Fluid compressibility in the hydraulic system used is very small, in general, the oil may be considered to be incompressible . But compressibility of air is large, about ten thousand times the oil , so even if the system contains a small amount of air , its influence is great, dissolved air in the fluid at low pressure from the oil will fluid escaping bubbles, voids are formed phenomenon , to the high pressure zone , the action of the oil under pressure , these bubbles is quickly crushed , compressed sharp , noise generated in the system , while the gas discharged sudden compression a lot of heat , causing localized overheating, accelerated temperature rise , makes hydraulic components and hydraulic oil damage. Large compressibility of air , but also lead to working organ produces crawling fault , destroyed the stability of work , sometimes vibrate.

Prevent air from entering the hydraulic system is:

① suction pipe and oil return pipe should be inserted below the surface, and to maintain sufficient depth;

② various parts of the system should remain filled with oil, is to install a check valve in the outlet of the pump, set the way back to the oil back pressure valve;

③ oil tank to be as large as possible, the suction side and the back side to use the partition separating the oil, in order to achieve the purpose of elimination of air bubbles;

④ discharge hole provided at the highest point of the piping and the cylinder, wherein in the starting air should miss;

⑤ should maintain a good seal pipe joints, particularly the suction pipe, if the seal is not good, then the air is sucked hydraulic system.