Modeling of Mixed Friction Interactions Occurring in Axial Piston Pumps

Axial piston pumps are integral to a variety of hydraulic systems due to their versatility and reliability, but most attempts to model them for design purposes have failed. However, software which fully models the fluid structures within these pumps, such as the FSTI gap design program, has improved significantly. Unfortunately, current discrepancies between simulated and measured friction forces on the cylinder due to the fluid drag and piston movement need to be eliminated so that more accurate simulation of these pumps in operation is possible. Friction measurements and FSTI simulation outputs were analyzed with MATLAB filters and graphs, resulting in the conclusion that mixed friction, a form of partially lubricated contact friction, is the best explanation for the discrepancy. To model and predict the mixed friction, the Stribeck curve, a relationship between piston velocity, fluid viscosity, contact pressure, and the coefficient of friction between the piston and cylinder, was used, allowing a computational model to be developed to post-process the FSTI results and determine the mixed friction. The greatly improved model resulted in simulated total friction increases of nearly 300 percent in cases involving rough surfaces and high pressures. Though this module still needs to be fully integrated into the FSTI gap design program, the developed mixed friction model greatly increases the accuracy of the simulation by bringing the magnitude of the simulated friction much closer to realistic measured values