[Cuog]

U mean the hex shaft?

Basically instead of the shaft being round it is a hexagon shape so that when u slide on a wheel, gear etc. it can slide along the shaft but cannot spin, for a great look at it open up the transmission FIRST sent you this past year and you will see a small hex shaft going through one or two of the gears.

for a hub its easy:
These are from jeeps since there are the most taken apart vehical, you dont need anything this beefy on your robot though

http://www.thedieselstop.com/content...nt/image24.jpg
http://jeephorizons.com/tech/xj44/axle9.jpg
http://jeephorizons.com/tech/xj44/axle10.jpg
http://jeephorizons.com/tech/xj44/axle11.jpg