Beyond starvation: Amplitude-dependent grease replenishment mechanisms in oscillating rolling bearings

Oscillating bearings find applications in various industries, including robotics and wind energy systems. Damage due to starvation can lead to system failures in highly precise positioning mechanisms due to unpredictable torque fluctuations. Moreover, wear damage can trigger secondary failure mechanisms. The occurrence of starvation is dependent on operating parameters, such as oscillation frequency, amplitude, load, and lubricant characteristics. Among these factors, amplitude has been identified as the most critical parameter affecting grease starvation and can even overshadow the effects of other variables. Therefore, investigating the impact of amplitude on lubrication performance is essential for optimizing the lubrication design of oscillating bearings. Hence, this experimental study addresses grease replenishment mechanisms under varying oscillation amplitudes for 6008 deep groove ball bearings with different cage structures. This is complemented by in-situ observations of film thickness formation using an optical elastohydrodynamic lubrication tribometer. Thereby, three distinct lubrication mechanisms are identified. At small amplitudes, lubrication of the contact area is primarily determined by the lubricant supply from the grease side bands. Under medium-amplitude conditions, interaction between the cage and the side bands on the rolling elements substantially enhances lubrication. At large amplitudes, when the rolling track on the rolling elements connects both the inner and outer rings, grease flow between the rings emerges as a dominant mechanism, effectively reducing wear. Understanding the above mechanisms provides a theoretical foundation for the selection of lubricating grease based on amplitude, and the structural optimization of bearings, including cage or raceways.