Lubricant design for oscillating rolling bearings: Greases, ionic liquids, and friction torque

Oscillating rolling bearings are susceptible to accelerated raceway damage triggered by a lack of proper lubricant retention and replenishment within the ball-raceway contacts. A low-consistency grease with high oil release from the thickener matrix is desirable for wear attenuation under these challenging operating conditions. However, these physical grease parameters cannot be taken to extreme values in view of other practical considerations. To address this freedom-of-design limitation, greases formulated with added ionic liquids are experimentally probed at the component level under oscillating bearing conditions for the first time. An in-house-built frameless motor test rig allows for precise control of the oscillations of the angular contact bearings and the monitoring of frictional torque. Our results show that ionic liquids, when used as grease additives, can delay the onset of starvation effects and reduce the associated increase in friction torque in harsh operating conditions (high oscillating frequency); while yielding a narrower range of torque amplitude variations in mild conditions (low oscillating frequency). These improvements hinge on the chemical nature of the ionic liquid. By changing only the anion structure, drastic differences in bearing frictional torque are observed. Surface analysis shows that magnetite and hematite, formed in damaged contact tracks on the bearing raceways, make the affected surfaces softer than the bearing steel. Our findings indicate that the use of ionic liquids constitutes a promising pathway for the development of greases for oscillating bearing applications.