Lessons Learned  When analyzing root cause of friction anomalies in rotating mechanisms, it is important to consider the  possible effects of electrical discharge. The following recommendations are presented to facilitate this,  based on recent experiences in reaction wheel applications:  1. If the friction anomaly occurs in space, review the timing of the initiation of the friction anomaly against the local space weather. Coronal mass ejections, geomagnetic storms, and specifically the direct observation of aurora may indicate a correlation with space charging events. 2. If the friction anomaly occurs in the lab, review the timing of the anomaly relative to motor current changes, or specifically test equipment failures or grounding circuit changes or transients. 3. Measure the shaft voltage and bearing electrical resistance during various operating conditions. This can be easily done with a brush applied to the shaft. Several companies offer pickups specifically designed to measure rotating shaft voltage to detect electrical discharge. 4. Measure the electrical properties of the lubricant and consider how they may evolve over time. Resistivity greater than 2.5 G Ω-m (conductivity less than 400 pS/m) has been considered to be a possible danger zone for electrical discharge. This is more likely with synthetic or refined lubricants. 5. Directly test the sensitivity of a sample mechanism to electrical discharge by applying an alternating voltage directly across the bearing. This can be done with a brush used to measure shaft voltage. The rate of change of the voltage is an important variable, as slow voltage changes will bleed off. 6. When inspecting bearings that may have been affected by electrical discharge, look for the following physical features which may be associated with electrical discharge: a. Pitting or small craters from electrical discharge, usually between 5 to 10 microns. b. Countless microscopic pits on the balls or races known as “frosting” of the surfaces. c. Wear grooves, with depths ranging up to 1 micron or more. d. Blackened or discolored lubricant and staining of the retainers at the expected rubbing points. e. Repetitive micro-denting of the ball track on the race, which may be a result of a hard electrical discharge crater formed on a ball. Conclusions  Analysis of anomalous friction in ball bearings in laboratory and space environments should include  electrical discharge as a possible cause. Potential sources of electrical discharge, plausible explanations  for response of various friction signatures to electrical discharge and supporting observations from the  author’s experience have been presented for consideration.  References  1. Busse, D., J. Erdman, et. al. (1996) System Electrical Parameters and Their Effects on Bearing Currents, IEEE Transactions on Industry Applications, Volume 32 , Issue 6, p 71-85. 2. Bialke, W. and Hansell, E. (2017), A Newly Discovered Branch of the Fault Tree Explaining Systemic Reaction Wheel Failures and Anomalies, 17th European Space Mechanisms and Tribology Symposium 3. Bialke, W. and Hansell, E. Inventors (2017), USPTO Patent Applications 15668663 titled Bearing Assemblies with Electrodynamically Matched Races  and 15668669 titled Electrodynamically Finished Plain Bearings . United Technologies Corporation, Owner. 4. Hacker, J.M., J. Ying, P Lai (2015) Reaction Wheel Friction Data Processing Methodology and OnOrbit Experience, Journal of Astronautical Science, 62:254 5. Boesiger, E.A. and Warner M.H. (1991) Spin Bearing Retainer Design Optimization, NASA/ CP3113, 25th Aerospace Mechanisms Symposium 6. Bialke, W. (1998) Experiences with Differentiating Ball Bearing Retainer Instabilities, NASA/CP1998-207191, 32nd Aerospace Mechanisms Symposiu m NASA/CP—2018-219887 68