Application of a DLC-Coating for Improving Hydrostatic Piston Shoe Bearing Performance under Mixed Friction Conditions

The pumps of electro-hydrostatic actuators are frequently subject to mixed friction, since they operate as a control element compensating for position control errors. Therefore, they should be capable of enduring more extreme tribological conditions than conventional pumps operating constantly at high speeds. Especially, when conventional swash plate type piston pumps are applied to electro-hydrostatic actuators, their performance under mixed friction conditions should be examined and supplemented. The frictional power losses, as well as the wear rate of the sliding components such as piston shoes, can significantly increase under mixed friction conditions. In this paper, a DLC-coating was applied to the swash plate and ball joint of pistons, and its ability to reduce the power losses from the frictional contact and the leakage flow rate of the hydrostatic piston shoe bearing was investigated. The DLC-coated swash plate was able to effectively reduce the friction force on the piston shoe and the wear rate, while the leakage flow rate could also be reduced using the DLC-coated ball joint. Using the DLC-coated ball joint and swash plate together the total power loss from the hydrostatic piston shoe bearing could be reduced by more than 40% in the pump speed range below 100 rpm.