[sanddrag]

It all depends on the material. In 2004 we used a 5/8" aluminum keyed axle (3/16" key) , turned down to 1/2" on the ends to go in bearings. Over time, the keyway got a little sloppy and it got some play. But keep in mind there were large 12" wheels on this axle.

In 2005, we used a 1045 carbon steel 1/2" keyed axle (1/8" key). We had the wheels external (cantilevered out) and we bent the actual wheels but not the axles.

The problem when you go smaller than 1/2" is that your key size goes down (less than 1/8") unless you do an oversize key.

If you had a dead axle, or a hex axle, or a woodruff key maybe, then you could probably go down to 7/16 or 3/8.

As for calculations, I have no idea. In the short six weeks of FIRST Robotics build, I'm not even sure that something like that needs to be or should be calculated. Go with what you know will work, and try a little less if you are feeling brave. That is my rule of thumb. Of course it would probably never apply in the real world of engineering, but it FIRST it works beautifully.

Also, can you describe a little bit more exactly what your fabrication abilities/resources/capabilites are? With that info we can give you some more ideas for bearings and axles and stuff.