Torsion Spring Design During a sample acquisition operation the load plate on the ISAD is preloaded against the target by the  RA, and the rasp bit retracts into the scoop body, applying a preload (or “weight-on-bit”) to the soil. This  weight-on-bit is produced by a torsion spring. In its free state the rasp cutting bit protrudes from the back  of the ISAD scoop and is preloaded by the spring against an internal hard stop that provides 5.0 N of  weight-on-bit at the bit tip. When loaded against a soil target the bit pivots into the back of the ISAD to a  maximum deflection of 15° and applies 6.6 N of weight-on-bit at the bit tip. This weight-on-bit range was  selected based on the ability of the bit to penetrate into frozen Mars soil analog targets to the full depth of  the tool and to meet the actuator startup torque margin requirement. Generally, increasing the weight-onbit of the rasp decreases torque margin but increases the ability to penetrate hard icy soils, so a  compromise between the two was required.  The spring is made from 1.5-mm (0.059-in) diameter 17-7 PH stainless steel wire that has been age  hardened, shot peened and passivated. The spring has 12 coils with an 11.1-mm (0.438-in) coil outer  diameter. One leg of the spring is formed tangential to the coil and the other axially aligned with the coil.  Since the spring assembly is directly in line with the soil particle transfer path, both the coil diameter and  leg geometries were constrained in volume and shape so as to allow unhindered particle flow underneath  the assembly. To reduce the potential for jamming by foreign debris, the spring coils were covered by a  protective housing that was integral with the spring mount. The spring coil inner diameter is constrained  by an aluminum arbor with a General Magnaplate TUFRAM wear resistant surface treatment.  Spring Axial LegTorsion SpringSpring Tangential LegSpring Mount (Cutaway) Spring ArborPivot Assembly Cutting Bit Torsion SpringSpring Arbor        Figure 11. (Left) ISAD Pivot and Spring Assemblies; (Right) ISAD Spring Test Fixture  The spring was designed to produce a change in weight-on-bit over the 15° bit pivot stroke of 0.7 N. Such  a narrow range of weight on bit was desired to maintain sample acquisition performance over the entire  stroke of the cutting bit. The ISAD flight unit spring produces a change in weight-on-bit of 1.6 N over the 15° stroke, a difference of 0.9 N between the theoretical and actual values. This difference is a combined  result of the spring manufacturing and integration processes. The spring rates of the springs provided by  the manufacturer were slightly higher than that called out in the manufacture drawing (0.023 in-lb/° actual  compared to 0.019 in-lb/° theoretical), which yields an increase of approximately 0.3 N over the stroke of  the bit. The remaining 0.6-N difference between the actual and theoretical weight-on-bit values is  attributed to friction in the spring assembly and the accuracy of the test method. Some of the springs also  had non-linear torque versus deflection curves and there was some variability in both the slope and offset  of this curve. In addition to spring rate differences, there were small variations in the axial leg geometries  of the springs, such as leg straightness, the angle at which the leg protrudes from the coil, and the radius of the axial leg bend. The leg straightness and bend radius had an effect on the fit of the spring into the  outer housing and the geometry of the assembled spring when wound up. As seen in Figure 11, when  300