JPH0988906A - Fluid hydraulic drive device with fly wheel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid hydraulic drive device with a flywheel being small and light weight, with enough endurance against the variation of load and capable of being controlled easily and accurately. SOLUTION: An electric motor is connected directly with a flywheel 3 and a one direction delivery type fixed capacity liquid hydraulic pump 8A is driven through the flywheel. A liquid hydraulic pump with the flywheel 3 is constituted so as to drive a liquid hydraulic cylinder 10 through a directional control valve 9 by the fixed capacity liquid hydraulic pump 8A.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic drive device having a flywheel suitable for a drive device such as a press device and a material fatigue tester.

[0002]

2. Description of the Related Art Most of conventional large-sized press machines, bed knock-out machines attached to the press machines, material fatigue testing machines, etc., reduce the rotational motion of the output shaft 1a of the electric motor 1 as shown in FIG. Decelerate with machine 2 and flywheel 3
Is rotated, and this rotation is converted into a linear motion through the crankshaft (or eccentric shaft) 4 and the connecting rod 5 to reciprocate the slide 6. In a large press device, a crank press or a knuckle joint press is used. What is called is predominant. These press machines are shown in FIG.
As shown in FIG. 3, if the crankshaft 4 and the flywheel 3 are connected via the clutch 7 and the clutch 7 is left connected, one press shot is performed by one rotation of the flywheel 3. That is, accumulation and release of kinetic energy are repeated for each revolution of the flywheel 3.

[0003]

As described above, when the rotary motion of the electric motor is converted into the linear motion by the mechanical link mechanism, it is difficult to arbitrarily set the stroke of the slide and the top dead center and the bottom dead center. Is. Also, the kinetic energy of flywheel K. E is expressed by the following equation. K. E = (1/2) Iω ² where I: moment of inertia ω: rotational angular velocity, that is, the kinetic energy of the flywheel K.K. It can be seen that E changes in proportion to the square of the rotational angular velocity ω.

From this, it is understood that the faster the rotational movement of the flywheel, the smaller the moment of inertia I is. However, since the rotation speed of the flywheel of the conventional large-sized press machine is usually about 50 RPM, the diameter of the flywheel is very large, and the weight is also large. Therefore, the rotation speed of the motor is 1500RP
Let M be (50/1500) ² = (1/30) ² = 1/900 by directly connecting the flywheel to this electric motor. Therefore, the inertia moment I of the flywheel is It can be 1/900.

Therefore, as shown in FIG.
By directly connecting the flywheel 3 and the flywheel 3 and connecting the flywheel 3 and the reduction gear 2 via the clutch 7, both the flywheel 3 and the clutch 7 can be small in size, and the weight of the device can be reduced. Although it should be useful, such a structure has a problem that a very large forward and reverse torque is directly applied to the speed reducer 2 for each revolution of the crankshaft 4, resulting in destruction of the device. To prevent that,
Conventionally, as shown in FIG. 11, the flywheel 3 and the crankshaft 4 are connected to absorb a large torque fluctuation of the crankshaft 4, and the flywheel 3 is absorbed as a shock absorber.
The electric motor 1 and the speed reducer 2 had to be structured so as to always output a constant torque.

Further, in the case of a fatigue tester for a spring, it is necessary to store energy by applying a large compressive force to the spring and release the energy at the next moment, so that energy is stored in the flywheel. It is most effective to let them do it. For this reason, the conventional device uses a flywheel having a very large moment of inertia, and since the heavy electric motor, speed reducer, and flywheel are placed at the top of the device, the center of gravity moves upward. There was a problem that the structure was biased and unstable.

[0007]

In order to solve the above-mentioned problems, in the present invention, a flywheel is directly connected to an electric motor, and a one-way discharge type hydraulic pump is driven through the flywheel. A hydraulic drive device having a flywheel is configured such that the hydraulic cylinder is driven by the hydraulic pump via a direction control valve.

Further, a sensor for detecting the displacement of the piston rod is connected to the piston rod of the hydraulic cylinder of the hydraulic drive system of the above-mentioned first invention, and its output is fed back to the directional control valve via the controller. You may

In the present invention, the flywheel is directly connected to the electric motor, and the bidirectional discharge type hydraulic pump is driven through the flywheel, and the hydraulic cylinder is driven by the hydraulic pump. Thus, the hydraulic drive unit having the flywheel is constructed.

