Film thickness in grease lubricated cylindrical roller bearings

Cylindrical roller bearings are widely used for their high radial load-carrying capacity. Grease is commonly employed in rolling bearings because of its sealing properties and ease of application. The service life of grease-lubricated cylindrical roller bearings is strongly influenced by lubrication conditions. Film thickness serves as a key parameter for characterizing lubrication performance. Despite its importance, reliable prediction of film thickness in grease-lubricated rolling bearings remains challenging, owing to the complex interaction between grease rheological behavior and bearing dynamics.

Building on existing knowledge of grease lubrication in deep groove ball bearings, this thesis systematically investigates lubrication mechanisms from a single contact to a multiple contact cylindrical roller bearing. At the single contact level, various analytical film thickness equations are first reviewed and applied to examine the evolution of film thickness during the transition from elliptical contact to line contact under fully flooded conditions. The results show that, for identical operating conditions, film thickness increases with increasing contact ellipticity due to reduced side leakage at higher ellipticity. For ellipticity values exceeding $k>7$, both wide elliptical contact and line contact film thickness equations yield comparable predictions. These analytical results are critically assessed using a tapered roller-on-disc optical interferometry (WAM) setup.

The investigation is then extended to starved lubrication conditions in a single contact, with particular emphasis on the influence of contact ellipticity on film thickness decay. Using the van Zoelen film thickness model, it is shown that film thickness decays more rapidly for small ellipticity values, while increasing ellipticity slows the film thickness decay. Experimental measurements performed with the tapered roller-on-disc setup (WAM) using different greases and entrainment speeds are compared with the model predictions. Under mild starvation, the film thickness decay in wide contact is speed-dependent, and the experimentally observed decay is faster than that predicted by the model. In addition, the effect of contact misalignment, specifically roller skew, on film thickness is examined. The results indicate that skew can either accelerate film thickness decay or enhance lubricant replenishment, depending on the availability of lubricant at the sides of the contact track.

Based on the insights obtained at the single contact level, an electrical capacitance measurement method is developed to quantify film thickness in grease-lubricated cylindrical roller bearings. The method is calibrated using base oil and subsequently applied to grease-lubricated bearing. The measurements exhibit fluctuations within a range, which is attributed to bearing dynamic effects. The circumferential distribution of film thickness is found to be nearly uniform in the loaded zone, with a local reduction at the transition from the unloaded zone to the loaded zone. Furthermore, analysis of the circumferential film thickness profile enables indirect identification of bearing misalignment.

Finally, the developed electrical capacitance method is used to investigate the influence of operating conditions and lubricant properties on film thickness in the most heavily loaded point of grease-lubricated cylindrical roller bearings. The effects of entrainment speed, load, grease type, and temperature are systematically analyzed. The results reveal distinct, grease-dependent trends with increasing speed and load, while the degree of starvation is found to be independent of temperature. Two lubrication regimes in film thickness are identified: a semi-fully flooded regime, and a starvation-limited regime, in which film thickness reaches a plateau. Overall, the findings of this thesis demonstrate that film thickness in grease-lubricated cylindrical roller bearings is governed not only by base oil viscosity, but also by grease properties, such as oil bleeding behavior, consistency and mechanical stability.