257 To both maintain the desired thwack impact speed and converge on a structural  solution, the range  between minimum and maximum release had to be closely controlled.     A computational mechanism simulator was created t hat modeled the specific mechanism components  and forces between them to accurately predict the point  of thwack release. Incorporated into this model  was a variable range of friction coefficients applied to all sliding surfaces and rotary joints. Because the  release angles between the latch and tang depended heav ily on the machined final dimensions of the  internal mechanism components, a method of inducing variation due to tolerances was incorporated into  the mechanism simulator. A standard tolerance stac k-up across the mechanism was fed into the  simulator to account for how small deviations in part tolerances affected the release point of the  mechanism. Finally, the change in mechanism release as  a function of component wear at end-of-life was  incorporated into the model. The mechanism simulation additionally aided the design because it was able to predict the specific contact angles and surfaces that were frictionally compressed at all moments of the latch release. This allowed internal radii to be dialed in  to ensure that local yielding from contact stress did  not occur (this was later verified in subcomponent  mechanism life test). It was only through this  computational design synthesis, which was able to  account for frictional dependency, manufacturing  tolerances, spring variability and internal component wear, that the range of mechanism release was well  understood (Figure 12) and the convergence of a so lution that met all functional and traditional  requirements of the PTM was possible.                  

Figure 12. Output of Mechanism Simulator   When assembled and tested, the PTM showed a release point within two-tenths of a degree of where  expected and the actual thwack impact speed was pr edicted within ten percent. During the development  and assembly of the PTM, three lessons were lear ned. 1) Developing a sufficient understanding of the  sensitivities in the functional requ irements of a mechanism is worth the testing time to find them. This  allows changes late in the design phase to be incorporated without re-dialing in the entire mechanism. It was likely that a reduction in the thwack impa ct speed could have been made and still met unclogging  performance metrics, however, the mechanism was unable to capitalize on that possibility due to lack of test data. 2) Mechanisms that have tight requirements on internal alignment should not be split across an interface. As can be seen in the cross section of the mechanism, the upper and lower portions of the  mechanism are not explicitly connected. This drove s ubstantial assembly effort to ensure that these two  parts were able to be aligned to each other with  a high degree of accuracy. 3) The trial-and-error  fabrication technique of hand-winding st eel springs was not compatible with the high degree of accuracy  required for this mechanism. In hindsight, a feature to  vary the initial spring preload within the mechanism  should have been included. Spring variations had been accounted for in the design by having a settable  latch release position, but it would have been far more efficient to modify the initial spring preload when installed in the mechanism.     NASA/CP-2010-216272