Retainer (Cage) Instability  Retainer instability is a severe type of bearing friction anomaly, which can ultimately lead to bearing  failure if the retainer fractures. It is characterized by an almost instantaneous increase in friction and  audible noise, which periodically comes and goes, and is universally attributed to the chaotic motion of  the retainer in unstable energetic modes resulting from increased coefficient of friction between the  retainer and the other bearing parts. Reference 5 concluded that ball pocket friction is more critical to  stability than retainer land friction for the bearing investigated. The increase in ball pocket friction has  been widely attributed to lubricant starvation, despite the trend that occurrences do not necessarily seem  to increase with accumulated operating time, and the contradictory evidence that it is sometimes  observed in conjunction with ‘oil jogs’ which would tend to suggest ample, or even excessive, lubricant  supply. However, electrical discharge can be an obvious source of increased ball pocket friction by  increasing the surface roughness of the balls, rather than the more popular theory of starving the surfaces  of the retainer of lubrication. Therefore, it is suggested that electrical discharge increasing the surface  roughness of balls may be a plausible path to the initiation of cage instability, in the lab due to electrical  currents, and in space due to space charging.  In Reference 6, this author cited an example of “Vibration-Induced” retainer instability in the lab with  brand new well-lubricated bearings, in which the retainer instability could literally be turned on and off with  an electrical switch that changed the motor from delta to wye. Figure 13 shows a waterfall chart from an  accelerometer during a run-up from 0 rpm to 3600 rpm, with data sampled every 200 rpm. A 36x per  revolution disturbance dominates the plot. The retainer instability is clearly visible in the 1-kHz range  above 3000 rpm. The source of the 36x disturbance was the prototype motor used to drive the reaction  wheel. Circulating currents in the delta-configured motor were caused by excessive amplitude modulation  of the magnetic flux density in the motor gap. To prove this, the motor was rewired to a wye configuration,  which eliminated potential circulating current loops and the waterfall measurement was immediately  retaken. The results are shown in Figure 14, which clearly shows elimination of the 36x per revolution  disturbance and no signs of retainer instability. The modified motor winding was the only change to the  reaction wheel between the data taken in Figures 13 and 14. At the time Reference 6 was written, the  observed retainer instability was assumed to be due to micro-vibrations from the circulating currents in the  motor, since no other connection with the circulating currents could be imagined. In hindsight, it is more  likely that the large circulating currents were resulting in high voltage electrical discharge across the  bearings above 3000 rpm, which would tend to roughen the balls and increase the friction coefficient  between the ball and ball pocket, and which increased the potential for retainer instability.  Figure 13. Run-up with large circulating motor  currents at a frequency of 36X per revolution and  resulting cage instabilityFigure 14. Run-up without circulating motor  currents – no cage instability NASA/CP—2018-219887 66