Hydraulic cylinder is the hydraulic energy into mechanical energy, do linear reciprocating motion (or swing motion) of the hydraulic actuator. It has simple structure and reliable operation. When it is used to achieve reciprocating movement, deceleration device can be removed, and there is no transmission gap, the movement is stable, so it is widely used in various mechanical hydraulic systems. The output force of the hydraulic cylinder is proportional to the effective area of the piston and the pressure difference on both sides; The hydraulic cylinder is basically composed of cylinder barrel and cylinder head, piston and piston rod, sealing device, buffer device and exhaust device. Buffering device and exhaust device depending on the specific application, other devices are necessary.