Abstract
The service life of rolling bearings is significantly affected by the film formation in elastohydrodynamic (EHD) contacts, which depends on the operating conditions, like rotational speed or temperature. In grease lubricated EHD contacts, the film formation is determined by the grease consistency and composition, i.e., thickener and base oil type as well as properties of the bleed oil, which is released from the grease during operation. Thus, the film formation of grease lubricated contacts as compared to base oil lubricated contacts can be different. With increasing rolling speed, the film thickness of oil lubricated contacts usually grows. However, in case of grease lubricated contacts, which are not fully flooded, the film thickness remains constant or even decreases with further increasing rotational speed. This effect is referred to as starvation. Since the onset of starvation depends on the grease composition, the film formation of two different grease compositions is investigated in this study. The film thickness measurements are performed on a ball-on-disc tribometer for each grease, as well as the corresponding bleed and pure base oils. Thereby, the characteristic rotational speed leading to the onset of starvation has been identified in dependence of the grease composition and the differences in the lubricating film formation of base oil, bleed oil, and grease lubricated EHD contacts have been discussed. The investigations should help to establish an advanced understanding of the physical mechanisms leading to the onset of starvation to encourage future work with focus on a method to predict the film formation in grease lubricated EHD contacts.
Highlights
The service life of rolling bearings is significantly affected by the lubricant and the lubricating film formation in elastohydrodynamic (EHD) contacts
The bleeding tests for the investigated greases show that the oil separation from PAO-lithium complex (Li)-100 grease with 4.14 wt % is higher in comparison to the polyalkylene glycol (PAG)-Li-140 grease with 2.20 wt %
The results of the film thickness measurement of the PAO-Li-100 grease, its base oil PAO and bleed oil are illustrated in the following
Summary
The service life of rolling bearings is significantly affected by the lubricant and the lubricating film formation in elastohydrodynamic (EHD) contacts. The film formation is affected by parameters, such as pressure and temperature, but significantly by the rolling speed. The physical mechanisms affecting the lubricating film formation in grease lubricated rolling bearings are not satisfactorily clarified [1]. Recent work by several authors like Cousseau et al [2,3], Goncalves et al [4,5], De Laurentis et al [6,7], and Cen et al [8,9] compare the film formation under fully flooded lubrication between grease and oil lubricated contacts. The film thickness measurements were performed on a ball-on-disc tribometer under variation of rolling speed. To achieve a fully flooded grease lubrication during the grease measurements, Lubricants 2018, 6, 32; doi:10.3390/lubricants6020032 www.mdpi.com/journal/lubricants
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