© 2016 Sierra Nevada Corporation   121 Extended Life Testing of Duplex Ball Bearings 

Jeffrey Mobley*, Mich ael Robertson * and Charl es Hodges * 

Abstract    Sierra Nevada Corporation’s  Space System s perform ed bearing life testing for the Scan Mirror  Motor/Encoder Assembly (SMMA), part of the Scan Mirror Assembly on- board the Aerosol Polarimetry  Sensor (APS) on the NASA Glory Spacecraft . The baseline bearing life test duration extended beyond the  launch date for the Glory Spacecraft ; a risk that the program was willing to undertake with the  understanding that if any anomalies or failures occurred before the required life was achieved, then the mission objectives or operating profile could be modified on orbit to take those results into account. E ven  though the Glory S pacecraft failed to reach or bit during its launch in March of 2011, the bearing life  testing was continued through a mutual understanding of value between Sierra Nevada Corporation  and  our customer ; with a revised goal of testing to failure rather than completing a required number of  life  cycles . Life testing thus far has not only exceeded the original mission required life, but has also  exceeded the published test data for Cumulative Degradation Factor  (CDF)  from NASA/CR -2009- 215681   [1]. Many lessons were learned along the way regard ing long life testing. The bearing life test has been  temporarily suspended due to test support equipment issues .  

Introduction  

Since the A erosol Polarimetry Sensor is a continuous scanning sensor, the SMMA was required to  provide a constant rotational speed to the S can Mirror Assembly  while the APS collected earth  atmospheric data for analysis . The APS had a design life of 5 years , encompassing a continuously  operating mission life of 3 years at 40.69 RPM  resulting in 64.2 million shaft revolutions.  As the Scan  Mirror Assembly was extremely sensitive to torque variation, there were specified  requirements on  maximum break -away torque, minimum  and maximum drag torque, and torque disturbance that had to be  met over the lifetime of the mechanism.  The bearings were required to meet those torque requirements at  the specified operating speed over an equiva lent 2X operating mission life equal to 128.4 million  revolutions . Based upon the bearing geometry, preload of the bearings, and the number of operating  cycles , the lubricant life of the bearing was uncertain when the CDF was evaluated against the data  published in NASA/CR -2009 -215681 for Pennzane 2001- based lubrication. Since the bearings are a  single point failure in the mechanism and c ould not  be qualified through similarity to existing test data, an  in depth bearing life test was required to gain mission confidence . The SMMA Actuator is shown in Figure  1. 

Lubricant Lifetime Prediction  

As detailed in NASA/CR —2005- 213424 [2], the bearing lubricant Cumulative Degradation Factor (CDF) is  a method used to determine and compare the relative lubricant lifetime of new applications based upon parameters and test data obtained from heritage programs. The CDF is calculated based on the product  of the bearing mean hertzian stress and the number of times a ball passes across a given spot on the  raceway and is expressed in units of ball passes •psi (bp •psi). The rotor be arings within the SMMA have a  mean hertzian contact stress of 521.7 MPa (75.66 ksi) at nominal bearing geometry and preload. Based  upon the bearing geometry, the SMMA rotor bearings experience 22.167 ball passes per shaft revolution. The 1X APS program lif e requirement of 64.2 million revolutions resulted in a calculated CDF of 107.7 x  10 12 bp•psi and the 2X APS program life of 128.4 million revolutions resulted in a calculated CDF of 215.4                                                              * Sierra Nevada Corporation’s Space Systems, Durham, NC  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016