Liquid is the working medium of hydraulic transmission. Hydraulic transmission uses liquid as the working medium to transfer energy. At the same time, the liquid also has the function of lubrication, corrosion prevention, rust prevention and cooling. The different nature and quality of the working fluid will greatly affect the working performance and reliability of the hydraulic system.
First, the quality requirements of hydraulic fluid in hydraulic transmission:
In the hydraulic system, in order to better transmit power, the hydraulic fluid should meet the following requirements:
(1) Appropriate viscosity and good viscosity-temperature;
(2) Has good chemical stability (including good oxidation stability, thermal stability and not easy to oxidize, metamorphic, etc.)
(3) Good lubricity to reduce wear between relative moving pairs;
(4) Good anti-foam (less foaming, fast defoaming), anti-emulsification and good compatibility with metals;
(5) Low volume compression coefficient, flow point and freezing point, high specific heat capacity, heat transfer coefficient, flash point and ignition point;
(6) The composition is pure, does not contain corrosive substances, and has sufficient cleanliness;
(7) Harmless to human body, small pollution to the environment, cheap price.
Second, the choice of hydraulic fluid:
The correct selection of hydraulic fluid is very important to improve the working performance and reliability of the hydraulic system and extend the service life of the system and components. The choice of hydraulic fluid includes the choice of hydraulic fluid type and the choice of viscosity.
1. Type selection:
Hydraulic fluid can be divided into three categories: combustible hydraulic fluid (mineral oil type, synthetic hydrocarbon type), inflammable hydraulic fluid (including emulsion type, water, water-glycol type and synthetic forming) and special hydraulic oil (aviation, ship, gun and vehicle brake hydraulic oil, etc.). The main varieties, characteristics and uses of hydraulic oil are shown in Table 2-4.
Introduction and selection of hydraulic media
When selecting the type of hydraulic oil, we should first consider the working environment of the hydraulic system and the requirements of working fire resistance and anticoagulation, and whether the system has the requirements of lubricity, extreme pressure anti-wear, viscosity-temperature and anti-creep (see Table 2-5). You can refer to the "Hydraulic Fluid Products and Main Applications table" (see Hydraulic Engineering manual) to select and determine the type of hydraulic fluid used in the system. At present, more than 90% of hydraulic systems use petroleum-based mineral oil as the working medium.
Introduction and selection of hydraulic media
2, viscosity selection:
After the variety of hydraulic oil is determined, the viscosity level of the oil is then selected. Viscosity has a significant impact on the stability, reliability, efficiency, temperature rise and wear of the hydraulic system, and is the most important factor affecting the performance of the hydraulic system. The viscosity of the selected hydraulic fluid is too high, which will cause the power loss of the system due to friction. The viscosity of the selected hydraulic fluid is too low, which will increase the power loss caused by leakage of the system. When determining the viscosity of the hydraulic fluid required by the hydraulic system, the following factors are mainly considered:
(1) Work pressure. When the working pressure is high, the hydraulic fluid with higher viscosity should be selected to reduce the volume loss.
(2) Working environment temperature. When the ambient temperature is high, the hydraulic fluid with higher viscosity is selected to reduce the volume loss.
(3) Mainly with the relative motion speed of the pair. When the relative motion speed is high, the hydraulic fluid with lower viscosity should be selected to reduce friction loss.
