Search

In this paper a feasibility study has been conducted to determine a technique for the measurement of lubricant fluid film thickness in the piston-cylinder interface of an axial piston pump. The thickness of the lubricant film has a significant impact on the efficiency of the piston pump yet it still remains an uncertainty. If the lubricant film is too thin then friction will cause excessive damage resulting in deformation of interacting surfaces resulting in further uncertainty in thickness measurement. If the lubricant film is too thick then there is excessive leakage which will compromise the efficiency of the system. Acoustic, inductive, capacitive and optical techniques were investigated through literature review and it was decided to investigate further into acoustic techniques. Experimental procedures using ultrasound to measure the lubricant fluid film thickness are investigated and an experimental procedure is explored further into in this paper. The governing factors of measuring fluid film thickness were determined. The effect of surface deformation on lubricant thickness is also investigated and a technique of imaging using ultrasonic sensors is presented. A CAD model displaying sensor locations and mounting apparatus in piston-cylinder interface is also presented. A feasible method for the measurement of lubricant fluid film thickness is proposed. These measurements will be used to validate existing models which predict the behavior of lubricant fluid film.

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.