Film thickness variations in elasto-hydrodynamically lubricated circular contacts induced by oscillatory entrainment speed conditions

Abstract

In this paper, the effects of an oscillatory entrainment speed on the film thickness in an elasto-hydrodynamically lubricated circular contact operating in the piezo-viscous regime as predicted by a conventional elasto-hydrodynamic lubrication model are described in detail. The relative variation of the film thickness on the centreline of the contact mainly depends on two dimensionless parameters: relative entrainment velocity amplitude and dimensionless entrainment velocity frequency. For small to moderate values of dimensionless frequency, the results can be accurately predicted through a simplified ‘engineering approach’: propagation of inlet modulations along oscillatory characteristics. For values of the dimensionless frequency of O(1), film thickness modulations occur on a sub-contact scale. This work was initiated by the experimental results of Glovnea and Spikes obtained for two lubricants PAO and 5P4E, which have the same ambient viscosity but a different viscosity—pressure coefficient. During the experiments significant difference in behaviour was observed. In this paper, it is shown that a conventional model assuming Newtonian rheological behaviour predicts the same relative film thickness results for both the lubricants. This agrees well with the behaviour observed for PAO, which can be considered as a standard oil that is well characterized by its bulk viscosity, density, and their pressure dependence alone. Nevertheless, further experiments need to be carried out to validate the predictions for larger values of the dimensionless frequency. The behaviour observed in the experiments for 5P4E is significantly different. For accurate prediction of friction, the need to include non-Newtonian lubricant behaviour in models is well known. Film thickness predictions are often less sensitive to the specifics of the lubricant behaviour. However, the results presented here indicate that for film thickness calculations, under rapidly varying conditions in time, more advanced rheological models, including visco-elastic behaviour, may have to be considered as well.