Hydraulic system leakage can be divided into internal leakage and external leakage according to the flow direction. External leakage mainly refers to the leakage of hydraulic oil from the system into the environment, resulting in the hydraulic system of hydraulic pipeline, hydraulic valve, hydraulic cylinder and hydraulic pump (hydraulic motor) external; Internal leakage refers to the hydraulic oil flowing from the high-pressure side to the low-pressure side of the system due to the pressure difference between the high-pressure and low-pressure side and the failure of seals, etc., such as the leakage of oil in the hydraulic transmission from the high-pressure cavity to the low-pressure cavity. Leakage from the pressure channel in the reversing valve to the return oil channel, etc. The main forms of leakage include gap leakage, polypore leakage, adhesion leakage and power leakage.

1. Gaps leak

There are mainly two kinds of gap leakage in hydraulic system of construction machinery: fixed seal (static joint surface) leakage and moving seal (dynamic joint surface) leakage. The leaking part of the fixed seal mainly includes the joint of the hydraulic cylinder head and cylinder cylinder, etc. The moving sealing part mainly includes the inner wall of hydraulic cylinder piston and cylinder cylinder, piston rod and cylinder head guide sleeve. The size of gap leakage is related to pressure difference, gap and other factors.

2. Porous leakage

Various cover plates in hydraulic components, due to the influence of surface roughness, it is impossible to completely contact between the two surfaces, in the two surfaces do not contact the microscopic depression, the formation of many cross section shapes and sizes of the gap, gap cross section size and surface roughness related. Multi-void leakage, the liquid needs to flow through a lot of curved pores, in the sealing performance test, need a certain pressure holding time, leakage can be revealed.

3. Adhesive leakage

There is a certain adhesion between the viscous liquid and the solid arm. After contact, a thin layer of liquid will be adhered to the solid surface. If the film on the solid surface is thicker, the oil film will be scraped off by the sealing ring due to mutual movement, resulting in adhesive leakage. The basic method to prevent adhesive leakage is to control the thickness of liquid adhesive layer.

4. Power leakage

On the sealing surface of the rotating shaft, if there is a trace of spiral processing, when the shaft turns, the liquid flows along the groove of the spiral trace under the action of rotating force of the rotating shaft. If the direction of the screw trace is consistent with the rotation direction of the shaft, power leakage will occur due to the "oil pumping" effect of the screw trace. The characteristic of power leakage is that the higher the rotational speed of the shaft, the larger the leakage. To prevent power leakage, it is necessary to avoid the machining trace of "pumping oil" effect on the sealing surface of the rotating shaft and the lip of the sealing ring or to use the principle of power leakage, and to use the pumping oil effect of screw trace to return the leaking oil pump to prevent the leakage.