is essential. See figure 2-17. The threaded taper pin
is used with a taper pin washer and a shear nut if the
taper pin is drilled, or with a self-locking nut if
undrilled. When a shear nut is used with the threaded
taper pin and washer, the nut is secured with a cotter
pin.
FLAT-HEAD PINS.—The flat-head pin is used
with tie rod terminals or secondary controls, which do
not operate continuously. The flat-head pin should be
secured with a cotter pin. The pin is normally
installed with the head up. See figure 2-17. This
precaution is taken to maintain the flat-head pin in the
installed position in case of cotter pin failure.
SNAP RINGS.—A snap ring is a ring of metal,
either round or flat in cross section, that is tempered to
have springlike action.
This springlike action will
hold the snap ring firmly seated in a groove. The
external types are designed to fit in a groove around
the outside of a shaft or cylinder. The internal types
fit in a groove inside a cylinder. Special pliers are
designed to install each type of snap ring.
Snap rings can be reused as long as they retain
their shape and springlike action. External snap rings
may be safety wired, but internal types are never
safetied.
STUDS.—There are four types of studs used in
aircraft structural applications. They are the coarse
thread, fine thread, stepped and lockring studs. Studs
may be drilled or undrilled on the nut end. Coarse
(NAS183) and fine (NAS184) thread studs are
manufactured from alloy steel and are heat treated.
They have identical threads on both ends. The
stepped stud has a different thread on each end of the
stud. The lockring stud may be substituted for
undersize or oversize studs. The lockring on this stud
prevents it from backing out due to vibration, stress,
or temperature variations. Refer to the Structural
Hardware Manual, NAVAIR 01-1A-8 for more
detailed information on studs.
HELI-COIL
INSERTS.—Heli-coil
thread
inserts are primarily designed to be used in materials
that arc not suitable for threading because of their
softness.
The inserts are made of a diamond
cross-sectioned stainless steel wire that is helically
coiled and, in its finished form, is similar to a small,
fully compressed spring. There are two types of
heli-coil inserts. See figure 2-18. One is the plain
insert, made with a tang that forms a portion of the
bottom coil offset, and is used to drive the insert. This
tang is left on the insert after installation, except when
its removal is necessary to provide clearance for the
end of the bolt. The tang is notched to break off from
the body of the insert, thereby providing full
penetration for the fastener.
The second type of insert used is the self-locking,
mid-grip insert, which has a specially formed grip coil
midway on the insert. This produces a gripping effect
on the engaging screw, For quick identification, the
self-locking, mid-grip inserts are dyed red.
FASTENERS (THREADED)
Although thousands of rivets are used in aircraft
construction, many parts require frequent dismantling
or replacement. For these parts it is more practical to
use some form of threaded fastener. Furthermore,
some joints require greater strength and rigidity than
can be provided by riveting. Manufacturers solve this
Figure 2-17.—Types of aircraft pins.
2-14
