A Numerical Model That Predicts Lubrication Fluid Film Thickness and Friction in the Piston-Cylinder Interface of Hydraulic Piston Pumps/Motors

Abstract

Hydraulic piston pumps and motors are commonly used in low-speed-high-torque (LSHT) applications. Side load is always present in such piston-cylinder geometry which causes friction and wear. Sufficient lubrication in the piston-cylinder interface is essential to reduce wear and friction and increase efficiency. In this thesis, a computationally efficient numerical model that predicts the lubrication fluid film thickness and friction in the piston-cylinder interface for a generic piston-cylinder geometry is presented. The model focuses on LSHT applications. A specific Reynolds equation for the geometry is derived which includes the fluid film pressure buildup caused by the squeeze, the wedging, and the rotation of the piston. The model is validated by comparing the model prediction of friction results in the piston-cylinder interface of an axial piston pump/motor with the results presented in the literature. The model is then used to simulate lubrication conditions of an axial piston pump/motor and a novel hydraulic motor (Variable Displacement Linkage Motor) under different cylinder pressures (2 MPa, 10 MPa, 20 MPa) and shaft speeds (1 rpm, 5 rpm, 12 rpm, 60 rpm). The cylinder pressure and shaft speed are directly proportional to the side load and the piston axial velocity respectively. The simulation results show that higher cylinder pressure (or side load) causes the lubrication film to be depleted faster. Larger shaft speed (or piston axial speed) helps the piston maintain a lubrication film for a larger fraction of the piston cycle. Reversal of side load direction enables redevelopment of lubrication film. Minimizing overturning moments on the piston decreases solid contact forces. The findings of the simulation show that the optimal design of piston motor for LSHT application should select the lowest cylinder pressure that satisfies the torque requirement. For the VDLM, when the output shaft rotational speed is low, a good linkage module design would maintain a high piston axial speed. Reversal of the side load direction within one piston cycle should be ensured. In addition, a preliminary study shows that chamfer on the piston can also enhance lubrication.