Exploring Compatibility Limits for Lubricating Grease Formulations

Grease compatibility is an important consideration for both grease manufacturers and end-users. When switching from one lubricating grease to another, the concern is whether the two greases can safely coexist or if mixing them will degrade performance and potentially harm equipment. Almost 60% of the global grease demand is based on lithium soap thickener chemistry, so finding viable alternatives and understanding their compatibility is vital. In this study, the compatibility limits of different greases were experimentally examined by preparing binary blends of a conventional lithium complex grease (LiX) with three alternative thickener types: aluminum complex (AlX), polyurea (PU), and calcium sulfonate complex (CaSX) greases. Three mixing ratios (10:90, 25:75, 50:50) were tested for each combination to simulate partial or nearly complete grease replacement. A total of nine blended samples, along with the four neat (unmixed) greases, were evaluated according to the ASTM D6185 protocol for grease compatibility. Primary test criteria included dropping point, shear stability, and high-temperature storage stability, while secondary evaluations covered oxidation stability and anti-wear performance. The results revealed that the lithium complex–polyurea combination achieved the highest compatibility score overall, while the lithium complex–calcium sulfonate complex blends were the least compatible by ASTM criteria. The study demonstrates the importance of both standardized tests and practical performance limits in judging grease pair compatibility.