On grease redistribution, inlet replenishment, starvation, and cage effects in rolling contacts: Insights from LED-induced fluorescence

Lubricant supply at the inlet of rolling contacts is critical in elastohydrodynamic lubrication (EHL), as it governs starvation and film thickness. In grease-lubricated bearings, replenishment is controlled by grease properties and bearing components. However, due to the lack of direct visualization, the starvation process and its mechanisms remain unclear. Especially the influence of the cage in lubricant transport and redistribution and its role in regulating inlet grease supply are still not fully understood. In this study, the LED-induced fluorescence (LED-IF) technique was developed to analyze grease distribution at the inlet in a ball-on-disc setup, enabling direct observation of starvation and its mechanisms. The residual grease layer height H_a was quantified and correlated with the dimensionless meniscus length m to construct a lubrication supply map (H_a–m), which was further used to analyze the effect of cage clearance on grease redistribution and replenishment. Results show that grease is mainly supplied via a stringing mechanism. Over time, grease becomes fragmented, preventing the reformation of strings, thus limiting replenishment and causing starvation. EHL film thickness depends not only on the residual grease layer height but also on its spatial distribution, whereby the H_a–m distributions show reasonable consistency with film thickness results. Cage clearance substantially affects the residual grease layer height by altering grease redistribution. An appropriate clearance enhances grease entrainment, while the optimal value depends on operating conditions and grease properties. Excessively large and small clearances reduce replenishment. Therefore, lubrication is governed by the combined effects of entrainment speed, grease type, and residual grease layer height.