Influence of transfer- and transformation processes on service life of dry lubricated ball bearings

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Solid- or dry lubricants are applied in a number of technical systems, where conventional lubricants, such as oil or grease, could not be used because of environmental conditions (i.e. vacuum, high temperature, hygiene regulations). Typical applications are fluid-free lubricated rolling bearings. For their function, a sufficient lubricant supply over the complete service life is indispensable. With the help of material-specific optimized rolling bearing cages, consisting of a polymer-compound with embedded dry lubricant, it is possible to improve service life significantly in comparison to bearings with conventional MoS2-coated rings. The consumption of dry lubricant during use is essential for the durability. Models have been developed in the past to calculate the duration of service life. However, for the real technical design of dry lubricated ball bearings they approach their limits. This is the case in particular with regard to the physical modelling and the systematic description of dry lubricant transfer. So far, therefore, it is not possible to predict reliably the extension of the service life due to transfer - lubrication. The project focuses on an improved service life model to achieve a deeper knowledge of the complex physical and technological processes involved in dry lubricated bearing systems. Modern methods of surface- and thin film analysis, such as electron microscopy, electron-spectroscopic and mass-spectrometric techniques will be the main tools to reach this objective. It enables precise and quantitative analysis of tribologically stressed bearing surfaces providing the necessary empirical input data for an advanced service life model. Among others, the following quantities are measured with respect to each element of the lubricant materials involved: Friction coefficient, friction energy-wear-factor and transfer – factor. For this purpose tribological tests will be conducted in vacuum using a four-bearing test bench with different rotational speeds and loads. Multi-body system models for solid lubricated bearings will be used for the analysis of the mechanical load spectrum in the bearings. Besides the material-transfer, transformation processes of the dry lubricant due to tribological induced chemical reactions will be taken into consideration. Furthermore, new approaches will be pursued to understand the influence of load collectives on the still empirically determined input values.