Efficient numerical models for the tribological analysis of textured journal bearing systems

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The reduction of friction and wear in sliding bearing systems can significantly contribute to the efficient design of a drivetrain. Surface texturing is an emerging technique to reduce friction and wear. Numerical models for the design of sliding bearing systems for applications with sequential operation under mixed friction conditions, e.g., wind power drives or internal combustion engines, commonly only incorporate the effect of surface roughness on friction and wear. The application of these models to textured systems leads to significant modeling errors or requires time-consuming simulations with highly-resolved surface-texture discretizations. Consequently, determining the optimal texture design for the reduction of friction and wear in a given application is nearly impossible. The main objective of this project therefore is the development, implementation and validation of an efficient numerical method for the optimal design of bearing systems with rough, textured shaft surfaces. In contrast to existing software solutions that incorporate Patir and Cheng’s empirical average flow model, the modeling in this project is based on the mathematical concept of homogenization. Homogenization is based on an asymptotic expansion, which - in contrast to the average flow model - provides correct results for any configuration of surface roughness or texturing. In addition, a simple upscaling of the averaged solution allows for capturing local effects. The concept can be extended by reiterated homogenization in such a way that roughness and textures can be taken into account without resolving the textures by fine computational meshes. By developing and implementing individual modules, a comprehensive model for transient simulations is assembled. This model incorporates (i) hydrodynamics taking into account cavitation, (ii) mixed friction by computing the elastic-plastic asperity contact pressure and elastic deformations (EHL) and (iii) temperature effects through energy equations for the fluid and the two solids. The resulting thermo-elastohydrodynamic/mixed friction model is finally extended by physical models for the prediction of wear. The latter are enhanced to account for the conformity of shaft and bearing developing during running-in on asperity contact scale. In addition to the numerical studies, experiments are conducted to validate the numerical model and to systematically investigate the uncoupled and synergistic effects of roughness and texture on the tribological characteristics of sliding bearing systems. Furthermore, the durability of surface textures is studied. This involves the use of different bearing materials that tend to smearing as well as tests under mixed friction conditions. Running-in/wearing-in effects such as the shift of the transition speed from mixed to hydrodynamic lubrication are studied as well.