Many Elastohydrodynamic lubricated (EHL) contacts, such as rolling element bearings, gears or cam systems work under transient conditions due to changes in speed, load, or geometry. A better tribological understanding is necessary in order to give further insights and guidelines to engineers who need to design systems with EHL contacts experiencing transient conditions However, due to the complexity a lot of fundamental research done in the field of EHL is mostly concentrated on steady-state conditions.
In this work, experimental investigations with an interferometric test rig as well as numerical studies have been carried out. At the University of Pisa, the existing experimental setup was modified in order to carry out EHL tests under transient conditions. This involves also the development of new tools in order to analyze efficiently the results such as the film thicknesses and friction coefficients. For example, an image-processing tool was developed in order to reconstruct out of a white light interferogram the deformed surface of an EHL contact.
In particular the influence of thermal effects combined with transient effects was investigated. Thermal effects are caused by high slide-to-roll ratios, or contacting materials with different properties, whereas transient effects by changing the velocity over time. If the contacting materials have different thermal properties, at some contact conditions non-standard film shapes different from the usual EHL shape with a so-called “dimple” in the center of the contact exist. This can have some influence on the film thickness and hydrodynamic friction coefficient under steady state as well as transient conditions.
In collaboration with the University of Kyushu in Japan, as a study object for a numerical investigation a reciprocating EHL rolling point contact was chosen. In this field few studies exist and are of interest because the lubrication state of oscillating machine elements is often influenced by cavitation phenomena. Due to the complexity caused by the non-steady state and cavitation phenomena, there is no design equation for the oil-film thickness under oscillatory motion, while reliable estimations can be done for steady state conditions. Based on recent numerical solution methods a Multilevel Multigrid (MLMG) algorithm was developed in C/C++, which made it possible to carry out parametric studies relatively fast. With the help of the algorithm, as an intermediate step to obtain a future design equation, a first equation to predict the length of cavity at the outlet of an EHL contact has been developed. The knowledge of the cavity length is of importance, because after reversal of the rolling direction the outlet region becomes the new inlet region and thus the cavitation produced beforehand in the outlet region can be entrained into the conjunction. This can lead to starvation effects, which in turn can cause a decrease in film thickness and increase in wear. In addition, first complete simulations of reciprocating EHL rolling point contacts have been carried out and shown good agreement when comparing with experiments.