Interpreting Experimental Bearing Oil Film Thickness Data

To verify the relationship between bearing oil film thickness and high-temperature, high-shear (HTHS) engine oil viscosity, accurate values for bearing oil temperature and shear rate are needed. Techniques for making these determinations are presented using data collected from engine tests with both single- and multigrade oils. A regression model is developed for estimating bearing oil film temperature as a function of engine operating variables. Bearing oil temperatures can be more than 20°C greater than sump temperatures. The relationships between film thickness and viscosity evaluated at several bearing shear rates are described. If oil viscosity is evaluated at the maximum bearing shear rate, previously published findings which conclude that multigrade oils provide a thicker oil film than single-grade oils are verified. However, if viscosity is evaluated at an average shear rate in the loaded portion of the bearing, film thickness values provided by single- and multigrade oils are equivalent.