Further, in the present invention, a flywheel is directly connected to the electric motor, and a bidirectional discharge type hydraulic pump is driven through the flywheel, and a hydraulic motor driven by the hydraulic pump is provided. The crankshaft is rotatably driven via the crankshaft, and the hydraulic cylinder is driven by the crankshaft to form a hydraulic drive device having a flywheel.

Further, the hydraulic cylinder of the hydraulic drive device of the above-mentioned fourth invention may have a small diameter, and this hydraulic cylinder and a large diameter hydraulic cylinder may be connected to form a hydraulic power booster.

Further, another hydraulic pump may be connected to the small-diameter hydraulic cylinder of the hydraulic drive system of the fifth invention.

The hydraulic pressure booster of the fifth aspect of the invention may be constructed by directly inserting a small diameter piston rod reciprocating by a crankshaft into a large diameter hydraulic cylinder.

Further, in the present invention, the flywheel is directly connected to the electric motor, and the one-way discharge type hydraulic pump is driven through the flywheel, and the discharge side of the hydraulic pump is a hydraulic cylinder. A hydraulic drive device having a flywheel by connecting both liquid chambers of the hydraulic cylinder to a tank and connecting a pilot check valve to each of these four circuits to form a bridge circuit. Configure.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to FIGS. In the figure, the same symbols as the above-mentioned symbols indicate the same items. FIG. 1 shows a first embodiment of the present invention. In this embodiment, a flywheel 3 is directly connected to an electric motor 1 and a one-way discharge type fixed displacement hydraulic pump 8A is provided through the flywheel 3. The fixed capacity hydraulic pump 8A drives the hydraulic cylinder 10 via the direction control valve 9 to form a hydraulic drive device having a flywheel. 1 in the figure
Reference numeral 1 is a relief valve, and 12 is a tank.

FIG. 2 shows the fixed displacement hydraulic pump 8 of FIG.
A variable displacement hydraulic pump 8B is used instead of A, and the other configurations are the same as those of the apparatus of FIG.

Further, a linear actuator 13 is connected to the piston rod 10a of the hydraulic cylinder 10 of the hydraulic drive system of FIGS. 1 and 2 as a sensor for detecting the displacement of the piston rod 10a, and its output is transmitted via the controller 14. Feedback is provided to the directional control valve 9. In the figure, 9a and 9b are solenoids of the directional control valve 9, 1
Reference numerals 5 and 16 are conductors connecting the controller 14 and the solenoids 9a and 9b.

FIG. 3 shows another embodiment of the present invention. In this embodiment, an electric motor 1 and a flywheel 3 are provided.
And a flywheel so as to drive a two-way discharge type fixed capacity hydraulic pump 17A through the flywheel 3 and drive the hydraulic cylinder 10 by the fixed capacity hydraulic pump 17A. A hydraulic drive device is configured. In the figure, S is a leaf spring connected to the lower end of the piston rod 10a of the hydraulic cylinder 10, and in this case, this device acts as a fatigue tester for the leaf spring S. In the figure, 18 is a pilot check valve, 19 is a check valve, and 20 is a tank. Further, FIG. 4 uses a variable displacement hydraulic pump 17B instead of the fixed displacement hydraulic pump 17A of FIG. 3, and the other configuration is the same as the device of FIG.

FIG. 5 shows another embodiment of the present invention. In this embodiment, an electric motor 1 and a flywheel 3 are provided.
And a two-way discharge type fixed capacity hydraulic pump 17A is driven via the flywheel 3, and the crankshaft 4 is connected via a hydraulic motor 21 driven by the fixed capacity hydraulic pump 17A. It is rotationally driven, and the hydraulic cylinder 2 is driven by the crankshaft 4 through the connecting rod 5.
A hydraulic drive device having a flywheel is constructed so as to drive 2. In the figure, 19 is a check valve and 20 is a tank. Further, FIG. 6 shows the fixed capacity hydraulic pump 17A of FIG.
Instead of the above, a variable displacement hydraulic pump 17B is used, and the other configurations are the same as those of the apparatus of FIG.

