A hydraulic cylinder is a hydraulic actuator that transforms the hydraulic energy into a mechanical energy, and does a straight line reciprocating motion (or swinging motion). It is simple in structure and reliable in work. When it is used to achieve reciprocating motion, it can avoid the deceleration device, and has no transmission clearance and smooth movement, so it is widely applied in various mechanical hydraulic systems. The output force of the hydraulic cylinder is directly proportional to the effective area of the piston and the pressure difference between the two sides. The hydraulic cylinder is basically composed of cylinder and cylinder head, piston and piston rod, sealing device, buffer device and exhaust device. The buffer and exhaust devices depend on specific applications, and other devices are essential.