How many kinds of common hydraulic pumps are there

How many kinds of common hydraulic pumps are there?
Expand the definition of all hydraulic pumps: a machine that can change mechanical energy into hydraulic energy by changing the closed working volume to achieve liquid suction and pressure.
Types of commonly used hydraulic pumps:
1. According to whether the flow can be adjusted, it can be divided into variable displacement pump and fixed displacement pump.
The output flow can be adjusted according to the needs is called variable pump, and the flow that can not be adjusted is called quantitative pump.
2. According to the commonly used pump structure in the hydraulic system, it can be divided into gear pump, vane pump and plunger pump.
Gear pump: small volume, simple structure, lax requirement on oil cleanliness, low price; but the pump shaft is seriously worn and leaked due to unbalanced force.
Vane pump: divided into double acting vane pump and single acting vane pump. The pump has the advantages of uniform flow, stable operation, low noise, higher working pressure and volumetric efficiency than gear pump, and more complex structure than gear pump.
Plunger pump: it has high volumetric efficiency, small leakage, can work under high pressure, and is mostly used in high-power hydraulic system; however, it has complex structure, high requirements for material and processing accuracy, expensive price, and high requirements for oil cleanliness.
Generally, the plunger pump is used when the gear pump and vane pump cannot meet the requirements. There are some other forms of hydraulic pump, such as screw pump, but the application is not as common as the above three.