Further, the hydraulic cylinder 22 of the hydraulic drive system shown in FIG. 5 has a small diameter, and the hydraulic cylinder 22 and the large diameter hydraulic cylinder 23 are connected by pipes 24 and 25 to form a hydraulic power booster E. Good. Further, another hydraulic pump 26 may be connected to the small-diameter hydraulic cylinder 20 of the hydraulic drive system of FIG. In the figure, 27 is an electric motor for driving the hydraulic pump 26, but the hydraulic pump 26 may be a manual type.
Further, the hydraulic pump 26 is a low-pressure large-flow pump,
It is also possible to use the output of the crankshaft 4 for the pressurizing process requiring a large force, and to have the pump take charge of the process of high speed ascent and descent at no load.

Further, as shown in FIG. 6, the hydraulic pressure booster E
Alternatively, the small-diameter piston rod 30 that reciprocates by the crankshaft 4 via the connecting rod 5 may be directly inserted into the large-diameter hydraulic cylinder 31. In the figure, 31a is a piston of the hydraulic cylinder 31, and 31b is its piston rod.

FIG. 7 shows another embodiment for reducing the size of the hydraulic booster E. In this case, the piston 31a and the piston rod 3 of the hydraulic cylinder 31 having a large diameter are shown.
A small diameter piston rod 3 is formed by forming a cavity 31c in 1b.
0 allows entry into the void 31c.

FIG. 8 shows still another embodiment of the present invention. In this embodiment, the flywheel 3 is directly connected to the electric motor 1, and the one-way discharge type fixing is performed via the flywheel 3. The displacement hydraulic pump 8A is driven, and the discharge side of the hydraulic pump 8A is connected to both the hydraulic chambers 10A and 10B of the hydraulic cylinder 10 through the pipes 32 and 33, and both the hydraulic cylinder 10 and the hydraulic chamber 10A are connected. Liquid chamber 10A, 1
0B is connected to the tank 20 through the pipes 34 and 35, and the four pipes 32, 33, 34, and 35 are sequentially provided with pilot check valves 36, 37, 38, and 39 to form a bridge circuit. A hydraulic drive having a flywheel is constructed. Further, FIG. 9 uses a variable displacement hydraulic pump 8B in place of the fixed displacement hydraulic pump 8A of FIG. 8, and other configurations are the same as those of the device of FIG.

Further, FIG. 10 shows a pilot check valve 36.
~ 39 and an example of the directional switching valve 40 for controlling pilot pressure. When the pressure liquid from the hydraulic pressure source 8 is supplied to the external pilot port P through the directional switching valve 40, this check valve closes. When the direction switching valve 40 is energized and the pilot pressure is released to the tank 20, the valve opens. This hydraulic pressure source may be supplied from the pump 8 shown in FIGS. 8 and 9, or a separately prepared hydraulic pressure source may be used.

The operation of each of the above-described embodiments will be described. In the embodiment of FIGS. 1 and 2, the flywheel 3 directly connected to the electric motor 1 rotates at a high speed (for example, 1500 RPM), and a fixed displacement hydraulic pump 8A of one-way discharge type.
Alternatively, when the variable displacement hydraulic pump 8B rotates, the tank 1
The liquid in 2 is sucked up and flows in the direction of arrow A. When the directional control valve 9 is in the state shown in the drawing, the hydraulic cylinder 10 does not operate, but when the directional control valve 9 switches to the pattern 9A, the piston rod 10a of the hydraulic cylinder 10 is indicated by the arrow B.
The piston rod 10a operates in the direction of arrow C when operated in the direction of and switched to the pattern 9B. Therefore, the hydraulic cylinder 10 can drive, for example, a press machine. In this case, when the fixed displacement hydraulic pump 8A is used, the driving speed of the pressing device is constant, and when the variable displacement hydraulic pump 8B is used, the driving speed of the pressing device can be made variable.

The relief valve 11 shown in FIGS. 1 and 2 is
It determines the maximum pressure of the liquid pressure. The linear actuator 13 connected to the piston rod 10a of the hydraulic cylinder 10 is a sensor that detects the operating position of the piston rod 10a, and feeds this output back to the directional control valve 9 via the controller 14. The operation of the hydraulic cylinder 10 becomes accurate, and the top dead center, the bottom dead center and the stroke of the hydraulic cylinder 10 can be stabilized.

