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Technical Committee Lubricants

Study on the influence of different grease formulations on the tribological behavior of hybrid bearings

Although grease lubricated hybrid spindle bearings are widely used, there is only little knowledge about these bearings as a tribological system. In particular, the influences of the steel-ceramic-contact and the high speeds on the operating behavior have only little been analyzed. Thus, the correlations between the operating behavior and the operating conditions, the grease formulation and the properties of the boundary layers on the bearings‘ raceways have been investigated here.

Short-cycle tests to determine the speed ratings were carried out with four simply formulated sample greases and one high performance grease as a reference. In long-cycle tests the service lives of these greases at highest speeds were determined. After the long-cycle tests samples were cut from the bearings‘ raceways and electron microscopy methods were used to determine the structure and composition of the boundary layers. Samples from fresh and used greases were analyzed to determine changes in the greases’ properties caused by the long-term test runs.

Aside from the reference grease no clear correlation between the operating behavior and the grease formulation could be established. The same can be said for the structure and the composition of the boundary layers. However, it could be proofed that for-life grease lubricated hybrid spindle bearings can be operated for considerable times far above the speed ratings given by manufacturers. Furthermore it was discovered that the fundamental correlations between the thickness, the smoothness and the structure of the boundary layers and the failure performance that have been established for steel-steel-rolling contacts in standard bearings are also valid for steel-ceramic-contacts.

The IGF project (17093 N) of the research association DGMK Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e.V., Überseering 40, 22297 Hamburg was funded by the Federal Ministry for Economic Affairs and Energy via the AiF within the framework of the programme for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag.

Christian Brecher, Joachim Mayer, Marcel Fey, Alexander Hassis
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