Shafting size, keyways, and Dr. Joe's Drill Shaft

[Alan Ing]

Last year, our team coupled a 1/2" shaft to the output of the drill transmission locked in low. From the 1/2" shaft, we used No. 35 sprockets and chains for a further reduction of 3:1 driving 8" wheels. This simple combination gave us a lot of torque which enabled us climb the center divider, push or pull a fully loaded stretcher across the bridge, and balance the goals with relative ease.

I'm far from an expert when it comes to the strength of materials, but do you guys think we could have used a 3/8" shaft, No. 25 chains and sprockets (with a keyway) without breaking or bending anything? In other words, can I stall the drill motors locked in low with a 3/8" shaft with sprockets mounted with keyways without breaking anything? What if I used a dual motor design increasing torque by 40%? Would it be recommended to stick with a 1/2" shaft?

If the bosch drill is available this year, we would like to consider using Dr. Joe's drill shaft. On the plans, I noticed that the maximum shaft diameter is 12mm. Unfortunately 12mm bearings are not legally available through SPI. 12mm is .4724" which is too small to use with .5" bearings. The closest SAE bearing that would work is .375" (3/8"). In order to use these, I would have to turn the shaft down to .375" which is considerably smaller and weaker than 12mm. Worse yet, I imagine that a keyway would make the shaft even weaker.

Anyone have any thoughts? Am I missing something? Is there some practical way to support a 12mm shaft with bearings legally? Would home made bearings out of bronze be adequate if we squirted a little oil in them?

With the competition closing in, I'm starting to have bad dreams of sleepless nights and weekends.

Here we go again!

Regards,
Alan