The embodiment of FIGS. 3 and 4 shows an example in which the device of the present invention is applied to a fatigue tester for leaf springs S. In this embodiment, the flywheel 3 directly connected to the electric motor 1 is rotated at a high speed. At the same time, the fixed-capacity hydraulic pump 17A or the variable-capacity hydraulic pump 17B capable of discharging in two directions by rotating in one direction is driven, and the hydraulic cylinder 10 is operated by the discharged pressurized liquid, so that the leaf spring S It compresses and repels.

That is, the leaf spring S is attached to the tip of the rod 10a of the hydraulic cylinder 10 by using the fixed displacement hydraulic pump 17A or the variable displacement hydraulic pump 17B capable of discharging in two directions by the rotation in one direction. In the compression type fatigue tester, most of the energy for compressing the spring S is obtained from the flywheel 3. Also,
When the discharge direction of the pump 17A or 17B is reversed, the reaction liquid of the spring S causes the pressure liquid on the head side of the cylinder 10 to flow back to the pump 17A or 17B. In this case, the pumps 17A and 17B are transformed into hydraulic motors, and the output torque thereof is used for increasing the speed of the flywheel 3.
Therefore, the hydraulic pumps 17A and 17B in this case are hydraulic pump motors, and energy is exchanged between the flywheel 3 and the leaf spring S, and the electric motor 1 supplies a small amount of energy so as not to stop the exchange. All you have to do is continue to supply.

3 and 4, the pilot check valve 18 inserted between the tank 20 and the cylinder head side pipe 28 and the check valve 19 between the tank 20 and the cylinder rod side pipe 29 are the cylinder 10 Is a valve for absorbing the liquid amount difference between the head side and the rod side. That is, when the rod 10a is pushed out, a shortage of hydraulic fluid is sucked up from the tank 20 via the check valve 19,
When is drawn in, the pilot check valve 18 opens and has a function of returning excess liquid on the head side to the tank 20. Any method can be used to absorb the difference in the amount of liquid in the cylinder 10 because a shuttle valve or a relief valve can be used.

Further, in the embodiment shown in FIGS. 5 and 6, the two-way discharge type fixed displacement hydraulic pump 17A or variable displacement hydraulic pump 17B is combined with the hydraulic motor 21 to form the hydraulic continuously variable transmission D. The hydraulic continuously variable transmission D reduces the rotational speed of the flywheel 3 to rotate the crankshaft 4. In this case, since there is no difference in the amount of liquid between the inlet and the outlet of the hydraulic motor 21, two check valves 19 are inserted as shown in the figure, and only the internal leakage (drain) of the pumps 17A, 17B and the motor 21 is stored in the tank. Two
You can suck it up from zero.

Further, in FIG. 5, E is a hydraulic power boosting device formed by combining a small diameter hydraulic cylinder 22 and a large diameter hydraulic cylinder 23, and this device is driven by the small diameter hydraulic cylinder 22. It is performed by reciprocating the input rod 22a by the crankshaft 4 via the connecting rod 5. In this case, the output rod 2 of the large diameter hydraulic cylinder 23
The stroke of 3a is shortened, but the power is increased accordingly.
For example, in the case of a forging press of about 400 tons, the connecting rod 5
When the stroke of the output rod 23a is set to 20 mm when the stroke of the hydraulic cylinder 2 is 200 mm,
The thrust of No. 3 is 10 times the thrust of the small diameter hydraulic cylinder 22.

Further, as shown in FIG. 5, if a small pump 26 is connected to the small diameter hydraulic cylinder 22, the stroke of the output shaft cannot be changed, but the height of the rod can be freely set. be able to. The pump 26 may be a manual pump instead of an electric motor drive. Also, this pump 26
It is also possible to use a low pressure, large flow rate pump, and use the output of the crankshaft 4 for the pressurizing process that requires a large force, and this pump is responsible for the high speed ascending or descending process under no load.

If the hydraulic pressure booster E is formed as shown in FIG. 6, the structure is simplified, and if it is formed as shown in FIG. 7, the hydraulic booster E can be further downsized. it can.

