NCR 00040


Root Cause:

            The joint was analyzed with the shoulder of the bolt acting as a pin carrying all the shear load from the tower.  However, alignment tolerances required up to 4 mils of clearance between the shoulder and the flexure and up to 2 mils between the shoulder and the hole in the Grid.  Friction from the bolt preload proved to be not nearly enough to prevent movement within this tolerance during random vibration.  The movement caused the bolt to loosen. 


Contributing Factors:

Contributing factors were a low torque specification and the fact that no secondary locking mechanism was used.  The root cause of the bad torque specification was the lack of any reviewed assembly procedure documentation in Italy and lack of an assembly drawing.  The root cause of the lack of secondary locking was a deliberate position within the Tracker group that the test phase should be passed without such locking.



            Redesign of the flexure-Grid interface impacts the Grid design and the flexure design, plus tooling for drilling the Grid and flexures.  It impacts the Tracker schedule by delaying completion of the EM vibration test, delaying the EM thermal vacuum test, and possibly delaying production of the bottom trays.


Corrective Actions

  1. Redesign joint to act as true shear pins, with no slop.  This has been done, including a complete re-evaluation of the tolerance in the interface, as well as installation and verification methods.  The new design uses adjustable cones to take out any misalignment between Tracker and Grid while maintaining tight joint.  See drawings LAT-DS-50-1, 417-4, 422-4, 851-1, 1803-3, 1921-3, 2786-1, 2787-1, 2788-1, 2789-1, 2790-1, 2791-1, 2792-1, 2793-1, 2794-1.
  2. Complete the assembly drawing and procedure for the Tracker tower, including mounting on the vibration fixture.  A draft, LAT-DS-00722, now exists in LATDOCs.  It will be reviewed and finalized in preparation for the tower assembly PRR, to be held in mid April.  The assembly procedure, LAT-PS-01854, is being drafted and also must be finalized in preparation for the PRR.
    1. Include torque specifications.
    2. Include dry lubricant on the bolts to assure a reliable preload.
  3. Introduce a secondary locking mechanism for the bolts, even during any future test phase.  The Grid inserts in the new design include a locking feature, and the nut on the Tracker side will be staked with epoxy.
  4. The new joint design will be verified by a set of static tests and a new vibration test, either with an engineering model or during Tower A protoqual tests.  The plan for verification will be posted by 31 January 2004 on the Tracker website.  The decision whether to use an EM or Tower A for vibration verification is still under discussion with Project Management and Tom Venator of GSFC.