(a) Theory of pipe thread seal. The
threaded portion of a pipe fitting Is tapered so that when
it is tightened, the part with the external thread (male) Is
forced into the internally threaded (female) part, creat-
ing a seal.
(b) Tightening fittings with pipe threads. Ex-
treme care shall be taken when tightening pipe fittings.
Overtightening causes distortion, cracking, and leaks.
Tighten 1/8-inch fittings to a torque of 125 to 175 Inch-
pounds, 1/4-inch fittings of 200 to 300 inch-pounds, and
3/8-inch fittings to a torque of 300 to 400 Inch- pounds.
(2) Tubing fitting thread lubricants. Various
lubricants and antiseize compounds are used on
threads to prevent seizing and difficulty In disassembly.
Threads that give the most trouble are male and female
cast aluminum alloy tapered pipe threads Pipe threads
secure their sealing features by interlocking of threads.
Threads being tapered become highly stressed In final
torqued condition Due to this high stress and peculiar
stress-strain characteristics of aluminum alloy, threads
may deform permanently, causing sufficient misalign-
ment to make it difficult to disassemble unit. It is highly
desirable that male parts be made of different material
and properly lubricated with antiseize compound.
Federal Specification TT-S-1732, or equivalent Low
pressure lines with low torque anodized aluminum alloy
fittings and with antiseize compound used, may be
disassembled under normal conditions.
c . Straight Pipe Thread. Straight pipe thread is
used on fittings In hydraulic components connected to
tubing It Is shown in figure 4-26.
d. Universal Fittings AN 832. AN 833. and AN 834
Low Pressure. These fittings are used for connecting
tubing to components of the system and for connecting
sections of tubing where the line passes through a
bulkhead and fitting is secured to the bulkhead These
fittings are satisfactory for pressures up to and including
1000 psi For Information pertaining to systems of over
1000 psi, refer to paragraph e Bulkhead fittings shall be
installed on system components as follows:
(1) Assemble nut on fitting end and screw all the
way back to clear gasket groove.
(2) Place gasket or equivalent performed pack-
ing In gasket groove.
(3) Screw fitting into boss until gasket contacts boss.
(4) Unscrew fitting to approximate position
desired but not more than 360 degrees
(5) Tighten locknut lightly.
(6) Assemble flared tubing to fitting.
(7) Tighten locknut against boss
e. UniversalFittingsAN832. AN833, andAN834
High Pressure. It is necessary to use a backup washer
in the check nut when combination bulkhead and univer-
sal fittings are installed In bosses on systems where
pressures exceed 1000 psi This method eliminates the
possibility of high pressure extending the rubber gasket
into threaded areas of fitting. A leather washer
(MS28777) is used to prevent this extrusion of the
rubber gasket A Teflon backup ring (MS9058) Is used
in lieu of the leather washer in systems where tempera-
ture in excess of 1600 F (710 C) are anticipated The
method of installation is as follows:
(1) Install nut as high up on fitting as possible.
Add leather washer and gasket in position shown in
detail A, figure 4-27.
(2) Screw fitting assembly into boss until first
thread at top of fitting groove is from 3/16- to 1/8-inch
from top surface of boss, as shown In detail B, figure
(3) Hold fitting stationary and screw nut down
until it seats on boss. Slight extrusion of leather washer
between surface of boss and nut is considered satisfac-
tory, as shown in detail C, figure 4-27.
(4) Tighten locknut against boss
f. Rosan Fluid Fittings. The Rosan fluid fitting is a
one-piece, thin wall insert with an external lockring The
Rosan insert is fabricated from 6AL-4V titanium per
AMS4967. The lockring is fabricated from A-286 or-
rosin-resistant steel per AMS5734 and is retained on
the insert by a controlled interference fit. The Rosan fluid
fitting is used to provide semi-permanent port mounted
installations and is compatible with Denature or flared
tube ends. The unique features of the Rosan fluid fitting
are as follows.
(1) Lockring provides positive lock.
(2) No lacquering required.