286 force at the fully compressed pivot position and the spring rate was selected to be as flat as possible such  that the contact force was about 0.4 N at the lower hard-stop.  The brush block subassembly was dynamometer tested across the operational temperature range (-70°C to +70°C). Instead of using a torque watch to directly measure the torque supplied by each pivot axis over the range of motion, a test was designed to measure the net contact force delivered by both brushes against a contact plate instrumented with a load cell. At each temperature set point, the instrumented contact plate was translated toward the DRT brush block as the brush block was spinning at low RPM – this mimicked the flight operational concept. The DRT and contact plate translation stage were configured horizontally such that gravity should not have been a factor. Force measurements were taken at 21 mm (1 mm beyond the brushes reach), 16 mm, 11 mm, 6 mm and 1 mm between the center post tip and the contact plate. Measurements were not taken in the reverse order to quantify friction drag on the pivot axis. The results of this test, shown in Figure 8, were nominal (i.e., the springs produced forces that were within the expected range).   Brush Pivot Problem/Failure Report   Following the dynamometer testing of the DRT Flight Model (FM) and Engineering Model (EM) units and the subsequent delivery of the FM, the DRT EM was performance and life tested across the operational temperature range. During these tests, the EM motor was energized to spin up the brush block while it was positioned above a rock surface in the thermal vacuum chamber. Once at speed, a linear translation stage (ground support equipment playing the part of the RA) moved the DRT to a position of engagement with the rock. After a period on the rock, the motor speed was decreased to a lower setting and the DRT was retracted off the rock. This test procedure was consistent with the manner in which the tool would be used during the mission.  During the -70°C tests, it was observed the EM Inner Brush did not return all the way to its hard-stop following the low speed retraction from the rock. Instead it stopped a few degrees away from the hard-stop. However, the Inner Brush did return to its hard-stop position following the nominal 10-second  900 rpm run. The EM performance and life testing continued until the EM had successfully met its dust removal performance requirements after 2x life (greater than 300 brushing operations).    The Problem Failure Report process was initiated to capture the anomalous Inner Brush behavior and subsequent root cause investigation. The concern was that the pivot (unassisted by centripetal force) was not meeting its torque margin requirement. Per the DRT equipment specification, all actuators were required to demonstrate a minimum margin of 100% on torque required for operation under worst-case conditions where Margin = (Actual/Required – 1)*100%. In this case, the EM Inner Brush pivot spring (the actuator) had apparently failed to supply enough torque to overcome friction and return the brush to its hard-stop. The requirement in this case is that the spring be able to supply twice the torque required to overcome the friction opposing a return to hard stop – this would be considered a margin of 100% or factor of safety (FOS) of 2.  The first thing the team did was to revisit the pivot analytical model used during the design process as well as the as-built in-process test data and dynamometer test data. According to the manufacturing documentation the measured seal drag on the EM’s Inner Brush pivot was 50% higher than that of the EM’s Outer Brush and both the FM Inner and Outer Brush. This appeared to be the smoking gun. Unfortunately however, the team realized that the dynamometer test approach inadequately demonstrated the torque (or force) margin and it was impossible to draw any further conclusions about the pivot spring’s torque margin. Pursuing the seal drag theory, it was hypothesized that seal contraction at cold temperatures had caused the drag torque to spike on the EM Inner Brush seal-shaft combination because it was perhaps past the “knee in the curve” known to exist in these types of configurations. So measurements of seal drag versus diametral interference were made at room temperature using gage pins to create a model (see Figure 9) and the maximum theoretical seal contraction was calculated. According to the model, the EM Inner Brush would have a margin less than 100% (FOS < 2) but the