wound up the coil tends to pivot about the axial leg anchor point and move out of alignment with the  arbor. This misalignment can cause the spring to contact the arbor, housing or both, which increases  friction in the assembly, resulting in a higher spring rate.   It was anticipated that there would be variability among the as-received springs. Therefore, five times as  many springs were fabricated than were required in the delivered ISAD units. This provided the ability to  choose the best springs out of the lot to be integrated into the ISAD units. The springs integrated into the  ISAD units were selected based on having a minimal and constant spring rate and acceptable axial leg  geometries. Since there is significant variability from spring to spring, each spring had to be “tuned”  appropriately for each ISAD unit. To achieve a desired range in weight-on-bit from a particular spring, the  mounting hole location in the spring mount for the spring’s axial leg was determined following testing of  that spring in a custom test fixture. As stated, the ISAD flight unit weight-on-bit range is 5.0 N to 6.6 N,  which was measured at room temperature. Tests conducted with the mechanism soaked at -80°C yielded  weight-on-bit values of 5.5 N to 8.2 N over the 15° stroke of the bit, an increase of 1.1 N over that  measured at room temperature. This increase is attributed to added friction in the spring assembly at cold  temperatures as well as a potential increase in the spring stiffness.  When considering a custom torsion spring to provide a narrow range of output torques at specific  deflection angles, there are many sources of error between the theoretical design and the actual  performance. If a torsion spring is required to operate within a precise performance range it is important  that sufficient margin be accounted for in the design so that deviations in spring performance due to the  manufacturing process, mounting method and assembly process do not adversely affect mechanism  performance. For future mechanisms, it is recommended that the use of an axially aligned torsion spring  leg be avoided when variability in performance is undesirable. An axially mounted leg is sensitive to both  the spring mounting scheme and the assembly procedure. In the case of the ISAD, spring preload directly  affected the actuator torque margin and bit productivity. A specific range was desired, but the spring was  successfully implemented in the design by fabricating a relatively large batch of springs that could be  selected from to find the best match for each unit and by waiting until the spring was selected before  locating mounting holes for the axial leg.  Conclusion  The ISAD is a first of its kind device to be qualified for space flight. Many aspects of the design are  unique, but can be applied to future spacecraft mechanisms. The highly constrained requirements space  resulted in the use of a number of unconventional design features and methods, while schedule and  budget restrictions played a large factor in many design decisions.  Designing a low torque mechanism while preventing debris migration from damaging sensitive drive  components requires knowledge of the effects of the dust seal on the overall frictional drag of the  mechanism. Depending on available torque margin, shafts that interface with dust seals may require  modification to minimize seal drag. Seal behavior across the environmental range should also be characterized before design finalization.  A cam-actuated vibration mechanism is an effective means of inducing vibration to help fluidize both dry  and ice-bearing soils and powders. The limitation on the overall life of the mechanism is the interaction  between the cam and the bumper. Proper material, surface treatment and geometry selection is critical to  extending the lifetime of the device.  Designing a small form factor braze joint for brittle material to sustain high load conditions involves careful  consideration of the connection geometry. Materials such as tungsten carbide require gradual changes in  geometry, and reinforcement with braze filler material to sustain high loads. Abrupt geometry transitions  can result in failure of the joint when load is applied.  Use of torsion springs in a mechanism where a narrow range of output torque is required should be  avoided. If space or packaging constraints dictate the use of a torsion spring, the spring should be  designed using legs that exit the spring tangentially, rather than aligned with the spring axis. An axially  aligned leg imposes a high degree of unpredictability on the output torque due to frictional effects  301