561 On the structural  side, the solar arrays had a 0.6 Hz minimum fundamental frequency  requirement . Since  the first mode of the array  is cross- axis bending ,  it was designed to maximize the stiffness in that axis as  much as practical . To this purpose, numerous technical interchanges  with the actuator manufacturer were  conducted to modify the actuator design in order to develop an actuator that could meet our requ irement  for a bending stiffness of at least 339,000 N-m/rad. To maximize the bending stiffness, the output bearing  was changed from a tradi tional duplex back -to-back pair with a set of spacers between the bearings to a  Super Duplex™  bearing with a one- piece outer ring to minimize the number of components and sliding  interfaces.      The Super  Duplex™  thin section bearing has a common ring, either a single outer (as in GPM’s  HGA S and  SADA actuator output bearings), or a single inner, which has two bearing paths in it . Because this type of  bearing had never been used before on a GSFC mission, an evaluation and comparison of Super Duplex™   vs the standard duplex DB pair  was conducted. Stiffness: Havin g a “one piece” rigid outer ring provides  improved ring stif fness and reduced distortion,  as there no longer is a two- piece assembly; Geometric  Dimensioning and Tolerancing ( GD&T ): two key GD&T  parameters that will affect the performanc e of thin  section ball bearings are the radial and axial runout. The radial runout is a measurement of the thickness  variation of  the bearing rings. The outer ring is measured from the ball path to the outer diameter of the  ring, and the inner ring is measured from the ball path to the bore. The radial runout is defined as the wall  thickness variation of the rotating ring. The axial runout on the other hand, is measured from the ball path  to the face of the bearing rings. The variation in thickness measured is the  axial runout.  The Super Duplex™  bearing has a common ring , which  has two bearing paths in it. As these are ground at the same time, they  “run- out” together. This helps  smooth out potential torque variations that would be caused by the variations  in axial and radial runout adding torque into the bearing; Assembly & Installation: the disadvantages of  using two separable bearings are potential errors in misalignment and orientation during assembly. If   however , a SuperD uplex ™ bearing is used, since the assembly is not separable, the possibility for  installation  orientation errors is eliminated [2].  

Figure 10. SuperDuplex™  (left) vs. Duplex DB (right)  [2]  To maximize  the bending stiffness of the actuator  the internal contact angle of the bearing was increased  by 50% and the internal preload was increased by 300% . The machined components that support the inner  and outer ou tput bearing raceways were chang ed from Titanium to Stainless Steel for a hig her modulus of  elasticity and the inner raceway support was thickened to add rigidity . Finally, to minimize the influence of  bearing mounting clearances, the tolerances on the mating journal diameters were tightened, the minimum  installation clearance was reduced, and bearings were matched with the mating machined components to  minimize the clearance fit across all deliverable units and provide more consistent results . Detailed bearing  analysis was performed to ensure the final bearing parameters and fit -up resulted in acceptable stresses  for operating and static loading conditions . A detailed Finite Element Model of the actuator was correlated  with preliminary subassembly stiffness testing to gain confidence that the final design configuration would  meet ov erall actuator stiffness goals . The as -delivered actuators demonstrated bendin g stiffness in the  range of 341,000 to 370,000 N-m/rad, above the target goal to meet the desired fundamental frequency .  

The increased contact angle and preload result in increased bearing friction and contact stress relative to  the SDO/LRO heritage. Therefore,  a successful 6.5x  life testing was  performed to verify the expectation  that lubricant life would still be more than adequate for the mission. One of the main lessons learned from  the TRMM anomaly to ensure that mission life requirements  are met is that the harmonic  drive  materials