(1) the lubrication condition is not good, the cylinder position is near the rolling line, the average temperature is 50-60 degrees. The temperature is high and the lubrication condition is poor. The cylinder is in dry or semi dry friction state, and the looper seal produces high temperature deformation in such an environment, so that the sealing resistance is stuck between the piston and the cylinder. This is the main cause of the cylinder wear. (2) the upper part of the cylinder is subjected to high pressure and the action is not vertical, causing the cylinder wear to be heavier and lighter. The piston is tightly pressed on the cylinder wall under the action of its own elasticity and back pressure. The greater the positive pressure, the more difficult the formation and maintenance of lubricant film is, and the mechanical wear is aggravated. During the movement, with the piston descending, the positive pressure gradually decreases, so the cylinder wear is heavy, light and uneven. (3) entering mechanical impurities makes the internal wear of cylinder become more and more serious. The dust in the air, the oxide scale in the rolling process, and so on, when the piston moves back and forth between the piston and the cylinder wall, it aggravates the wear of the piston and the inner wall of the cylinder. The use of compressed air is unclean. The impurities in compressed air enter the cylinder through pipes, wear the cylinder and affect the cylinder work. Compressed air pressure is unstable, it will also lead to unstable cylinder work.