A Great Challenge in EHD Lubrication: Toward Quantitative Predictions Based on the Actual Rheological Behavior of the Lubricant
Résumé
Since the elastohydrodynamic lubrication (EHL) concept was developed in 1950-1960, considerable progress was accomplished in the understanding of the complex mechanisms that manifest there. This was made possible thanks to progress achieved in the field of modelling and numerical simulation, and thanks to the development of more efficient and accurate experimental techniques. In the meantime, the operating conditions of lubricated mechanisms have changed dramatically: higher contact pressure, higher operating temperature, lower lubricant viscosity etc. As a consequence the critical feature of these lubricated contacts, i.e. the thickness of the thin lubricant film confined between the two solid surfaces in relative motion, has evolved significantly. Typically during the 50s film thickness was considered on the order of several micrometers and nowadays, it comes to the order of 10 nanometers or less, a huge reduction of at least two orders of magnitude. Regarding central film thickness, indeed greater than that occurring over the EHD contact lobes, it becomes clear that the film thickness prediction in application should be much more accurate because the room for manoeuvre becomes increasingly narrow to cope with surface roughness or possible unforeseeable change (hazards) in the lubrication conditions (thermal effects, non-Newtonian behaviour of the lubricant, inlet starvation, etc.). To address this challenge, authors proposed a modern approach that breaks with the practices of the field and based on two fundamental ideas: - Consider the real properties of the lubricant, obtained through independent characterizations carried out over representative conditions, - Take into account the multi-physics aspects of the EHL problem, by using physically based models to describe the lubricant behaviour. This is the purpose of this presentation to tackle this issue by presenting into details this initiative, with two particular focuses on (i) the physical behaviour of the lubricants under conditions representative of the application, and on (ii) the development of a proper multi-physics modelling of EHD contacts. In other words, this is an attempt to show how one can achieve a quantitative prediction of EHL based on the actual rheological behaviour of the lubricant.