© 2020 Sierra Nevada Corporation   406 environment  prior to the beginning of life testing.  Although they had the s ame baseline bearing  configuration,  SN032 performed its life test at the high end of the operational speed (7000 rpm), and was  completed within three weeks , whereas SN019 performed its life test at the low end of the operational  speed (1500 rpm), and took two months to complete.  In reality, the actual motor speed in mission operation  on the rover could be at any speed in between. These two speeds were selected to bound mission  operation, since speed could have a potentially significant impact in the bearing lubrication film parameter   and overall bearing life.  

In parallel to these efforts, the program team chose to procure an alternate crown phenolic retainer in the  “refurbish” path in lieu of the baseline crown steel retainer. This decision was made due the determination   that the root cause of failure was the compounding deterioration  of the crown steel retainer in the absence  of effec tive lubrication  at cold temperatures.  The premise was that the porous phenolic retainer had more  published heritage success and itself was a potential lubricant reservoir, inherently resistant to lubricant  starvation.  To this end, JPL was able to successf ully modify four bearing sets to include crown phenolic  retainers in place of the crown steel version, each set having a different lubrication scheme, as follows:  

• 58.1 mg ( 30% void volume fill)  of Braycote 600EF grease only   • 38.7 mg ( 20% void volume fill) of Braycote 600EF grease only   • 30 mg ( 15.5%  void volume fill) of 2:1 slurry of Braycote 600EF grease :  Brayco 815Z oil   • 22 mg ( 11.5%  void volume fill) of 2:1 slurry of Braycote 600EF grease :  Brayco 815Z oil  

The purpose of the various lubrication schemes was to compare cold temperature performance. This  performance test ing was used to determine  the drag of each bearing configuration  with respect to  temperature in order to assess the torque uncertainty associat ed with these alternate configurations.  The  JPL Motion Control Subsystem (MCS) team evaluated the results of this testing  with respect to torque  uncertainty goals . The cold performance test results for these lubricant configurations are shown  in Figure  10, with the following observations:  

• SN012 motor with 58.1 mg grease reached the 0.6 A current limit and could not operate at or below  -50°C. Even at warmer temperatures , the no- load current was above 0.5 A and this lubricant  configuration was deemed unusable by the JPL MCS team.   • The other three configurations had acceptable performance but only the SN016 motor with 38.7  mg grease and SN012 with 22 mg slurry proceeded to life testing.    • The data show ed a low torque characteristic at temperatures at or below  -60°C. This characteristic  is believed to be from a starvation condition where the lubricant is so close to its pour point that its  high viscosity does not allow for it to effectively flow  within  and relubricate the bearing components .  While this produces low drag torque, it is not acceptable for a long life application.   • Peak torque was found to occur around -50°C indicating that the lubricant viscosity was still high  due to the low temperatures but was able to flow within the bearing.