The high-pressure rheology of gear oils

Abstract

Fifty years of generating pressure-viscosity data and flow curves have provided this author with much data on commercial gear oils. Here, four commercial gear oils are thoroughly characterized for the temperature, pressure, and shear dependence of viscosity. Like all supercooled liquids these oils display the super-Arrhenius previtreous pressure dependence which is ignored in the classical approach to elastohydrodynamic lubrication, EHL. The parameters of the Hybrid model are presented. Time-temperature-pressure superposition has been employed to generate a master flow curve for each oil with Vinogradov-Malkin shifting. Flow curves clearly show that there is more than one component contributing to the viscosity of the whole. The double modified Carreau-Yasuda model describes this constitutive response. These models are essential for quantitative predictions of film thickness and friction in real machines. The fictional narratives being taught in tribology textbooks regarding high-pressure rheology have not been helpful and must have contributed greatly to the slow progress in EHL over the last decades.