Film Forming Behavior of Greases Under Starved and Fully Flooded EHL Conditions

ABSTRACT

Improving knowledge on the film forming behavior of greases is essential to be able to develop efficient greases. This article examines how operating conditions (e.g., temperature, lubrication condition [fully flooded/starved]) and base oil viscosity influence the film forming properties of greases by comparing the behavior of two lithium-based greases and their respective base oils in rolling point contact. It is found that the onset and degree of starvation is controlled by speed (u) × viscosity (ν)/load (W) factor (uν/W) and temperature and that low uν/W values promote entrainment of thickener into contact. Thus, grease with low base oil viscosity shows significant thickener entrainment in the low speed region compared to the one with high base oil viscosity, which leads to the formation of thickener-rich viscous material during extended running with the low base oil viscosity grease. The results suggest that the shape of the film thickness versus speed curve is viscosity and uν/W range dependent. Furthermore, for the test conditions used in this study, grease-lubricated contacts appear to shift from the initial fully flooded condition to starved condition over a prolonged running of 2 h. The results from this study concur with those reported in the literature that fully flooded oil elastohydrodynamic lubrication (EHL) theory or film thickness cannot be directly applied or taken as a guideline in grease-lubricated contacts.

KEYWORDS:

• Grease
• film thickness
• starved lubrication
• fully flooded lubrication
• low/high speed
• EHL
• rolling element bearings

Acknowledgements

The authors thank Dunja Scheibal, Klüber Lubrication München SE & Co. KG, for preparing the test samples and Dr. Günther Bodesheim, Head of Tribology & Chemistry, Klüber Lubrication München SE & Co. KG, for permission to publish the work described in this article.