Modelling and simulation in tribology, and more specifically of friction in lubricated contacts, has gained increasing attention over the past years. In a lubricated contact, the dissipation of energy is due to the relative motion of the mating surfaces and arise due to direct contact as well as shearing of the lubricant film. The presence of a thin lubricant film is crucial for the operation of various machine elements, e.g., for the concentrated contact between the rolling element and the raceway in a bearing. The contact in this type of applications is typically exhibiting substantial elastic deformation which together with hydrodynamics governs the formation of the lubricant film. Therefore, these type of contacts are said to operate in the Elastohydrodynamic lubrication (EHL) regime. An elastohydrodynamically lubricated contact can be classified as line, circular or elliptical. The line contacts can also represent a truncated ellipse or be of finite length. The line contact that appears between two cylindrically shaped bodies of infinite length does of course not exist in reality. It does, however, constitute an important type of simplification of the contact in real applications where the contact length perpendicular to direction of motion is comparatively large. The reason for this is that it permits a 2D-model for the flow and there are analytical solutions, at least in the most elementary cases. The circular and the fully elliptical contacts are more complicated. The case where the surfaces are fully separated by the lubricant film has, however, been addressed by many researchers and there are quite a few papers reporting numerical predictions validated by experimental data. The finite length line and truncated elliptical contact are even more challenging, but these are also the only physically reasonable models for EHL contacts exhibiting edge effects, created by profiled rollers or in cases where the contact ellipse becomes larger than the physical size of the contacting elements.
This thesis presents the development of a fully coupled model that can be used to predict the pressure build-up and lubricant film formation in finite length line contacts. More precisely, in EHL contacts where the rolling element have profiled edges (fillets) and the surface of the counteracting element is wider than the roller, e.g., in a typical cam-roller follower contact. Hereafter this type of contact will be referred to as a 'finite EHL line contact'. The numerical analyses, conducted with the present model, were designed so that generic knowledge about friction in cam-roller follower applications would be generated, but also to provide for the development of a semi-analytical for fast and efficient estimation of friction.
There are quite a few parameters that affect the friction in EHL contacts and it is already a challenge to include 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. When modelling a cam-roller follower application, which typically results in a finite length EHL line contact, the size and geometry of the contacting parts further complicates modeling of EHL. The main objective, with this thesis project, was to design a low degree of freedom model that can be employed in a multibody dynamics model, to estimate EHL friction in milliseconds and yet capturing the most important features of a lubricated contact including edge effects. This resulted in a semi-analytical low degree of freedom (LDOF) model taking thermal effects into account and that include lubricant shear thinning and compressibility, in order to estimate the viscosity and volume of the lubricant.
In addition, this LDOF model was extended to perform friction prediction covering the mixed lubrication regime where colliding asperities partially influence shearing of the lubricant. The extended, mixed lubrication LDOF, model was utilized to perform friction predictions covering a range of operating conditions, which were also covered in an experimental investigation using a ball-on-disk test device. The results turned out to compare well, suggesting that the model established in this project, is a usable tool that can be employed when designing lubricated devices and that it constitutes a suitable foundation for further developments.
Luleå tekniska universitet, 2016. , 222 p.