413 The first cryogenic temperature test at 35K was comple ted and the results were satisfactory in terms of  overcoming the high torque of the bearings when initially turned at 35K. The torque ratio available to  initiate rotation of the wheels (some missed steps pe rmitted), at 35K, was 5.0 for the PW and 3.0 for the  FW. The torque ratio is defined as the maximum motor current available divided by the minimum motor current required to simply achieve wheel rotation.  After a run in of the bearings (25 revolutions of eac h bearing) the torque ratio at the start of science  operations (no missed motor steps per mitted) at the beginning of life of the mechanism was 2.75 for the  PW and 1.83 for the FW. The difference in the torque ratio values was attributed to the fact that the PW geared motor demonstrated a higher pull in torque (31 N-cm) at 35K than the FW geared motor (22 N-cm).     The temperature gradient measured across the beari ngs (during the cool down) was only 30K. A much  lower value than that measured during the cool down of “bearing only” tests. This was attributed to the larger thermal inertia of the all up DW mechanism asse mbly. It was learnt that ea ch different configuration  of hardware cooled to 35K produced very differ ent thermal gradients within the assembly.    The repeatability of positioning each optical element was demonstrated at 35K. The repeatability of the FW was ±0.133 mm and the PW was ±0.183 mm, versus a requirement for each wheel of ±0.2 mm. 

Conclusions and Lessons Learned 

Item (1)  – The bearing retainer suffered an interference condition with the inner race at the cryogenic  temperature. This was attributed to an inappropriate CTE design value for the PGM-HT material. The  interference was eliminated by enlarging the inside diameter of the retainer. Visual evidence of adequate  clearance was obtained by adding a window to the beari ng test fixture. It is recommended that precise  knowledge of the CTE value of non-metallic reta iners be used for designs intended for cryogenic  applications.  

Item (2)  - A non-metallic bearing retainer may be in a non-optimal location when a bearing is initially  rotated at cryogenic temperature. Th is causes a higher than normal torque. This may be caused by the  relatively large dimensional change of the retainer  cooling down to the cryogenic temperature. The  retainer being under a 1G load and being offset from an ideal location may magnify this problem. This  problem was rectified by a 10 degree rotation of the bearing provided the bearing was cooled in an  isothermal manner. A metallic retainer may eliminate this problem. 

Item (3)  – When a bearing is cooled in such a way t hat a large thermal gradient occurs across the  bearing, the bearing preload is lost. The loss of preload allows the ball and race system to be in a nonoptimal location during the bearing cool down. Also, it is possible that the 1G load pulls the balls and the  retainer off center. The balls re-register with the ra ces in a non-optimal location when the preload is reapplied at the cryogenic temperature. The non-optimal location of the balls causes a higher than normal torque when initially rotated at cryogenic temperatur e. This problem can be rectified by up to four  rotations of the bearing. It is recommended that me chanisms be cooled in an isothermal manner. The  requirement to cool isothermally is problematic if  accelerated cool downs are required for supporting  ground tests.   

Item (4)  – The mechanism level tests cool the DW mechanism  in six days. This invites item (3) to occur.  Item (2) will always occur even if the mechanism is  cooled isothermally over a long period of time.    Item (5)  – Bearings behave in a normal manner after items (2) and (3) have been overcome. Tests  confirmed that a mechanism torque ratio of greater than 3.0 was available to overcome the problem from  items (2) and (3).   

NASA/CP-2010-216272