The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

N De Laurentis,P M Lugt,A Kadiric,P Cann

doi:10.1007/s11249-017-0908-7

Abstract

This study investigates the influence of base oil type and viscosity on the frictional behaviour of lithium-thickened bearing greases. A series of model lithium greases were manufactured by systematically varying viscosity and type of base oil, so that the influence of a single base oil property could be studied in isolation. In addition, selected greases were blended with oleic acid, with the purpose of evaluating its effectiveness in further reducing grease friction. Friction coefficient and film thickness were measured in laboratory ball-on-disc tribometers over a range of speeds and temperatures. For a specific oil type, the influence of base oil viscosity on friction was found to be closely related to its effect on film thickness: greases formulated with PAO oils covering a wide range of viscosities gave very similar friction at the same nominal film thickness. For a given base oil viscosity, base oil type was found to have a strong influence on grease friction under all test conditions. PAO-based greases generally produced lower friction than mineral- and ester-based greases. Addition of oleic acid to the test greases did not significantly affect friction within the range of test conditions employed in this study. The results provide new insight into the frictional behaviour of greases, which may be used to help inform new low-friction grease formulations for rolling bearing applications.

Highlights

Grease is a multi-phase lubricant consisting of two main components: a fluid, which is typically a lubricating oil, and a solid ‘thickener,’ usually a metal soap
Lithium greases display two operating regions: at relatively high speeds, grease behaviour is dominated by the properties of its base oil; below a certain transition speed, there exists a thickener-dominated region where grease friction decreases and film thickness increases with decreasing entrainment speed, trends that are opposite to those exhibited by the corresponding base oils
The transition between these two regions occurs at a characteristic value of nominal film thickness, which appears to be affected by the size of the lithium thickener fibres in a given grease;

Summary

Introduction

Grease is a multi-phase lubricant consisting of two main components: a fluid, which is typically a lubricating oil, and a solid ‘thickener,’ usually a metal soap This composition provides grease with some inherent benefits over lubricating oils including a certain rigidity, which allows grease to stay in place without leaking, as well as sealing abilities. The primary role of grease in a rolling bearing is to lubricate the highly stressed, concentrated contacts at the interface between the rolling elements and the raceways of the bearing, where a large contribution to the overall friction is generated. Such concentrated contacts operate in the elastohydrodynamic lubrication (EHL) regime. A number of different mechanisms have been proposed for rolling bearings [2,3,4,5,6], a comprehensive, universally accepted theory does not yet exist

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