Experimental Observation of the Lubricant Replenishment Using Trust-Ball-Bearing-Structured Device

The lubricant is displaced to both sides of the rolling track under over rolling of the rolling element. Basically, the displaced lubricant will replenish to the rolling track to maintain an available amount and lubrication safety. However, due to successive motion of the rolling elements, the replenishment interval is too short for efficient replenishment of the lubricant, resulting in insufficient supply and subsequent kinematic starvation. To investigate both starvation and replenishment mechanisms, a simple ball-on-disc configuration is commonly employed. By understanding these mechanisms, measures are developed to regulate replenishment and enhance lubrication efficiency. Even when operating under starvation conditions, the rolling bearing demonstrates a prolonged service life compared to the duration of model tests, suggesting that certain inherent features of the bearing contribute to lubricant replenishment. This is primarily due to the oversimplification of the conventional ball-on-disc single contact configuration, which overlooks crucial bearing features such as contact geometry, cage design, and bearing kinematics and dynamics. As a result, it becomes challenging to extrapolate the obtained results to complete bearing performance.

Alternatively, a thrust rolling bearing test device with a comprehensive bearing structure is employed, wherein the upper race of the rolling bearing can be directly replaced by the transparent disc. Using this test device, experiments can be conducted to explore the various factors that affect replenishment such as replenishment time, speed, lubricant supply amount, and lubricant viscosity. In this study, the influence of replenishment interval on the lubricant state was quantitively observed by regulating the numbers of the rolling elements and arrangement. The findings demonstrated that an increase in the replenishment interval leaded to improvements in both the supply of inlet lubricant and lubrication states, resulting in a reduction in friction coefficient. The investigation revealed the existence of an optimal lubricant supply amount that minimized the friction coefficient while maintaining an acceptable film thickness for effective separation of the bounding surfaces. Additionally, the presence of a cage significantly contributed to the replenishment of lubricant on the ball surface. This was primarily due to the fact that as the lubricant passed through the clearance between the ball and cage, it caused a squeezing effect on the lubricant at sidebands, redirecting it back onto the rolling track. The replenishment mechanisms under mechanical action mitigated the impact of lubricant viscosity on replenishment, resulting in a positive correlation between higher viscosity and increased film thickness. From the investigations, it was evident that the inherent bearing features had potential contributions to the lubricant replenishment and improvement to the film formation. This study was beneficial to understand the negative and positive factors that impact the lubricant replenishment, which was helpful to regulate lubrication state through bearing design.