201 Testing shows that gold, as a solid lubricant, is capable of maintaining a low coefficient of friction (0.2) for  mistracking angles higher than the maximum conservatively predicted angle for the TBAs [4,5]. Testing reproduced several visible features of the on-orbit damage [7]. Testing also shows loss of gold adhesion  [6]. Using the same roller material as the SARJ TBA, the ion-sputtering process used for the starboard SARJ rollers was re-created and the gold finish was tested. The tests exonerated the gold plating process. However, aging humidity tests indicated that, in time, the gold film would delaminate from the  parent material due to corrosive growth on the substrate.   Metallographic analysis was performed to compare the imperfections between the race rings used for the starboard SARJ and the port SARJ. The data indicate that the defect concentrations in the starboard ring nitrided case are approximately six times that of the port nitrided case. The difference in the nitride case defect concentrations makes the starboard SARJ race ring more susceptible to damage initiation and damage propagation caused by high loading. It also may explain the survivability performance of the port SARJ race rings [10] .    Anomaly Most Probably Root Cause

The kinematics of the TBA and DLA mechanisms require that the roller thrust loads (related to friction coefficient and mistracking angle) be controlled to ensure stable roller line contact with the race ring surfaces. Inadequate lubrication of the roller/race ring interface combined with roller mistracking angles within specification resulted in thrust loads high enough to cause at least some of the TBA or DLA rollers to edge load as the SARJ rotated. When a roller is edge loaded, the preload on that roller is concentrated on a reduced contact area resulting in high contact stresses and shear stresses in the race ring case and core. These stresses exceed the allowable bearing strength capability of the race ring case and core leading to brittle fracture and spalling of the nitrided layer from the starboard SARJ race ring.   Lessons Learned

As part of the root cause investigation, the build paper was reviewed . Unlike the Port SARJ which was  tested in vacuum, the Starboard SARJ was not due to cost considerations . A complex mechanism such  as the SARJ cannot be analyzed for break-in performance . Instrumented vacuum testing, particularly for  the break-in period, might have yielded indications that the as-built mechanism was not operating  nominally . Since the degradation of the Starboard SARJ took place over a short period, during  accelerated testing the current increase would have been evident in a very short period of time.  The build-paper investigation also indicates that the testing decisions made for the Starboard SARJ did  not accurately account for the friction differences between an ambient and vacuum test environment . The  root cause investigation highlighted the sensitivity of the SARJ to small changes in friction . The SARJ  Structural Test Article was tested at ambient to verify drive pinion life. The test was not intended to verify  system life. However, since the entire system was utilized during the test , successful completion of the  test gave a false sense of security regarding system-level life . Recent testing indicates that the vacuum  coefficient of friction between 15-5 PH nitrided steel and 440C steel roller is approximately 0.6 [5] . It is  now understood that operating the SARJ with coefficients of friction greater than 0.3 increases the risk of roller tilting and resulting damage to the race ring surface.  The root cause investigation also highlighted the importance of correlating testing with analysis and dynamic simulations . Cost-effective and cost-saving simulations can aid the mechanism designer in the  understanding of the mechanism performance prior to its fielding . Moreover, mechanism testing should  have as a stated objective the correlation of the critical variables of the mechanisms performance . This  process can increase the chances of finding phenomena that may be time consuming to test without prior knowledge . The lesson learned here is two-fold and inextricable . A complex mechanism should not be  flown into space without testing, nor should it be flown only having been tested. Analysis must be integral to the testing but should not be used in lieu of testing. There are many conditions for mechanism failure that could be understood in a reasonable and cost-effective basis only by analysis. 

NASA/CP-2010-216272