There are retaining rings provided in the kit. They look like a washer with teeth. They slide on one way but don't slide (easily) the other. I would suggest making spacer out of the aluminium tubing they sent you to space the gears off of the bearings inner race. This is what it looked like first did. They don't show it in the expanded view but if you took pictures you can see it on the robots from first.
http://www.archergeeks1022.org/photo...off/index.html
Also don't forget to have clearance for the inner race of the bearing that goes up against the aluminium channel. I hope that helps.