International audienc

The micro-elastohydrodynamic lubrication of a single transverse ridge is revisited using an experimental technique, which combines an optical interferometry technique and a high-speed color video camera. The purpose of this study is to augment prior experimental analyses, by providing a complete and detailed history of the ridge associated with changes in film thickness as it passes through a high-pressure conjunction. An enhanced experimental procedure has been developed to enable an automatic analysis of the interferograms. In particular, the methodology allows abrupt changes in film thickness and rapid variations of interference orders to be taken into account. The observations presented in this paper exhibit interesting and fascinating features that have not been previously reported. In particular, it is observed that under rolling/sliding conditions the ridge undergoes further deformations as it proceeds to the exit to the contact. In addition, there appears to be an important contribution of pressure flow to the transport of lubricant and, contrary to current understanding, entrapped lubricant is seen to accompany the ridge as it passes through the contact, therefore appearing not to move at the entraining velocity.

A direct comparison between experimental and numerical results for the passage of an array of 3D flat-top, square shaped surface features through an EHL point contact is presented. Results for pure rolling conditions show that the features’ deformation in the high-pressure region is governed by their ability to entrap lubricant both underneath and in the grooves during their passage through the inlet zone. Film perturbations associated with each defect occur as locally enhanced regions of lubricant and film thickness micro-constrictions. Under sliding conditions the features sustain further deformations as they traverse the high-pressure conjunction and meet the highly viscous lubricant entrapped in the grooves, which moves at a different velocity. Lubricant is also seen to accumulate just in front or behind the features depending on the slide-to-roll ratio. Overall, the results highlight the importance of understanding the effects of the defects structure and the lubricant rheology on the film thickness to unravel the effects of real roughness patterns.

A simplified semi-analytical solution for the film thickness fluctuations in the normalapproach problem of elastohydrodynamic lubrication (EHL) contacts has been obtained. The model is based on an inlet analysis to include squeeze-film effects together with the variation of lubricant entrainment speed that is induced by the changes of contact radius as the load oscillates. The results obtained are in excellent agreement with numerical solutions and suggest that the fluctuations in entrainment speed are the main cause for the observed film modulations. The modified semi-analytical solution seems applicable in many practical cases. The amplitude of the film thickness fluctuations produced by fluctuations in the load is in this way related to a single parameter.

Recent experimental observations by Fe lix-QuinÄ onez, Ehret and Summers (Trans. ASME, J. Tribology, 2003, 125, 252±259) for a single,¯at-top transverse ridge passing through an elastohydrodynamic lubrication (EHL) point contact under various slide±roll ratios have revealed some unexpected features. In particular, under sliding conditions the ridge was observed to sustain additional deformations in the high-pressure region of the contact and an entrapped amount of lubricant was seen to accompany the ridge during its passage through the conjunction and, contrary to current knowledge, appeared not to move at the entraining velocity. This paper analyses the experimental observations by means of a direct comparison with numerical simulations. The numerical results show that the main contributing factor towards the experimentally observed accumulation of lubricant is a reduced lubricant viscosity via shear thinning effects. For the operating conditions considered in this study, internal heating of the lubricant is found to be negligible. In addition, it is shown that the amount of lubricant accumulation is dependent on the geometry of the ridge. Compared to a defect with a Gaussian shape, it is further shown that a greater accumulation of lubricant occurs with the¯at-top ridge due to the generation of larger pressure gradients.