Actuator

An actuator, comprising:

• a cylinder;
• a piston slidably inserted into the cylinder;
• a rod inserted into the cylinder and coupled to the piston;
• a rod-side chamber and a piston-side chamber partitioned by the piston in the cylinder;
• a tank;
• a first pump capable of supplying liquid to the rod-side chamber;
• a second pump capable of supplying the liquid to the piston-side chamber;
• a first control passage allowing communication between the rod-side chamber and the tank;
• a second control passage allowing communication between the piston-side chamber and the tank;
• a first variable relief valve provided at an intermediate position of the first control passage and capable of changing a valve opening pressure for permitting a flow of the liquid from the rod-side chamber toward the tank by being opened when a pressure in the rod-side chamber reaches the valve opening pressure;
• a second variable relief valve provided at an intermediate position of the second control passage and capable of changing a valve opening pressure for permitting a flow of the liquid from the piston-side chamber to the tank by being opened when a pressure in the piston-side chamber reaches the valve opening pressure;
• and
• a center passage allowing communication between the tank and the interior of the cylinder.