[Kevin Sevcik]

pulling out my ridiculously huge handbook for mechanical engineers I see that the allowable shear for a fillet weld is:

tau = 0.30 x electrode tensile strength

various other magic happens and you get:

allowable force per linear inch = 0.707 x weld size x 0.30 x electrode strength.

so... with a half in shaft, you figure the torque and diameter and all and multiply out a lot of constants and you can figure out how big the weld needs to be. I get:

torque thru shaft / (0.083291 X electrode strength) = min weld leg size

that's all in lbs and inches, by the way. So you ask the welder what kind of electrode he's using and you can figure out how big a bead he needs to make.

Electrodes are usually labelled in thousands of psi. an E60 has a strength of 60,000 psi. just tossing in some numbers... I'll assume the torque is 75 lb-in from your 300 lb figure and an E60 electrode....

I get a minimum bead size of 0.015 in. Which is ridiculously small, and took entirely too much work, but now you know that the weld should be plenty strong.