Experimental Study on the Frictional Resistance between Rolling Element and Cage of Grease-Lubricated Bearings

The cage is a crucial component of the bearing. It not only evenly separates the rolling elements but also influences the redistribution of lubricant within the bearing. The sliding contact between the cage and the rolling element leads to energy loss due to friction. Friction and wear are inevitable; however, friction can be minimized by altering the contact conditions between the rolling elements and the cage. Therefore, this study aimed to experimentally investigate the sliding friction between the rolling element and the cage under various spatial positions and structural variables. To achieve this goal, a cage simulation unit with free movement and displacement had been designed and installed on a ball-on-disc testing machine. This setup allows for the measurement of the cage in different spatial positions, and the cage simulation unit could accommodate various types of cages for experimentation, providing a range of data references for the interaction between the rolling element and the cage. The test results indicated that the design of the cage has a significant impact on the performance of the grease-lubricated bearing. It had been determined that the rolling element speed, cage position, cage height, and the clearance between the rolling element and the pocket were the primary factors affecting the sliding friction of the ball cage. By varying the rolling element speed, it was observed that the speed impacts the viscosity and thixotropy of the grease within the cage pockets. As the speed increased, the thixotropic effect of the grease became more pronounced, resulting in greater changes in yield stress and corresponding shear stress, which in turn increased the friction. Additionally, altering the position of the cage revealed substantial differences in friction across various spatial positions. An excessively high or low relative position of the cage could cause it to contact the rolling element closely, leading to a sudden increase in friction. However, based on the bearing’s orientation, there exists an optimal relative position for the cage that created a converging gap in the pocket, which reduced the friction. By altering the height of the cage, it had been determined that the cage height influences the contact area of the pocket surface. The friction was the integral of the shear stress over the area of the friction pair, indicating that the friction was proportional to the height of the cage. And adjusting the gap between the rolling element and the cage pocket directly affected the flow and distribution of grease within the pocket. When the gap is small, there was less grease stored in the pocket, leading to an increased likelihood of shear friction between the rolling element and the cage, which in turn raises the friction. Conversely, a larger gap allowed more grease to adhere to the rolling element, resulting in insufficient extrusion shear and a reduction in friction. However, if the clearance became excessively large, the rolling element and cage may not make contact. Therefore, it was essential for the rolling element and cage to operate within a specific range of clearance to effectively manage friction changes. In summary, the research presented in this paper establishes a foundation for reducing internal friction in bearings and optimizing cage design, which was crucial for understanding the interactions between rolling elements and the cage in bearing systems.