223 All the while, a simultaneous deeper inspection of the Circular Splines revealed that their black oxide  coatings did not demonstrate the expected physical properties. Namely, it  seemed unnaturally thick for a  hot black oxide conversion coating, and instead could be scraped off with a dental pick, turning to powder  in the process and revealing a bare surface. It appeared to be a room- temperature, or cold, black oxide  application, which is a deposited layer of copper selenium as opposed to a conversion coating, which is not  included in the MIL- C-13924 specification as an acceptable black oxide application process.  

This understanding of the black oxide type provided the clue to the rogue copper discovered in the previous  XRF grease analysis results. The cold temperature black oxide coating had been slowly scraped into the  grease by the sliding tooth action during the Endurance Test, and, in addition to generating debris in the  tooth mesh grease, it was certainly not performing as a corrosion inhibitor for the Circular Splines, as it was  intended.  

This meant the Circular Splines had to be abraded as well, to remove all of the undesirable black oxide  coating. After this was completed,  the units were re- cleaned with Heptane, re- grease plated, and slurry  filled as before. This time, when the completely renovated HD was reassembled into the Drive Output  subassembly, rotation was smooth, and lubricant was clean. See Figure 15 for Circular Spline images  before, during, and after cleaning and re- assembly.  

Figure 15.  Panel 1: Harmonic Drive with Room Temperature Black Oxide on Circular Splines, Dynamic  Spline Shown, Installed into Drive Output Subassembly, with Braycote 601EF at Tooth Interface. Panel 2:  Clean Circular Splines with Black Oxide R emoved. Panel 3: Clean Harmonic Drive Circular Spline,  Dynamic Spline shown, Installed into Drive Output Subassembly, with Rheolube 2000 at T ooth Interface.  

The Final Endurance Test   The new Pennzane lubricant configuration needed to be vetted by running it through a successful  Endurance Test. A refined test profile was created, reducing the loads and varying them based on the three  dither timings. The target speed was reduced to be the flight maximum value of 7.75 deg/s, instead of  8.0 deg/s. Cycle counts were scrutinized, and the definition of an actuator mechanical life was reduced from  4.5 to 3.7 million input revolutions. The test would be run for a total of two time one life instead of three  times , reflecting a hesitation against over -conservatism, since the JPL standard policy calls for a 2x  mechanical life test for wear -life limited elements. This brought the required total HD input revolutions for a  successful test declaration down from 13.5 to 7.4 million. The fixture was also modified to include a pulley  and weights at the output, such that the brake would be used to apply different bi -directional output drag  loads, and the pulley system would simulate the uni -directional load of the flight ant i-backlash mechanism,  present in the BGA design to facilitate pointing accuracy. A simple control system was also implemented,  with a speed controller and position- based sensor feedback to make the testing automatic and consistent.