© 2016 Sierra Nevada Corporation   365 developed an analytical method to predict the load at which a brush would gap (‘lift -off’) away f rom the  rotor. The underlying theory was that a brush that does not gap cannot  inadvertently displace to adjacent  slip ring circuits. The plan was to compare the recovered brush tip reaction loads to the as -built brush  preload and establish margins against  gapping. With this approach in mind, efforts shifted to predicting  gapping events rather than trying to understand complexities of brush dynamics once gapped. Finite  element analyses  became the focus in this next phase of the investigation with the primar y goal to refine  and test -correlate an existing SADA finite element model (FEM) and develop a high- fidelity predictive tool.  

Figure 7.  Accelerometer Response Data - Time History Recorded During Vibration Testing    This proces s to refine the existing SADA FEM started with a fundamental update to incorporate individual  brushes around and along the slip ring rotor . Several power ring brush groups as well as a seri es of  single signal brushes were placed in strategic  locations  including areas  where known short circuit events  had occurred. Each brush was modeled to closely match the shape, cross- sectional geometry, and  mechanical properties of the brushes used. Most importantly, t hese brushes were restrained at the  contact point s between the brush tip and rotor. The purpose of these FEM updates was to devise  a  method of recovering brush/rotor reaction loads that could be then be compared to as -built brush preload.   This comparison could then be used  to establish gapping margin estimates for  SADA exposure to various  input spectrum and random vibration test conditions. With this goal to accurately predict the ‘lift-off’  threshold, further FEM development  and refinements c ontinued .   The next phase of this FEM development  involved test -correlating the analytical model  to the as-built  hardware . The process  began with comparison of  FEM  results to prior test data. Comparison confirmed  that the refined model was closely predicting the SADA response,  but two issues surfaced and  complicate d the task of further FEM refinement. First, prior data review and data mining efforts found  minor variations in test setup, SADA design, and boundary constraint s associated with the various tests .  These  minor test and hardware configuration differences  were not unexpected given th e progressive  development of the SADA  spanning several years and following many different test programs, each with  its unique assembly and test history . Variables included minor differences in cable bundle boundary  constraints, rotational alignment, rotor axial preload, brush contact preload, accelerometer placement, and vibration test equipment/facil ities. Table 1 summarizes some of these configuration differences.   Because the historical data sets were complicated by these differences, precise tailored FEM correlation  was extremely challenging  and it became clear that additional test data would be required to continue the  FEM model -correlation and refinement effort .  

   Time   Level (Gs)   -250  +250