Lifting devices used in lifting chairs can be divided into three categories-hydraulic, mechanical and pneumatic. Mechanical use of lead screw, reducing or increasing the height of a pitch by one turn, is more troublesome to adjust, and often requires many turns to reach the required height. The height adjustment of oil pressure and air pressure is much simpler. There is a handle to control the height. When the chair is empty, pull the handle and the chair surface will rise to the highest position. When people sit on it, pull the handle and the chair surface will slowly descend. When it reaches the required height, just loosen the handle.