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How To Select A Suitable Viscosity Grade For Hydraulic Oil?

How to select a suitable viscosity grade for hydraulic oil?

If the hydraulic oil is too viscous, hydraulic pressure loss will be large, and the system efficiency will be low, it’ll also be hard for the oil pump to pump the oils. If the hydraulic oil is too thin, hydraulic pumps may have leakage problem with loss of oil volume, and the volume efficiency will be low. Therefore a suitable viscosity should be selected to meet the requirements of the hydraulic system and working environment, to keep the volume efficiency and mechanical efficiency at a balance.

The selection mainly considers the starting condition, working temperature and types of pumps. Normally, HV and HL indoor fixed hydraulic system working environment is 30~40 higher than the ambience. At this temperature, the oil has a viscosity value between 13~16mm2/s; if the value is lower than 10mm2/s, erosion will be increased significantly, therefore the hydraulic oil’s viscosity index should be no lower than 90. HL outdoor (pressure greater than 20.0MPa) hydraulic system working environment is 50~60 higher than the ambience; at the temperature, the oil has a viscosity of 25mm2/s. Due to the large difference in temperature, the oil should have good viscosity-temperature performance, and the viscosity index should be higher than 130.


Viscosity cannot be too low to protect parts from wear. The gear pumps should have a viscosity not lower than 20mm2/s,vane pumps ≥10mm2/s, piston pumps ≥8mm2/s. Oil cooler could be installed when necessary, to regulate the oil’s temperature for the oil to have a viscosity within the safety range. 

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