Influence of Thermal, Oxidative, Catalytic, and Mechanical Effects on Thickener Degradation and the Associated Lubricating Performance of Greases

Continuous advancements in application technology aimed at higher efficiency and power density place ever-increasing demands on mechanical components and construction elements—and, consequently, on the lubricating greases employed. This is particularly true for rolling bearings, where greases are exposed to high mechanical loads and wide temperature ranges. A current example can be found in the bearings of hybrid vehicle powertrains, which are subjected to extreme thermal and mechanical stress due to engine downsizing, high rotational speeds, and radiant heat from the combustion engine. A collaborative project between the Competence Center for Tribology (KTM) at Mannheim University of Applied Sciences and the OWI Science for Fuels gGmbH (OWI), affiliated with RWTH Aachen University, demonstrated that the loss of lubricating performance—which ultimately leads to bearing failure—is directly linked to changes in the thickener structure. Various degradation processes reduce yield stress and viscosity, thereby eliminating the typical grease characteristics. Mechanical, thermal, oxidative, and catalytic processes all play decisive roles. This paper presents analytical methods that enable these individual influencing factors to be investigated and evaluated independently. These approaches can significantly reduce the need for time-consuming and costly laboratory tests in grease development and qualification.