During recent verification testing of the electrical discharge hypothesis, just 28 minutes after applying  ±6 volts across a bearing, one RWA bearing exhibited a 20 mN-m friction spike and produced a rare  audible rumbling sound. This large friction increase (10x the normal bearing friction) at the same time as  the noise is characteristic of severe retainer instability. The author had never observed cage instability in  that size RWA bearing prior to that event. This test result implies the correlation between electrical  discharge and retainer instability.  Another point regarding the possible connection between electrical  discharge and retainer instability is that in addition to roughening  the balls, electrical discharge can also result in instantaneous local  raised defects, which could also result in a cyclical radial excitation  of the retainer at the ball rotation frequency, as depicted in Figure  15. This vibration excitation would seem to be more destabilizing than static increases in frictional coefficients. The intent here is not to try to oversimplify something as complex  as cage instability, but to include electrical discharge as a variable  which should be considered as a possible factor during  investigation of retainer instability.   Stiction (Breakaway Torque) Increase  Breakaway torque and rolling friction are two performance parameters typically measured on RWAs  during acceptance testing. Breakaway torque is the minimum amount of torque required to start the wheel  from rest, and rolling friction is the minimum amount of torque required to keep the wheel turning. They  are intended to be indicative of the health of the bearing, and their magnitudes can affect spacecraft  attitude control during zero crossings if they are excessive.  Anomalies of breakaway torque occurred for a new high torque RWA design. Previous RWAs with the  same bearings and lubricant had expected values of breakaway torque. When a change to a heritage  design was made to significantly increase the current in the motor, breakaway torque values skyrocketed  during the initial phases of acceptance testing. A root cause analysis initially concluded that it was due to  a variable in the bearing lubrication process, but corrective actions based on this hypothesis were  ineffective. The breakaway torque improved to acceptable limits with accumulated operating time, but the  mystery of why the torque went up compared to the heritage product was not resolved. Increase of the  motor torque capability inevitably increases the quiescent motor-induced voltage across the bearings. In  retrospect, it is believed that the motor torque increase affected the properties of motor-induced electrical  discharge during extended low speed operation encountered just prior to the breakaway torque test.   Preventative Measures  The likelihood of electrical discharge can be reduced using bearing design features intended to prevent  the chain of events presented. The use of ceramic balls blocks the electrical discharge through the  bearings via the isolation provided by ceramics. It is noted in Reference 2 that 72 ITHACO RWAS with  hybrid ceramic bearings have been flown with no known anomalous friction events. However, once the  electrical path from reaction wheel housing to flywheel through the bearings has been eliminated, another  means for providing the flywheel an electrical path must be provided. It must be considered that if the  flywheel were perfectly isolated, very large potentials could develop possibly leading to very large  discharges, which could damage spacecraft electronics. Alternately, with steel bearings, a grounding  brush can be used to bleed off any charge across the lubricant film, or conductive lubricant additives can  reduce the resistance across the bearing. All of these measures are used routinely in terrestrial  applications to mitigate electrical discharge in bearings. Figure 15. Top View, ball pocket  depiction of effect of raised crater  on cyclical radial excitation of  bearing retainer NASA/CP—2018-219887 67