96 manufacturing flaw that resulted from the inner ring becoming loose in the machining fixture. Thus this  post-test ring crack was at tributed to the known material and machining flaws [11].    For the bearing failure that is the subject of the current paper, the material was known to be of the highest  quality and absent of similar dimensional errors (low spots) or manufacturing flaws. To fully understand  the present bearing failure more detailed information regarding the application and installation are  needed.    The bearings were dimensionally characterized two times:  once during manufacture by the bearing finisher and once immediately prior to installation by the NASA MSFC Tribology Lab. In general, both  bearings were found to be within specification for all maj or diameters and widths, raceway curvatures and  surface finishes. All of the construction materials (balls, cages, race material, etc.) were fully certified for  quality. Visual inspection of the bearings revealed no observable flaws and both bearings turned smoothly. The only significant discrepancy between the bearing finisher’s dimensions and those from the MSFC lab pertained to the bearings’ outside diameter and roundness. The MSFC lab noted that the bearing outer race roundness values were affected by t he installation of the grease shield retaining wires.  These “C” shaped retaining wires are pre- compressed and then installed in a groove machined in the  outer bearing race adjacent to the shield. The wires apply a small expansion load on the outer race causing it to become non- circular (slightly oval). This rather surprising observation is likely the result of  the relatively low elastic modulus of the 60NiTi. In any case, the magnitude of the roundness variation   was small (less than 0.0002 in or 5 µm ) and deemed unlikely to affect the bearing operation once  confined (pressed into) by the DA hub structure.  

Unfortunately, the failed bearing could not be removed from the DA intact. Disassembly forces fractured  the outer race into several pieces making forensic  interpretation of the damage impossible. Further, since  the DA was operated long after significant bearing damage (wear debris detection on filters) began; it is  difficult to judge the failure mechanism from the severely damaged bearing. The second test b earing (the  motor side) that did not fail was in the same condition as prior to test.    Dimensional measurements were then made of the DA bearing- machine interfaces and compared to  specifications and to the pre- test bearing dimensions to assess whether the bearing- machine fits were  correct. Table III shows the major dimensions of the bearings and their relation to installation fits in the  DA. 

Table III.   DA Bearing Overall Fit and Housing Dimensional Measurements , inches   Parameter  Specification  Hub-side  bearing  Hub-side  bearing fit  Motor -side   bearing  Motor -side   bearing fit   Outer race  outside  diameter  3.14960 max   3.14910 min  3.14987  +0.0011  interference  3.14956  +0.0007  interference   Inner race  inside diameter  1.9685 max   1.9680 min  1.9681  –0.0007  clearance  1.9681  –0.0005  clearance   Bearing width  0.630 max   0.629 min  0.6296  ---- 0.6298  ----  Radial  clearance  0.0014 max   0.0007 min  0.0008  ---- 0.0008  ---- 

Evaluation of the data in Table III reveals that the failed hub- side bearing’s outer diameter was not within  specification. Careful measurements were made at two angular locations and at three different width locations. The average is shown in the table. T he assembled bearing was under the maximum allowed  diameter by approximately 0.0003 in (7.6 µm) at one angular location and over the maximum by 0.0009 in  (23 µm) at the other location. The averaged diameter of the hub- side bearing was greater than the  maxi mum allowed by approximately 0.0003 in (7.6 µm). In addition, measurements of the mating DA hub