280 at 60 °C and 314 rad/s ( 3000 rpm) . These conditions were selected to accelerate the test and introduce a  large number of bearing revolutions and associated stress cycles on the lubricant by operating in the  boundary  lubrication regime.  Current from the DC test motors was converted to bearing torque to track and  trend the health of the bearings. The baseline torque was comprised of bearing plus motor torque, and was  recorded at the beginning of the life test  and monitored throughout the testing with any observed increases  in torque attributed to bearing drag since the motor torque was  constant over time.  

Phase II Testing   Phase II testing was initiated shortly after the failure of motor 22 (52% curvature) which took place at 436  million revolutions  (Figure 4) . The temperature control for Bell J ars #1 and #2 was first lowered to a plate  temperature of 45°C, temporarily , to avoid any potential damage due to dropp ing the temperature too  quickly, then adjusted to 25°C resulting in motor temperatures ranging from 26- 28°C. The final plate  temperature of 25 °C was selected to operate the motors at a temperature mor e typical of RWA s in flight .  At this point , the motors with all steel bearings were categorized into two groups based on curvature and  the operating mode of  each group subcategorized and adjusted as follows : 1.) 105 rad/s ( 1000 rpm)  no  zero crossings, 2.) 105 rad/s ( 1000 rpm)  with zero crossings, 3.) 52.4 rad/s ( 500 rpm)  no zero crossings,  4.) 52.4 rad/s ( 500) rpm with zero crossings . The motors with hybrid bearings were allowed to continue   running at 314 rad/s ( 3000 rpm)  and 60° C until they achieved ~5.3 billion revolutions where the temperature   and speed on the hybrid motors w as adjusted to 27- 29°C and 105 rad/s ( 1000 rpm), respectively , with pair s  running with and without zero crossings . A summary of the testing configuration is shown in Figure 5. T he  speed was reduced periodically to 31.4 rad/s ( 300 rpm ) on all motors then ramped up to set  speed in order  to estimate the coulombic and viscous components of friction comprising the total torque. The viscous  component was extrapolated to 314 rad/s (3000 rpm) for the motors running at 105 rad/s (1000) and  52.4 rad/s  (500 rpm) . This measurement w as useful in tracking the state of lubrication in the bearings  over  time since the viscous component of friction would slowly decrease while the coulombic friction would  increase, providing an indication that the  lubricant was being depleted.  Microphones were added to detect  any noise associated with cage instabilities. In some cases, as the bearings reached end of life, the total  torque would suddenly step- up (Figure 6) with the bell jar microphones recording an i ncrease in the  background nois e. One example where the background sound amplitude increased 12 to 20 dB in the 1400  to 2400 Hz range, a signature indicative of a bearing experiencing dry cage instability , is shown in Figure  7. 

Figure 4. Motor 22 torque signature at point of failure in phase I testing (436 million revolutions).  

Results  

The test ing described in this paper  has been running for over 4 years. T here have been four  440C and two  hybrid bearing failures  and one motor  with 440 C bearings fully recovered.  Hybrid motor 40 testing was  suspended when its torque versus speed curve showed no viscous response.  All of the steel bearing  failures, occurred at less than 1.4 billion revolution s, whereas the hybrid bearing failures in motors 33 and   36 occurred at greater than  5.3 billion  revolutions, a significant finding in this work. I t is important to note