© 2016 Sierra Nevada Corporation   124 Design Considerations for Long Life  

In order to achieve extremely long life out of a lubricated bearing,  special attention has  to be paid to the  bearing materials, accuracy, design parameters , lubrication, verification, and mount ing techniques.  

Materials   To maximize reliability and minimize the chance of impurities causing surface weakness or contamination,  materials should be of the highest purity grade feasible . For this application , Consumable Electrode  Vacuum Melted 440C Stainless was chosen for the balls and raceways . Phenolic was chosen as the ball  separator material due to its high level of heritage in vacuum operation.  

Accuracy   To minimize inherent torque variation and accuracy  error, b alls and raceways should be held to the  highest accuracy requirements feasible. For this application the raceways were held to ABEC 7 or better  and the balls were held to AFBMA Grade 3 or better.  

Surface Roughness   To maximize the lubricant film p arameter (λ) , the surface finish of the balls and raceways should be held  to the finest finish feasible. For this application, the balls were held to a maximum Ra of .013 µm (0.5 µin),  as controlled by the AFBMA Grade 3 requirements, and the raceways were held to a maximum Ra of  .076 µm ( 3 µin). 

Lubrication   Finally, lubrication type and amount should be carefully selected to provide the longest life possi ble within  program requirements such as outgassing restrictions and operating temperature range. The program  team solicited inputs from several leading lubrication experts within the space industry  and concluded that  for this application, Pennzane 2001- 3PbNp oil was the best candidate for selection. Oil was chosen over  grease since the thickeners and other additives in grease tend to create more inherent torque variation  over oil alone. Pennzane 2001 was chosen over Braycote 815Z because it has proven to have longer operating life, as can be seen in the CDF data listed in NASA/CR -2009- 215681. Pennzane 2001 was  deemed acceptable for this application since the cold operating temperature of only +5°C did not get into  the realm of significant viscosity increase . Lead Naphthenate was chosen as the high pressure additive to  provide an additional layer of wear  protection for boundary lubrication. A wettability test was performed on  100% of the raceways and balls to ensure that the surfaces would yield a uniform coating of lubricant with no signs of beading, gaps, or dewetting. The ball separators were desiccate d and vacuum impregnated  with the same Pennzane 2001- 3PbNp oil.  

Component Testing   For bearings with tight requirements on starting torque, running torque, and torque variation; each preloaded bearing pair should be tested for those parameters prior to installation . For this application, the  bearing vendor performed those tests on the fully lubricated and internally preloaded bearings and provided that data with each bearing.  

Mounting Techniques   To maintain minimal torque variation and consistent loadi ng in the mounted and assembled bearing  configuration, the controlling bearing mounting features have to be precisely controlled. For this  application, in addition to applying tight tolerances to the housing and shaft mounting shoulders, the bearing inner and outer races were precisely located and bonded in place to minimize misalignment,  wobble, and free play while avoiding the inherent variation and residual stresses resulting from interference fits.