173 or increase vibration to the “take action” criteria as observed during CMG3 on-orbit operations. These  observations led to the following redesigns;   1. Change the sleeve material to a harder dissimilar metal than the bearing outer race to prevent  fretting and galling  2. Add Rheolube grease to the bearing/steel sleev e interface to improve long term lubrication,  reduce wear/galling/fretting, and improve sliding   3. Change the preload nut material from A286 to harder 52100 to prevent wear/record grooving   4. Add flats to the Belleville preload spring to reduce contact stresses and prevent wear/record  grooving of the preload nut  5. Improve the inner gimbal cover/inner gimbal in terface to reduce gimbal distortion and maintain  bearing bore alignment  6. Improve the cleaning process of  bearing housing sleeves to minimize/eliminate hard particles that  may have contributed to the r epetitive marks in the bearing ra ceway and that may have lodged  between the outer race and steel sleeve resulting in locked outer races   The design improvements identified from these invest igations were successfully demonstrated in a redesign verification test program and have been implemented in the rebuild of CMG1 and CMG3 to minimize the most probable failure mechanisms and in crease the life of the spare CMGs. These CMGs  will be stored on the Space Stati on as “ready to use” spares. 

Conclusions 

Determining the cause for failure was the primary focus of the investigation team s. The same teams were  utilized to develop modifications to eliminate the design susceptibilities. From these two CMG failures,  valuable engineering and programmatic lessons learned surfaced to improve futu re space programs. The  lessons learned from these failures are:  1. Sliding fits should be properly designed and lubricated to minimize wear and galling/fretting over  the life the device. This can be accomplished th rough proper selection of materials at the  interface. In general, the use of dissimilar metals at sliding interfaces will reduce the likelihood of galling/fretting. Galling/fretting/wear tests should be performed in the design phase to optimize the sliding fit interface.  2. Ensure all structure distortions are totally analyzed in the design phase. Do not underestimate the  importance of thermal-stress analysis.  3. Ensure a cleaning validation and verification process are present for the parts in the critical sliding  fit applications. Sliding fits can deteriorate quickly if hard particulate residue remains at critical fits  from manufacturing operations.  4. As part of the qualification program a disa ssembly evaluation should be planned. The Space  Station CMG program did not include a disassembly of the qualification unit; acceptance was based on performance after environmental exposu res. A disassembly and evaluation of the  qualification unit may have identified these su sceptibilities at the sliding interfaces.  5. New designs based on heritage are a good starti ng point; however, incorporation of present day  technology should not be discouraged. A thorough risk analysis should be performed before a deviation from a heritage design is accepted. Eval uation tests using “design of experiments” must  be performed in the development phase to demonstrate the innovative configuration provides a low risk improvement over heritage.    The ISS, an orbiting laboratory, is providing a learning opportunity not only for the planned scientific experiments but also for spacecraft design and operation within the International Aerospace Community. The lessons learned from the CMG failures are only a few of the many that have been identified during  the assembly and operational phases of the ISS.      NASA/CP-2010-216272