Table 1. Ratchet wheel rotations to failure, MoS2 on gears, no MoS2 on bearings  Powder at gear teeth  Initial investigation  The energized stroke of the solenoid is on the order of five milliseconds and the spring (de-energized)  return of the drive arms is also on the order of five milliseconds. The initial investigation by high-speed  video of the mechanism in operation showed erratic operation times for the inboard rotor during energized  strokes when the unit was close to failure. Failed units were disassembled and visually examined. Drive  arm bearings and drive springs showed no problems, and no rubs were detected between the arms and  adjacent surfaces. The wear zone on the gear teeth showed a powdered layer rather than a burnished  appearance as shown in Figure 3. The powdered layer was 0.1 to 0.2-mm thick at the edges and 0.05 mm  or thinner in the center. The powdered layer was composed of many micron size particles and was readily  scraped or wiped away from the gears. In addition, many small dark particles were observed on the  mounting plate under the gear teeth and at stop pins. The quantity of particles was much greater than  observed in previous safety mechanisms.  An additional troubling test was a higher than expected minimum operate voltage. The minimum operate  voltage occurs when the solenoid torque is nearly equal to the drive arms spring torque. The solenoids  were designed to operate the mechanism as low as 16 volts and up to the maximum supply voltage of 22  volts. The ratchet wheel cycle tests are operated at 22 volts to give more solenoid torque and greater  forces and loads on the bearings and pins. From the measured solenoid torque and the measured drive  spring forces, the ratchet wheel units were expected to operate at 14 volts but when initially assembled  were measured to operate at about 15 volts. This higher minimum operate voltage indicated a small loss,  in the solenoid bearings, gear teeth or drive arm bearings, and the gear teeth contact between the  solenoid and the drive arms was suspected as the probable cause. The powdered layer that was  observed at the gear teeth contact in the first wheel cycle tests gave more evidence than the gears had  substantial sliding contact and not the rolling contact desired in well-formed gear trains. The solenoid  torque at 22 volts is approximately double the solenoid torque at 15 volts. The failure to operate after  dozens or a hundred ratchet wheel cycles at 22 volts means an apparent frictional loss increased from  near zero at the start to approximately 50% of the solenoid torque. If the large frictional loss was solely at  the gear teeth, then slower and erratic spring return times of the drive arms and solenoid rotors were also  expected but not seen in the high-speed videos.  The powdered layer at the gear contacts was obviously troubling, but it did not appear that the gear teeth  interaction caused such high loss of torque. From the failed units, a solenoid rotor was manually stroked  and no high friction was measured. Several failed units were cleaned of the powdered layer at the gear  teeth contact, and the minimum operate voltage improved somewhat to 18 to 20 volts, but was still  significantly higher than the initial 15-volt measurement. The powdered layer at the gear teeth did not  appear to be the high fiction loss that caused the units to stop at 22 volts.  27