In article 1JKib.12573$Eh3.6300408
@news4.srv.hcvlny.cv.net, says...
"Ned Simmons" wrote in message
...


Oh, come on, Ed. What's so different about pipe threads
that they don't "exploit the strength of the material"? The
sharp thread form likely causes some stress concentrations,
but other than that, I don't see much difference.

It isn't the sharp threads, Ned. It's the fact that the walls are weak at
each end of the taper, because one side or the other is thin at either end.
Only a couple of threads in the middle of the joint can produce the full
strength you would get with straight threads, and that isn't enough to
exploit the strength of the material. There aren't enough threads where the
material on *both* elements is equally thick.

If you look at an actual pipe fitting, unless it's one of
the thin "merchant" couplings, the wall is much thicker
than that of the pipe. I still maintain that the weakest
point is the pipe wall where it exits the fitting, per my
exchange with Jim.


You probably could approximate the strength if you calculated the strength
of three straight threads, more or less, of the same size in the same
material.


Here the NPT thread has an advantage over a UN thread. The
shear area is a function of the minor diameter of the
external thread, and for a tapered thread the minor dia of
the ext and internal thread are equal. Measuring in the
middle of the thread engagement on a 3/4 fitting, I get a
minor dia of about .92

(.92 in X pi) x (3 threads / 14 threads/in) = .62 in^2

Assuming a shear strength about 1/3 of UTS (very
conservative, I think)...

..62 in^2 x 20000 lb/in^2 = 12400 lb.

Which is pretty close to the yield of the full (unthreaded)
wall of the pipe.

Ned Simmons