With increasing time and wear, we are finding that wheels, formerly held on by friction, routinely fall off. While it would be far more robust to secure the wheels by affixing a collar at the end of the axel, 3-inch axels aren’t long enough to do the job. We are thinking of cutting our long axels into 3.5 or 4-inch lengths, but were wondering if anyone has a cheaper solution.
How about attaching your sprocket to your wheels instead of to the shaft. This way your wheel and sprocket would be a free spinning entity on the shaft. This is how 229 has done it for at least the past 3 years. Example
Hope this helped,
JT
229
I think ManicMechanic was looking for a solution to this problem on the Vex robots. 
Since you cannot really have dead axles in Vex, I would suggust trying to “shift” the axle outward if you need an extra quarter inch or so to get the shaft collar on. If there is any “extra” shaft on the inside of the robot frame, try to get this extra onto the outside.
Also, by rearranging the location of the shaft collars on the inside of the Squarebot you can minimalize the amount of axle needed there, so that you should be able to have enough axle on the outside of the robot next to the wheel in order to get a shaft collar there.
Wow, I guess I really spaced out on that one. Sorry about that.