Prediction and Measurement of Space Lubricant Lifetimes in Angular Contact Bearings

Abstract

The design of spacecraft mechanisms normally involves consideration of lubricant degradation as lubricant life can limit the operational life of machine elements for long-duration space missions. However, accurately estimating lubricant lifetime, even in simple mechanisms, remains a significant challenge, as there is a poor correlation between data extrapolated from basic lubricant life tests and bearing performance. This paper compares two approaches to predicting lubricant lifetime with data from accelerated tests to shed light on this challenge. 

CABARET [1] is a software package which is used by the space industry to predict details of mechanism operation. The authors have used the lubricant lifetime modelling capability of this package to derive a relationship between lubricant life and bearing designs, revealing an inverse fourth power dependence. This level of sensitivity suggests that even small increases in contact pressure result in very rapidly reducing lubricant lifetime. Power loss predictions for these contacts have also been conducted. This has revealed distinct behavioural regimes across different ball complement configurations, and lubricant volumes with a transition in behaviour between low and high lubricant volume. CABARET data for lifetime and torque predictions across the speed ranges 50 rpm to 400 rpm with three different Perfluoropolyether (PFPE) based lubricants Z25, Z60 and 815Z will be presented.

A second predictive model, also based on the Hertzian contact stress, formulated by Palladino et al., [2] has also been implemented. This model was applied to examine lubricant lifetime at equivalent Hertzian pressures and operating conditions as the CABARET analysis and was found to predict substantially shorter lifetimes . The difference between the results from these two well-established models highlights the complexity of translating contact models to full bearing systems and emphasizes the need for validation through component level testing. To investigate this gap and attempt to validate predictions, data from both simulations will be compared to experimental data for the same conditions from laboratory tests with the aim of improving lubricant lifetime prediction in rolling element bearing systems.

REFERENCES

[1] A. Vortselas, "CABARET v.3 user manual," ESR Technology Ltd, Warrington, UK, Tech. Memo. ESA-ESTL-TM-0320, Issue 02, June 2022

[2] M. Palladino, J. Murer, S. Didierjean, and L. Gaillard, “Life prediction of fluid lubricated space bearings: A case study,” in Proc. 14th European Space Mechanisms and Tribology Symp. (ESMATS), Constance, Germany, Sep. 28–30, 2011, ESA SP‑698