The main purposes of oil/grease lubrication in rolling element bearings are to reduce friction and to resist wear. To basically understand the lubrication mechanisms of rolling bearings, the contacts between rolling elements and outer or inner ring were reduced to a ball-on-disc (BOD) configuration, which is widely used as a simulation device. With this classical configuration, the lubrication features of concentrated contacts were described by measuring film thickness and friction coefficient. However, in absence of real contact geometry, the lubrication behaviors of practical rolling bearings cannot be properly reflected. For instance, the micro gaps surrounding the contact zone between BOD configuration and real contacts were different, resulting in a different capillary force. Thus, the local lubricant replenishment and oil reservoirs were different, which consequently affected the lubrication regime. This became more pronounced under a restricted oil supply. In addition, the direction of centrifugal force at the high speed of real bearing was also different from the situations of BOD configuration. That meant in BOD cases, the lubricant was thrown away from the flat disc surface, whilst the lubricant adhered to the outer ring surface of a real bearing. Unfortunately, studies associated with these effects were rarely conducted. Therefore, it was necessary to develop a ball-on-ring (BOR) configuration to further simulate the contacts of rolling elements and the outer ring of the real bearing, by which both effects of contact geometry and centrifugal force can be introduced. The main purpose of this study was to compare the lubrication behaviors between two configurations of BOD and BOR. To implement it, optical interferometry was employed to measure film thickness and to reconstruct the film profile under single lubricant charge conditions. The lubrication behaviors determined by the contact geometrical parameters were analyzed.
The results showed that at low speeds the lubrication behaviors between BOD and BOR contacts were quite different. It can be apparently recognized from the interferograms that the configurations of BOD and BOR produced circular contact and elliptical contact, respectively. The film thickness in BOD contacts was higher than that of in BOR contacts. The hydrodynamic effect induced by inlet contact geometry contributed to the BOD film formation. On the other hand, the side leakage induced by a higher-pressure gradient suppressed the BOR film formation. That meant in the present studies a larger ellipse ratio contributed to a higher film thickness. It was also found that the onset of starvation was earlier in BOD contacts. At high speeds, the inlet starvation was more prone to take place for BOD contacts, while starvation did not occur for BOR contacts in the whole speed scope. Moreover, for BOD contacts, the critical speed corresponding to the onset of starvation was lower for thicker PAO20 than that of PAO6. There was also a critical entrainment speed beyond which the film thickness in BOD contacts became small than that of BOR contacts mainly due to the occurrence of starvation. The centrifugal forces for the two types of contacts were responsible for the observation. In the case of BOD contact, the centrifugal force aggravated the lubricant loss from the disc surface, resulting in progressive starvation. But the viscous force suppressed the lubricant loss to some extent, thus the occurrence of starvation for PAO20 was relatively later. However, the centrifugal force for the BOR contacts utilized the lubricant supply and hence improved the lubrication. The influence of viscosity on the film formation of the two types of contacts was also analyzed, at low speeds, high viscosity lubricant tended to cause inlet starvation for BOR contact. At high speeds, high viscosity lubricant delayed the occurrence of starvation for BOD contact.
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