Hydraulic cylinder is an executive element in hydraulic transmission system, it is an energy conversion device that converts hydraulic energy into mechanical energy. The hydraulic motor realizes the continuous rotary motion, while the hydraulic cylinder realizes the reciprocating motion. There are three types of hydraulic cylinders: piston cylinder, plunger cylinder and swing cylinder. Piston cylinder and plunger cylinder realize reciprocating linear motion, output speed and thrust, and swing cylinder realizes reciprocating swing, output angular velocity (rotation speed) and torque. In addition to being used singly, hydraulic cylinders can also be used in combination of two or more or in combination with other mechanisms. to perform 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.