WINTER 2016

120 THE DISTRIBUTOR’S LINK
GUY AVELLON WHAT FASTENER DISTRIBUTORS NEED TO KNOW ABOUT PIPE FITTINGS from page 28
After the shape of the bar stock is completed, the
bar is then cut into small sections that will represent the
body of the fitting. The ends are machined to form male
threads or ports, then drilled and tapped or threaded and
chamfered where necessary.
Typically, the type of brass used is a high grade SAE
C36000 brass which provides a tensile strength of
58,000 PSI and a yield strength of 50,000 PSI. Since
brass is a soft and malleable material, its yield strength
becomes extremely important. Even though the tensile
strengths of the forged fittings and the extruded fittings
are very close, the yield strength of the forging is only
23,000 PSI compared with 50,000 PSI for the extruded
fittings. This difference results in product dependability,
reduced chance of thread failure and leaking and
provides total optimum performance.
Pipes
Pipes are used for long runs, usually in a straight
line. It is a rigid connection for large flow volumes and
where disassembly is infrequent of not expected. There
are three types of pipe; galvanized steel, black steel and
brass pipe.
Galvanized steel pipe is used of air and water systems.
Due to the coating, it cannot be used for hydraulic fluid
flow. Pipes conforming to ASTM A120 may be used for
air brake lines as per SAE J1149 Type 2.
Black steel pipe is used for gas and hydraulic
systems.
Brass pipes are used for water lines and low pressure oil
and air systems.
Pipe Threads
A pipe thread is the most commonly used thread
form in manufacturing brass fittings as well as many
hydraulic hose end fittings. Although common, pipe
threads are rather confusing. Misuse of this thread form
can result in leakage in a system which can cause
catastrophic damage to equipment.
When attempting to measure pipe threads, it
appears that there is no relationship between pipe size
and the actual measurement of the pipe itself. This is
when pipes and pipe fittings were first introduced in the
late 1800’s, pipe size was originally determined by the
inside diameter. The original pipes were made of cast
iron: a very easy and available process then. When the
mating thread sizes were established, the cast iron had
to have a thick wall section to provide adequate strength,
since cast iron is not that strong.
As methods improved for producing stronger pipe, it
was no longer necessary to retain the thicker walls
because the welded seam or seamless pipes were much
stronger. However, it was necessary to retain the original
outside diameter (OD) of the pipe and thread dimensions
of the newer pipes and fittings to be compatible with the
pipes already in service. Therefore, the manufacturers
increased the inside diameter (ID) of the pipes to make
up for the thinner walls. To make sure that all pipes and
pipe fittings mated, all pipe sizes are now measured by
the OD.
When trying to measure a tapered pipe fitting at the
very end of its taper, it is easy to see a 3/4" fitting does
not measure 3/4" anywhere. To determine the pipe size
for all NPT and NPTF fittings, take a measurement of the
thread diameter at approximately the third thread from
the end of the fitting, and then subtract 1/4". That will
determine the pipe size.
Pipe Thread Design
Most pipe threads are tapered. There are some
permanent crimp hydraulic fittings that are tapered and
some that are straight. Know the difference to avoid
damage. The taper is necessary since the sealing occurs
in the threads themselves. When installing a fitting into
a port (tapped pipe threads into a body), the threads of
the fitting are becoming wider. If improperly installed,
hair line cracks around the port thread will occur because
the fitting was installed too far and was overtightened.
The taper thread form helps to create a seal but the
major sealing effect is due to the threads being crushed
together. The first type of thread design was the NPT,
National Pipe Thread Tapered and is still in use.
The NPT threads are similar to the mating threads of
a nut and bolt inasmuch as there is a gap tolerance
between the thread roots and crests. When the threads
engage, the thread flanks may provide an interference fit,
but there is that small area at the crests and thread
roots where there is no metal-to-metal contact and spiral
leaking occurs. This can only be prevented by using
some type of pipe sealant.
CONTINUED ON PAGE 164