562 and lubricants  must be properly selected . This was particularly import ant given that the life requirement of  GPM SADA and HGA gimbal actuators significantly exceeded that of previous GSFC missions that had  used the same actuator: SDO had a life requirement of 2500 cycles/900,000°; LRO’s was  5700  cycles/2million degrees; GPM’ s SADA is 17500 cycles/8.75million degrees and GPM HGAS is  48,600 cycles/18.5million degrees . To ensure the SADA and HGAS actuators would meet their life  requirements Sierra Nev ada Corporation  (SNC) Durham,  NC facility  and NASA GSFC performed a series  of harmonic drive accelerated life tests with different combinations of circular splines and flexible splines  materials in both ambient  (2x life) and vacuum environment (100,000 cycles)  [3 ].  

An exceptional  description of  the testing process  and results  obtained  is provided in Reference 3, and main   results are summarized here. Based on discussions between Harmonic Drive LLC and SNC, the material  combinations shown in Table 1 were selected for comparative life testing. All HD gear configurations were  customized HDT -25, 200:1, T -cup component sets manufactured by Harmonic Drive LLC and all were of  the same dimensional design. All  combinations used the same wave- generator bearing material and  design, and each was identically  lubricated with Pe nnzane 2001- 3PbNp oil and Rheolube 2004 grease per  standard procedures . For the ambient test (conducted in air at room  temperature),  the input to the harmonic  drive gear was set at 500 revolutions per minute ( rpm) and a co nstant 2.93 N*m of friction load was applied  to the output . The main results for this test were as follows:  the two Melonite Harmonic Drives  showed a  reduction in torsional stiffness at the end of life and the presence of relatively large particles of metallic  debris in the gear mesh and were eliminated from consideration. The two non -Melonite gears were relatively  comparable . The 15- 5 vs. 15 -5 gear visually looked slightly better with regard to the circular spline teeth  and gear lubricant but the Nitronic vs. 15- 5 gear was also acceptable. This latter one was selected as the  least risky p ath forward due to concerns over how the 15- 5 vs. 15 -5 would perform during vibration and in  a vacuum environment [3] . 

Table 1. Harmonic DriveTM Materials Selection Ambient  (2x life) & Vacuum  (100K cycles)  Test Results [3]  

Based on the above results it was decided to perform a second round of testing for the two leading  candidates but in a vacuum environment under thermal cycling. The vacuum test consisted of  100,000 cycles of reversing 180° output revolutions in a <5.0x10-5 Torr vacuum enviornment cycling  between 0 and 40°C . Input to the harmonic drive g ear was set at 500  rpm and a cons tant 2.93 N*m of  friction load was applied to the output . Results showed that innthis case, the vacuum environment had little  to no effect on the wear rate of the two harmonic drives tested with small differences in wear between the  units . The vacuum test confirmed the program’s decision to use the 15- 5PH flexible spline against the  Nitronic 60 circular spline as the f inal configuration to be used on the GPM SADA and HGAS actuators  [3].  

As it can be seen on the cross sectional view  Figure 9,  the Spacecraft Interface Plate (SIP) is used to attach  the Solar Array Drive Assembly (SADA) to the Lower Bus Structure panel . The design and analysis of the  SIP was driven by the deployed fundamental frequency requirement of the Solar Array which translated in  the need to have a very stiff SIP . The Cable Wrap attaches to the bottom of the SIP and both are stationary .  The mounting flange of the actuator is attached to the bottom of the SIP and is also stationary . The Hinge