deformation of the threads or the tape?
The crests & valleys of internal & external NPT pipe threads NEVER
Look at the thread form description in Machinery's Handbook.
The spiral leak path ALWAYS exists, this spiral MUST be seal with a
You can tighten lubricated NPT pipe & fitting joints 'til you run out
of torque ..the joint will leak unless sealant is used.
ter/TheDosandDontsThreadedPlas....- Hide quoted text -
That's a good find. What they are saying makes sense and they
should know. I've used tape for years and rarely had a problem.
But part of it is probably because I know how much to tighten them
and don't over tighten a fitting to crack it. For the future, I'll
Also, the part about Sched 80 vs 40 was interesting. Particularly
the part about how any threaded PVC joint reduced the overall
pressure rating by 50% compared to all glued connections.
On 5/11/2013 10:43 AM, firstname.lastname@example.org wrote:
While basically true there are some fallacies there, particularly wrt to
the Sch 80 vs Sch 40 discussion on stresses.
The stress on a Sch 80 fitting is _not_ the same as that for a Sch 40
fitting--the extra thickness _DOES_ make a difference because the
remaining cross-sectional area remaining after the threads are cut is
thicker and that increases the denominator (the per sq in portion of the
stress which has units of psi).
I've not gone and looked up actual dimensions, but the percentage
increase in remaining thickness will be quite significant.
I was wondering if they were only looking at stress induced from
& doing their tightening stress approximation as merely an E * radial
strain based wedging action?
Would that not be independent of wall thickness?
Total fitting load is tightening load + pressure load.
Of course the system pressure capacity would be higher with Sch 80 vs
but perhaps they were trying to emphasize the point that sch 80 was
not more over-tightening resistant than sch 40?
Am I missing something here?
The _loading_ is same because the dimensions are the same but the stress
is per unit area and there's more material so the actual stress is lower.
Meself, I think they just blew it on that part...
The weakest link in a Sch 40 threaded joint w/ a Sch 80 fitting will
still be the section of threaded Sch 40 not inside the fitting (think of
a female coupling/tee, say, w/ a male pipe for example).
Certainly it is true one doesn't want to over-torque a plastic fitting,
particularly the female that is in tension but it certainly is not true
that the same stress at the same thread depth on a Sch 80 has the same
stress as a Sch 40 because it does have that extra thickness.
On 5/11/2013 2:32 PM, dpb wrote:
Being as the following was my posting I'll edit it a little in place...
The above previously wasn't written very well, unfortunately. I recast
it some while writing and didn't get all the edits that could intended
done, sorry. I made a couple of minor fixes above that at least help
but I'll not worry about it further...
I'll just add that loading-->force; stress--> force/unit area. The
geometry (hence tightening) controls loading of this portion of the
total load; they're correct there. But loading isn't the same thing as
stress; it's only the numerator.
They're just saying that using Schedule 80 fittings is treating the
symptom, not the disease. They're saying that the disease of plastic
female threaded fittings splitting when tightened can be cured by using
a proper thread sealant instead of using pipe dope or teflon tape on
They're saying that the use of either teflon tape or pipe dope on
plastic threaded fittings will increase the wedging action of the male
pipe threads, thereby increasing the liklihood that the female fitting
So, the admonition against switching to Schedule 80 female threaded
fittings is that it's not addressing the real problem. If people used a
thread sealant instead of pipe dope or teflon tape, there wouldn't be
the increased splitting force on the female fitting, and there wouldn't
be any splitting problem. So, switching to Schedule 80 female plastic
fittings to avoid splitting is not really solving the problem. The real
solution is found in using thread sealant instead of pipe dope and/or
teflon tape to avoid the splitting problem altogether, rather than buy
stronger fittings that will stand up to the greater splitting force.
Basically, if the square peg won't go into the round hole, you need to
realize that you need a round peg, not a bigger hammer. In this case,
the analogy is that if your female plastic fittings are splitting, you
need to realize that the cause is the pipe dope or teflon tape your
using, and you need to use thread sealant instead, not use a stronger
or they're being over-torqued if not using tape or metal pipe dope.
