As shown in Figure 3, the ISAD drivetrain utilizes a dust seal near the cutting bit to protect the critical  internal components from dirt and dust migration. A custom canted coil spring energized seal was used  for the dust seal, with Glass Filled Polytetrafluoroethylene Molybdenum Disulfide Reinforced (GFPM)  polymer as the seal material, and 316 Stainless Steel as the backing and spring material. The shaft  material on which the seal mates is 6Al-4V Titanium alloy.   When specifying a custom dust seal for a low torque application, it must be understood that  manufacturers typically fabricate this type of seal to withstand high-pressure liquids and gases. As a  result, spring energized seals are typically designed to be tight on the shaft, causing drag torques that  can be a significant portion of the available torque when dealing with a mechanism such as the ISAD.  With this effect in mind, the manufacturer was instructed to design the seal to a shaft diameter of 6.53  mm, when the actual shaft diameter was 6.34 mm. The difference was based on previous experience with  high speed, low torque sealed mechanisms. When the seals were delivered, they were qualitatively  determined to have an unacceptable amount of drag torque on the 6.34 mm Titanium shaft. This was  expected, and material was removed from the shaft to reduce the frictional drag of the drivetrain. The  required shaft diameter was determined by pressing the seal into the seal housing, and using hardened  gage pins of varying size to find the optimal shaft size. By gently rotating the gage pin by hand, the  technician could “feel” the difference in drag torque. This qualitative assessment of the drag torque  resulted in the final diameter of the Titanium shaft being modified to 6.09 mm, a difference of 0.44 mm  from what was originally specified to the manufacturer. This diameter produced the minimum amount of  drag torque while maintaining a dust seal around the shaft. Subsequent testing performed on an identical  seal revealed that the seal drag torque at room temperature on the 6.09-mm shaft was approximately  0.0035 N•m, while a 6.53-mm shaft would have produced a drag torque of about 0.018 N•m. After thermal  dynamometer testing the Flight Model ISAD unit, the total drag torque of the mechanism was determined  to be approximately 0.013 N•m at the low temperature operational extreme of -80 °C, and 0.007 N•m at  room temperature (+20 °C). Had the manufacturer-recommended shaft diameter been used, the  drivetrain drag torque would have increased by a factor of three at room temperature, leading to greatly  reduced startup torque margin, and possible stalling of the mechanism at low temperatures.  When designing a custom spring seal for a high-speed low torque mechanism, it is difficult at best to  determine the size of shaft, which will mate with a particular seal without having the seal in hand.  Similarly, there is a large amount of risk associated with fixing the shaft diameter, and attempting to specify to the manufacturer the seal diameter to provide a dust seal while minimizing the amount of  mechanism drag torque. The risk inherently lies with schedule and cost constraints, which were  significant drivers in the development of the ISAD.  To minimize the impact on development schedule and project cost, the spring seal should be specified  initially for a nominal shaft diameter. The shaft itself should be designed with the intention that material be  removed once the spring seal, shaft and housing are in hand. This involves leaving enough material on  the diameter of the shaft to allow for machining prior to final assembly of the mechanism. In this torque  regime, it is critical that the shaft be matched to the actual seal press fit into the housing, as a slight variation in shaft diameter can make a substantial difference in the drag torque.  An additional observation related to the seal was noticeable change in frictional drag after exposing the  mechanism to the high end of the required temperature range during thermal vacuum contamination  control bakeout. While GFPM is rated for temperatures safely exceeding the +110 °C maximum that the  ISAD was subjected to during thermal cycling, a noticeable difference in drag torque was observed after  soaking for 8 hours at this temperature. Specifically, an additional 0.004 N•m of torque (approximately  10% of actuator output capability) was measured indirectly by considering the additional motor current to  drive the mechanism under “no-load” conditions with the vibration feature disengaged. This observation  was consistent for both the engineering and flight units. Initially it was not clear that this additional  frictional drag was due to the GPFM seal. Since the change in drag did not warrant disassembly of the mechanism for inspection of this component, the seal could not be completely confirmed to be the root  cause. However, all other possible scenarios were deemed highly unlikely .The seal hypothesis was  supported by the fact that the drag slowly decreased to normal levels after several minutes of operation.  Assuming the seal was the source of the additional drag, the reason for its change in behavior could be:  (1) a slight tightening of the GFPM seal on the shaft due to pressure applied by the canted spring; (2) a  change in the frictional properties of the GFPM-on-titanium material combination due to out gassing of  294