The hydraulic cylinder is the executive element in the hydraulic transmission system. It is an energy conversion device that converts hydraulic energy into mechanical energy. The hydraulic motor is continuous rotary motion, and the hydraulic cylinder is reciprocating motion. The structure of the hydraulic cylinder has three major types: piston cylinder, plunger cylinder and swinging cylinder. Piston cylinder and plunger cylinder realize reciprocating linear motion, output speed and thrust. The oscillating cylinder realizes reciprocating swinging, output angular velocity (speed) and torque. In addition to single use, hydraulic cylinders can be combined in two or more ways or combined with other institutions. To accomplish special functions. The hydraulic cylinder is simple in structure and reliable in operation, and has been widely used in the hydraulic system of machine tools.