Effect of Complex Bio-Thickener Concentration on Thermal, Rheological, and Tribological Properties of a Novel Bio-Based Grease for Rolling Element Bearing Applications

The recent motivation for sustainable lubrication has driven the development and advances of bio-based and potentially environmentally favorable alternatives to petroleum-based greases. Yet, their industrial adoption is largely hindered by the thickener weak network or inconsistency leading to grease unacceptable degradation under applied loads and operating temperatures in rotating machinery. This study investigates a novel grease formulated from 80% palm oil and a 20% complex thickener system from carnauba wax (CW) and glycerol monostearate (GMS). The effect of thickener composition on grease performance was investigated by testing their X-ray diffraction (XRD) spectra, Fourier transform infrared (FTIR) spectra, penetration level, oil separation percentage, viscosity, thermal properties, and tribological behavior. GMS-rich blends achieved up to 70% lower friction than lithium grease. However, they showed high wear rates and excessive oil separation ranging from 0.07% at room temperature for the 20% GMS blend to above 9% at 40 °C for softer formulations. The blend of 15% CW + 5% GMS showed only 0.113% and 3.145% oil bleed at room temperature and at 40 °C, respectively, with suitable consistency (NLGI 3) and acceptable dynamic viscosity rates. Regarding thermal behavior, CW-based samples revealed an enhanced melting point compared to GMS. For validation, investigations were conducted on rolling element bearings on a customized test setup operating at 1400 rpm under selected radial loads. The results demonstrate that CW/GMS bio-thickeners achieved lower vibration levels compared to the GMS thickener, approaching the performance of lithium grease.