7) Finally, while not necessarily a problem, a very faint motor ‘’ticking” sound was found wherein the  tick corresponded to the commutation frequency of the motor (6-step commutation) and the  intensity of the sound was proportional to motor current. This was first noticed during Windsat  EM1 in the “soundproof” EM1 chamber. The drive had been listened to with a mechanics  stethoscope earlier as standard practice but the faint tick was not noticed. However, the  stethoscope test was done in a clean room next to the air handling system so it is quite possible  the “tick sound was always there but not noticed until the dead quiet EM1 chamber. The tick  might have been just the magnetostrictive contraction of the armature and normal - it is not  known for sure. The ticking sound was tracked through system level vibe, TVAC, etc and it never  changed and the source was never isolated. A lesson learned is to turn off all noise sources (air  handlers, etc) when listening to something and to use a microphone to provide an audio record  that is subsequently analyzed by sound /frequency analysis software if possible.  Value Of Screenina Toraue Traces On Bearinas - EsDeciallv Thin Section  The Windsat slip rings were built as a standalone assembly with their own bearings. This was done for  ease of integration and to minimize potential conflicts with the slip ring integration and test schedule. The  bearings used were angular contact thin section (50.8-mm (2) ID, 63.5-mm (2.5”) OD, 6.35-mm (0.25)  cross section, 42 balls). Since the slip ring vendor had limited bearing design/analysis capabilities, NRL  helped them with bearing design and analysis. Additionally NRL provided the bearings and assisted with  installation. During bearing screening torque tests conducted at NRL prior to installation, several bearing  pairs were rejected as a result of torque spikes at the bearing cage frequency (Figure 7). The bearing had  very thin “rubber band like” one-piece phenolic retainers, which were difficult to dimensionally control.  Many of the cages were out-of-round and would pinch the riding land of the bearing, resulting in torque  spikes at the cage frequency. The torque screening tests ensured that these bearings were rejected and  after several iterations acceptable bearings were obtained and flown.  I HSG S/N 22 Vettical -30 RPM 20:47 PM  0246 8 10 12  Time (=e)  Figure 7. Screening Torque Trace (left) & Thin Section Bearings (right)  Analvsis & Life Test Data For Couplinas Cannot Be Overlooked  The Windsat BAPTA design used two different bellows shaft couplings. Shaft couplings are often a  “necessary evil” when multiple shafts must be connected and small amounts of misalignment would result  in large/unacceptable bearing loads. On the Windsat BAPTA the shafts in question were aligned with high  precision, however, there was no solid analytical prediction for coupling life. One of the couplings was a  “catalog” design with a published fatigue life/misalignment capability. When the vendor was pressed to  back up the published data, however, it was found that it was extrapolated off tests of the first few  couplings made 50 years ago and that “you really should run a test”. A life test was run on one of the  couplings with the flight misalignment duplicated. The life test was stopped after the required design life of  3 years / 50 million revolutions as it was deemed unlikely that a fatigue failure would occur past that point  (and because the life test gearbox wore out). There are many different styles/designs of shaft couplings,  the more common being Oldham and Bellows. The Oldham can wear and have backlash issues whereas  the bellows can fail by fatigue (either in vibration testing or operation). An assessment of the life-limiting  21 4