Electric oil pump

Figure 1: Electric oil pump.



When choosing a hydraulic pump, the following aspects can be considered.

Pressure, Flow Rate and Efficiency

Pressure, flow rate and efficiency are the basic parameters for selecting a hydraulic pump. Generally speaking, in low pressure conditions, any type of pump can be used.

Under high pressure condition (pressure less than 60Mpa), select hydraulic piston pump; in medium pressure condition (pressure less than 32Mpa), select hydraulic gear pump; under low pressure (pressure less than 7Mpa), choosing hydraulic vane pump and hydraulic screw pump is more economic.

According to whether the flow rate can be changed, the hydraulic pump can be divided into fixed displacement pump and variable displacement pump. If requiring constant speed, choose fixed displacement pump, and hydraulic gear pump and double-acting vane pump are optional; if requiring variable displacement, choose single-acting vane pump and hydraulic piston pump.



Type

 

Performance parameter

Gear pump

Vane pump

Piston pump

Sing acting

Double acting

Flow regulation

×

×



Table 1: Types of hydraulic pumps.



The efficiency of a general hydraulic pump is affected by the following factors:
Pump size, the clearance and matching accuracy of pump parts;
The viscosity and properties of the hydraulic oil, such as lubricity and the effect of operating temperature on viscosity;
Working pressure and speed.

The overall efficiency of a hydraulic pump is determined by mechanical, frictional and volumetric losses. In general, axial piston pumps have the highest overall efficiency, followed by vane pumps, and finally gear pumps. Among hydraulic pumps with the same structure, the pump with large displacement has high total efficiency. The pump with the same displacement has the highest total efficiency under rated operating conditions.



Types of hydraulic pumps

Gear pump

Vane pump

Piston pump

Screw pump

Volume efficiency

0.7~0.95

0.8~0.95

0.85~0.95

0.7~0.98

Overall efficiency

0.63~0.87

0.65~0.82

0.81~0.88

0.65~0.85



Table 2: Volume and overall efficiency of hydraulic pump.



Power Density

Hydraulic pumps are compact and have a smaller gravity-to-power ratio than conventional energy conversion devices. This kind of ratio is called power density. Power density is an important index when applying hydraulic pumps in the aviation and vehicle industries.
Power density mainly depends on the type of hydraulic pump and the materials used. Among ordinary hydraulic pumps, the hydraulic vane pump has the smallest power density, about 2N/kW, the power density of hydraulic piston pumps is 3~6N/kW, and that of most hydraulic gear pumps is 5~7N/kW.

Noise and Service Life

The noise levels generated by the hydraulic pump varies greatly depending on the following factors: the type of the pump, the material of the pump parts, the matching of the parts, the installation of the pump, the vibration elimination method used, the stiffness of the pump, flow rate, pressure, speed, and pressure pulsation and the influence of other components connected in the circuit.

Experience has proved that the external gear pump and the hydraulic piston pump have the largest noise, while the screw pump has the smallest noise, and the noise level of the vane pump and the internal gear pump is medium.
Hydraulic pumps with a sound pressure level of more than 90dB(A) are very noisy, and around 60dB(A) is considered relatively quiet. The intensity of the noise generated by the hydraulic pump increases more significantly with the increase of the speed than with that of the pressure or displacement.

Experience shows that the screw pump is very economical when its use pressure is 2~3MPa. This pump is the quietest and pulsation-free, and has a high reliability when the oil viscosity is appropriate.
The pressure pulsation and noise of the vane pump are also smaller. In the case of fixed displacement and medium pressure, it is more suitable than the external gear pump, and its total efficiency is lower than that of the piston pump.



Internal gear pump diagram

Figure 2: Internal gear pump diagram.



Modern internal gear pumps have low noise when used in medium and high pressure conditions, with an expected life span of 20,000 hours and a volumetric efficiency of 97%. But internal gear pumps are more expensive than external gear pumps.
In terms of its economy, the external gear pump is cheaper than other pumps, but with the increase of pressure and the prolongation of use time, its noise value will increase sharply.

Radial piston pumps have a long life expectancy and are suitable for high pressure applications. When the working pressure of the axial piston pump is 20~25Mpa, its life expectancy is 40000h, and when the working pressure is 30~35Mpa, its life span will be reduced to less than 15000h.

Application and Cost

The gear pump features simple structure, low price, good self-priming performance, and anti-pollution, but it has very low pressure and efficiency, and large noise, and it can only be made into a fixed displacement pump. It is mostly used in some low-pressure systems and occasions with low efficiency requirements and relatively poor environment.

The piston pump has high efficiency and pressure, and can realize variable displacement, but it is expensive, very sensitive to pollution, and has high requirements for oil quality. It is mostly used in systems with high pressure and high power requirements.

The vane pump features medium pressure, high efficiency, poor self-priming ability and it is more sensitive to pollution. The single-acting vane pump can be made into a variable displacement pump, such as a pressure-limiting variable pump, and the double-acting vane pump can only be made into a fixed displacement pump with low noise.

The screw pump has high efficiency, low noise, and can realize variable displacement, but it is expensive. It is mostly used in precision equipment, and also used in oil exploration and production.



Twin screw pump diagram.

Figure 3: Twin screw pump diagram.



Conclusion: The gear pump has the best anti-pollution ability, followed by the vane pump. Piston pumps have the highest demands on oil cleanliness. Generally speaking, gear pumps cost the least, internal gear pumps are more expensive than vane pumps, screw pumps are more expensive than internal gear pumps and vane pumps, and piston pumps are the most expensive.

General Considerations

When choosing a hydraulic pump, the designers and the maintenance personnel of the hydraulic system must understand and consider the following basic factors:

1.Safety pressure and the maximum working pressure of the system;
2.The allowable speed of the hydraulic pump;
3.Standard characteristics of hydraulic pump;
4.Flow rate required by hydraulic system;
5.The relationship between pressure, speed and flow rate;
6.The adaptability of variable control;
7.The tolerance of pressure shock;
8.The degree of leakage loss;
9.Volumetric efficiency and total efficiency;
10.Pollution tolerance;
11.Operational reliability and durability;
12. Expected life under various loads and speeds;
13. The characteristics of the oil and its influence on the wear rate of the hydraulic pump;
14. Noise generated by hydraulic pump running under different pressure, speed and flow;
15. Hydraulic system temperature;
16. Maintainability, and availability of maintenance and spare parts;
17. Filtering requirements;
18. Drive form and installation method;
19. Special coating on the sliding surface;
20. Oil suction conditions;
21. Manufacturing characteristics, the clearance and fit of parts;
22. Compactness and power density;
23. Compatibility and cost of the overall system.

When selecting a pump, the above factors should be considered one by one to make sure the pump you choose has corresponding adaptability and can operate reliably in the system, otherwise various failures will occur.

Hydraulic Pump Selection

According to the main machine operating conditions, power required and the system's requirements for working performance, first determine the type of hydraulic pump, and then determine its specifications and models according to the pressure and flow required by the system. In addition, if you have any problem with hydraulic pump selection, you can contact us. Okmarts has well trained and professional salesmen to serve you.

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