A Discussion of Friction Anomaly Signatures in   Response to Electrical Discharge in Ball Bearings  William Bialke*   Abstract  Friction anomalies and failures in spaceflight ball bearings have commonly been attributed to lubricant  dynamics, but can also be a result of electrical discharge. Plausible scenarios of the root cause of various  bearing friction signatures in response to microscopic changes in bearing surface finishes resulting from  electrical discharge are discussed, along with supporting anecdotes, test results and observations in the  laboratory and in space. The objective of the discussion is to inform and possibly shed a new light on  previously unexplained or unverified bearing anomalies and failures.  Introduction  Electrical Discharge in bearings is a well established source of bearing degradation and failures in  terrestrial applications as discussed in Reference 1, resulting in the growing practice of grounding shafts  or using ceramic balls in many industrial motors. Only recently has electrical discharge been associated  with spaceflight bearing anomalies and failures as discussed in Reference 2, which concluded that there  is a remarkable correlation between on-orbit Reaction Wheel Assembly (RWA) bearing friction events and  large geomagnetic storms, strongly supporting electrical discharge as a likely root cause.   Conventional lore has frequently resulted in assigning blame for friction anomalies to the Goldilocks  syndrome: either too much lubricant or lubricant starvation. We examine the possible connection of a  variety of sources of electrical discharge with examples of ball bearing friction signatures specifically  observed in RWAs. Mysterious friction anomalies discussed herein occurred frequently over a lifetime of  trending on-orbit reaction wheel friction and testing hundreds of reaction wheels. The signatures made  the most logical sense to the author when electrical discharge was recently considered as a variable.   The scope of this paper is not to present scientifically proven models or conclusions, but only to open the  discussion of anomalous friction signatures to include the effects of electrical discharge and introduce this  possible root cause into the paradigms of aerospace mechanism bearings and tribology.  Background  RWA bearing friction anomalies and failures have plagued many missions since the dawn of the space  age. The ITHACO RWA product line, purchased by United Technologies Corporation as part of the  acquisition of Goodrich in 2012, had failures on a number of spacecraft over the years, starting with two  RWAs on the Far Ultraviolet Spectroscopic Explorer (FUSE) Spacecraft in November of 2001, and  eventually the failure of all four of the RWAs on that spacecraft. A series of failure investigations  performed for over a decade ultimately failed to successfully determine root cause of the bearing failures.  United Technologies, after their acquisition of Goodrich Corporation, which included the ITHACO RWA  product line, embarked on a Relentless Root Cause Analysis (RRCA) to resolve the RWA failures and  anomalies. A rigorous test program accompanied detailed modeling and analysis of every aspect of the  RWA design. A very large number of initially credible items were positively eliminated from the fault tree  by test, inspection or analysis, including many previously eliminated items, which were revisited and  exonerated with fresh objective evidence.  * Reaction Wheel Consultant, Lochridge Farm LLC, Trumansburg, NY Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018 NASA/CP—2018-219887 55