142 During the Seal Activation sequence , the Sample Tube enters the Sealing Station laterally at an elevation  higher than its seated position. The Gripper c loses around it, enclosing but not squeezing, and the t ube is  pulled down by the SHA,  until the t ube bearing race is seated against the top of  the Gripper with  a preload  of about 50 N. Because of position accuracy challenges with the SHA in the Martian thermal environment  there can be residual load between the Sample Tube and the Sealing Station due to deflection in the S HA’s  end effector , which  features a remote center of compliance mechanism. Because of this residual load the  Gripper must exert a torque high enough to center the tube under the Ram, and hold it there passively,  necessitating the need for a passive brake described later. The Ram moves to activate the seal, retracts,  then the Gripper and t ube reverse the ingress motions to extract the now sealed Sample Tube. For tube  drop-off a similar ingress procedure is used, after which the SHA detaches from the tube leaving it in the  Sealing S tation. T he G ripper then opens , allowing the t ube to fall to the s urface of Mars. To prevent the  tube from sticking in the non- moving half of the Gripper , a kicker mounted to the gripper rotational output   motivates the tube out of the Gripper. To prevent t he tube from sticking to the moving half of the Gripper  output, the Ram extension hardstop is used as a lateral guide to ensure  the tube remains on center of the  egress path .  

Rotary motion of the gr ipper output is accomplished through a spur gear set  supported by angular contact  bearings , which are in turn driven by a motor with  a planetary gearhead and a magnetic detent brake. The  motor is described in [ 2]. The gripper output, green in Figure 7, is positioned to rotate into the closed positio n  just above the supportive shelf meant for reacting seal activation forces from the ram. This gap is intended  to prevent jamming of the m echanism. The output is flexurized to allow the gap to close while still controlling  rotation. The flexures permit ver tical motion of the gripper output, allowing  the high seal activation loads to  be reacted by the static gripper housing as well as preventing the output shaft , colored magenta in Figure  7, and bearings from being in the ram load path. The rotational axis of the gripper output shaft and the bore  of the gripper, used for tube locating,  and tube support features are offset from one another, making the  mechan ism self -closing during seal activation and tube seating.  

Figure 8. Hardstop/detent interaction plots for theoretical cases. Blue line depicts active motor torque at  mechanism output with variation due to the passive detent brake. Grey line depicts the passive holding  torque of the mechanism, see also Figure 9. Orange line depicts reacted torque from hardstop contact  and preload.  

The Gripper uses a passive magnetic detent brake to maintain a preload between the Gripper output and  the Gripper housing during both launch, rover traverse accelerations , and loads during seal activation. The  interaction of the detent step size and the hardstop stiffness is an important design element that was not  well understood until flight har dware testing. For high hardstop stiffness  relative to the detent step size it  may be nearly impossible to guarantee preload against the hardstop. To ensure preload, it is desirable to  have a relatively soft hardstop to ensure that the detents can “catch” the reacted preload. See Figure 8.  The intersection of orange and blue is the maximum torque at which the motor, with detent brake, can push  into the hardstop,  denoted by a red mark. When the motor is powered off, system load follows the orange