In addition, the embodiment of FIGS. 8 and 9 is a motor 1
A hydraulic liquid is produced by the flywheel 3 and the fixed displacement hydraulic pump 8A or the variable displacement hydraulic pump 8B directly connected to the hydraulic cylinder 10 and the relief valve 11 is for adjusting the pressure of the circuit. And 4 pilot check valves 36
.About.39 form a bridge circuit. FIG. 10 shows an example of the pilot check valves 36 to 39 and the direction switching valve 40 for controlling the pilot pressure. That is, when the pressure fluid from the fluid pressure source 8 is supplied to the external pilot port P via the direction switching valve 40, the check valve is closed.
When the solenoid of the direction switching valve 40 is energized to release the pilot pressure to the tank 20, the valve opens.

When the pilot check valves 36 and 39 shown in FIG. 9 are opened and the pilot check valves 37 and 38 are closed, the working fluid flows into the liquid chamber 10A of the cylinder 10 and flows out from the liquid chamber 10B. The rod 10a is retracted. Then valves 36 and 39 are closed and valves 37 and 38 are
When is opened, the piston rod 10a is projected.
Further, when the four valves 36 to 39 are all opened at the same time, the cylinder 10 stops, but both liquid chambers 10A, 1A of the cylinder 10 are stopped.
Since 0B is in communication, it freely moves when an external force is applied to the rod 10a. Even if the four valves 36 to 39 are closed at the same time, the cylinder 10 is stopped, but both liquid chambers 10A and 10B are closed.
Is closed, the piston is restrained and does not move even if an external force is applied to the piston rod 10a. In this way, the pilot pressures of the pilot check valves 36 to 39 configured in the bridge are individually controlled to control the opening and closing of the valves 36 to 39, respectively, thereby instantaneously selecting the direction switching modes of various types. Can be done.

As shown in FIG. 10, the pilot check valves 36 to 39 allow the working fluid to flow from IN to OUT, and the pilot pressure is applied as shown by the dotted line, so that the check valves prevent free flow. Since it is closed, it is possible to prevent the pressure impulse from being generated. Also 4 valves 3
Since it is possible to control with a slight time lag without simultaneously operating 6 to 39, it is possible to further reduce the pressure shock.

[0037]

As described above, according to the present invention, the flywheel 3 is directly rotated at a high speed by the electric motor 1 without rotating the flywheel via the speed reducer as in the prior art. And the effect that other devices can be made significantly smaller and lighter than conventional ones is obtained.

Further, in the hydraulic drive system of the present invention, the fluctuation of the load directly becomes the pressure fluctuation of the hydraulic fluid, but even if there is a large pressure fluctuation, the hydraulic pump of the apparatus of the present invention and the hydraulic pressure communicating with this pump. No problems occur in the system. In other words, mechanical deceleration transfers energy by point contact,
In the case of hydraulic pressure, the mechanical portion and the contact portion of the pressurized liquid may all be considered to be surface contact according to the principle of Pascal. Therefore, even if there is a large change in load, the pump will not be damaged within the rated pressure range. In short, the hydraulic pump functions as a fluid converter. Therefore, according to the present invention, it is possible to sufficiently withstand a large load change such as in a press machine.

Further, if the hydraulic pump used in the hydraulic drive device is of a fixed displacement type, if the rotation of the flywheel is constant, the speed of the cylinder is also constant and it is considered that a speed reducer with a fixed gear ratio was used. You can do it. On the other hand, if the hydraulic pump is a variable displacement type, it becomes a continuously variable transmission, and if the discharge amount of the pump is reduced, the speed of the cylinder decreases, but the pressurizing force can be increased correspondingly. Therefore, by using the hydraulic drive device of the present invention, the size and size of the device can be reduced without compromising the kinetic energy of the flywheel.
It is possible to significantly reduce the weight. Further, for example, by using a simple hydraulic pressure control system having the structure of the present invention instead of the mechanical link mechanism in the press device, the top dead center, the bottom dead center and the stroke of the slide can be quickly set when the die is replaced. The effect of being able to do is also obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a first embodiment of the device of the present invention.

FIG. 2 is a schematic diagram of an embodiment in which the hydraulic pump of the apparatus of FIG. 1 is of a variable displacement type.

FIG. 3 is a schematic diagram showing a second embodiment of the device of the present invention.

