Abstract
Grease lubrication performance prediction is challenging. Only recently that empirical equations to predict grease film thickness for prevailing rolling conditions under fully flooded lubrication taking into account thickener properties and content for low, moderate, and high speeds were developed. At starved lubrication, although new insights about the supply and loss mechanisms that govern film formation have been published, contact replenishment and, consequently, film thickness predictions for long-term operation are still not available. Prediction of components efficiency requires film thickness values and properties, including film’s molecular structure, which makes it even more challenging. When it comes to prevailing sliding conditions, the literature is scarce and most of the knowledge developed for prevailing rolling conditions is not applicable. During the sliding of the contacting bodies, boundary and mixed lubrication regimes are expected. In this situation, the tribological response is primarily defined by grease thickener and additives physicochemical interaction with the surface. This complexity leads many researchers to seek simpler relationships between grease formulation and properties with its performance. This review aims to present the state-of-art on grease lubrication and update some of these relationships.
Highlights
The distribution of these components depends on the application but typically is 65–95 wt % base oil, from 5 to 35 wt % thickener, and from 0 to 15% additives. These separate components form a multiphase matrix, in which the thickener forms a structure that holds the base oil and the additives. This multiphase matrix endows the grease with a certain consistency and a yield pseudoplastic behavior that gives it many advantages over lubricant fluids, such as Tribology ease of use, inherent sealing action, low friction, and protection against corrosion [2]
It makes it quite difficult to characterize or model the individual influence of each of its components on the grease overall performance because their effectiveness depends on their interaction [3, 4], manufacturing process [5, 6], and system operating conditions, as observed with calcium sulfonate thickened greases that show excellent wear resistance in pure sliding tests [7, 8] but reduced rolling contact fatigue life in prevailing rolling conditions [9, 10]
It is well known that under fully flooded conditions, grease does not always follow the film thickness predicted by the traditional elastohydrodynamic lubrication (EHL) theory [13] nor the coefficient of friction is depicted by the Stribeck curve [14]
Summary
The most common definition of lubricating grease is the one put forward by ASTM D 288: “A solid to semifluid product of a thickening agent in a liquid lubricant, other ingredients imparting special properties may be included” [1]. The available work modeling lubricating greases as a multiphase fluid is very promising and may bring significant new insights about grease performance, such as the grease lubricating phase (variation of volume fraction distribution of oil and thickener in the contact) as a function of time and position [11], and the rheological changes of lubricating greases as a function of thickener distribution and deformation [12] Such models were not yet employed to compare different grease formulations with their flow and tribological responses. It is well known that under fully flooded conditions, grease does not always follow the film thickness predicted by the traditional elastohydrodynamic lubrication (EHL) theory [13] nor the coefficient of friction is depicted by the Stribeck curve [14] This divergence is attributed to the presence of thickener in the contact [15] and depends on the operating conditions, in particular the lubrication regime [16]. In order to clarify the role of thickener type and base oil viscosity on grease film thickness, friction and wear, this review presents a compilation of the literature for prevailing rolling and prevailing sliding conditions through schematic graphics along with a ranking of the thickeners and base oils that contribute the most on the tribological response of the greases
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