Hydrodynamic lubrication of pocketed thrust washers.

摘要

Modifications to the thrust washers' surfaces utilize the geometric wedge effect to generate pressure in the lubricant, separating the bearing surfaces and supporting the load. These modifications in the form of dimples, pockets, grooves or undulations create a hydrodynamic lubrication film that allows relative motion to occur with no surface contact, minimizing both friction and wear. A series of studies were performed to analyze the lubrication mechanisms provided by these surface modifications. Various test rigs were designed, developed and implemented to measure cavitation, pressure, and temperature distributions in the thrust washer contact. Visualizing the lubricant flow allowed the study of gaseous cavitation occurring on the leading edge of a surface pocket. The size and shape of the cavitation air bubble was found to be a function of rotational speed, contact pressure, viscosity, and pocket geometry. Experiments were then designed to map the load bearing pressure generated by the pockets. High resolution, thin-film pressure transducers were installed beneath the surface of the thrust washer pad and used to study the pressure distributions within the pockets. Interactions between adjacent pockets were shown to have a strong effect on the load carrying capacity and friction of the bearing. Temperature of the lubricant film was measured using a thermochromic sheet that changed color as a function of temperature. Video recorded during operation showed the thermochromic material dynamically adapting to the temperature rise within the bearing. The viscosity of the lubricant correlates directly with the temperature rise and was shown to have a strong effect on friction and film thickness. The gaseous cavitation, pressure and temperature distributions within the contact were also modeled numerically. Initially, ANSYS FLUENT computational fluid dynamics models were developed to corroborate experimental results. These simulations used various cavitation algorithms and pressure solvers to predict lubricant film conditions, but FLUENT was found to lack the adaptability and access necessary for high accuracy modeling. Therefore, a finite volume numerical model was developed using the thermal Reynolds equation coupled with the energy equation. The finite volume formulation was developed in FORTRAN and used to determine pressure and temperature distribution within the thrust washer contact. The experimental results were corroborated with the results obtained from this numerical model, their comparison shows that they are in good agreement.