Torsional Deflection of shafts?

[coreyjon]

I am partly in charge of our drive train, myself being included in the construction for 3 years (this being my fourth) but only my second year with design. In an interest to save time, we have opted to use the kit gear box as stage one of a cut down four speed gearbox converted to a two speed. We initially planned on using 3/8 shafts in the "2nd stage" two speed addition, however what crossed my mind was, "Will that be strong enough to put up with the riggers of a FIRST match. I did a little research, and I found a chart with recommended loads on shafts, and it recommended only 15 inch-pounds (it actually had it written lb.in.), with an output torque at 40 amps being 17.7 Newton-meters, I get about 156.7 inch pounds of force, and that puts me at a shaft size of 3/4 inch. Now I know the output shaft on the kit gear box is a large diameter, just a little larger than 5/8 (evil metric sizes), which the chart recommends only 125 inch pounds of torque. now, I don't think that small difference will be a problem, in fact I’m sure the chart I’m referring to is using a safety factor, but my question is, 3/8 inch shaft, is it worth the weight saved, my own personal feelings are that the shafts may not hold up, and repairing an old 3/8 axle shaft on one of our older robots, with a weaker drive train leads me to believe that even if the distances are kept short, that I may be taking a bit of a risk. Or, my lack of sleep and coffee could be throwing my numbers off, and maybe I’m just paranoid, but I really like to have 100% confidence in the drive train, we've won matches just being able to drive around when we lost our arm functionality last year, so I maybe taking an "if all else fails" approach, but I would rather be able to rely on one of the more basic functions a robot should have.