Created on：2021-06-01 09:59

# Selection of hydraulic pump and motor

(3) Selection of basic parameters

① The rated pressure of hydraulic pumps with different working pressure types and specifications is also different.

The output pressure PP of the hydraulic pump shall be the sum of the pressure P1 required by the actuator and the total pressure loss ∑△ P (including the pressure loss of the pipeline and the pressure loss of the control valve) on the oil inlet circuit of the system, i.e

pp≥P1+∑△p (1-42)

The pump output pressure PP shall not exceed the rated pressure on the sample. In general, the normal working pressure of the hydraulic system in the fixed equipment can be selected as 70% - 80% of the rated pressure of the pump, and the working pressure of the hydraulic system in the system or walking equipment (such as vehicles and construction machinery) requiring high working reliability can be selected as 50% - 60% of the rated pressure of the pump.

The maximum working pressure on the product sample is the allowable pressure in case of short-term impact. If the impact pressure occurs in each cycle, the life of the pump will be significantly shortened, and even the pump will be damaged.

The maximum pressure and the maximum speed of the pump should not be used at the same time to extend the service life of the pump.

② The flow QP of output flow hydraulic pump is related to working condition. The output flow QP of the hydraulic pump should include the required flow of the actuator (when there are multiple actuators, the total flow can be calculated from the flow time cycle diagram) and the sum of the leakage of each element, that is, it can be determined by the following formula:

qp≥K(∑q)max (1-43)

Where k is the leakage coefficient of the system, generally 1.1-1.3 (small value for large flow and large value for small flow);

(∑ q) max -- the maximum flow rate of hydraulic actuator acting at the same time, m3 / s.

For the system which always uses the flow valve to regulate the speed during the working process, the minimum overflow flow of the overflow valve should be added, generally 2-3l / min. Sometimes, it is necessary to consider the flow reduction caused by motor loss (usually about LR / s) and the flow reduction caused by the efficiency reduction of hydraulic pump after long-term use (usually 5% - 7%). The displacement, speed range and output flow under different pressure of typical speed are often given on the sample.

③ Prime mover prime mover has two forms: motor and internal combustion engine. The requirements of prime mover and the determination of prime mover power.

④ Speed and displacement speed are related to pump life, durability, cavitation and noise. Although the allowable speed range is indicated in the technical specification sheet of the product, it is better to use it at the best speed suitable for the purpose, and it shall not exceed the maximum speed. Especially when the hydraulic pump is driven by internal combustion engine, it is difficult to absorb oil at low speed when the oil temperature is low, and there is a risk of seizure failure due to poor lubrication. At high speed, the possibility of cavitation, vibration, abnormal wear and flow instability should be considered. The drastic change of rotational speed also has a great influence on the strength of the internal parts of the pump.

When the required flow of the system is known, the speed and displacement of the hydraulic pump should be considered comprehensively. Generally, the reference value should be calculated first according to the required flow rate of the system QV (L / min), the primary hydraulic pump speed n1 (R / min) and the volumetric efficiency of the pump η V (which can be taken as η v = 0.9 according to the product sample)

Vg=1000qv/(n1ηv)(1-44)

For the quantitative pump, the final pump flow should be consistent with the flow required by the system as far as possible to avoid excessive power loss.

⑤ The efficiency value of efficiency pump is the embodiment of pump quality. The higher the pressure is, the lower the speed is, the lower the volumetric efficiency of the pump is, and the volumetric efficiency decreases when the displacement of the variable displacement pump is adjusted. When the speed is constant, the total efficiency of the pump is the highest under a certain pressure, and the total efficiency of the variable displacement pump is the highest under a certain displacement and pressure. The total efficiency of the pump has a great influence on the efficiency of the hydraulic system. We should choose the pump with high efficiency and try to make the pump work in the high efficiency working condition area.

(4) When the self-priming capacity is used in the open circuit, the pump needs to have a certain self-priming capacity. Cavitation and cavitation may not only damage the pump, but also cause vibration and noise, resulting in bad action of control valve and actuator, which has a bad impact on the whole hydraulic system. While confirming the self-priming capacity of the pump, it is necessary to determine the installation position of the pump relative to the liquid level of the oil tank and design the oil suction pipeline on the basis of calculating the resistance of the oil suction pipeline by considering the service temperature condition of the hydraulic device and the viscosity of the hydraulic oil. In addition, the calculated value of the pump's self-priming capacity should have sufficient margin.

(5) Noise hydraulic pump is the main noise source of hydraulic system. In the case of limited noise, low noise pump or low speed should be used.

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