In article zoGib.8878$Eh3.4447339
@news4.srv.hcvlny.cv.net, says...
"Ned Simmons" wrote in message
...
In article LIzib.6306$Eh3.2408645
@news4.srv.hcvlny.cv.net, says...
"Ned Simmons" wrote in message
...

The argument that tapered pipe threads are not intended to
carry a tension or compression load is nonsense. (I don't
mean to imply you said this, Jim.) If there is pressure in
the pipe, then there is an axial force on the joint equal
to the pressure x area of the pipe ID.

Ned Simmons

And that may run a couple of hundred psi.

Which gives us the lower bound on the axial strength of the
threads, but says nothing about the upper bound. Much
higher pressures are acceptable with forged fittings and
heavy wall pipe. 3000 psi fittings are readily available up
to 4" NPT(F), 6000 psi up to at least 2".

Those loads on plumbing pipe from
internal pressure are nowhere near the strength expected of mechanical
tubing in tension or compression, Ned.

I don't see how that's relevant. The question is not what
the optimum solution is, but rather, can pipe threads carry
significant axial loads. Clearly they can.

Ned Simmons

Then the job is to quantify strength of a joint that was never designed for
much strength. Pipe threads are designed to seal, not to exploit the
strength of the material. Straight threads are.

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.

Sure, they have to seal, but there's gonna be no seal if
the pipe is ejected from the fitting with 6000 psi behind
it. That doesn't happen in a properly made up joint.


As I said, it may well do the job here. But don't expect it to perform like
a straight thread, for the reasons given by several people here. And don't
expect to find a spec on it. There may be such a thing, but it's largely
incidental if it is.

Of course it's not going to behave *exactly* like a
straight thread. Why is that a problem? Perhaps the
analysis is a bit more involved than for a straight thread
on a solid member(though other than taking into account the
thin wall at the root of the thread, and the effects of the
sharp vee, I'm not convinced of that), but if all the
components of every new design had to be used in a mannner
that was anticipated and pre-calculated we wouldn't see
much progress, would we?

As far as finding a spec, I expect you're right, and I said
as much in a post to the OP back on 10/2. My objection is
to the several (unsubstantiated IMO) assertions that pipe
threads are unsuitable for carrying axial loads.

Ned Simmons