In high-speed applications, such as machine tool main spindles, grease-lubricated hybrid spindle bearings can now achieve speed characteristics that until a few years ago were only possible with oil-air lubricated bearings. While dispensing with compressed air as the pumping medium brings ecological and economic advantages, the central disadvantage of grease lubrication is the disproportionate reduction in grease operating life with increasing speed. This problem can basically be solved by using grease relubrication systems. However, the greases available today with the highest speed suitability are only suitable for use in grease relubrication systems to a limited extent. At the same time, it has not been fully researched how the grease supply to the bearing can be optimally designed.
The objective of the research project was to extend the application range of grease-lubricated hybrid spindle bearings by using a grease relubrication system. To achieve this goal, an improved grease relubrication system was developed and the effect of the retention time of the grease in the relubrication system was investigated.
In the first step of the project, three greases were subjected to the loads of a grease relubrication system for up to six months using a test rig. The test greases were found to be partially severely deoiled, which can lead to blockage of the lubrication lines. Damage to the grease itself could not be detected. In tests on a rolling bearing test rig, no strong influence on the short-term speed suitability could be detected, even though some tests showed signs of an imminent failure.
De-oiling as a result of the loads caused by the relubrication system was reduced by optimizing a lubricating grease. The optimization measures included a shortening of the thickener chain length and the addition of additives. Damage caused by the relubrication system could not be proven with this grease either.
The experiments to develop the lubrication strategy showed that the introduction of grease into a bearing at 24,000 rpm is possible. It further became clear that relubrication in open rolling bearings as well as high speeds must replace worn grease rather than replace aged grease. It also became apparent that there is a risk of aged grease being conveyed back into the rolling contact by an axial grease supply, so that a radial supply into the bearing is preferable.
With the lubrication strategy developed and the optimized grease, it was possible to achieve a significantly longer operating life than with the use of lifetime lubrication.