THE EFFECT OF EFFECTIVE VISCOSITY OF GREASE ON THE ELASTOHYDRODYNAMIC FILM THICKNESS

The optical interferometry technique is used to measure the elastohydrodynamic film thickness of grease. Tests are carried out at varying speeds, constant load and at ambient temperature. Three lithium soap greases of the same consistency, but made with mineral oils of different viscosities, were tested using a steel ball and a glass disc to simulate the contact in a lubricated ball bearing. The measured film thickness enables a comparison to be made between the different greases. This is of practical significance as many ball bearings are lubricated with greases. The measured film thicknesses are compared with the theoretical values given by earlier researchers. From the experiments, it was observed that the grease film thickness was thicker than its corresponding base oil under the same elastohydrodynamic conditions. The effective viscosity (high shear rate viscosity) of the grease has a more significant effect on the elastohydrodynamic film thickness than its base oil viscosity. Results indicate that the grease film thickness increases with an increase in the high shear rate viscosity and speed. The results suggest that it is essential for process industries to select a grease based on its effective viscosity, rather than its base oil viscosity, for application in rolling contact bearings and gear drives as this can lead to improved machine life and significant cost saving.