The selection of hydraulic system oil viscosity and the influence of oil temperature

The viscosity of hydraulic oil in hydraulic system is generally recommended by hydraulic  oil manufacturers.

① Oil viscosity changes under the influence of temperature. When the oil temperature increases, the viscosity decreases, so the oil temperature must be kept below 60°C. In order to make the hydraulic pump work stably at different working temperatures, the selected oil should have oil temperature characteristics with viscosity less affected by temperature changes, as well as better chemical stability and anti-foam performance. It is recommended to use L-HM32 or L-HM46 (GB1118. 1-94) anti-wear hydraulic oil.

②Use domestic 32# or 46# hydraulic oil or IS0 3448-1992 viscosity 32~68 wear-resistant hydraulic oil, and use other hydraulic oils with a viscosity index greater than 90. The moisture, ash and acid value in the oil must conform to the relevant hydraulic oil. Provisions.

③Pay attention to the maximum allowable working temperature of the hydraulic pump. The maximum allowable operating temperature of a hydraulic pump depends on the nature of the oil and seal used. When the temperature exceeds the allowable temperature, the oil will become thinner and lower in viscosity, and the normal lubrication of high-load parts cannot be maintained, causing oxidative deterioration. According to the regulations of the manufacturer, the working oil temperature range of the plunger pump and hydraulic motor is -25~+80℃. The lowest viscosity of the working medium is 10mm²/s, and the highest viscosity is 100mm²/s.


④Pay attention to the minimum working temperature of the hydraulic pump. It is generally determined based on the increase in the viscosity of the hydraulic system oil as the temperature decreases. Cavitation will occur when the oil is so thick that the inlet conditions no longer guarantee that the hydraulic pump is fully filled. The specific gravity of flame-resistant hydraulic oil is greater than that of petroleum-based hydraulic oil, and sometimes the viscosity is higher at low temperature. Many fire-resistant hydraulic fluids contain water, and the water will evaporate if the pressure is low or the temperature is high. Therefore, when using these oils, the inlet conditions of the hydraulic pump are more sensitive. The commonly used solution is to use an auxiliary pump to boost the inlet of the main pump, or arrange the inlet of the hydraulic pump to be lower than the liquid level of the oil tank to fill the inlet of the hydraulic pump with oil.

⑤ Pay attention to the hazards of low-temperature operation. The viscosity of hydraulic oil is very sensitive to temperature. As the temperature decreases, the viscosity of the hydraulic oil will increase rapidly, and the viscosity of the hydraulic system oil has a great influence on the work of the hydraulic pump. Therefore, hydraulic pumps should try to avoid operating under low temperature conditions. If they must operate under low temperature conditions, the internal problems of the hydraulic pump due to temperature must be analyzed and improvement measures must be proposed.

Cold start In cold areas, in addition to improving the engine, it is also necessary to pay close attention to the failure of the hydraulic pump system. If the temperature is too low, the oil will become very viscous. At the same time, if the temperature is too low, the volume of the cylinder and the plunger will shrink, and the clearance between the two will be smaller than normal. After starting the vehicle, if the pump is operated at a high speed prematurely, take the plunger pump as an example. At this time, the internal component of the plunger pump is the most heavily loaded part of the plunger shoe. The clearance between the plunger and the cylinder is very large. Small, high-viscosity oil will increase the viscous force between the cylinder and the plunger a lot. Therefore, when the plunger shoe is forcibly drawn back and forth in the hydraulic pump, the shoe is fixed on the return plate, and the pulling force received during the twitching is very large. If the plunger is frequently cold-started and continuously loaded with cold oil, the plunger The gap between the head and the shoe will be enlarged. Generally, the required gap is 0.02~0.03mm. However, when the gap exceeds 0.15mm, the internal leakage of the hinge joint of the plunger shoe will cause the internal leakage to be too large, and the pressure will drop. If you continue to use it, the plunger will be pulled out of the shoe, causing a large amount of internal leakage or damage to the return disk and other components, causing a major accident.

Another failure of the hydraulic system that may cause low temperature operation is the problem of oil return. The oil temperature is low, the oil viscosity is high, and the damping of the hydraulic oil through the return line increases. Under the condition of the normal return oil volume, the return pressure will increase, and the abnormal internal leakage may cause the return The oil pressure is too high, causing damaged.