Pump is "heart" of any hydraulic system. Therefore, whenever there is a problem with oil pressure system, everyone will often blame problem on pump.
In fact, as far as causes of pump damage are concerned, phenomena caused by absence of pump itself are rarely seen. To be more precise, when working condition of pump is getting worse day by day, it shows a symptom of other problems hidden in hydraulic system.
Ninety to ninety-five percent of pump damage can be roughly summarized as following reasons:

Air entrainment refers to a phenomenon in which air bubbles disperse in hydraulic oil of system. This bursting effect occurs when compressed air bubbles in oil system are exposed to pump outlet pressure. And this cracking effect will cause pressure side plate, wear-resistant side plate inside pump and other metal surfaces close to rupture point to peel off, resulting in extremely high temperature there.
Phenomenon of air mixing, pump will produce a loud crackling noise. Pitch of this noise increases as pressure increases, and excessive air incorporation can cause hydraulic fluid to emulsification (due to high amount of moisture in air). Moreover, due to phenomenon of air compression in hydraulic oil, hydraulic components of various parts may not move smoothly.
Possible pathways that lead to air mixing into pump are:
1. Bad shaft seal.
2. Junction between fuel tank and pump inlet part leaks.
3. Oil level is too low or oil return pipe returning to oil tank is installed on oil surface, causing hydraulic oil in oil tank to generate air bubbles, which will be sucked by pump again before air bubbles are eliminated.

Cavitation refers to a localized gasification phenomenon that occurs in fluid of hydraulic oil when pressure is reduced below saturated vapor pressure. To put it more simply, when hydraulic oil does not completely fill space it should occupy, that is, there is a vacuum in oil circuit, which will cause this cavitation phenomenon.
Noise caused by cavitation is roughly similar to noise caused by air entrainment. Factors that cause cavitation are:
1. Matching problem between pump and electric motor causes pump speed to be too high.
2. Pump suction pipeline is too long, or suction port is limited.
3. Viscosity of hydraulic oil is too high.
4. Whether exhaust port on fuel tank is plugged.
5 Check filter screen of pump suction port.

Any foreign matter in hydraulic system circuit will have a negative impact on efficiency of hydraulic components and their circuits. Contaminated foreign matter may be solid, liquid or gaseous.
Most foreign matter contamination will cause frictional movement between workpiece and tight clearance of workpiece, which will lead to accelerated wear and tear of workpiece.
Pollution sources in oil circuit can be roughly divided into:
1. Casting oil cylinder, oil block, inside of oil tank, pipe joint, etc.
2. Oil seal or O-ring is brittle and broken.
3. Hydraulic oil deteriorates under high temperature for a long time, resulting in oil pollution.
4. Pump causes metal surface to peel off substances due to air mixing or cavitation.

Overheating is a thermal phenomenon that causes fluid viscosity to change due to exceeding specified upper limit temperature. Overheating will cause accelerated oxidation of hydraulic oil and deterioration of hydraulic oil viscosity, and cause a chain reaction. Therefore, real root cause of overheating must be eliminated to effectively correct this problem.
Causes of overheating are:
1. Overload operation and overpressure use.
2, caused by air mixing and cavitation.
3, caused by oil pollution.

Overpressure means that operating pressure is greater than pressure for which pump is designed. Overpressure can subject internal parts of pump to extreme forces and cause premature failure of internal parts.
All of reasons mentioned above will leave their characteristic signs of damage. It is important to recognize and understand messages that these signs bring. In this way, actual cause of damage to pump can be corrected before the other pump is damaged.