410 The retainer design for these bearings had to addre ss two modes of operation. The first, being a floating  retainer at room temperature and second, an inner race riding retainer at cryogenic temperature. The  extreme temperature change, combined with the large CTE value of PGM-HT, means that the retainer would have only a small clearance from the inner race at cryogenic temperature.     Differential thermal contraction between the cages and the bearing races must be taken into account in the design since operational range is 295K – 35K, hence fo r this design the validity of CTE data is critical.  When the retainer was originally designed, the CTE va lues used for calculating clearances at 35K was   52 x 10 -6 K-1 for PGM-HT and 8 x 10-6 K-1 for 440C [1] over the temperature range of 295K – 35K. A  review of more recent test data (provided by ESR Technology) revealed that the CTE of PGM-HT could  vary from 80 x 10-6 K-1 at 295K to 60 x 10-6 K-1 at 125 K. No data existed below 125K, so CTE values for  the temperature range of 125K to 35K were extrapolated from the most recent data.  

The larger values of CTE were used in a dimensional shrinkage analysis of the retainer and the bearing inner race. The results of the analysis showed an interference condition could occur between the retainer and the inner race at approximately 90K for the PW bearing and 60K for the FW bearing.   It was decided to confirm the findings of the anal ysis by test. The PW bearing was tested again at  cryogenic temperature, with torque measurements made at 50, 70, 90 and 100K. The test revealed that below 80K, torque was greater than 50 N-cm. When above 100K, torque was only 10 N-cm. This test confirmed our supposition that an interference was oc curring and it was decided to re-make new retainers  with a larger inside diameter. The new CTE value was used for the new retainer design. Thanks go to our vendor, ESR Technology for their timely assistance with this issue.          

With all bearings having a revised larger retainer installed, the test philosophy of the program was  revised. Both the PW bearing and the FW bearing would be tested at the bearing level, at cryogenic temperature. A bearing test fixture was designed by ESR Technology and COM DEV. A novel feature  was the addition of a “window” so that a direct view of the retainer could be seen at cryogenic  temperature. Temperature sensors were also adde d to directly monitor inner and outer race  temperatures.   The first cryogenic test of a bearing (with the new re tainer) gave us a surprise. The bearing still exhibited  a high torque when initially rotated at cryogenic te mperature (approx. 50 N-cm) but a lower value than  that seen before. The bearing recovered quickly to a normal value (10 N-cm) when rotated two revolutions. The good news was that the retainer  could be seen to have the design clearance of  approximately 0.4 mm from the inner race at cr yogenic temperature. The fixture “window” and the  clearance are shown in Figure 9.      The post test inspection of the bearing showed no flakes and particles had come off the retainer. The conclusion of the test was that t here was still a problem with the bearing, but not necessarily related to  the retainer interference issue. A solution to the problem had not yet been found, but the initial torque of  50 N-cm could easily be accommodat ed by the driving torque avail able for the geared motor. As a  precaution, it was decided to torque test all bearings at 35K. The tests revealed that all bearings had a higher than normal torque when initially rotated at  cryogenic temperature. However, bearings having the  lowest values of initial torque ( ≤50 N-cm) were selected for flight and the life test program.  

The investigation team was re-convened to establish a root cause for the problem. All evidence from the  tests to date was reviewed and it was decided to further investigate the effects of the large thermal gradient, which was occurring across the bearing during the cool down. It was already known at this time that the cool down of the DW mech anism over a period of 6 days or less would invite the large thermal  gradient across the bearing to occur and therefore may cause the high initial torque at cryogenic temperature to occur in the DW mechanism. A furthe r series of pupil wheel bearing tests were carried out  to further characterize the problem and investigate possible mitigation strategies.       NASA/CP-2010-216272