Fundamental analysis of the transfer lubrication in dry lubricated rolling bearings

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Solid lubricated rolling bearings are used in many technical systems in which conventional lubrications cannot be used due to the critical environment, e.g. vacuum or high temperature. Due to their excellent tribological characteristics, Tungsten disulfide (WS2) and Molybdenum disulfide (MoS2) have proven themselves as dry lubricants. WS2 and MoS2 are already successfully applied as surface coatings and as additives in lubricants for metal and ceramic surfaces under vacuum and high temperature conditions. Newest research indicates that WS2 and MoS2 offer extraordinary potential especially in case of rolling bearings to improve their lifetime, and these solid lubricants can be incorporated into the polymeric rolling bearing cage material and provide an intrinsic lubrication effect. Accordingly, with solid lubricant microparticles a lubricant transfer from the cage to the raceway is achieved, which maintains a tribofilm, that is essentially influencing the lifetime of the system. Previous works have concentrated on the coating of the raceway and the optimization of cage geometry. Then the cage material formulation was focused, which is based on a high performance polymer and lubricant particles. It was found that also the cage composite material exerts strong influence on the bearings' lifetime. Corporate exploratory works of the applicants then investigated each contact partner of the tribosystem in order to gain a more fundamental insight into the transport behavior of lubricants. Initial findings on the tribofilm formation in different zones at the raceway were obtained. This project aims to fundamentally understand and explain the lubricant transport mechanisms within the rolling bearing. Transport of lubricant occurs primarily from its depot in the cage towards the contact area between rolling element and raceway. For this purpose, the applicants systematically extrude compounds with polyamidimide (PAI) matrix that contain various sizes and concentrations of lubricant particles (MoS2, WS2). Test samples for mechanical and tribological characterization, as well as rolling bearing cages will be produced. A thorough characterization of friction coefficient and specific wear rate at varying loading conditions will be performed by instrumented block-on-ring model test rigs. Further information especially for the dynamic contact between ball and cage will be gained by simulation. Individual rolling bearing components will then be investigated by surface analytical methods. These will provide fundamental information on the influence of the cage material to the functionality of the rolling bearing. Especially by the detailed characterization of the chemical composition of the tribologically functional surfaces of cage, rolling element, and raceway it is aimed to better understand the assembly and degradation processes of tribofilms, and to describe the effect of transfer lubrication in solid lubricated rolling bearings.