At atmospheric pressure, about 10% of the air can be dissolved in the hydraulic oil, and more air or gas will be dissolved in the oil under the high pressure of the hydraulic system. Air bubbles form in the oil. If the hydraulic system of vertical roller mill changes rapidly between high and low pressure in a very short time during the working process, the bubbles will burst at high temperature on the high-pressure side.

If the surface of the hydraulic system has concave points and damage, the hydraulic oil will rush to the surface of the components to accelerate the wear and tear, causing leakage.

Due to the influence of humidity in working environment, the external water may enter the hydraulic system, water will react with hydraulic oil, forming acid substances and sludge, reducing the lubrication performance of hydraulic oil, accelerating the wear of components, and corrosion of valve stem caused by water, which makes the control valve difficult to operate, scratch the seal and cause leakage.