565 These two arm spiral wrap transitions to a round bundle to pass through the actuator quill shaft, and  communicates 16 6 wires (132 power & 34 signal) across the rotary joint to the Solar Array . The two main  belts are separated by a .25-mm ( 0.010 -in) thick sheet of  Kapton w hich minimizes the sliding friction  between the belts and their Dacron stiches . The GPM Cable Wrap has a rotation capability of  ±178° (where  it either winds completely on the inner diameter or out on the outer diameter)  in excess of the ±120°  requirement to minimize the bending stresses on the belts  and allow it to operate in the ‘sweet spot’ region  of low friction . Harnessing is clamped to a 3- piece spool assembly (for ease of manufacturing) made out of  Titanium . The spool is supported by two radial bearings slip fit to the housing and spool end fittings . The  forward bearing is axially constrained whereas the aft one is not, to allow for spool assembly tolerance  stack- up and thermal growth. Bearing inner races are clamped to the spool end fittings and forward bearing  outer race is clamped to the CW housing. Because there is a 20°C gradient across the bearing races a  problem ar ises due to the differences in Coefficients of Thermal E xpansion  (CTE) between the bearing  races (Stainless Steel 440C), CW housing (Al -6061- T6), and the CW spool (Ti -6Al-4V) which required a  0.0005 – 0.0008  in (13 – 20 µm) radial play  [4]. 

Figure 14. Cable Wrap Radial Bearings [4]  

Over large temperature gradients such as the ones that the CW  experiences, the clearances between the  components in contact with the bearing can be eliminated and excessive loads could be transmitted into  the bear ing, loads which could potentially cause brinelling of the its races . For of this reason it is very  important to make sure the right tolerancing is used at the inner and outer race interfaces . A thorough   bearing thermal stress analysis was performed that took  into consideration the compression of the bearings’  outer race due to the CTE  mismatch (6061- T6 vs. SS440C) at this interface; contraction of the bearing’s  outer race bore (where it contacts the ball ) due to this mismatch and associated loss of radial play; and loss  of radial p lay across entire temperature range (0 –  60°C) due to a 20 °C temperature gradient between inner  and outer race. Since there was  interest  in looking at possible interferences due to thermal growth because  of CTE mismatches that could generate stresses on the bearing, the worst scenario for this condition  was  chosen: max. spool O uter Diameter (OD)  vs. min bearing Inner diameter  (ID) and min CW housing ID vs.  maximum  bearing OD . Analysis showed that even after loss of radial play the  highest mean contact Hertzian  Stress that the bearing balls saw was 6.5*108 Pa which was  well below the 2.4*109 Pa that would cause  brinelling on the races .  

SADA On -Orbit Performance  

The GPM Core Observatory as seen in Figure 8 operates in a near circular orbit of approximately 407 km  with a 65- degree inclination . It is a three- axis stabilized spacecraft , nadir pointing for instrument observation  of the Earth and its atmosphere, with the X -axis aligned with the velocity vector. Depending on Solar Beta  angle, GPM flips 180 degrees in yaw such that it flies with either the +X or – X axis forward. However, since  it has thrusters on both sides, it can execute maneuvers in both orientations without slewing. GPM’s orbit