The system design was further optimized through careful consideration of load paths. This resulted in the  somewhat “organic” shape of the Axle  and how it “flowed” into the Boom. The lower Boom End Fitting  contributes significantly to the stiffness of the Axle and was developed integrally.  A slight exception was that a three-point semi-kinematic interface was implemented between the LDM  and the Primary Structure to minimize the potential distortions from bolt-up, because it provided the  easiest adjustment if ever required, and because it happened to offer the most direct load path into the Primary Structure at the time. While isolating the Hinge from the mounting interface in this fashion had its  benefits, it should be noted that it also had the potenti ally negative effect of “putting up blinders” to design  options that spanned the interface. In hindsight, though the semi-kinematic interface mitigated several potential  problems, the existing  problem of stiffness possibly could have been  remedied more quickly and  more efficiently had the Primary Structure been modified to help impart stiffness to the Hinge (like was  done between the Axle and Boom Fitting). A lesson learned was to not stop engineering upon reaching  an interface. To the extent possible, approaching an interface design from both sides may offer better solutions. The inertia supported by the UDM was only about one-tenth of the LDM’s, dramatically reducing the  stiffness needed to meet the frequency requirement. However, to minimize the number of drawings, parts, test configurations, and spares, it was decided to keep the two Mechanisms as similar as possible.  Therefore, the UDM was nearly identical to the LDM, except its deployment angle was shifted by about   20 degrees and it was ma ss-optimized by thinning most cross-sectional areas of the Axle and Clevis.  Bearings Every aspect of the Mechanism design encountered complications, and the Bearings were no exception.  The initial bearing selection was influenced by the availability of some size 107 spindle bearings left over  from another project that would expedite an early brassboard model. To maintain alignment, zeroclearance shaft and housing fits were needed to keep the Hinge axis from permanently shifting under the high radial loads experienced during launch and deployment. At the same time, the Hinge had a very  large temperature range because thermal control would be considered only as a last resort. The titanium  hardware and the 52100 steel bearings therefore faced a large CTE mismatch problem. Consideration also had to be given to the radial temperature gradient, which strongly affects preload in a DF bearing [5].  Further adding to the challenge, schedule didn’t allow for custom-toleranced bearings, nor fabricating the  hardware to match the bearings that were available, resulting in an increased tolerance stack-up. Lastly,  the bearings would be lubricated with a light grease-plate of Castrol Braycote 601EF, both for simplicity  and schedule. At the lower limit of Braycote’s recommended temperature range, the high lubricant viscosity would be the dominant source of drag. Lubricant drag calculation had the largest uncertainty,  requiring additional conservatism.  Bearing analysis software [6] was used to evaluate the wide range of worst-case scenarios. Maintaining a  line-to-line fit at one temperature extreme  would result in excessive interference at the other, leading to  unacceptably high ball stresses. Therefore, some amount of clearance had to be accepted. Further complication then derived from the software’s race clamping inputs. Axial clamp force and coefficient of  friction determined if the bearing race was constrained radially by the axial clamping. This effected  bearing preload as a function of temperature, depending on whether the race could slip and therefore behave as steel, or if the race was con strained and forced to have a CTE closer to that of the titanium it  was clamped between. Given the unreliable nature of the inputs (and somewhat questionable accuracy of  the calculated effect), clamping was assumed to always work to the detriment of the system. The final result was that bearing preload and stiffness rapidly dropped off at each end of the temperature range.  After many iterations and lots of fine tuning, ID and OD fits were established that just barely maintained  the necessary preload at the temperature extremes, without excessive drag elsewhere. While the worst-case analysis was extremely con servative, it was seen as prudent to balance the uncertainty associated  with significantly modifying the ground-in preload of duplexed bearing pairs th rough heavy interference  fits (over 25 μm on a 62-mm diameter). The bearings were baselined with a “medium preload” for  minimum drag, but with so much hinging on them (!) stiffness testing was planned, and “heavy preload”  models were procured just in case additional stiffness was needed. The lesson learned was that bearings  are complicated items with long lead times, and should be one of the first elements designed. 238