Abstract
Grease is highly susceptible to degradation due to regular usage and the severity of the operating conditions. Degradation can negatively impact the performance of grease-lubricated machinery, demanding frequent maintenance to avoid premature failure of machine elements. Quantification of grease degradation has proven to be a formidable task, for which no accepted standards are currently available. In this paper, we extend the results of a novel approach developed recently for the evaluation of the water-resistant property in grease to quantify degradation. The methodology is based on measurements of the contact angle of a water droplet on the surface of a sample of grease. We report the results of extensive tests performed on different grades of lithium complex greases to evaluate the variation of contact angle values with the composition of grease. The measurements were compared with penetrometer readings to quantify a relationship between the grease consistency and contact angle. Detailed study results are also presented on three types of greases sheared in a grease worker for a different number of strokes: contact angle and the yield stress values were measured and compared. Finally, the tribological characteristics were determined for two greases that exhibited a low or high change in their contact angles.
Highlights
Grease has widespread application in machinery components such as rolling element bearings, pin bushings and journal bearings [1,2,3,4,5,6,7], gears [8,9,10], slide-ways [11,12], and the like
Grease degradation occurs due to physical changes, chemical changes, or a combination thereof [13,14,15]
Physical degradation occurs due to bleed-off and/or evaporation of base oil and contamination by particles and/or water
Summary
Grease has widespread application in machinery components such as rolling element bearings, pin bushings and journal bearings [1,2,3,4,5,6,7], gears [8,9,10], slide-ways [11,12], and the like. The industry typically measures the physical changes in grease by evaluating its consistency through the worked penetration test provided in the American Society for Testing and Materials (ASTM) standard D217 [16]. ASTM D217, requires a large amount of grease, which is not practical when trying to assess the consistency of small amounts of grease taken from roller element bearings or other lube points To overcome this complication, Rezasoltani and Khonsari [17] employed a rheometer for assessing grease degradation by monitoring the change in the rheological properties and correlating it to the mechanical degradation of the grease. Lijesh and Khonsari [18] extended the approach proposed by Rezasoltani and Khonsari [17] for developing a predictive model for determining the degradation of grease from their operating NLGI grade and thereafter estimating their remaining useful life They based their degradation assessment on a relationship between the change in the grease consistency and entropy generation.
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