Hydraulic oil is not only the medium of power transmission, but also lubricant and coolant in hydraulic system. To ensure that the hydraulic system can work reliably and economically in all situations, the hydraulic oil must have the proper viscosity and good viscosity temperature characteristics. Viscosity is an important index of hydraulic oil.

For a certain hydraulic system, only working of vertical roller mill medium with limited viscosity range can be used to work normally. The viscosity is too large, the resistance increases, the temperature rises rapidly, the energy loss is large, and the system efficiency is reduced; The viscosity is too small, which increases the volume loss of the pump and reduces the oil film support capacity, which leads to dry friction between the moving pairs, and increases the leakage.