43 Mid-hinge Testing   To test the Mid-hinge thermal latches, the preload in the spring latches needed to be monitored as the  hinge assembly was taken to cryogenic temperat ures. The two aluminum spring latches were  instrumented with thermistors and strain gages that  were calibrated down to 80K. The latch gap between  the spring latch and the catch was set for a desired preload of 2335 N +  111 N (525 lb +  25 lb). The  latches were then cycled down to ~88K (-300 °F) and back to room temperature 3 times to characterize  any creep due to loading and unloading the assembly. The results of the testing in the form of preload  verses temperature curves are shown in Figure 26.    Several things were apparent from t he test results that will help in the design of the flight latches. First,  the latches proved to be very repeatable with the cool- down curves for the three cy cles lying nearly on top  of each other. Also, the stiffness of  the latch preload paths for latch 1 & 2 were very well matched, but  only after the initial cool-down fr om room temperature to ~255K (0 °F). The Bellville washer stacks  demonstrated good repeatability and we ll matched stiffness during cool-d own, but the non -linear behavior  and large amounts of hysteresis that is often associated with Bellville wa shers was evident as the latches  warmed back up to room temperature.   In addition to setting the initial gap of the latches, the preload in the Bellville washer stacks is set to just  below the desired operational preload. This was ac complished for the DOTA testing by measuring the  desired compression of the washer stack by counting  turns on the preload nut. Since this method lacked  precision, the resulting initial preloads of latch 1 & 2 were off by roughly 445 N and 663 N (100 lb and 140 lb) respectively. This, however, was balanced by the added compliance in latch 1 and both latches  achieved the desired flight preload.    The Bellville washers served their purpose for the DOTA test and demonstrated the advantage of having  a compliant member in the latch stiffness path. Howe ver, we are currently looking into replacing the  Bellville washers with a machined spring in order to  increase the compliance, further reducing the  sensitivity to thermal changes, and to re duce the non-linearity of the latch.     End Hinge Testing

The end hinge DOTA testing was very similar to th e mid-hinge test. The clamping pin was instrumented  with thermistors and a strain gage and taken down to 88K. The hinge was cycled 5 times and demonstrated very good repeatability as shown in Figure 27.    In addition, a cable with a spring and a load cell was attached to the end of the end hinge strut and  tensioned with a motor until the latch broke loose. An  LVDT was used to measure displacements of the  strut near the latch and clearly indicated when the torque generated by the motor exceeded the torque  carrying capability of the latch. Th e test was repeated 3 ti mes at six temperatures (preloads), and as  expected, proved to be very repeatable. The results ar e plotted in Figure 28 as applied slip force verses  latch preload. The flight preload of 13,345 N (3000 lb f) provided an 8.47 N-m (75 in-lb) torque carrying  capability, which is 10x greater that the expecte d on-orbit loads.