third embodiment of the present invention.

FIG. 6 is a diagram of a fourth embodiment of the present invention.

FIG. 7 is a diagram of a fifth embodiment of the present invention.

FIG. 8 is a diagram of a sixth embodiment of the present invention.

[Explanation of Codes] 1 hydraulic pump 2 tank 3 hydraulic motor 3a hydraulic piston 3c mechanical brake 3f disc spring 4 switching valve 5, 6 conduit 7, 8 relief valve 9, 10 check valve 11 brake valve 12 brake release pressure source 14 Pilot pressure source 15 Mechanical brake control means 15a Spool valve 15b Spool 15c, 15d Piston chamber 15e First spool valve 15f Second spool valve 17 Throttle check valve 18 Return spring 25 Circuit 26 Connection path 27, 28, 29, 3
0 position 31 on-off valve 32 open position 33 closed position 36 second closed position

Claims (5)

[Claims] 1. A switching valve is provided for switching and connecting a hydraulic pump and a tank to a reciprocating pipeline of a hydraulic motor having a mechanical brake having a hydraulic piston, and the fluid of the pipeline is supplied to the reciprocating pipeline. By providing a brake valve composed of a relief valve for discharging to the tank and a check valve for allowing the fluid to flow into the pipeline, the hydraulic motor is caused to relieve the fluid in the reciprocating pipeline by the relief valve. A hydraulic circuit device for applying a brake, the brake release pressure source having a pressure equal to or higher than a reference pressure for releasing the mechanical brake and independent of a hydraulic system of the hydraulic pump, and a low pressure side of the relief valve. And a pilot pressure source utilizing the pressure of the mechanical brake, and when the pressure of the pilot pressure source that exceeds a certain value acts on the circuit to the hydraulic piston of the mechanical brake or the solution of the mechanical brake. A release electrical signal or pressure signal is introduced to introduce the pressure of the brake release pressure source to release the mechanical brake, the pressure of the pilot pressure source is below a certain value and the release electrical signal. When a pressure signal is not input, a reciprocating motion of the hydraulic motor in a swing-back prevention device provided with a mechanical brake control means for excluding the pressure acting on the mechanical brake to operate the mechanical brake The connection circuit that connects the pipelines of
The connection circuit includes a closed position that closes the connection circuit when the pressure of the circuit to the hydraulic piston of the mechanical brake is equal to or higher than a set pressure, and the connection circuit when the hydraulic motor is stopped by the brake operation of the mechanical brake. An anti-sway return device, which is provided with an on-off valve having an open position for opening. 2. The swing-back prevention device according to claim 1, wherein the on-off valve has an open position for connecting the connection circuit to a tank. 3. The on-off valve has a second closed position for closing the connection circuit when the pressure of the circuit to the hydraulic piston of the mechanical brake becomes a pressure within a certain range or lower than the set pressure. The shake-back prevention device according to claim 1. 4. The low pressure side of the relief valve is connected to a tank via a throttle check valve, and the mechanical brake control means is composed of a first spool valve and a second spool valve, and the first spool valve is It has two positions which are switched by an electrical signal or a pressure signal to connect the circuit to the hydraulic piston to the brake release pressure source or to a second spool valve, the second spool valve having a pressure before and after the throttle check valve. Have a spool acting in opposition to each other, and a spring force acts on the side of the spool on the rear side of the throttle check valve on which the pressure acts to connect the first spool valve and the second spool valve to the pilot pressure source. Alternatively, the rocking-back preventing device according to claim 3, which has two positions for connecting to the tank. 5. A spool valve having a spool in which the low pressure side of the relief valve is connected to a tank via a throttle check valve, and the mechanical brake control means has a spool on which pressures before and after the throttle check valve act in opposition to each other. The spring force acts on the side of the spool on the rear side of the throttle check valve on which the pressure acts, and the side of the spool on the side on which the pressure in front of the throttle check valve acts on the spool along with the switching operation of the switching valve. 2. The anti-sway return device according to claim 1, wherein the anti-sway device comprises a spool valve on which the pressure of the brake release pressure source acts.