277 Recovery and Operational Best Practices for Reaction Wheel Bearings  

Michael J. Dube*, Jeff Fisher**, Stuart Lo ewenthal+ and Peter Ward++ 

Abstract  

Left unaddressed, t he observation of sustained high torque signatures in bearing applications ultimately  leads to failure.  This paper describes v acuum bearing testing of 40 motors fitted with R4 angular contact  bearings  with 52% and 54% race curvatures , in all steel and hybrid configurations,  runni ng in different  lubrication regimes. Testing was intended to identify  any differences in bearing performance attributed to  operation and configuration,  and evaluate approaches  to recovering bearings  in distress  with the goal of   restoring bearing performance. This paper will present data and describe approaches employed, including  resting and heating, that have in some cases resulted in successful bearing recovery, avoiding the onset  of hard failure and restoring performance for v arying periods of time.  Data addressing p erformance  differences between motors equipped with all steel R4 bearings  with 52% and 54% curvatures , as well as,  performance differences  between all steel bearings  and their hybrid counterparts will be presented.  

Introduction  

Over the past decade, t here have been several reaction wheel assembly (RWA) in -service bearing related  failures. The Kepler s pacecraft, launched in March 2009, experienced failures  of two of its RWAs in 2012  and 2013  [1]. The Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite  suffered an  RWA failure in 2007. The Dawn space probe experienced failures of one of its reaction wheel s  in 2010 and another in 2012, and w hen a third reaction wheel stopped working in 2017, Dawn resorted to  its hydrazine t hrusters for attitude control  [2]. Similarly, the Far Ultraviolet Spectroscopic Explorer ( FUSE )  spacecraft experienced three bearing- related RWA failures  [3]. In the commercial sector, a  second  generation Globalstar satellite failed  to spin up one of its RWAs, despite high torque commands . This event  was attributed to a stuck bearing,  and shortly after regaining control with the three remaining RWAs , a  second RWA exhibiting a similar torque signature failed [4]. 

Efforts  to maintain  reaction wheel performance and  extended life have included attempts at  bearing  recovery employing techniques such as rest, increasing temperature, and reversing direction.  These  actions have had limited success with  reliance on the remaining reaction wheels as the only recourse. This  approach suffers from t he potential for similar life- threat ening bearing related issues. As a resul t, the NASA  Engineering and Safety Center initiated a multi -year assessment that included an extensive bearing test  program to evaluate the failure modes and effectiveness of potential on- orbit recovery techniques for R 4  bearings experiencing high torque arising from cage instabilities and lubricant depletion  leading to failure.  This investigation is one of the most comprehensive efforts of its kind with 40 test motors fitted with R4  angular contact duplex bearings in all stainless steel and hybrid (ceramic balls) configurations, tested in  hard vacuum at 3 different test speeds and 2 different temperatures. Motors incorporating R4 bearings with  52% and 54% race curvatures were include d because there is  anecdotal  evidence that tight ball -race  curvatures  can lead to early failure,  a claim  that to the best of our understanding has never been  documented. In addition, the testing performed in this  investigation and its associated findings are of value  to designers of bearings for scanners, gimbals, and other rotary spacecraft actuators.  

* NASA Langley Research Center, Hampton, VA   ** Fisher Aerospace, Sunnyvale, CA   + Lockheed Martin Space (retired) , Sunnyvale, CA   ++ The Aerospace Corporation, El Segundo, CA  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020