[Cptn Patches537]

We used a turret for our arm this year, and it is fairly similar to what you want to do except that you have one more joint thrown in there. Our turreted design has the carrier plate flush with the top of the chassis. A steel shaft runs through 2 bearings down to a pulley which is connected via timing belt to the nippo denso motor. The 2 bearings are about 8 inches apart, and the steel shaft gives us excellent stability. Our turreted arm has worked amazingly, giving us a clear advantage when people try to play defense against us. See us in action here:
http://www.youtube.com/watch?v=ydGq45HHCuM
All our primary driver has to do is move back and forth.
If you would like more detailed drawings post and i will try to find them for you.
-Eric