That's a better summary of the point they're trying to make than
Now granted the _strain_ is the same as it's the dimensionless ratio of
the distance moved/turn and the geometry of a Sch 40 or Sch 60 or 80
fitting is the same so the geometries do cancel out. But the _stress_
in psi induced by that same strain is less for the fitting with more
material in it--if that weren't so it wouldn't be possible to design for
higher pressures by adding material because it wouldn't matter is the
conclusion that statement leads to. That clearly just isn't so so
they've confused the writeup in that regard.
Granted the weak point is as they state w/ the notching effect of the
threading but there's still significantly more material left behind in a
heavier fitting than the lighter and that does make a difference.
Certainly it shouldn't be/isn't necessary to use heavier fittings if
properly assembled (or the matching fittings are under-designed) is
their main point and that anything that actually causes deformation of
the fitting is detrimental. Those are certainly common sense
conclusions as well.
On Sat, 11 May 2013 23:37:04 +0200, nestork wrote:
Your summary was fantastically well written and easy to follow!
(much easier than the original article)
It's clear I should not be using Teflon tape.
But which of the non-lubricating "sealants" should I be using then?
I did not use that RectorSeal No. 5 stuff for my PVC.
In fact, it appears NONE of the "pipe stuff" I own is
correct for PVC; so I will try to find the Spears Blue 75
thread sealant at the irrigation supply places on Monday.
And, in the future, if I'm forced to use Teflon tape,
I'll use this method of wrapping the tape properly:
On Sun, 12 May 2013 16:57:56 +0000, Danny D wrote:
I should correct that slightly, by saying I read and
understood Oren's and DD_BobK's recommendation of the
best being the Tplus2 pipe thread sealant.
The only proscription against that stuff, based on
the Vic Smith article, was the fact it also lubricates
as it seals.
However, if I keep to the finger-tight-plus-two-turns
rule, I should be safe with the Tplus2 stuff.
I'm going to remove all the fittings, and try again.
I typically just use a little permatex blue silicone or the like. Yeah,
it's a little slick but just don't overdo tightening and you're fine.
Another that works fine is a white lithium; same thing as for usage. I
have even just used a little ol' gun grease when other wasn't handy and
Please quit saying that's "my article." I didn't write that, I just
cited it. I have no experience at all with threaded plastic pipe.
And I'm not buying all this emphasis on "lubricate vs non-lubricate."
As long as it's compatible with the plastic, seals the threads, and
doesn't harden up, it should work.
"Overtightening" because threads are lubed makes no sense to me.
One guy's "feel" for tight isn't the same as another's. You use a
different length wrench, the feel changes.
Likewise, these guys are saying stuff like "2 turns past
finger-tight." That fine if there's no burrs on the threads, the
threads are consistently formed, and everybody has the same fingers.
That's not realistic. What's more realistic is a consistent taper and
consistent thread count.
You want to look at how many threads should still be exposed for a
good fit, and tighten to there. lube or no lube. I can see that with
galvanized and know when the male is inserted far enough .
The force used to get there can vary depending on the thread
condition, but threads exposed is the best indication of what force
the taper is putting on the female, and when to stop cranking down.
Anyway, that's how I'd approach tightening plastic too.
But until I did it, it's still a guess to me.
It does to me.
The less force needed to overcome friction, the more force you have
available to tighten the joint. Try slipping a bicycle handle bar grip
onto bare chrome plated handle bars and you'll see what a fight it can
be. Now, lubricate the handle bars so the rubber grip slides over a
thin film of oil, and you'll see what a difference friction makes.
It's like a car. The more power used to drive the power steering pump,
the air conditioning compressor, the water pump and the alternator, the
less power you have left over for turning the wheels.
Except if you follow instructions and tighten the specified 1.5?
turns after contact it doesn't matter how much or how little friction
there is in the joint. the specified number of turns after contact
will ALWAYS give the same fit - and the same amount of thread
deformation or fitting swell.
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