Some important aspects of thermal elastohydrodynamic lubrication

Abstract

Thermal effect in elastohydrodynamic lubrication (EHL) has been the subject of study for the last four decades; however, some important aspects related to the physical behaviour of the lubricant in response to pressure, temperature, and shear rate remain largely neglected. This paper presents a brief review of the thermal EHL literature and sheds light on the importance of accurate characterization of the lubricant properties such as viscosity, density, rheology, and thermal conductivity. Full thermal EHL line contact simulations under steady-state and transient conditions show that using the ambient value of thermal expansivity, which has been the usual trend, may overestimate the central film thickness and introduce unrealistic features in transient EHL characteristics. Also, it is demonstrated that the most extensively used rheological equation – the sinh law – for characterizing the behaviour of shear-thinning lubricants underestimates the effect of viscous heating on EHL traction and film thickness.

Speed of Sound and Ultrasound Absorption in Ionic Liquids

“…The unique characteristics of ILs make these materials promising candidates for use as industrial lubricants, especially under hydrodynamic and elastohydrodynamic regimes. ,, Kumar et al observed that not enough attention is paid to the importance of high-pressure lubricant properties, such as the density and thermal expansibility. Pensado et al also proved that the pressure profile of viscosity should consider the pressure–temperature path of thermal expansibilities and the isothermal compressibilities of IL lubricants to model ILs.…”

“…The significance of this problem has also been noted by Franca et al 333 The unique characteristics of ILs make these materials promising candidates for use as industrial lubricants, especially under hydrodynamic and elastohydrodynamic regimes. 320,334,335 Kumar et al 335 observed that not enough attention is paid to the importance of high-pressure lubricant properties, such as the density and thermal expansibility. Pensado et al 336 also proved that the pressure profile of viscosity should consider the pressure−temperature path of thermal expansibilities and the isothermal compressibilities of IL lubricants to model ILs.…”

Pressure–viscosity response in the inlet zone for quantitative elastohydrodynamics

“…Beginning with the first full EHL simulation employing real pressure and shear dependent viscosity, measured in viscometers [8], there have been revolutionary advances in the understanding of friction and film-forming. See [9][10][11][12] for examples. While it is apparent that the Newtonian film thickness solution is much less widely applicable than previously thought, Newtonian inlets do exist in practical problems and, even when the viscosity is shear dependent in the inlet, it remains necessary to describe the pressure dependence of the low-shear viscosity to apply any shear-thinning solution using superposition.…”

Analytical model for predicting friction in line contacts

“…6,7 Furthermore, it was found that heat generated by friction can produce a local temperature rise, leading to analysis of the thermal effects, and therefore giving rise to the so-called thermal elastohydrodynamic lubrication. 8,9 Many other studies [10][11][12] contributed to a better understanding of the main factors which influence the behaviour of the lubricant and the quantitative effects of each.…”

“…Numerous analyses observed that under the severe operating conditions of elastohydrodynamics, the rheology of many common lubricants varies from Newtonian to pseudoplastic . Furthermore, it was found that heat generated by friction can produce a local temperature rise, leading to analysis of the thermal effects, and therefore giving rise to the so‐called thermal elastohydrodynamic lubrication . Many other studies contributed to a better understanding of the main factors which influence the behaviour of the lubricant and the quantitative effects of each.…”