Performance Prediction of Rolling Bearing Greases in Robotic Applications

Today, the friction and thus also the operating temperature of grease-lubricated bearings can only be calculated using part-empirical approaches. The reasons for this are on the one hand the starvation in the rolling contact at high speeds and on the other hand the losses in the grease structure outside the actual rolling contacts, especially at low speeds and start-up/reversal processes. There are no physically based models for the description of these influences on the friction torque yet. The aim of the research work is to investigate the processes outside the rolling contacts which lead to increased losses in grease lubrication and can thus disqualify the greases for applications with frequent starting and reversing processes, to model them, and make them calcuable. In the long term, this should qualify a statement about the suitability and performance of the lubricating greases in the application of robot technology (frequent starting and reversing processes). The corresponding models are to be verified experimentally in several stages.

Based on previous experimental and simulative work, physically founded models are to be developed that enable an estimation of the loss and heating of grease-lubricated rolling bearings. For this purpose, a close interlocking of CFD simulation, laboratory measurements and experimental investigations on model and component test rigs is planned. For this purpose, measurements on model systems (e.g. cone-disc rheometer) as well as on model systems that may have to be newly developed are used. These help in the formulation, verification and implementation of suitable rheological models for greases. Furthermore, component tests on rolling bearings are to be carried out, in which the losses and steady-state temperatures are measured for different operating conditions and grease filling quantities, and tests are to be carried out on a model contact (rolling element & ring), which allow the grease distribution to be determined. Furthermore, fluorescence-optical observations of grease distribution and movement are carried out on a single bearing. The results of all these experiments will be used to expand and verify the developed CFD model. The models are to be used to identify the properties with which the losses or dominant factors in the grease can be described. Finally, a method is to be developed to be able to draw conclusions from the rheological properties about the flexing work and the resulting temperature increase and breakaway torque. Based on this, a procedure will be developed which will allow a statement to be made about the suitability of a grease for use in robot technology or under complex operating conditions (frequent stoppages, starting, reversing).

Cooperating Research Association: FVA Research Association Drive Technology