on the friction between the mating gear teeth. The action of spur gears creating a thrust force is  analogous to the friction-dependent thrust forces created by misaligned rollers (Ref. 5).   Shaft speeds and total number of shaft revolutions were measured using encoders on each shaft. The  encoder pulses were counted and recorded via a digital pulse counter. The encoders provide 6000 pulses  for each shaft revolution.  A linear variable differential transformer (LVDT) measures the tilting position of the drive motor plate. The  tilting of the plate changes the pinion-to-gear center distance, and so the LVDT output thereby measures  the operating center distance. As the gears operate, the operating center distance changes slightly  because of the gearing action. As the tooth contact position on the pinion moves from the dedendum,  through the pitch point, and to the addendum region, the friction force changes direction. This changing  friction force causes slight changes in the instantaneous center distance, because of elastic deflections,  and thereby the friction force affects the output of the LVDT.   The gear teeth surface conditions were photographed at regular intervals during testing through a  viewport. The images were captured digitally using a single-lens reflex camera with a 150-mm micro lens  and a 12-million effective pixel image sensor.  Experiment Method  The experimental approach was to conduct an equal number of tests using unexposed and exposed  surfaces. The test matrix of Table 1 was selected for the study. The pinions and gears were assigned  randomly as 6 test pairings per Table 1. With the ability to test both front (side “A”) and back (side ”B”) of  each tooth, 12 tests were possible. Test article pairings 1 through 4 were assigned to have side “A” tested  with zero exposure to humid air. There was some minimal exposure time to air during installation  procedure into the rig, but in this report such minimal exposure is considered as zero exposure. For test  article pairings 1 through 4, once “side A” of the teeth were tested, the pair was placed into the humidity  exposure chamber to begin the exposure time for the tooth sides “B”. For gear pair 5, there was zero  exposure and sides “A” of the teeth were tested. Then the test chamber was opened long enough to  remove the gears from the shafts and remount immediately for testing of teeth sides “B” as also  unexposed. For test article pair 6, the pinion and gear were placed into the exposure chamber at the  beginning of the test program to obtain a long exposure time while testing the other gears. Both sides “A”  and “B” of pair 6 were tested after an exposure time of 77 days. The exposure times for other test were  less than 77 days as was dictated by the testing pace and sequence.   As the test sequence progressed, it was decided that photo documentation of the conditions of the teeth  prior to test may prove insightful. Beginning with the fifth test in the testing order sequence (test  MoS2 4-A, per Table 1) the first step of the testing sequence was to document the visual condition of  each gear by digital photographs. Next, the gear pair was then mounted onto the test shafts, the vacuum  chamber closed, and then the chamber vacuum condition was established over several hours, and  typically overnight, prior to applying torque and motion. The chamber pressure was 7x10-7 Torr or less at  the beginning of each test. Figure 3 shows a pair of the MoS 2 coated test gears out of sealed bags just  prior to test and in the test chamber just prior to closing the vacuum chamber door.  NASA/CP—2018-219887 131