Rheological Challenges and Opportunities for EHL

There can be no doubt that the assumptions used in modeling elastohydrodynamics for the temperature, pressure and shear rate dependence of viscosity are at odds with empirical fact. As a result, there has been relatively little progress since the classic Newtonian film thickness solutions toward relating film thickness and traction to the properties of individual liquid lubricants and it is not clear at this time that full numerical solutions can even be obtained for heavily loaded contacts using accurate models. One central issue is the validity of Reynolds equation, derived under the isoviscous assumption, for conditions where the local pressure-viscosity coefficient can approach 100 GPa

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. Some pressing problems are reviewed in this paper, including the effects of shear-thinning on film thickness and traction, and the proper definition of the pressure-viscosity coefficient for film thickness calculations. Wherever possible, for credibility, sources from outside of Tribology will be used. Then some opportunities for the field will be discussed. These opportunities result from advances in molecular dynamics simulations and improved techniques for high-pressure measurements and should shed light on the relationship between molecular structure and performance in contacts.