97 bore showed that its diameter was undersized by approximately 0.0005 in (13 µm). As installed, this  bearing experienced an interference fit with the hub- side ho using bore of 0.0011 in (28 µm). This heavy  interference fit was outside the design range (0.0001 in (µm))  clearance to 0.0006 in (15 µm)  interference)  and may be a contributing factor in the bearing failure.    Like the hub- side bearing, the motor -side bearing also had an outside diameter that varied by  approximately 0.0006 inch (15 µm) but with an average diameter slightly below the maximum allowed.  The motor -side bearing housing diameter was larger than the hub- side bearing housing diameter and was  within the specified design range. Taken together, the hub- side bearing, which did fail, was substantially  tighter in its DA housing than the motor -side bearing, which did not fail. To evaluate whether these  bearing fit differences alone contributed to the failure requires a closer examination of the mating hardware and the stresses and deformation that arise from the installed fits. In particular, it is elucidating to examine any other structural differences between the two bearing locations to determine if other differences such as operating loads exist that could have led to one bearing failing while the other performed well.  

Other Differences between  Hub and Motor Side Bearing Locations  

The hub- side bearing is axially loaded against the motor -side bearing using a calibrated wave spring set  to approximately 40 lb (178 N) . This is the primary bearing load and results in an average Hertz ball -race  contact stress level of approximately 180 Ks i (1240 MPa) . In ground testing, the weight of the drum rests  on the bearings and can add approximately 20 lb (89 N) . In addition, a very small radial load (10 lb   (44 N) ) is imparted by an O -ring drive belt located slightly outboard of the motor side bear ing. Taken  together, these loads are considered very low for such a large ball bearing. In effect, both bearings  experience similar and low loads that are not likely a contributing factor in the failure.  

Aside from the differing levels of interference fit s, the most obvious difference between the motor and hubside bearing locations is the structural design of the housings. As shown in Figures 1 and 5, the motor -side  bearing housing has an axial symmetric bowl shape while the hub- side bearing housing is comprised of a  shallow cylinder supported by a three- spoke flat plate. The spoke design provides ready physical access to  the central portion of the drum to enable the installation of piping and other internal elements. Figure 5(c),  for example, shows a clos e-up view of the hub- side of the drum where a fluid pick -up tube is visible. To  determine whether the differences between the hub and motor side housing structures were a contributing factor in the bearing failure, a finite element based analysis was under taken.  

Bearing Installation Fit Considerations  - Structural Analysis    The interference (press) fit of a bearing into the bowl -shaped motor -side housing results in the  circumferentially uniform compression of the bearing outer race. For steel bearing races, the typical rule of thumb is that the bearing diameter will be reduce d by approximately 80% of the fit interference [12].  Since the elastic modulus of NiTi is half that of steel, it is expected that the bearing diameter reduction will be at least this amount, or possibly more. For the motor -side bearing which was known to have an  interference level of 0.0007 i n (18 µm),  we estimate a reduction of about 0.0006 in (15 µm) . This level is  slightly less than the bearing’ s internal clearance (0.0008 in (20 µm) ) thus leaving an installed clearance  of about 0.0002 in (5 µm) .