Therefore, thickener type, concentration and thickener particle dimensions appear to have a strong influence on film formation and consequently affect friction by determining the lubrication regime. For example, certain types of thickeners have been reported to be able to adhere on the surface and form a thin residual layer of deposited thickener particles, while other studies suggest that, during the lubrication process, grease is mechanically degraded and transforms into a Newtonian suspension of thickener particles in oil, with a higher viscosity than that of the base oil alone. This is particularly true for grease friction which is the subject of significantly fewer studies than grease film thickness.Įxisting grease studies utilising standard laboratory single-contact tests strongly suggest that grease thickener properties have a significant influence on grease friction. Although a number of different mechanisms have been proposed specifically for rolling bearings, a comprehensive, universally accepted theory does not yet exist. However, largely owing to the complex composition of greases, the equivalent mechanisms for greases are still relatively unclear. The mechanisms of lubricant film formation, and to a lesser extent friction, in these EHL contacts are relatively well known for oils. Such concentrated contacts operate in the elastohydrodynamic lubrication (EHL) regime. 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. Using a suitable ‘low-friction’ grease represents one of the most versatile ways of reducing frictional losses in a rolling bearing. As a result, the cumulative impact of bearing frictional losses on the energy efficiency and sustainability of mechanical systems is significant. It is reported that over 50 billion bearings are operating in the world at any time. Rolling bearings are ubiquitous machine elements used in almost all engineering systems, and across many different sectors of industry and transportation. Indeed, this is by far the most common application of lubricating greases, with more than 90% of rolling bearings employing grease as the lubricant. These properties make grease particularly suitable for application in rolling bearings. 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. 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. 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. Addition of oleic acid to the test greases did not significantly affect friction within the range of test conditions employed in this study. PAO-based greases generally produced lower friction than mineral- and ester-based greases. For a given base oil viscosity, base oil type was found to have a strong influence on grease friction under all test conditions. 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. Friction coefficient and film thickness were measured in laboratory ball-on-disc tribometers over a range of speeds and temperatures. In addition, selected greases were blended with oleic acid, with the purpose of evaluating its effectiveness in further reducing grease friction. 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. This study investigates the influence of base oil type and viscosity on the frictional behaviour of lithium-thickened bearing greases.
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