Created on:2021-05-31 11:45

Selection of hydraulic pump and motor

(6) Others

① The connection mode of oil port should be considered. Generally, there are two kinds of oil ports, thread and flange. The selection should be made according to the application situation and conditions and the convenience of maintenance.

② Size and weight with the miniaturization and lightweight of modern mechanical equipment, in many applications, whether the required power of the hydraulic pump can be installed in a certain space is an important factor in the choice of pump, weight may also play the same role, such as aerospace equipment.

③ The quality and cleanliness of the working medium is the key to ensure the normal operation of the hydraulic pump and the whole hydraulic system and extend the service life of the pump. The working medium of hydraulic pump is usually the same as that of the whole hydraulic system. At present, the working medium of hydraulic system mostly adopts mineral hydraulic oil (mechanical oil, turbine oil, common hydraulic oil, etc.), flame retardant hydraulic oil (oil in water emulsion, water in oil emulsion, water glycol liquid and phosphate liquid, etc.) and special hydraulic oil. The general requirements of working medium are shown in the schedule. The factors to be considered in the selection of hydraulic working medium include working environment (flammability, toxicity, odor, etc.), working conditions (viscosity, system pressure, temperature, speed, etc.), oil quality (physicochemical index, compatibility, antirust, etc.) and economy (price, service life, etc.). Among the above factors, the most important is the viscosity of the medium. Although all kinds of hydraulic components have specified the hydraulic fluid to be used, the hydraulic pump is the most severe part in the whole system (not only the pressure, speed and temperature are high, but also the liquid is sheared when it is sucked in and pressed out by the pump). Therefore, the viscosity and variety of the hydraulic fluid can be determined according to the requirements of the pump. At this time, the anti-corrosion property should be mainly considered Wear resistance requirements. The order of anti-wear performance of hydraulic pump is vane pump > plunger pump > gear pump, so HM oil should be selected for hydraulic system with vane pump as main pump, regardless of pressure; HM oil should be selected for hydraulic system with plunger pump as main pump, and HL oil can be selected for low pressure. According to the requirements of the pump to determine the viscosity and variety of hydraulic oil, you can refer to the following table. The oil selected according to the pump is generally applicable to hydraulic motors and other hydraulic components (excluding proportional valve and servo valve).


General requirements for working media

Serial number


Serial number



Suitable viscosity: little affected by temperature change, general kinematic viscosity v = (11.5 ~ 41.3) × 10-6m2 / s


Good anti foam and anti emulsifying properties, good compatibility for metals and seals.


Good lubricity: the oil film produced by oil lubrication has high strength to avoid dry friction


The results show that the volume expansion coefficient is low, the specific heat capacity and heat transfer coefficient are high, the flow point and freezing point are low, and the flash point and ignition point are high


Pure texture, no corrosive substances and other debris


Good filterability: particulate pollutants in the working medium can be easily filtered through the filter screen to ensure high cleanliness


Good chemical stability: the oil is not easy to oxidize, not easy to deteriorate, so as not to produce viscous sediment, affect the system work, and the oil will become acidic after oxidation, which will corrode the metal surface


It is cheap and harmless to human body


According to the type and viscosity of the hydraulic oil selected by the hydraulic pump

Hydraulic pump


Pressure / MPa

Kinematic viscosity V / mm2 · S-1 at 40 ℃

Applicable variety and viscosity grade



Vane pump




HM oil: 32, 46, 68




HM oil: 46, 68, 100

Screw pump




HL oil: 32, 46, 68

Gear pump




HL oil (km for medium and high pressure): 32, 46, 68, 100, 150

Axial piston pump




HL oil (HM for high pressure): 32, 46, 68, 100, 150

Radial Piston Pump




HL oil (HM for high pressure): 32, 46, 68, 100, 150



The user can also select the working medium according to the oil selection chart provided by the pump manufacturer in the product sample. Figure g shows the oil selection diagram of a hydraulic pump, which gives the temperature range (abscissa) (- 25 ~ 90 ℃), the limit range of kinematic viscosity (ordinate) (LO ~ loo mm2 / s) and the best working viscosity range (Vopt) (the shaded part in the figure) (Vopt = 16 ~ 36 mm2 / s). The minimum viscosity Vmin = 10mm2 / S is the viscosity under 90 ℃ oil temperature for a short time, and the maximum viscosity Vmax = 1oomm2 / S is the viscosity under short-time cold start. In order to select the hydraulic fluid correctly, the relationship between the working temperature (open circuit) and the ambient temperature of the oil in the oil tank of the hydraulic system must be known (calculated) in advance. The selected oil should ensure that the working viscosity is in the best range (Vopt) within the working temperature range. Generally, the possible high viscosity grade should be selected in every situation. For example, under a certain ambient temperature, the working oil temperature of the oil tank is 60 ℃, and the corresponding viscosities in the best working viscosity range (Vopt) are HM46 and hm68, then hm68 hydraulic oil is selected. When using the oil selection diagram, it should be noted that the drain temperature of the shell is not always higher than the oil temperature of the oil tank, which is affected by the working pressure and speed of the pump. However, the maximum temperature of any part of the system shall not be higher than 90 ℃.

Due to the advantages of water hydraulic technology in energy saving, environmental protection and flame retardant, pure water hydraulic technology has been developed rapidly in recent years. There are a series of hydraulic pumps and hydraulic motors available abroad, such as Danfoss hydraulic pump and hydraulic motor (Fig. h), Hauhinco hydraulic pump (Fig. I). When using hydraulic pump and hydraulic motor, we should understand the performance difference between pure water working medium and hydraulic fluid (table below) and their advantages. At the same time, we should pay attention to the problems of material corrosion and aging, leakage and wear, cavitation and impact, vibration and noise caused by water medium.



Performance comparison between pure water and hydraulic fluid


Hydraulic fluid

pure water

Density / kg · M-3


one thousand

Compressibility / MPa




(6.3~7.8) ×10-3

























④ Economy (purchase cost) the purchase cost of hydraulic pump will be taken as a selection condition to weigh with other conditions. Usually, under the condition of a certain displacement, gear pump is the cheapest, plunger pump is the most expensive, and vane pump is between the two.

⑤ Adaptability refers to whether the hydraulic pump adapts to the habits of users and whether it can be interchanged with similar products.

⑥ The convenience of maintenance should be fully considered. The hydraulic pump used should be easy to maintain in the workshop and on site, easy to find Repairers and have sufficient supply.

⑦ Supply and product history should consider whether the selected hydraulic pump can be obtained quickly, how long it will take to obtain spare parts, the performance of the pump in similar or similar applications, the performance of the product and the historical status of production, use and acceptance. As the designer, user and maintainer of hydraulic system, we should have a comprehensive understanding of the distribution of domestic and foreign manufacturers (companies) of hydraulic pump and their product varieties, performance, service, reputation and replacement of new and old products, so as to realize the correct and reasonable choice of hydraulic pump.

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