1. The oil level of the tank is lower than that of the oil standard, and the oil absorption, oil filter or suction pipe is exposed on the oil surface.

2. Oil tank back to the oil pipe and the oil suction pipe is too close, and between the two are not separated by a partition (or not broken bubble network), the oil back to the foam produced by the bubble is not too late to be sucked into the pump.

3. The sealing of the pipe joint between the oil inlet and the oil inlet of the hydraulic pump is not good, or the pipe joint is loose due to vibration, or the pipe is cracked, and the air is sucked into the air.

4. Because the pump shaft seal is broken, the gear pump body and the seal between the seal and into the air.

5. The oil suction pipe is too thin, too long, the oil absorption filter is blocked by dirt or the oil filter is designed to be too small to cause the increase of oil resistance.

6. Deterioration of the oil, due to the emulsification of water, the bubble breaking performance is poor, the dispersion of bubbles in the oil layer or in the network bubble floating on the oil surface, the pump suction system when working.

7. Hydraulic cylinder without exhaust device for exhaust.

8. A mixture of volatile substances (such as gasoline, ethanol, benzene, etc.) in the oil field, which is formed from the oil in the low pressure zone. In the back pressure valve is not installed on the back of the road, while the cylinder and sometimes negative pressure.

9. Hydraulic cylinder head seal is not good, sometimes intake, and sometimes oil spills.