[SEL] pipe taps, threads, and fittings

Richard Allen linstrum55 at yahoo.com
Tue Dec 28 10:11:53 PST 2004


Hi, guys,

I have been absent from here for about two years but through the good
offices of John Culp I have returned. My old Pentium 1 computer was so
slow I couldn’t participate in a lot of things in general on the
Internet simply because the processor speed was so slow. I upgraded to
a 2.6 GHz Celeron computer and now it is a lot better, just my
extremely rural phone line now being the limiting factor by not
supporting a decent modem speed. 

When I use a pipe tap I run it in about half way and then check for
thread engagement of the fitting of the part that screws into it.
According to Machinery’s Handbook the minimum number of turns of pipe
thread engagement is five BY HAND. After five turns by hand, I use a
rule thumb to find the final tightening torque by using the torque that
is applied to a coarse pitch low strength soft steel bolt that
corresponds to the actual outside diameter of the pipe in question.
Remember that the pipe size and its actual inside and outside diameters
are not very closely related, especially at smaller sizes! With brass
and aluminum fittings the torque has to be considerably less and a lot
of care has to be used. I use the torque table at:

http://www.thelenchannel.com/1torque.html  

Teflon thread sealant has been an absolute godsend for pipe threads and
I have had perfectly good hand tight leak-proof seals for ¼” pipe at
2100 psi on oxygen tank lines. A word of warning, with oxygen the only
safe thread sealant is Teflon, if a sealant burns it will explode under
pressure with oxygen!

Pipe threads used for pipes that carry gases and liquids are cut on a
taper so that the DIAMETER decreases by ¾” per 12” or a 16 to one
taper. A round rod with a diameter of ¾” will come to a point at a
length of 12 inches with this taper. Pipe threads used for electrical
conduit have the same outside diameter and pitch as the starting
diameter of tapered pipe threads and they will screw together quite
nicely onto PROPERLY manufactured tapered pipe fittings despite the
apparent conflict in geometry that indicates that they won’t. This is
due to the female tapered thread having a larger starting diameter than
the male thread to make engagement easier. Where sealing is of no
consequence they may be used interchangeably. Pipe threads have very
little actual strength to resist bending forces applied at the joint
and the pipes will break off at the fitting when pried, and this is a
quick and easy way to take apart a pipeline. I shudder at the sight of
a huge heavy muffler hanging out at the end of a long horizontal
exhaust pipe. I worked at a place where there was a high school kid
beekeeper next door who made a crane for lifting his beehives in and
out of his pickup truck with the upright and reach arm made out of two
1-1/2” pipes screwed into a 90° elbow. I told him it was going to break
off but you know how high school kids are when listening to adults. A
few days later I saw him loading them by hand again.

Commercially manufactured pipe threads have become the bother,
nuisance, annoyance, irritation, pest, curse, blight, and all around
bane of those who work with threaded pipe joints. I base my deprecatory
litany from having to use commercially manufactured pipe fittings that
quite usually screw together only one or two turns by hand and with a
wrench perhaps three turns before collapsing or tearing off the end of
the pipe (properly cut tapered pipe threads will not strip because of
the thread tips bottoming out and halting rotation due to interference
binding before enough longitudinal force is developed to shear the
thread from its root. Well drilling pipe strings are screwed together
with tapered threads in part because of this feature). Before using
commercially manufactured pipe fittings I chase the threads before
attempting to assemble the parts just as a matter of course. The reason
why commercially manufactured pipe threads are so abominable is because
those who maintain the thread cutting machines at pipe fitting
factories have their necks under the hobnailed boots of the bean
counters who try to strip every last $0.0001 of value from a cutting
tool before discarding it. When the pipe and fitting threading tools
become worn they are re-sharpened far beyond design limits and the
threads produced are outside of specifications for proper fit,
consequently making them unsuitable. So, before buying pipe fittings
screw them together first and if they don’t go five complete turns by
hand they are out of spec. What you do after that is up to you. We all
hear news reports about mysterious gas explosions, “broken” water
mains, and flooded apartments all the time.

Richard Allen




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