Elastohydrodynamic Lubrication of Cam and Roller Follower
Abstract: Modelling and simulation of friction is a research issue that still requires an extensive amount of input from the scientific community. In a lubricated system, the dissipation of energy is connected to the direct contact between the surfaces, or more precisely the tribofilms, as well as of the shearing of the lubricant film. Elastohydrodynamic lubrication (EHL) is a lubrication regime which is characteristic for contacts found in machine components such as in roller bearings, gears and cam mechanisms. These contacts have in common that they carry load on a very small/concentrated area and exhibit elastic deformations that are much greater than the thickness of the hydrodynamically formed film. There are a vast number of parameters that affect the friction in EHL contacts and it is a challenge to include other than the most basic ones in the model. The most advanced and sophisticated models are very complex with millions of degrees of freedom and are, therefore, not yet feasible to conduct parametric studies with. The extreme conditions associated with EHL, i.e., nm thin films, with phase transition from liquid to solid, GPa pressure, temperature increase with considerable implications on lubricant flow and surface chemistry, etc., makes it even more difficult to model these systems. The shape of the contacting parts further complicates modeling of EHL. More precisely, an EHL contact can geometrically be either of a line, circular, elliptic or truncated contact type. Since the line contact appears between two cylindrical shaped bodies of infinite length, it permits a 2D-model for the flow and there are analytical solutions, in the most elementary cases. The circular and the elliptic contacts more are complicated. The case when the surfaces are fully separated by the lubricant film has, however, been addressed by many researchers, who also have presented numerical predictions validated by experimental data. The finite line contact appears to be the most challenging type, but it is also the only physically reasonable model for EHL contacts where the edge effects cannot be neglected. In this work, both steady state and time dependent, fully deterministic models are utilized and further developed to enable the study edge effects under variable operating conditions in cam and roller follower systems. The numerical investigations were specified so that generic knowledge about friction in these systems would be generated and also to provide validation data for the development of a semi-analytical, low degree freedom model, for rapid estimation of friction. The main objective was to design such a low degree of freedom model so that it can be employed in a multibody dynamic model, requiring friction estimation in milliseconds. The semi-analytical low degree of freedom model developed in this work, takes thermal effects into account and is built on an advanced and well-characterized rheological model, including lubricant shear thinning, in order to estimate the viscosity and volume of the lubricant. The model was utilized to perform friction prediction covering a range of operating conditions, which were also run in an experimental investigation using a ball-on-disk test device. The results turned out to compare well, suggesting that it constitutes a suitable foundation for further developments.
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