Grease Degradation in Critical Helicopter Drivetrain Bearings

An investigation of critical aviation bearings lubricated with MIL-PRF-81322 grease was conducted to derive an understanding of the mechanisms of grease degradation and the loss of lubrication over time. Chemical analysis was performed on grease samples from fielded bearings and compared to fresh grease and samples taken from bearings run for extended times in a laboratory environment. Size exclusion chromatography and Fourier transform infrared spectroscopy were used to investigate the condition of the grease, and evidence of additive depletion, oil evaporation, and thickener degradation were seen, consistent with results reported by other authors. Given the relatively light loading conditions experienced by the test bearings, they were able to continue operating at high temperature despite having most of the original oil depleted from the grease.