Testing  Proper testing of mechanical systems is extremely important, and cannot be over-emphasized. A  comprehensive test program, including run-in tests, functional and environmental acceptance tests,  qualification tests, and design life verification tests, is recommended to ensure that mechanical systems  operate properly on-orbit. In addition, development testing, done early with prototype hardware, is  extremely valuable and has proven to be cost effective by catching problems early when there is time to fix  them.  One aspect of testing that is often overlooked, which will be discussed below, is the interaction of  hardware inspections and functional verifications.  Pavload Bav Door Mechanisms  Each Space Shuttle Orbiter Payload Bay Door (PLBD) is comprised of multiple sections that are permitted  to float along dry film lubricated shear pins. There are multiple requirements to inspect these expansion  joint shear pins after a predetermined number of flights. At the same time, there are multiple door  functional verifications for the operation of the PLBD mechanical systems (drive mechanism, bulkhead  latch mechanism, and centerline latch mechanism). The shear pins are critical to the proper operation of  the doors. In the event that the pins are replaced following a door functional, the door functional test must  be repeated with the newly installed pins in place. Over the life of a vehicle, unanticipated actuations like  these can add many cycles to mechanisms that were not necessarily accounted for during the original  design phase.  In an integrated vehicle of various systems, there will be some overlap between hardware inspections and  functional verifications. Numerous examples of this interrelation of maintenance requirements and  functional verifications exist within the space shuttle program, including the interaction between tile  replacement work and the cycling of the landing gear doors, star tracker doors, air data probe doors, vent  doors, external tank door, etc.  Lesson Learned: Design life of mechanisms should consider maintenance cycles of interfacing  systems.  Lesson Learned: Processing work should be planned such that the fewest number of functional  tests are performed, and cycles on mechanical systems should be tracked.  Summary  Several lessons learned have been presented in this report. A complete summary of the lessons are  presented in Table 1. These lessons may prove to be of benefit for anyone who designs, develops,  evaluates, or operates mechanical systems, regardless of the program.  Categon,  Binding, Jamming,  i and Seizing Table 1 : Summary of Lessons Learned  Lesson  Proper tolerancing and inspection are critical to preventing interferences in  mechanical systems.  Repair and rework of mechanical assemblies can cause collateral damage.  Dry film lubricant thickness is important and the appropriate surface preparation  must not be overlooked.  Fasteners:  Secondary Locking  Features Clear secondary locking feature requirements need to be specified in program and  project requirements documents.  Liquid locking compounds are very application process-sensitive and not  verifiable, and therefore are not recommended for use as a secondary locking  feature.  124