On the role of lubricant rheology and piezo-viscous properties in line and point contact EHL

Abstract

Accurate prediction of elastohydrodynamic lubrication (EHL) characteristics, i.e., film thickness and traction coefficient, requires the knowledge of lubricant rheology. The EHL analyses based on Newtonian fluid model may be useful primarily for film thickness prediction of mineral oils. However, for applications involving mineral oil/polymer blends and synthetic oils that exhibit shear-thinning behavior, the use of an appropriate non-Newtonian fluid model is required to predict the EHL behavior correctly. It is, thus, no surprise that characterization of non-Newtonian fluid behavior in EHL studies has been the focus of attention over last four decades. In this regard, several types of non-Newtonian EHL models have been presented and solved using different methods. Unfortunately, many of these studies suffer from various drawbacks such as use of inappropriate non-Newtonian model, approximations in the numerical solution and lack of sufficient experimental data pertaining to lubricant rheology as well as piezo-viscous properties. Over the years, some of these shortcomings have been transferred erroneously from one generation of researchers to the next. For example, notwithstanding the fact that the sinh-law lubricant model (commonly referred to as the “Ree–Eyring” model) was actually rejected for use as a shear-thinning model by Henry Eyring himself, it is still being widely used to describe the shear-thinning behavior of EHL lubricants. The present paper discusses the recent developments clearly indicating the incapability of sinh-law for the EHL applications. This paper also reviews the perturbation method often used to reduce the actual constitutive equation to a simplified form in order to derive a Reynolds-type equation to which normal EHL solvers can be applied. This approach is still in use even though appropriate procedures for implementing the generalized Newtonian approach are available that can conveniently and accurately allow the use of the exact constitutive equation in its original form. In the light of above facts, this paper presents a collective perspective in an effort to emphasize the importance of implementing realistic non-Newtonian and piezo-viscous models with accurate treatment methods in EHL applications.