© 2016 Sierra Nevada Corporation   130 Lubrication Film Parameter   Based upon nominal bearing parameters, nominal bearing preload, on-orbit speed of 40.69 RPM, and a  nominal on- orbit operating temperature of 17.5°C, the lubricant film parameter (λ) is calculated to be  0.811. Using those same nominal bearing parameters, nominal bearing preload, life test speed of   74 RPM, and a nominal  life test operating temperature of 28.6°C during the 2X portion of the life test, the  lubricant film parameter (λ) is calculated to be 0.810. Since the STLE lubricant life factor [4] changes with  λ, even within the same lubrication regime, this  close correlation in lubrication regime gives a high level of  credence to the life test results.  

Cumulative Degradation Factor   Taking into account the internal bearing geometry, a mean hertzian contact stress of 521.7 MPa (75.66  ksi) under nominal parameters , and 202 million shaft revolutions; the Cumulative Degradation Factor  is  calculated to be 3 38.8*1012 bp•psi for this test . This data point  significantly exceeds 88*1012 bp•psi, which  is the highest tested value of CDF  published in NASA/CR -2009- 215681 [1] for Pennzane- based oil  formulation.  

Lessons Learned  

Test Quantity   In order to obtain statistical data more representative of the entire production lot and to cover for any test  anomalies that arise with ground support  equipment (GSE) , it is best practice to t est as many articles as  feasible. As experienced in this life test, two of the six bearings under test had noisy load cell  data.  Fortunately, the data sample for this test was large enough to clearly identify that those two setups were  outliers  allowing them to be sufficiently investigated to determine cause. The investigation benefitted  greatly from comparing the abnormal test data to the test data from the remaining four test setups,  implicating the load cells . Rather than halt the test and wait for replacement load cells, the life test was  able to be continued with the data from the remaining four bearings. If the sample size selected for this  test was smaller, noisy data would have likely led to a lengthy and cos tly investigation, jeopardizing the  targeted completion date for 1X life testing. Additionally, higher test quantities increase the likelihood that  normal manufacturing variations are accounted for in the test data, further validating the test results.  

Test Duration  It is highly  desirable to test articles to failure  whenever possible. Many of the data points listed in the  NASA/CR -2009- 215681 had the notation “Test Suspension (no failure)” meaning that the  bearings/lubricant were functioning normally  at the completion of their required life test and could have  been operated longer providing better data for future use.    With higher quantities, consider grouping test articles into different life test durations  to maximize data  usage for future applicat ions. For example, test a portion to 1X mission life, a portion to 2X mission life,  and a portion to failure . Multiple test durations are  especially important for life test articles that use  acceptance criteria that cannot be fully verified during the life test. Such criteria include visual inspection  of internal components that require disassembly to perform and measurement of performance parameters  which require breaking the life test setup to verify ; either of which could jeopardize the validity of  additional life  testing . In the case of the bearing test described herein , a change in friction torque in the  test setup is  being used as the pass/fail criteria rather than rely on visual inspection ; so testing the  bearings  to failure results in the most useful data obtainable. Whenever possible, it is preferred to avoid  or minimize test pauses so that lubricant temperature and viscosity remain in a steady -state condition and  transient data does not have to be rationalized.  

Test fixture component selection   Ground Support Equipment (GSE) component selection and required maintenance should be carefully  evaluated against the life test duration to minimize test interruptions and erroneous data. In reality, not  only is the test article undergoing a life test, but every component in the life test setup is undergoing that