FIG. 4 is a schematic diagram of an embodiment in which the hydraulic pump of the apparatus of FIG. 3 is of a variable displacement type.

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

FIG. 6 is a schematic diagram showing a modification of the hydraulic power booster, while the hydraulic pump of the apparatus of FIG. 5 is of a variable displacement type.

FIG. 7 is a sectional view showing another embodiment of the hydraulic pressure booster.

FIG. 8 is a schematic diagram showing a fourth embodiment of the device of the present invention.

9 is a schematic diagram of an embodiment in which the hydraulic pump of the apparatus of FIG. 8 is of a variable displacement type.

FIG. 10 is a schematic diagram illustrating an example of a pilot check valve and a direction switching valve for controlling pilot pressure.

FIG. 11 is a schematic diagram of a conventional device.

FIG. 12 is a schematic diagram of a device in which an electric motor, a flywheel, and a speed reducer are sequentially connected.

[Explanation of symbols]

1 Electric motor 2 Reduction gear 3 Flywheel (flywheel) 4 Crankshaft 5 Connection rod 6 Slide 7 Clutch 8A One-way discharge type fixed displacement hydraulic pump (hydraulic pump) 8B One-way discharge type variable displacement hydraulic pump (liquid) Pressure pump) 9 Directional control valve 10 Hydraulic cylinder 11 Relief valve 12 Tank 13 Linear actuator (displacement detection sensor) 14 Controller 15, 16 Conductor 17A Two-way discharge type fixed capacity hydraulic pump (hydraulic pump) 17B Two-way discharge Type variable displacement hydraulic pump (hydraulic pump) S leaf spring 18 pilot check valve 19 check valve 20 tank 21 hydraulic motor 22 hydraulic cylinder (small diameter hydraulic cylinder) 23 hydraulic cylinder (large diameter hydraulic cylinder) ) 24,25 piping 26 hydraulic pump 27 electric motor 28,29 piping D hydraulic pressure Variable transmission E liquid-pressure force device 30 a small-diameter piston rod 31 large-diameter hydraulic cylinders 32, 33, 34, 35 pipe 36, 37, 38, 39 pilot check valve 40 direction switching valve

Claims (8)

[Claims] 1. A flywheel is directly connected to an electric motor, and a one-way discharge type hydraulic pump is driven through the flywheel, and the hydraulic cylinder is driven by the hydraulic pump through a direction control valve. A hydraulic drive device having a flywheel. 2. A sensor for detecting displacement of the piston rod is connected to the piston rod of the hydraulic cylinder of the hydraulic drive system according to claim 1, and its output is fed back to the directional control valve via a controller. A hydraulic drive device having a characteristic flywheel. 3. A flywheel is directly connected to the electric motor, and a bidirectional discharge type hydraulic pump is driven through the flywheel, and the hydraulic cylinder is driven by the hydraulic pump. A hydraulic drive device having a characteristic flywheel. 4. A flywheel is directly connected to an electric motor, and a two-way discharge type hydraulic pump is driven through the flywheel, and a crankshaft is connected through a hydraulic motor driven by the hydraulic pump. A hydraulic drive device having a flywheel, which is rotationally driven so that a hydraulic cylinder is driven by the crankshaft. 5. A flywheel, wherein the hydraulic cylinder of the hydraulic drive system according to claim 4 has a small diameter, and the hydraulic cylinder and a large diameter hydraulic cylinder are connected to form a hydraulic power booster. Hydraulic drive having a. 6. A hydraulic drive device having a flywheel, wherein another hydraulic pump is connected to the hydraulic cylinder having a small diameter of the hydraulic drive device according to claim 5. 7. A hydraulic wheel having a flywheel characterized in that the hydraulic booster according to claim 5 is configured by directly inserting a small diameter piston rod reciprocating by a crankshaft into a large diameter hydraulic cylinder. Pressure drive device. 8. A flywheel is directly connected to the electric motor, and a one-way discharge type hydraulic pump is driven through the flywheel, and the discharge side of this hydraulic pump is connected to both liquid chambers of the hydraulic cylinder. A hydraulic drive device having a flywheel, characterized in that both hydraulic chambers of the hydraulic cylinder are connected to a tank, and a bridge circuit is constructed by providing pilot check valves in each of these four circuits.