285 Conclusions  

The testing described in this paper will continue since only 6 of th e 40 test motors have failed. P ost-test  inspection to confirm f ailure modes will also be performed. At this point in the investigation,  some key  preliminary findings have emerged (Figure 5 and Table 1):  

1) Of the four 440C and two hybrid bearing failures, five  of the six failed motors had bearings with 52%  race curvature suggesting that tight curvatures, while improving load capacity, may possess shorter  life. It has long been suspected that tight curvatures can lead to torque irregularities adversely affect ing  bearing  lubricant life [7]. While our results are preliminary, t his study may be the first to quantitatively  confirm this  point.   2) To date,  only one out of the four motors that failed with all steel bearings occurred at 52.4 rad/s (500  rpm). The other three failures occurred at 105 rad/s ( 1000 rpm) . Of the 16 motors that have been  running at 105 rad/s (1000 rpm), three are currently in recovery protocol, three have failed, and one  has fully recovered. There has been only one motor failure out of the 15 motors  running at 52.4 rad/s  (500 rpm). Although,  bearings running at the 105 rad/s ( 1000 rpm)  had roughly twice the running  distance at the time of failure as their 52.4 rad/s (500 rpm) counterparts,  it is interesting to point out that  the failures of motors that were running at 105 rad/s (1000 rpm) occur red at total revolutions less than  currently observed for  the 14 motors still running at 52.4 rad/s ( 500 rpm). B earings running at 52.4 rad/s  (500 rpm) have less favorable lubricant film thickness and should theoretically fail sooner.   3) Hybrid bearings exhibited smoother, steadier torque signatures  and achieved longer lives compared to  the 440C bearings. Of particular note, motor 22 with 52% curvature and all steel configuration failed at  436 million revolutions while running under the same conditions as motors 33 and 36 with hybrid  bearings which failed at 5.37 and 5.30 billion revolutions, respectively, providing a direct comparison  between the performance of steel bearings and hybrid bearings of equivalent curvature.   4) All hybrid motors achieved an excess of 5 billion revolutions with only two failures, while the remaining  5 hybrid motors are still running. One motor with hybrid bearings, motor 40 (Figure 8), was suspended  prior to the onset of failure in order to prevent dry cage instability and eventual cage failure. It is  important to note that prior to initiating phase I motor 40 was previously run in prototype testing in an  all steel configuration at 105 rad/s (1000 rpm) and 30 °C where it failed ~250 million revolutions and  resulted in a broken cage due to dry cage instability. While the conditions of operation differed, the 314  rad/s (3000 rpm) and 50° C conditions selected for motor 40 in phase I and phase II testing approached  the same lubrication regime affording an indirect comparison of motors with steel bearings versus  hybrid bearings  (Figure 10). 

References  

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