© 2016 Sierra Nevada Corporation   131 same life test . For this test in particular, test equipment was selected based on the initial requirement of  128.4 million shaft revolutions for approximately 40 months . However, once the decision was made to  operate the bearings to failure, some of those GSE selections were not sufficient for extended life without  additional maintenance, as evidenced by the number of test interruptions seen in Figure 8 after the  completion of the 2X portion of the test. Had the life test been planned to operate to failure from the start,  then different GSE components may have been selected.         There should be planned maintenance of any test equipment which may fail during the life test  and  impact the cost and total duration of the test . During this test, there were numerous stoppages resulting  from maintenance issues arising with the test equipment.    A DC Bru sh Motor/Gearhead was selected to drive the bearing test . There is required maintenance to  periodically replace the brushes  and they were s wapped out twice during th is life test . Additionally, the  entire Motor/Gearbox was swapped out once due to an oil leak between the motor and gearbox causing  concerns about insufficient lubrication in the gearbox  over the duration of the test . If an indefinite  test  duration was the original goal, a direct drive motor with closed loop speed control c ould have been   selec ted to rotate the test shaft due to better control bearing speed over a long duration test , to alleviate  concerns about gearhead problems over the duration of the life test , and to minimize rotations on the  drive motor bearings.    The cryopump has to be regenerated periodically to clear out collected material  in order to be able to  maintain the appropriate vacuum level. Regeneration is a fairly quick process, only resulting in a test interruption of approximately 48 hours . Eventually, the cryopump needs  to b e returned to the  manufacturer to be rebuilt resulting in a longer interruption of the test program of approximately 3- 4  weeks . Periodic regeneration of the cryopump is inevitable and needs to be factored into the overall test  duration. For a longer life test, having a spare cryopump  on hand would have reduced the down time due  to rebuild.    The vacuum gauge is another maintenance item on vacuum chambers . A cold cathode vacuum gauge  was selected over a hot cathode vacuum gauge for this test to obtain longer g auge life at the expense  of  reduced pressure accuracy . Even so, the cold cathode vacuum gauge had to be replace d after  approximately 7 years, well beyond the original planned duration of the life test . Having a spare vacuum  gage on han d would have reduced test down time.    Over time, t he weight of the metallic hose from the roughing pump connected to the top of the chamber  created a pinhole crack in the feedthrough tube allowing external air to leak inside the chamber . This  pinhole was able to be filled with epoxy without breaking the test setup. The leak occurred while the test  was halted due to another equipment issue, so the slight drop in vacuum level did not impact the validity  of the life test. The roughing pump hose was moved to a more robust feedthrough to prevent recurrence.  

The chamber was configured without thermal control as a cost savings feature. Lack of thermal control  reduced both the initial cost  and schedule of chamber equipment as well as reducing the potential for  maintenance issues on the thermal control system . However, the downside to that selection was that the  temperature of the bearings under test was at the mercy of self-heating and any temperature variations  within the test lab ambient environment . The t est chamber also had to be moved midway through the test  to a new location within the test facility which resulted in a shift in the external thermal environment that was noticeable in the bearing temperature plots . If the natural self -heating of the beari ngs from operation  combined with the external chamber thermal environment had resulted in a bearing temperature outside  of the defined operating condition, then there would have been minimal opportunities for correction without significant delay to add thermal control or significant cost to control the chamber using room  ambient temperature.     The length of the subject life test exceeded the calibration due dates on a majority of the test equipment .  There was a mutual decision made to not stop the test to calibrate equipment , but rather verify equipment