Thermohydrodynamic Analysis of Journal Bearings Lubricated by Non-Newtonian Fluids—Theory and Experiments
Viscosity index improvers cause the lubricants to exhibit non-Newtonian flow behaviour and display shear thinning and normal stress differences. Shear thinning behaviour is studied by using a rotary shear viscometer. Owing to the non-availability of a rheogoniometer (for the measurement of normal stress differences), the first normal stress difference is calculated from the viscometric data using the Carreau viscosity function. The influence of the first normal stress difference on the hydrodynamic lubrication is analysed and shows that most of the commercial oils are inelasticoviscous in nature. Regression analysis shows that a large number of commercial lubricants follow the inelasticoviscous cubic law fluid model. Hence the cubic law fluid model is considered for the theoretical analysis. An experimental programme is developed to measure the effect of test parameters on the performance of a journal bearing lubricated with different types of non-Newtonian fluids. The experiments mainly include the measurements of the steady state characteristics like film thickness and fluid film friction. The experimental film thickness values are compared with the respective theoretical ones and are in good agreement. The theoretical performance characteristics are obtained through the simultaneous solution of the modified Reynolds equation using the cubic law fluid model and energy equation. The fluid film friction in a hydrodynamic journal bearing is experimentally determined through coastdown analysis. The results are presented in the form of an apparent Stribeck diagram of friction and are compared with the respective theoretical values.