Root Cause  The physical cause of the failure was the discrepant corner radius. The failed lot of pins was machined  with a sharp corner which violated the block tolerance. However, the discrepancy was not caught at  inspection since it was only a thread relief subject to block tolerances. Even though the parts were  discrepant per the drawing, it is clear that the critical radius was not adequately toleranced. Given the  exposure to cyclic impact loading, a specific radius callout was needed along with inspection  requirements for the feature.  A contributing factor to the failure was the high number of cumulative cycles on the test units, without  which the failure would not have occurred.  Corrective Actions  Corrective actions implemented as a result of the failure include addition of a healthy radius (.305-.432  mm or .012-.017 in) at the thread relief on the pin, and a countersink in the mating part. These were  flagged as critical inspection features on the drawings. The mechanisms in the failed production lot were  rebuilt with the improved geometry. Life tests were performed and confirmed essentially infinite fatigue life  with the revised radius callout. In addition, a cycle log is now maintained during production to monitor all  mechanical releases of the device, to preserve as-delivered mechanism life.  Discussion and Lessons Learned  During a failure investigation, unintended overstress of components can occur even with a careful  approach. Prior to this failure, we were not concerned about any life-limiting component in the  mechanism. The need to develop the process controls and characterize the hardware was real and the  cycles were not believed to be stressful to the units. The mechanisms in this lot were first produced as  single spring units (Gen l), then rebuilt as dual-spring units (Gen lA), and rebuilt yet again with the  stabilizing washer (Gen 1B). If the mechanisms had not been operated and tested so extensively in the  previous investigations, they would have easily passed the Delta-qualification test. Again, focused on  failure resolution and developing adequate process controls, we overstressed the hardware and  introduced another failure mode.  A more obvious lesson: always ensure a proper corner radius is specifically called out at corners of  thread reliefs - especially on shoulder bolts subject to cyclic loading.  The pin failure is an example of a vulnerability in the design which only required repetition (multiple lots)  to create the conditions for a failure. Even the most careful design reviews will not eliminate all areas of  vulnerability. In commercial manufacturing, process variation and scrap occur even on the best  production lines because of recurring opportunity for variations. The same is true in production of  aerospace hardware. Small lots of space hardware which pass a single qualification program are not  necessarily without areas of sensitivity in the design. With repetition in procurement and manufacture of  future lots, the failure modes will be revealed.  A new failure mode was not a welcome development in our path to complete the last of the Gen 1  production lots. There was concern for the impact to field units before the full lot history of the pins was  completed. Fortunately, no delivered units were affected.  Gen 2 Design and Roller Position  About the time the pin failure was resolved, the Gen 2 qualification test program was starting. The Gen 2  design incorporated significant revisions for usability and manufacturability, and was subject to intensive  review. During qualification level pyroshock testing on the Gen 2 design, 2 test units were exposed to out-  of spec shock levels, and released preload. Though the over-test condition was a clear physical cause, a  thorough investigation process revealed an underlying, physical root cause to the preload release. in the  process of changing a feature for the Gen 2 design, the nominal position of the 8 rollers was inadvertently  shifted relative to their detent slots in the outer race.  85