401 The FWA and PIL, along with all NIRCam components, are designed to support many demanding  requirements. The units must operate after being subjected to the stresses of launch loads and in the vacuum of space. These units are also required to have a minimum first mode frequency of 100 Hz, operate with a maximum power input of 0.6 mW, and all actuators must have appropriate torque margins. While these requirements are not unusual for space flight mechanisms, both the FWA and PIL mechanisms are required to operate at 37 K to allow imaging at the near infrared spectrum. Furthermore, the FWA and PIL are not allowed to generate debris larger than 300 microns over their lifespan to ensure minimal image degradation through contamination. The PIL was given a unique requirement that specified the mechanism shall include a fail-safe system that, given a set of reasonable failure conditions while in the deployed state, the PIL optic would be able remove itself from blocking the science beam. Previous papers [1], [2], and [3], are available for more information about the FWA and PIL mechanism mission requirements and original design.  The FWA and PIL are both rotating mechanisms that share common motor and bearing mount designs. The bearing and related bearing mount design ensures low friction rotation at ambient and cryogenic temperatures. The original designs for both mechanisms integrated a fixed titanium shaft, 440C cryo and space-rated bearings, and 455 CRES rotors. Due to different materials and the large operational  temperature excursion, a flex bearing mount was chosen as illustrated in Figure 4 . For operation at  cryogenic temperatures, a Teflon film bearing lubrication is employed through a transfer process from a  Teflon and fiberglass composite ball-bearing cage to the balls and races of the bearing. With the back-to-back bearing configuration, the bearing preload is set by clamping the inner race of the bearing. This bearing preload is critical to ensuring smooth running, low-friction rotation. Selecting a spring with a spring-force just higher than the bearing pre-load, and applying the spring force through a close tolerance slip-fit bearing sleeve, one ensures the bearings have constant pre-load but allows for slight changes from differential coefficients of thermal expansion and possible Teflon lubrication buildup.  While this design works well in quiescent environments, the forces induced during launch would exceed the capability of the spring system alone. Two features were added to the flex mount design to achieve the needed stiffness through launch. First, a hard-stop was added to the sleeve that only allows  75 microns of slip. The second feature is a cryo-release tube that effectively locks out the spring during the ambient launch temperatures but shrinks away at cryo temperatures, allowing the spring to pre-load the bearing. Through prototype and qualification testing, this design was proven to meet two-time-life testing with the required low friction rotation. 

Figure 4.  The spring clamped bearing mount design for the pupil imaging lens assembly (PIL).  Fixed titanium shaft Clam p nut and hardsto p  Cryo release tube  Bearing pre-load spring  Duplex bearin g pair Bearing sleeve