194 More consideration early on should have been given to halting operation of the mechanism until the  anomalous data signature was better understood. Cessation of nominal operations was not seriously  considered until just prior to the EVA inspection because the changes in operating conditions did not  represent an immediate threat to successful operation. The system was still effectively tracking the sun with ample torque margin. Instead of focusing on overall system capabilities, it would have been more  useful to focus on relative changes in the operational performance. For example, a change in drive motor  current from 0.25 to 0.50 ampere represents a change of only 17% in terms of overall torque margin .  However, it also reflects a 100% increase in joint drag . This jump in required current should have caused  significant enough concern to stop operating the mechanism. Changes relative to previous operational data, or data from the rest of the hardware fleet, are more indicative of hardware issues than changes relative to overall system capability .  

Anomaly Root Cause Investigation 

A team was formed immediately after the EVA inspection of the Starboard SARJ revealed significant  damage to the bearing surface. The team was made up of individuals from NASA Johnson Space Center  (JSC), Kennedy Space Center (KSC), Marshall Space Flight Center (MSFC) , Glenn Research Center  (GRC), Goddard Space Flight Center (GSFC) , NASA Engineering and Safety Center (NESC), The Boeing  Company, Lockheed Martin Space Systems Company (LMSSC), ATK, and Purdue University. The teams charge was to determine the root cause of the anomaly, determine a corrective action for the damaged mechanism, and determine appropriate recurrence controls for the undamaged (Port) SARJ.  The debris samples taken during the EVA inspections were analyzed in detail by Boeing Houston and NASA JSC and KSC materials and process teams [2] . The analysis yielded several key findings. First, the  debris was primarily composed of the case material . Second, the debris thickness showed that the  damage did not extend into the core material . And third, the morphology of the debris indicated that the  damage was initiated via subsurface spalling . It was not immediately clear to the anomaly investigation  team what conditions would generate sufficient stresses to cause the premature case spalling observed.  The anomaly team created a fault tree to aid in the search for the root cause of the damage and to focus in the areas that were critical for investigation. The focus areas were software, hardware, and operations .  The fault tree yielded over 350 events that were studied individually . The fault tree events were closed by  providing analysis, testing, simulation, or a combination of these. The closure process for the fault tree required that events be combined in a worst-case fashion. A review of fault tree events led to the  identification of a set of critical variables for this anomaly . The critical variables were determined to be  roller misalignment, lubrication selection and roller/race ring friction, bearing material properties, and applied loading . The team evaluated the interdependencies between the critical variables to determine  the most probable root cause of the anomaly.    TBA Roller Misalignment

The TBA roller design causes a pushing action on the roller as the SARJ rotates . This design does not  auto correct for roller misalignments as a castoring, or pulling, design would [3] . Misalignment is inherent  to any design and manufacturing process . The trundle bearings used match drilled assembly procedures  to minimize tolerance bui ld-up and associated roller misalignment. Actual misalignment was not  measured on individual units as part of hardware acceptance . After the anomaly had occurred, roller  misalignments were measured at MSFC using a coordinate measuring machine as part of the root cause investigation  [4]. The measurements showed that the trundle bearing misalignments were all within  tolerances  (±1 degree) . Nevertheless, coupled with high friction, the misalignments were large enough to  generate detrimental thrust loads on the roller bearings.   Lubrication Selection and Roller/Race Ring Friction

Gold lubrication was the design choice to mitigate the frictional loads caused by these misalignments in both the TBA and DLA rollers. Test data obtained as part of the SARJ Anomaly (discussed in further detail below) show that a gold film on the trundle bearing rollers could maintain a coefficient of friction of  NASA/CP-2010-216272