Research on the fluid film lubrication between the piston-cylinder interface

The fluid film between piston and cylinder works simultaneously as bearing and lubricating functions, its thickness determines the lubrication efficiency and performance of an axial piston pump. Because of the thickness is usually micron level, and its change in the whole cycle is difficult to observe directly. In order to reveal it particularly, this paper present a mathematical model which considering structure, piston kinematics and a dynamic piston load to predict the film thickness in a cycle. The minimum thickness is used as an index to evaluate lubrication performance and its variation are studied under different load pressure, rotating speed and length ratio between cylinder and piston. A special test rig with four displacement sensors placed through the cylinder surface allows measuring the film thickness at these points. The model was verified by comparing the numerical results with measurements taken on the test rig. Results indicate that: (1) The mathematical model could effectively predict the film thickness trend and the position of the minimum thickness; (2) The minimum thickness in pressure phase is greater than in suction phase, and increases with the increase of load pressure and decreases with the increase of rotation speed, the influence of load pressure on film thickness is much greater than that of rotational speed. (3) Increasing the length ratio reduces the piston eccentric and raises the minimum film thickness, and in order to obtain the better lubrication, the length ratio should be greater than 0.5.