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Posted by David Sherman [24.32.202.83] on Wednesday, December 23, 2009 at 03:33:31 :
In Reply to: how to weld posted by Dave Royal [76.182.149.93] on Wednesday, December 23, 2009 at 03:01:53 :
I'll be curious what the pros say, but I think your chances are excellent, mostly because it doesn't spin fast and thus doesn't have to be very well balanced. Is it possible to put the ends together, weld them, grind it smooth, and then put a sleeve (even just water pipe) over the whole thing and weld the ends of that to the shaft all around too?
There are people who weld drivelines while they're turning slowing on a lathe, but they are really good. I tend to think since balance isn't very important, add more metal to make up for the fact that the welded part won't have the strength per unit area (psi) that the undamaged parts have, so you need to add more "si" to carry the same "p". If you can't sleeve it, maybe build up the weld 1/2" or so thicker all around the joint, sort of like an old-fashioned wiped lead plumbing joint.
If you think you need extreme strength in the PTO shaft, think about the shear pin. There you have 5/16" dia mild steel, shearing in two places about as far from the central axis of the shaft as your weld will be. That's about .15 square inches total of steel in shear. So long as you have more than that much in weld cross-sectional area, taking a slice lengthwise, the shear pin will fail first. If, for example, you chamfer the ends 1/8" deep and build it up flush to the surface, with a diameter of 1 inch, you've got about .4 square inches of weld in the plane needed to resist torque, which is more than double what you need. You're obviously going to have more than 1/8" of weld penetration and/or build-up. A 1/4" chamfer would be easy to grind and easy to fill, and now you're up to a safety factor of 5, which is what civil engineers use on bridges and buildings. Add a sleeve and you'll more than double the strength because the sleeve is farther from the center of the shaft and thus has more leverage. What all this tells me is that it shouldn't take any exotic procedures, materials, or workmanship to make a weld that's much stronger than the shear pin. But heck, if you want to weld it perfectly all the way to the center, do like the railroad guys do -- use thermite.
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