256 is attached to the 150-micron sieve frame and is suppor ted by a lightly-preloaded, thin-section, back-toback duplex bearing pair. These two independently rotating elements of the PTM, the tang assembly and  mechanism output, are coupled together and attached to  a pre-wound spiral spring. In the nominal closed  configuration, the thwacker arm is preloaded agains t CHIMRA. When the tunnel is opened five degrees,  the mechanism output and tang assembly are engaged by  the latch cartridge fixed inside the tunnel base.  Additional rotation of the tunnel base forces the out put, sieve frame, and tang assembly to also rotate,  lagging five degrees behind the tunnel. This rotati on further winds the spring. At the end of the  mechanism range of motion, the latch is disengaged from the tang by a non-rotating stop fixed to the  static mechanism structure. Once disengaged, the wound spring accelerates the output and sieve frame  back into the closed position, generating the inertia impac t thwack that is used to eject stuck particles. As  the tunnel closes, the latch rides ov er the backside of the tang, snap s back into the armed state, and  resets the mechanism.                     Figure 11. Primary Thwack Mechanism 

The primary challenge to overcome in the PTM design was a poor initial test program that resulted in  ambiguous functional mechanism requirements. Co mbining the parameter space of multiple sieve  designs, different rock types, and alternating number  of thwacks into an efficient development test  program meant that only a rudimentary underst anding of the phenomena was developed before the  design moved on. Additionally, the single life requirem ent of 500 thwacks made it appear practical to only  perform a life test on a single parameter configurati on. Therefore, only one full life test was performed  early in the CHIMRA life cycle to define the thwack  impact speed requirements of the mechanism. This  design point was chosen to bludgeon sieve unclogging so that our limited experience was compensated  for with a heavily margined impact speed. Earl y in the CHIMRA design cycle, an impact speed was  selected that efficiently unclogged the screen, however left no option to compromise on performance to relax mechanism requirements.   The requirements of the PTM are interconnected in a manner that forced an advanced level of  computational design synthesis to meet all functional and traditional mechanism requirements. An inertial dynamic simulator was created in Simulink to allow the spring rate, preload and angular displacement to be modified and simulated. This was used in conjuncti on with empirical results from targeted cold bearing  and seal drag tests to converge upon an acceptable design. While the minimum functional requirements  had to be met at the earliest possible thwacker release point, all structural design loads had to account for  the last possible release point; this occurs when the spring is at its maximum angular displacement. As the structure was modified to survive these worst case design loads, the rotational inertia of the  mechanism increased, forcing the spring energy to increase to maintain the same minimum impact speed. This obviously boosted the impact energy going into the structure and forced additional iterations.  NASA/CP-2010-216272