Analysis of Textured Surfaces in Full-film Lubrication

One possible approach to reduce friction in lubricated contacts and to increase load carrying capacity is the application of surface modifications, called textures. Current publications often focus on parallel oil film gaps, although most technical applications involve converging lubricant gaps. Therefore, the present work investigates the behaviour of textured surfaces in convergent oil film gaps under hydrodynamic lubrication. Since the tribological performance of surface textures is influenced by a large number of parameters, a numerical simulation model was developed at the beginning of this work. This model was used to conduct comprehensive parameter studies, to analyse the influence of various geometric texture parameters. The results show that textures located in the region of the maximal pressure decrease the load-carrying capacity. To investigate the influence of texture geometry, nine different texture shapes were analyzed. The findings indicate that textures covering a larger surface area perform better than surface modifications that can only cover a smaller area. For example, rectangular or square dimples result in a higher load-carrying capacity than triangular textures. In addition to geometric parameters, the numerical results demonstrate that surface textures are more effective at higher velocities and viscosities, as well as at lower converging angles of the lubricant gap and decreasing loads. Since these findings were initially obtained purely through numerical simulations, a novel experimental test methodology for a converging oil film gap under hydrodynamic lubrication was developed to validate the results. In this methodology, the minimum lubricant film thickness is defined, and both friction force and load-carrying capacity are measured. To identify suitable textures, an optimization algorithm was applied to the simulation model, yielding optimal texture parameters. The resulting textures were then manufactured by milling and tested. The results show that textures can increase the load-carrying capacity by more than 35 % and, under certain operating conditions, also reduce the drag force by more than 10 %. Thus, the developed experimental methodology was successfully used to demonstrate that surface modifications can enhance load-carrying capacity and reduce friction in convergent oil film gaps.