Ed Huntress wrote:
"Ned Simmons" wrote in message
...
In article zoGib.8878$Eh3.4447339
, says...
"Ned Simmons" wrote in message
...
In article LIzib.6306$Eh3.2408645
, 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.
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.
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.
Just a minor nit... Wouldn't the strength be somewhat
dependent on how tightly the joint was made up? It seems
to me that the tighter you made the joint, the more load
you'd put on the material and that would have to come
out of your strength budget somehow. Not that I have
a clue how.
I still maintain that it's a bad application. I think
that any flexing could seriously weaken the joint over
time.