Shaft power calculations

[freedomlives]

Anyone know where there's some table of how much power a shaft can transmit? I've got to make an adapter for my two wheel tractor's PTO to attach a different manufacturers accesories, one of which is a cutterbar mower. I've got the two toothed couplings, and I was about to weld some thick pipe of the right length when it occurred to me that I'd rather use a thinner, mild steel rod, because if by chance something hard (like metal stake, particularly) gets caught in the teeth of the mower, its going to come to a hard stop, and better the PTO coupling breaks/bends than something inside the gearbox of the two wheel tractor (which I just replaced some gears that were stripped from the previous owner and presumably something with the snowblower, which had sheer bolts of a relatively high grade that probably hadn't sheered...)

I "feel" like 10mm rod ought to be about right, but what do I know?

 

[freedomlives]

Tried some other search terms, I guess I can go from this:
Relationship of torque and shaft size - Plant Engineering

D=sqrt( (P*105)/R )

My gut feeling seems about right. 5HP @3000RPM is the motor on the machine the attachments are from, so that works out to .41 inches, or 10.6mm