202 The damage sustained by the starboard SARJ highlights the importance of sound design and verification  practices in the development of complex rotating space machinery . The non-Hertzian contact mechanics  of the roller bearing to race ring interface proved to be a crucial detail of the system . Special attention  should be given to such effects in the design of bearing systems . The SARJ exhibited high vulnerability to  damage during the initial run-in phase of the mechanism’s life . The risks incurred during this period can  be mitigated through the use of adequate lubrication, crowned rollers, detailed screening of the nitriding process, the implementation of a pre-flight run-in period, and a through understanding of the differences  between ambient and vacuum performance . The addition of these elements to the design, verification and  operational plans for the SARJ forms the basis for the continued successful use of the mechanism in flight. 

On-Orbit Implementation of a Corrective Action  

Operations of the Starboard SARJ were severely restricted as soon as the damage was observed. The reduction in operation protected the ISS structure against the vibrations caused by SARJ rotations and against a stall of the mechanism. If the SARJ were to experience an unrecoverable stall, the operational impacts to the ISS would be significant enough to affect future missions and utilization of the station. These potential operational impacts to the ISS warranted corrective action.   The SARJ recovery team concluded that it was a reasonable action to remove debris, to the extent possible, from the damaged race ring and to add grease lubricant to all three bearing surfaces of the race ring. Debris removal was intended to decrease drive motor currents and improve torque margin. The  intent of the lubricant was to improve the lubricity between the roller/race interface in order to maintain a coefficient of friction below the critical roller tipping value; the improved lubricity, in turn, would protect the remaining undamage surface from experiencing degradation. Braycote 602EF © was chosen because the  base oil has the lowest vapor pressure of all available space greases. It also has molybdenum disulfide which is an excellent lubricant for sliding and capable of handling high loads.   Procedure Development

A preliminary cleaning and lubrication method was developed with inputs from the EVA tools and crew training teams. This method was successfully executed on a small section of  race ring during an on-orbit  test on the ISS 1J mission. Armed with the lessons learned from the cleaning trials, the team decided to clean and lubricate the entire damaged race ring during the STS-126/ULF2 mission. The team also decided to remove and replace all of the TBAs during ULF2. A significant amount of debris had accumulated on the TBAs, so their replacement would result in additional removal of debris from the joint.  This also allowed for the return the original TBAs for inspection to assist in the root cause investigation.   The cleaning and lubrication trials showed that the best method to clean the SARJ was to wet the surface with an EVA wipe pre-lubricated with Braycote 602 EF © and then scrape the surface with a scraper tool.  Lubricating the surface prior to scraping proved to be the best method to contain the loose debris particles and prevent them from being liberated and dispersed throughout the rest of the mechanism while scraping. The final application of lubricant to the surfaces after cleaning was done using a grease gun. The inner race ring surface is not visible to the crew so a unique lubrication tool, called the J-H ook  nozzle, was developed by the JSC EVA tool team to allow the crew to lubricate that surface. Photos of  the tools used are shown in Figure 16. 

NASA/CP-2010-216272