strain gauge functionality on demand and to keep track of how much torque should be applied. In addition  to these requirements, pre-load data showing variations under random vibration, de sign limit load (DLL)  application, thermal cycling, and time were collected to assure that pre-load would not go out of the  qualified range of 4448N to 6672N during the required duration of mechanism performance. A target pre-load of 5693N was formulated based on the recorded data.  Margin Assessment

The fault tree approach required the assignment of a probability of failure for each structural element. Positive margin assessment was therefore made at nominal and worst case geometry models. Since the  LL is an adjustable system, the geometry of the model was put into a “worst case” configuration as judged  by the author; who considered the development of moments due to DLL loading. Both high and low pre-load bounds were also applied from 4448N to 6672N since having lower level pre-loads could result in  lower contact and clamping forces causing stresses to re-distribute to other mechanism components. The  difference in Von-Mises stress b etween the two models was considered to be a 1-/g305 variation with respect  to possible deviations from the modeled stress assuming a Gaussian distribution. This assumption gave a  means of quantifying a probability of failure. For several parts, the difference from wo rst case to nominal  in stress was not negligible.     This process was an example of allowing reliability numbers that do not add value to drive considerable  effort. Carrying out this process was very tedious. The interpretation of actual worst case conditions is a very difficult problem that can be conflicting for different load cases. For example, a thicker member may  be best for structural loading but may cause problems for thermal loading. It is also very hard to interpret  stress differentials between mating parts because interface meshes were not exactly the same, requiring  manual inspection during post-processing. In addition, contact conditions sometimes varied significantly.  Development of simplification for this process or assigning 0% probability of failure for parts with positive margin in worst case conditions is recommended due to the tediousness and small real value of these  efforts.  Assembly and In-Process Testing

One hundred percent inspection data was used to build the three flight units based on which parts were closest to nominal. The entire process of assembly was carefully controlled by a procedure that included  all the necessary critical verifications that tie back to the fault tree. In the later stages of assembly,  thorough characterization of each unit was carried out through a process that was dubbed “in process  testing.” This testing involved careful measurement of the mechanism drag under various configurations  including a single torsional spring case. It quickly revealed that flight 1 and 2 easily met the required operational torque to friction torque ratio of 3 but that  flight 3 did not. This task was highly valuable in  understanding the mechanism and its performance. Coatings Issues  The failure mode that was hardest to obtain satisfactory results for was the spherical interface from the LL  main housing to separation element (Figure 2). The spherical interface was accepted because open  rotational d.o.f’s did not cause significant changes to the system level 1 st mode and because it allowed  the separation element to enter the pocket with an arbitrary orientation. This reasoning did not consider  the resultant interface. The moment arm of the LL sphere is 15.82mm and the moment arm of the gimbal is 596.9 mm (Figure 7). This thirty-eight-fold amplification makes sliding at the interface due to the gimbal  dynamic deformation inevitable even for very high friction and the highest pre-loads sustainable. The best  alternative to attempting to prevent sliding is therefore to allowing the sliding to occur while attempting to minimize wear. This is opposite to a “no-movement-no-wear” approach like those presented for recent  studies for rough diamond like nano-ca rbon (DLN) surfaces [6].    Configurations were not controlled to the degree necessary for exact sample-to-sample comparison and  are very specific to the test setup used for vibration testing. Cleanliness, surface preparation, coating  application methods, and lubricant application methods were not well documented except for the final  TiCN-MoS 2 case. The only precisely controlled variables were pre-load (5693N) and test setup  configuration. The random vibration (RV) environment generated was for a downsized test setup that  replicates the exit angle of the flight unit but is smaller by a scale factor of 2.5 (Figure 8). The fixture was  137