[AT] pipe threads

[Steve W.]

NPT   American Standard Pipe Taper Thread

NPSC   American Standard Straight Coupling Pipe Thread

NPTR   American Standard Taper Railing Pipe Thread

NPSM   American Standard Straight Mechanical Pipe Thread

NPSL   American Standard Straight Locknut Pipe Thread

NPTF   American Standard Pipe Thread Tapered (Dryseal)

BSPP   British Standard Pipe Thread Parallel

BSPT   British Standard Pipe Thread Tapered

The best known and most widely used connection where the pipe thread
provides both the mechanical joint and the hydraulic seal is the
American National Pipe Tapered Thread, or NPT. NPT has a tapered male
and female thread which seals with Teflon tape or jointing compound.
Pipe threads used in hydraulic circuits can be divided into two types:
a) Jointing threads - are pipe threads for joints made pressure tight by
sealing on
the threads and are taper external and parallel or taper internal
threads. The
sealing effect is improved by using a jointing compound.
b) Fastening threads - are pipe threads where pressure tight joints are
not made on
the threads. Both threads are parallel and sealing is affected by
compression of a
soft material onto the external thread, or a flat gasket.

Plastic injection molded thread forms are manufactured to ANSI B2.1 and
SAE J476 standards.
The word "tapered" in several of the above names points to the big
difference between many pipe threads and
those on bolts and screws. Many pipe threads must make not only a
mechanical joint but also a leakproof
hydraulic seal. This is accomplished by the tapered thread form of the
male matching the thread form of the
female tapered thread and the use of pipe sealant to fill any voids
between the two threads which could cause a
spiral leak. The bottoms of the threads aren't on a cylinder, but a
cone; they taper. The taper is 1?16 inch in an
inch, which is the same as 3/4 inch in a foot.
Because of the taper, a pipe thread can only screw into a fitting a
certain distance before it jams. The standard
specifies this distance as the length of hand tight engagement, the
distance the pipe thread can be screwed in by
hand. It also specifies another distance - the effective thread, this is
the length of the thread which makes the seal
on a conventional machined pipe thread. For workers, instead of these
distances, it is more convenient to know
how many turns to make by hand and how many with a wrench. A simple rule
of thumb for installing tapered
pipe threads, both metal and plastic, is finger tight plus one to two
turns with a wrench. Torque installation values
can be determined per application, but due to the variations involved in
pipe joints such as dissimilar materials of
male and female threads, type of sealants used, and internal variations
in product wall thickness, a standard torque
specification cannot be generically applied .

A number of variations of the NPT thread have been introduced to
overcome the problem of spiral leakage and are known as Dryseal threads
(See SAE standard J476). The best known is the NPTF (F for Fuel). With
this thread design, there are controls on the crests and roots of both
the
male and the female threads to ensure the crest crushes or displaces
material into the root of the mating thread. The interference fit
between the
crest of one thread and the root of the other, along with the thread
flanks matching, seals against spiral leakage.
A variation of the Dryseal thread is the NPSF (National Pipe Straight
Fuel). It is used for internal threads and a NPTF external thread can
be screwed into it to provide a satisfactory mechanical connection and a
hydraulic seal. The combination of a parallel and tapered thread is
not regarded as ideal but is widely used. High quality plastic quick
disconnect couplings typically use NPT threads.


Steve Williams
Near Cooperstown, New York


----- Original Message ----- 
From: "David Myers" <walking_tractor at yahoo.com>
To: "Antique tractor email discussion group"
<at at lists.antique-tractor.com>
Sent: Sunday, June 25, 2006 7:04 AM
Subject: RE: [AT] pipe threads


> Mike, that is my thinking on the letter designation
> also (just a kinda made sense thing to me).  And to
> add to my confusion is 'MTP' and 'FTP', I assume
> meaning male and female taper pipe? (I have seen these
> but are not sure if they're even a proper
> designation).
> JIC is a straight thread 37deg. taper SEAT fitting,
> used for medium presssure hydraulics.  JIC is being
> phased out in favor of the flat face O'ring fittings
> (FFOR, For-Seal, and other trade names).  There is
> also straight thread O'ring fittings, more commonly
> used to mate to valve blocks etc.  Of course, I have
> come across the dreaded 'Proprietary Fitting' WAY to
> many times!  Hope this isn't the case.
>
> Dave Myers
> Paw Paw, MI
>
>
> --- Mike Reggie <mrreg_99 at hotmail.com> wrote:
>
> >
> >   This is just a guess, but I always thought the "F"
> > after the NPT stood for
> > female, I've also seen it following the designation
> > NPS, which is generally
> > just used for electrical fittings and is the same
> > thread without the sealing
> > capability of the tapered thread. Again just a
> > guess, but could the thread
> > you are looking for be a JIC thread?? I don't know
> > how long that thread has
> > been in use, but it seems to be a finer thread than
> > the standard pipe
> > thread, and I think it tapers.
>
>
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