See my homebuilt elevator that 'hangs' on the threads of a single 3/4"
black iron pipe.


http://www.imagestation.com/album/?id=4288894713 Homebuilt Elevator


It looks very nice, Stu. About the strength issue: Pipe threads are made to
seal against fluid leaks, and are nowhere near as strong as a straight
thread, because first one wall (say, the pipe) and then the other wall (the
fitting) tapers to a thin section. So you only have real thread strength
over a short section near the middle of the thread.

Secondly, black iron pipe isn't spec'd for tensile or compression strength.
It's spec'd on wall thickness, and, if you want to get fussy about it, it's
the hoop strength (the strength against expansion of fluids inside) that's
implicit in its specification. That, and allowance for corrosion, uneven
construction, and ham-fisted plumbers.

Thirdly, even a straight thread on a hollow tube is a complicated strength
issue. Thread strengths and standards are based on threading solid bar or
rod. When you thread a tube, the standard strength calculations go out the
window. Your minor diameter now is based on effective wall thickness as well
as the depth of thread. There also is an issue concerning the tendency of
the inner threaded piece to collapse from the angular forces imposed on it
by the 60-degree thread angles. That's not an issue with bar or rod.

If you have to deal with calculated strengths, the first thing you want to
do is to go to a mechanical tubing, rather than plumbing pipe. Then you'll
have to consult someone who knows his stuff on the thread-strength issue for
tubing. Either that, or use another method of assembly. You may actually
have plenty of strength for the application with pipe-threaded plumbing
pipe. But you won't be able to prove it, and, if liability is an issue,
you'd be dead meat.

Good luck.

Ed Huntress