pic: Low Profile Gearbox Front View

[asid61]

Originally Posted by JorgeReyes

As is the gearbox weighs about 11.2 pounds according to Solidworks. I made the side plates 1/4" thick. The Shifting output shaft I am using is the one COTS shifting shaft and is the same one the is used on the DS.

It is this one:
http://www.vexrobotics.com/217-3635.html

The shafts are 3/8" hex shaft turned to 3/8" round but I might possibly use 1/2" hex and turn that to 3/8" because there isn't much material to constrain the shaft against the bearing with the 3/8" hex shaft.

I am in fact using the cims as nuts but I am curious to know why there is a problem with this. Would it be a structural issue?

That's a nice shaft.
3/8" hex turned to 3/8" will work fine. Look at the new Thunderhex usage examples to see how small a shoulder you can use. Bearings are very high precision, and you will have about a 1/32" shoulder on six points to constrain it.
As stated above, it's a maintenance issue. If you lose a CIM somehow, then having to take off the gearbox and reassemble it is a no-no.