Figure 4 . Initial Wear on Bushings: 8.487 MPa @ 40.1 mm/s for 1.78 km  Since  the cold  environment  proved  more  stressing  on the bushings  in this flight  lubrication  scheme,  the  program updated its life test temperature requirements to bias them colder than originally planned. A new  requirement was added to operate at least 25% of the life at an expected mission nominal temperature of  -55°C. These cycles were in addition to the existing requirements to operate at least 25% at cold (-70°C)  and 25% at hot (+70°C); with the remainder of cycles anywhere between hot and cold temperatures. Also,  the life requirements  of the dominating  gearbox  designs  (GB7  and  GB8)  were  iterated  in JPL’s  s pecifications  after  the initial  tests  on Bushings  A and B. The  peak  load  was  reduced  from  8.487  MPa  (1231  psi) to 8.173  MPa  (1185  psi),  but the required  Low  Load  duration  was  increased  from  122  km  (400,170 ft) to 301 km (987,101 ft), as can be seen in Figure 5. The remainder  of the component-level  bushing  test was  planned  to be completed  using  Bushing  A, but  biased  cold.  Having  already  completed  the required  operation  at hot, this bushing  would  be exposed  to  25% of the updated  High  Load/Short  Duration  load  case  at cold,  and then  exposed  to 100%  of the Low  Load/Long Duration cycles at a continuous -55°C, as indicated by Test #2 and Test #3 in Figure 5.  The operational  profile  was  also  updated  for the following  tests,  incorporating  a five-minute  pause  for  every 20 minutes of operation (two cycles of five minutes clockwise, five minutes counter clockwise). This  was in acknowledgement  that  the gearmotors  would  not be run continuously  on Mars,  and  that  the  periodic  pause  may  allow  for some  beneficial  oil reflow  from  the bushing  without  dramatically  increasing  the length of the developmental test. Finally, as the Low Load point would correspond to a lower gearbox  torque  transmission  and  higher  operating  speed,  the operating  speed  at 2.45  MPa  (356  psi)  was  increased to 192 mm/s (37.7 ft/min).  Unfortunately, this Low Load testing  in the fixture  with Bushing A resulted in a premature bushing failure  after only 25.58 km (83,914 ft) at 2.45 MPa (356 psi) shown in Figure 5. Furthermore, after the torque cell  data was  reviewed,  it was  determined  that  the bushing  began  operating  in a degraded  condition  after  19.20 km (62,977 ft). See Figure 6 for the failed condition. While this failure occurred short of the duration  required to envelope the life of all design configurations, it was sufficient to satisfy the life requirements of  the GB2, GB3, and GB9 designs.  It was  believed  that the test setup  led in part  to the premature  bushing  failure.  Clearances  in the fixture  could  allow  the radial  load  bar to apply  a slight  axial  load  into  the bushing  and  cause  edge  loading.  Concerns  regarding  the fixture  and  the negative  test result  led to the desire  for additional  testing  that  better represented a flight-like configuration within the context of a planetary gearbox. However, as flightlike gearboxes  of the GB7  and GB8  configurations  would  not be available to  utilize  in testing  for several  months, the results of this initial test led to a decision to begin a parallel procurement of components that  would  allow  the GB7  2nd Stage  and  GB8  2nd Stage  planet  supports  to be ball  bearings  instead  of  bushings. Without bushings in these locations, the enveloping required life of bushings at low load would  be significantly  reduced.  Additionally,  the possibility  of ball bearings  in these  locations  would  drive  the  requirements of component-level bearing testing.   © 2018 Sierra Nevada Corporation  NASA/CP—2018-219887 534