12 Drill Translation Mechanism   The Drill Translation Mechanism (DTM) provides the linear motion of the bi t, spindle, chuck and  percussion Drill subass emblies for the following functions: maintaining 120-N weight -on-bit (WOB) during  sample acquisition, generating a large retraction force to extract the bit from the hole, and mating to a  fresh bit in the bit box. The DTM is comprised of:   1. Aft housing assembly that forms the linear bearing rails (Figure 10).   2. A translation tube that is populated with various bearing elements (Figure 10).   3. A ball screw mechanism with an in tegrated force sensor (Figure 11).   The Percussion and Chuck / Spindle Sub-assemblies are installed into and integrated onto the end of the  translation tube, respectively. A welded metal bellows  mounted between the translation tube and the aft  housing seals the internal components to protect them from Martian dirt.  To support side and cross-moment loads, the linear bear ing system uses two sets of 6 pairs of needle  roller bearings mounted to the translation tube that ri de on the flat internal surfaces of the aft housing.  The arrangement of these bearing sets, which drives length and diameter of the Drill, was dictated by the  worst-case load scenario. Torsion loads are supported by a two-stage bearing sy stem: one set provides  low axial drag and the other supports  high loads. Testing across the qualification operation temperature of  ±70°C resulted in axial drag force less than 5 N for sample acquisition operation, thus yielding a low  disturbance to the WOB measurement.    The dual bridge force sensor provides redundant meas urement of the low weight-on-bit since the nominal  axial load is too low to be observed in the actuator current telemetry. The inner diameter of force sensor is axially clamped to the ball nut. The force sensor outer diameter is axially constrained between two preloaded wave springs. These springs serve a few functions:   1. Provide additional compliance to lower the WOB feedback control bandwidth requirement  2. Allow the high retraction load to be shunted around the sensor.  3. Isolate the science instruments mounted to the Drill aft housing from the dynamic environment  generated by the Drill.   

The force sensor and wave springs are housed in a gimbal assembly which couples the translation  mechanism to the translation tube. The gimbal isolates the ball screw and force sensor from radial and bending loads.    The ball screw mechanism consists of a custom ball sc rew supported by a high axial capacity bearing set  at one end. It is actuated by an equivalent electric ally-commutated gearmotor with a power-off brake as  the DCM. To reduce the overall length of the Drill, the ball screw bearings also support the output of the gearmotor. This not only reduced the volume by re moving a set of redundant bearings (in the actuator  gearbox), but also eliminated the need for a coupling component between the gearbox and the ball screw.  The ball nut torque is reacted by a flange mounted anti-rotation roller assembly. The range of the motion  of the DTM in the aft direction is limited by the c ontact of rotation hardstop features on the nut and the  screw.   

NASA/CP-2010-216272