The migration and distribution of lubricant oil in a rolling bearing strongly affect the elastohydrodynamic lubrication performance between the balls and rings. However, oil re-lubrication is highly dependent on the bearing design, which is different from the ball-on-disc model. This study directly observed the distribution of the lubricant film in a custom-made model-bearing rig, with an outer ring replaced by a glass ring to allow full optical access. The influence of the cage type and surface properties were presented. The physical origin of the re-lubrication mechanism, including capillary flow and mechanical redistribution, was discussed.

It is of great importance to develop an in-depth understanding of interfacial flow around a lubricated rolling point contact region (RPCR). Consideration of the flow patterns around an RPCR will be important for the lubrication, cooling, and cleaning of machine parts, such as rolling bearings and gearboxes. In this study, an experiment using laser-induced fluorescence and a simulation using computational fluid dynamics of interfacial flow around an oil-lubricated ball-on-disk RPCR are presented. The results show good agreement with each other, and the flow patterns are clearly classified. The forming mechanisms are analyzed in terms of force competition caused by inertia, viscosity, pressure gradient, and air–oil surface tension. Quantitative criteria are proposed to evaluate the behaviors at the air–oil interface and the transition of flow patterns. Analyses have shown that the competition between these forces drives the motion of the air–oil interface, and the formation of flow patterns can be considered a self-adjusting process for the air–oil interface toward the equilibrium positions of the forces. High surface tension is beneficial for maintaining interface stability and can prevent the meniscus from rupturing at an outlet and concaving at an inlet. High capillary numbers may increase the risk for outlet meniscus rupture and the degree of concavity of the inlet meniscus.

It is of great relevance to maintain an adequate and continuous lubricant oil supply to each contact region in successive rolling point contact systems, such as rolling bearings, rolling guides, and ball screws. As a case study, we implemented laser-induced fluorescence observations of the dyed oil in a ball bearing. It is found that there exists an oil layer evolution as the rotating speed increases, especially when it reaches some critical levels. The oil layer will break up and migrate out of the rolling contact traces, while the oil distribution changes. In addition, a series of air–oil two-phase flow simulations based on computational fluid dynamics models and the volume-of-fluid method are run to reveal the forming mechanism. Flow maps are of construction with the flow patterns and the oil distribution. The critical points of the flow pattern transitions and the oil distribution changes are highly coincident with the contour lines of Ca. The force extraction shows that the pressure gradient force caused by Sommerfeld pressures has different directions at the outlet and inlet zones. They tend to attract the outlet meniscus into the low-pressure area while pushing the inlet meniscus out of the high-pressure area. As a result, the oil layer will leave and migrate out of the rolling contact traces. The capillary force will always work against the pressure gradient force. Their competition and equilibrium may determine the flow pattern transitions, the oil distribution changes, and their close relationships with Ca.

Thin liquid films on free surfaces or inside restricted spaces are widely encountered and can have significant effects in heat transfer, coating, biofilm growth, and lubrication. The thickness and distribution of these thin liquid films are important determinants of performance, but cannot easily be measured accurately. This paper presents a methodological study on determining the thickness of thin liquid films by means of laser-induced fluorescence. A model of fluorescence imaging and measurement is established based on the fluorescence energy transmission. A novel pixel-dependent laser-induced fluorescence method is then proposed and compared with commonly used methods using experimental test results. The accuracy and precision of the measurements are discussed in detail. It is found that the measurement performance is greatly influenced by the nonlinearity, heterogeneity, and random noise of the optical system and imaging technique. The proposed pixel-dependent laser-induced fluorescence method effectively eliminates or ameliorates the effects of these factors and enhances the measuring accuracy.