[Al Skierkiewicz]

Aren,
We learned early on, that the crab modules (which are fully modular for ease of repair) need some additional bearing help at the bottom of the module. The delrin ring provides that bearing surface and is somewhat self lubricating. Any forces that would be transferred through the module top bearing are actually shunted by this bearing right to the base of the robot frame. By keeping the bottom movement to a minumum, the surface of the drive wheel stays flat on the floor and side forces do not cause the upper bearing to bind as the module is rotated during pushing from other robots.
Note: As photographed the module is upside down. As such, the bearing ring and plate are laying on the table. During operation both the ring and plate would be mounted in the same plane as(and mating with) the round base of the module.