Bearing Fatigue Life Tests in Advanced Base Oil and Grease for Space Applications

Abstract

Two synthetic base oils (815Z and 2001A) and two greases (601EF and R2000) used for space applications have been studied at ground level. Rheological tests were performed in order to characterize the behavior of each of the base oils versus the pressure and the temperature. Next, the effect of base oils and greases on ball bearing fatigue life was carried out using thrust ball bearings. The results of L 10 life tests showed a bearing life order from the highest to the lowest of grease 601EF (blended with base oil 815Z), base oil 2001A, grease R2000 (blended with the base oil 2001A) and, the lowest, the base oil 815Z. The general tendency of the base oils and the greases shows that the bearing life increases with the EHL film parameter. However, the anticipated beneficial effect of an apparently high film parameter for the base oil 815Z was not seen due to permanent viscosity loss in the EHL contact. The results showed that the viscosity of the base oil 815Z, which contains the acetal group (-OCF 2 O-), was decreased by mechanical shear at the high shear rate in the EHL conjunction and the EHL oil film was thinner than expected. Hydrogen fluoride is released with the decomposition of the acetal group. As a result, the permanent viscosity loss at high Hertzian pressure and the hydrogen fluoride generation shortened bearing life with the 815Z base oil. In the case of grease 601EF with base oil 815Z, the permanent viscosity loss did not occur in ball bearings and the bearing life is extended.