Characterization of edge-contact influence on tridimensional elastohydrodynamic film shape, pressure, stress and temperature distributions.

摘要

This doctoral project investigates edge contact influence on pressure, lubricant film thickness, temperature, and stress distribution of finite line contacts under an elastohydrodynamic lubrication (EHL) regime. This type of contact represents a common source of problems in engineering structures such as gears, cams and roller bearings, since non-conforming contact surfaces in such structures undergo intense stresses while transferring loads through relatively small contact areas. Additionally, they induce stress concentration zones at their extremities; as a result, profile modification becomes necessary. The present study investigates influence of free edges on EHL characteristics of finite line contacts. The initial stage of the research develops a 3D numerical model for the thermal, non-Newtonian EHL of general contact problems. A semi-analytical method (SAM), based on the Boussinesq half-space theory, is combined with a free boundary correction process to provide a fast and precise description of edge contact conditions. A modified finite difference expansion of the Couette term of the Reynolds equation guarantees computational stability, while the Carreau expression defines the shear-thinning response of the lubricant. Free boundary impact on tridimensional stress distribution is also investigated by extending the free-edge correction procedure to evaluate the levels of surface and subsurface stresses using SAM. The stress distribution data derived from this procedure are then contrasted with Finite Element Method (FEM) results using a two-level factorial comparison. Three dimensionless factors --- contact slenderness, contact length ratio, and load --- are examined. The comparison shows that the new model developed in this thesis provides a high level of precision in the evaluation of stress distributions, while computing more than 125 times faster than FEM simulations. This powerful model is then used to investigate and establish the influence of different roller profile modifications on EHL film shape, pressure and temperature distributions. Based on a series of detailed analyses of different roller profile corrections, it is found that a large radius crowning combined with rounding corners provides the most effective profile adjustment.;In the last step of this study, this newly developed model is combined with a multi-objective particle swarm optimization (PSO) to arrive at formulas establishing crowning and corner rounding radii, which can be applied to the rapid design of optimal rollers. The formulas take into account three dimensionless factors --- slenderness, load, and lubricant viscosity --- and coefficients for the formulas are derived from the PSO results using a five-level factorial design. By concurrently optimizing three objective functions --- contact pressure uniformity, film thickness stability, and maximum load capacity --- the predictions of these formulas guarantee optimal profile modifications. This study contributes to the understanding of edge influence on EHL characteristics of finite line contacts, while offering a robust model for axial profile corrections of lubricated contact problems.