• Using Geometric Dimensioning and Tolerancing significantly enhances ease of assembly and  reduces tolerance stackups.   • Work with gear manufacturers to establish pre- coating dimensions and tolerances to allow for  removal of recast layer or tooling marks,  case hardening, and/or dry film application.  • Work with materials and processes experts to select bushing, shaft materials and lubricants. 

PDU Lessons Learned   • Avoid using primer for corrosion protection on any of the m echanism  housings.  The PDU housings  use Super Koropon, which was found to flake and generate debris during assembly. Subsequent  housing designs used a hard anodize finish.  • Be sure to include alignm ent aids for features requiring high positional accuracy or that influence  geartrain perform ance.  The original design assumed the bol t holes’ positional accuracy for the  housings were sufficient to provide alignment aids for the gear train. Howeve r, it was found that the  hole sizes required to allow assembly of multip le fasteners allowed the housings to shift enough to  significantly reduce gear train efficiency.   • Design appropriate access for those design elem ents that m ay require rem oval. Many components,  including the gearmotors, could not be removed wi thout disassembling most of the gearbox. This  became very inconvenient when circumstances aros e which required replacement of the motors.  • Use caution when deciding to use COTS products . In this application, fixing shortcomings and  certifying COTS motors for flight was signific antly more expensive than originally considered.  However, COTS motors were useful for initial ground testing of the geartrain.   • For lower level components in a drivetrain, be sure to understand assem bly interdependencies at the  next higher level. Originally, the drivetrain torque tube pa ssed through a bushing in the housing of the  PDU. Upon assembly, it was determined that  this bound the torque tube and the bushing was  subsequently removed.   Torque Limiter Description   In order to protect components downstream of the PDU against overload caused by failures, jams, or  hardstopping, torque limiters were inserted into the drivetrain between the PDU and the each secondary  gearbox (See torque limiter in Figure 10). Sizing the downstream components to handle the contingency  loads, given such large gear ratios, proved not to be feasible from packaging and mass standpoints.  Based on the lesson learned during PDU developmen t, COTS candidates were carefully evaluated  before determining if this was an acceptable wa y to proceed. Detailed discussions with vendors  concerning materials, production methods, and feature m odifications were held prior to the decision. In  the end, a COTS vendor was sele cted, and a flight-like but undocum ented set of torque-limiters was  procured for testing. The product was COTS except for the use of Braycote 602EF in place of the  standard lubricant.    The torque-limiter is an adjustable ball-detent slip clutch set to slip at 40.3 N•m (357 in•lbf) and which  automatically re-engages after 360 degrees of rota tion or reversing the rotation so that the  synchronization of the two sets of mechanisms is not  lost if one clutch slips and the other does not. The  clutches are fixed to the torque tubes  through a friction-clamp interface. 

Torque Limiter Testing   Upon receipt, several torque tests were run on the tor que limiters to verify the correct slip setting before  using the torque limiters with the flight hardware.  The torque limiters operated with no problems during  and after exposure to all environments. For flight, a specification control drawing would need to be  released documenting the modified COTS hardware. 

Torque Limiter Lessons Learned   If you’ re careful, COTS can work well !  Significant cost savings were  achieved using COTS clutches  which only cost a few hundred dollars total versus a custom design effort to develop a torque-limiter.  Make sure that material vacuum compatibility, if r equired, is completely worked out prior to purchase.   Secondary Gearbox Description   371