Cylinder device

1. A cylinder device comprising:

a cylinder;

a piston slidably inserted into the cylinder;

a rod inserted into the cylinder and connected to the piston;

a rod-side chamber and a piston-side chamber partitioned by the piston within the cylinder;

a tank;

a first on-off valve provided in the middle of a first passage allowing the rod-side chamber to communicate with the piston-side chamber;

a second on-off valve provided in the middle of a second passage allowing the piston-side chamber to communicate with the tank;

a pump for supplying liquid to the rod-side chamber;

a discharge passage to connect the rod-side chamber to the tank;

a variable relief valve capable of changing valve opening pressure, said variable relief valve being provided in the middle of the discharge passage;

a suction passage permitting only the flow of liquid from the tank to the piston-side chamber; and

a flow straightening passage permitting only the flow of liquid from the piston-side chamber to the rod-side chamber, said variable relief valve being composed of a solenoid variable relief valve capable of adjusting the valve opening pressure by a proportional solenoid, said variable relief valve including the proportional solenoid, a damping passage connected to the middle of the discharge passage, a relief passage juxtaposed with the damping passage, a selector valve element biased to open the damping passage and set to close the damping passage in current-carrying to the proportional solenoid, and a relief valve element biased to close the relief passage, the valve opening pressure of the relief valve element being reduced according to the current-carrying amount in the current-carrying to the proportional solenoid, the selector valve element providing a resistance to the flow of liquid when the damping passage is opened, wherein thrust is controlled by controlling the valve opening pressure of the variable relief valve.

2. A cylinder device according to claim 1 , wherein the second on-off valve is opened and closed, in an extending operation for protruding the rod from the cylinder by the liquid supplied from the pump, while maintaining the first on-off valve in an opened state to thereby control the thrust in the extending direction, while the first on-off valve is opened and closed, in a compressing operation for penetrating the rod into the cylinder by the liquid supplied from the pump, while maintaining the second on-off valve in an opened state to thereby control the thrust in the compressing direction.

3. A cylinder device according to claim 1 , wherein the pump is driven by a motor, and the thrust is controlled by controlling the torque of the motor.

4. A cylinder device according to claim 1 , further comprising a limiting flow passage connecting the rod-side chamber to the tank and comprising an orifice provided in the middle thereof.

5. A cylinder device according to claim 1 , wherein a check valve is provided between the pump and the rod-side chamber to arrest the flow of liquid from the rod-side chamber to the pump.

6. A cylinder device according to claim 1 , wherein the tank is formed by an annular gap between the cylinder and an outer cylinder covering the cylinder.

7. A cylinder device according to claim 1 , wherein each of the first on-off valve and the second on-off valve is composed of a solenoid on-off valve, which is located at an interrupting position by a spring in no-current carrying, and a valve opening pressure of said variable relief valve is maximized in no-current carrying.

8. A cylinder device according to claim 1 , wherein said pump is connected to a supply passage, at least a portion of said supply passage being connected to said rod-side chamber in a first area of said rod-side chamber, at least a portion of said first passage being connected to said rod-side chamber in a second area of said rod-side chamber, said first area being located at a spaced location from said second area.

9. A cylinder device comprising:

a cylinder;

a piston slidably inserted into the cylinder;

a rod inserted into the cylinder and connected to the piston;

a rod-side chamber and a piston-side chamber partitioned by the piston within the cylinder;

a tank;

a first on-off valve connected at a position along a first passage to allow the rod-side chamber to communicate with the piston-side chamber;

a second on-off valve connected at a position along a second passage to allow the piston-side chamber to communicate with the tank;

a pump for supplying liquid to the rod-side chamber;

a discharge passage to connect the rod-side chamber to the tank;

a variable relief valve capable of changing valve opening pressure, said variable relief valve being connected at a position along the discharge passage, said pump being connected to a supply passage, at least a portion of said supply passage being connected to said rod-side chamber in a first area of said rod-side chamber, at least a portion of said first passage being connected to said rod-side chamber in a second area of said rod-side chamber, said first area being located at a spaced location from said second area.