Friction Forces Within the Cylinder Bores of Swash-Plate Type Axial-Piston Pumps and Motors

Abstract

In this research, the friction within the cylinder bore of a swash-plate type axial-piston machine is examined. Unlike previous research, this work develops a mathematical model for the friction based upon lubricating conditions which are described by the well-known Stribeck curve. Furthermore, a test device is built for measuring the frictional characteristics during low pressure and low speed operation and these results are compared with the mathematical model. For high pressure and high speed considerations, a numerical investigation based upon the model is conducted and it is shown that the friction associated with a pumping piston is greater than the friction associated with a motoring piston. It is also shown that increased piston speeds usually reduce the friction within the cylinder bore; however, a “cross-over” condition may exist where the increased speed will actually increase the friction as a result of increased fluid shear. Furthermore, it is shown that speed changes have a more significant impact on motoring pistons as opposed to pumping pistons due to a difference in the location of hydrodynamic lubrication within the cylinder bore. It is noted that this difference exits due to the bore geometry and the direction of piston travel.