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Figure 2 – Simplified M45S Motor Cross -Section (Brake omitted for clarity)  

The rotor magnets overhang the rear of the stator to allow efficient Hall sensor commutation. This overhang  increas ed the load on the front motor bearing due to the magnetic forces attempt ing to self -center the rotor  within the stator.  As a result, Hertzian contact stresses were expected to be higher  [0.99 GPa (144 ksi) ] in  the front bearing and lower [ 0.80 GPa (116 ksi) ] in the rear bearing. These are reasonable values  for a long  life application.  

The bearing lubrication film parameter ( λ), a ratio of the lubrication film thickness to the composite  roughness of the contacting surfaces  between the ball and raceway , was calculated using COBRA -AHS  analysis software across temperatures ranging from -70°C to +70° C and speeds ranging from 1, 500 rpm  to 7,100 rpm.  The analysis indicated a mixed boundary condition λ of 1.7 at slower speeds and higher  temperatures , and a  hydrodynamic (HD) condition (λ >3) for higher speeds and colder temperatures  [4] [5].  The lubricant ’s viscosity is a function of temperature and increases at colder temperatures. This increase  in viscosity inherently increases the lubrication film thickness, which i n general, results in longer bearing  life as the lubricant is able to insulate the bearing elements from direct contact [6] . However,  a simple  extrapolation of lubricant viscosity going cold may be insufficient for understanding impact on bearing life.  As will be shown in Figure 10, the lubricant ceases to function effectively when  approaching its pour point ,  resulting in starvation effects that dramatically reduce bearing life.  

In addition to Hertzian contact stress and the lubrication film parameter, lubricant stress cycles are us ed in  evaluating bearing life. These are defined as the number of times a ball passes across a gi ven spot on the  raceway,  a function of the bearing’s pitch diameter, ball diameter, and quantity of balls. See Eq. 1 in  reference [2] for calculation details  and an example in reference [5]. The lubricant stress cycles for the  bearings in the M45S motor were 469 million for the margined life requirement of 105.2 million motor  revolutions .  

Published and other available data summari zing acceptable limits  of Hertzian contact stress and lubricant  stress cycles indicated that the M45S design  was exceeding limits  of proven lubricant life,  but not in an area  of known failure [3] [7]. The data indicate s that at least 30-60 million lubricant stress cycles could be  expected for contact stresses around 1.0 GPa (145  ksi), with failure to be expected beyond 600 million  lubricant stress cycles . Being between the limits of proven success and known failure, bearing lubricant life  was identified as a significant program risk throughout the design phase.  

In response,  the bearing balls were coated with titanium carbide (TiC), shown to  quadruple the life of the  perfluoropolyether (PFPE) lubricant  at Hertzian contact stresses of  1.0 GPa or less [8]. Concurrent  with the  M45S design effort , developmental bearing life testing was being performed on similar  bearings being used  Stator   Rotor   Hall Effect Sensors    Motor Housing   Preload  Spring  Rear Bearing    Front Bearing