79 Efforts were made to find common ground between the experiences from t he bearing life test unit result s  and the on- orbit flight unit data. This effort was complicated by t he fact that the life test units  were focused  on only the mechanism bearing assemblies and that the on-orbit data were being obtained from a higher  level o f mechanism assembly . The telemetry obtained for the life test units were not directly relatable to the  telemetry available from on- orbit. As an example, the temperature telemetry in the test units was a better  indicator of average bearing temperature, whi le the on- orbit implementation was more of a drive motor  temperature. This type of data divergence made the evaluation of the on- orbit motor current spikes much  more difficult . We found that the best common data relationship was that of average motor current . We had  higher rate motor current data for both the test unit and the on- orbit instrument . However, the data rates  were vastly different and the motors were driven by very diff erent control systems . That made any useful  comparison of the high rate data from both sources almost impossible. Still, the high rate on- orbit data did  provide valuable information on what was going on within the flight mechanism bearing system.  

Figure 3 provides some information on the initial on- orbit observation. This shows the main torque trend  from the beginning of mission to end of February, 2014. The transient in late 2012 returns to the previous  nominal condition on its own with no intervention fr om the ground or otherwise.  

Figure 3 .  On-Orbit Main Motor Current –  showing the torque  spike in late 2012      ATMS Life Test Unit Experience  

The ATMS program initiated a bearing life test program on February 2003 with the first of three bearing test units . This was for the same toroid type separator bearings . All three units went on line by  August 2003.    Test unit 2 started experiencing higher torque spikes at about the 5 year mark of its ‘operational’ life. The  torque generated by the unit exceeded l imits at the 5.3 year mark of its life (2/20/2009) . At this point , the  unit was disassembled to inspect the bearings . It was found that the bearing toroids had experienced severe  wear . The toroids of one end of the bearing pair were almost totally destroyed and the other bearing showed  severe wear of the toroids . The temperature of the test units was initially set at 41 °C as a predicted worst  case temperature. In the September  2012 timeframe the temperature was changed to more closely match  that of the measured on-orbit values . The test unit temperature w as changed from 41 ° C to 18 °C. At the  time, it was thought that the accelerated wear of the test unit #2 bearings was the result of an overly  conservative temperature prediction leading to a premature loss  of lubrication.  

The higher test temperature did contribute to a very conservative test. The protective elasto- hydrodynamic  film thickness between the bearing balls and race differed by a factor of 2X . Also the chemical degradation  rate of the lubricant was accelerated by temperature approximately doubling for every 10° C according to  the Arrhenius relationship. However the sister unit #1 and #3 test bearings ran approximately twice as long  as the #2 bearings so there was yet another unidentified factor pr esent.  

Figure  4 show s the wear of the bearing toroids for test unit 2. The left picture shows the ‘heater’ end bearing  and the left shows the ‘encoder’ end bearing. The ‘heater’ end bearing was thought to run at a slightly  higher temperature than the other, leading to more severe wear.