365 Lessons Learned 

While the upgraded brush motor development and qualif ication was successful, but the methodology in  some areas could have been improved. The following paragraphs document the major lessons learned during development and qualification.   Understand derived requirements

A firm understanding of the requirements (actual and derived) is needed prior to development. At the  onset of the development process, the gear head bushings were identified as a cause of BGM  performance problems. A total redesign of the gear head was started, with heritage design practices,  processes, and software ut ilized in the new gear head. Gear design parameters were optimized to allow  for greater allowable tolerances and used compatible material combinations to reduce galling and thermal expansion issues. Optimization of the gear head fo r producibility adversely affected performance,  however.    Since the BGM motor torque output is relatively low, it is sensitive to drag torque. Valuable time was spent on developing a new gear head that had a gear drag greater than the motor could produce. If the BGM gear drag data had been available, it would have been realized that there was little chance to design a new gear head with significantly lower drag.  Understand test capabilities

At the start of development it was determined we would test all gear heads before they were integrated  into the BGM. It was assumed we would use our st andard test setup, tooling and test equipment. During  initial gear drag testing it was disc overed that minor misalignment caused major shifts in the drag torque  measurement. Thermal expansion of the tooling was enough to double or triple drag torque measurements. A standardized proces s was developed to consistently adjust the alignment before each  test.  Verify performance at every environment

During testing it was observed that  brush drag and wear were different in vacuum than at ambient  pressure. The leading brush material was eliminated after vacuum testing. Almost no wear was observed during ambient and initial vacuum testing, but during  extended vacuum testing the brush was completely  worn away.  Work with suppliers to understand procured part requirements

One brush manufacturer’s brush shunt  wire broke significantly more than the others. The brush shunt  attachment had to be redesigned to address yield issues. The initial design used an eyelet to keep the solder from wicking down the shunt wire, their intern al requirement. The eyelet damaged the wire strands  causing them to fail. The eyelet was removed and a braided shunt wire was used. The redesigned brushes were installed into the engineering model for functional and vibration testing. The redesigned brushes successfully completed testi ng without any broken shunt wires.   The issues involved with the lessons learned were not  catastrophic, but each o ne of them could have had  serious consequences. The upgraded brush gear motor was successful because the issues were handled early enough to meet program schedule dates.     Summary and Discussion   After development was completed a qualification unit  was fabricated to production paper work, using  production processes and tooling. The unit was subjec ted to qualification testing which included vibration,  thermal vacuum exposures and life tests. The qualif ication unit successfully passed all qualification and  life tests with no findings. After qualification and life testing the unit  was disassembled and cleaned. All  parts were inspected and showed minimal wear and no signs of damage.   NASA/CP-2010-216272