© 2016 Sierra Nevada Corporation   133   Automated testing over an extended life test also stores a tremendous amount of data (over 6 MB per  hour for this test) and maintenance of that data must be considered. The data acquisition computer  internal hard drive was not sufficient to store the entire life test data and as such would have to be  monitored to ensure the capacity was not exceeded resulting in inadvertent test interruption or loss of  data. To free up memory storage space on the internal computer hard drive , the test data was transferred  to redundant external hard drives approximately once a month for long term storage and backup.  Postprocessing of significant amounts of test data for trending summaries and reporting was cumbersome and time consuming. Addit ional forethought should have been given to utilize different tools to create useful  charts and reports with less effort.    Since the test was automated, t he test setup was manually checked once per working day to verify that  the speed was within tolerance,  that the daily rev count was as expected, that the vacuum level was  within limits, and that the bearing temperatures were within limits and within expectations . While there  were not any occurrences of catastrophic anomalies during the testing performed to date, there would  have been an additional level of risk reduction if automated safeties would have been incorporated to safely shut down the test in the event of test parameter exceedances such as torque, temperature, vacuum, or speed . These automated saf eties were beyond the budgeted scope of the test.  

Life Test Conclusions    The lubricated bearings in this life test  not only exceeded the required program life, but also  demonstrated the capability to greatly exceed the tested CDF values listed in NASA/CR -2009- 215681 [1],  showing the possibility to operate bearings for extended lifetimes under closely controlled design  parameters and similar operating conditions . For this application,  a CDF of 338.8*10 12 bp-psi has been  achieved with no torque degradation seen to date . In order to achieve this life, bearing parameters  including bearing geometry, materials, mounting methods, preloading, lubrication, and component screening should be carefully selected and designe d in order to optimize contact stress, assembly  alignment, and initial torque disturbance. Life testing has been temporarily halted since October 2014 due  to maintenance issues with the Vacuum Chamber, but the test articles are being held under rough  vacuum in the test chamber . The life test is planned to resume in 2016 with a continued goal of testing  until a significant change in bearing torque is witnessed.  

References   1. Lo, John C.; et al. “Use of Cumulative Degradation Factor Prediction and Life Test Result of the  Thruster Gimbal Assembly Actuator for the Dawn Flight Project.” NASA/CR —2009 -215681.    2. Jones, William R. Jr and Jansen, Mark J. “Lubrication for Space Applications.” NASA/CR —2005213424.   3. Zaretsky, Erwin V. “Lubricant Effects on Bearing Li fe.” NASA Technical Memorandum 88875.    4. Zaretsky, Erwin V. “STLE Life Factors for Rolling Bearings.” STLE SP -34, Park Ridge, IL, 1999.   5. Braza, Joseph; Jansen, Mark J.; Jones, William R. Jr.; “Lubricated Bearing Lifetimes of a Multiply Alkylated Cyclopentane and a Linear Perfluoropolyether Fluid in Oscillatory Motion at Elevated  Temperatures in Ultrahigh Vacuum .” NASA/CR —2009 -215637.