AIRCRAFT WASHERS
Aircraft washers used in airframe repair are either plain,
lock, or special type washers.
• Plain washer
• Lock washer
• Special type washer
Plain Washer
Plain washers [Figure 5-30], both the AN960 and AN970, used
under hex nuts. They provide smooth bearing surface as shim to obtain correct
grip length. They are used to adjust the position of castellated nuts in
respect to drilled cotter pin holes in bolts. Plain washer should be used under
lock washer to prevent damage surface. Aluminum and aluminum alloy washer may
be used under bolt or nut on aluminum alloy or magnesium (corrosion caused by
dissimilar metal). When used in this manner, any electric current flow will be
between the washer and the steel bolt. However, it is common practice to use a
cadmium plated steel washer under a nut bearing directly against a structure as
this washer will resist the cutting action
of a nut
better than an
aluminum alloy washer. AN970 steel
washer provides a greater bearing than the AN960 washer and is used on wooden
structure.
Lock washer
AN 935 and AN 936 are used in machine screws or bolts where
the self locking or castellated type nut is not appropriate. Spring action to
provide prevent loosening nut from vibration. Never be use lock washer at
condition:
• With fasteners to
primary or secondary struc-tures
• With fasteners on
any part of the aircraft where
failure might result in damage or danger to the
aircraft or personnel
• Where failure would
permit the opening of a joint
to the airflow
• Where the screw is
subject to frequent removal
• Where the washers
are exposed to the airflow
• Where the washers
are subject to corrosive conditions
• Where the washer is
against soft material without a plain washer underneath to prevent gouging the
surface
Shakeproof Lockwashers
Shakeproof lockwashers are round washers designed with tabs
or lips that are bent upward across the sides of a hex nut or bolt to lock the
nut in place. There are various methods of securing the lockwasher to prevent
it from turning, such as an external tab bent downward 90° into a small hole in
the face of the unit, or an inter-nal tab which fits a keyed bolt.Shakeproof
lockwashers can withstand higher heat than other methods of safetying and can
be used under high vibration conditions safely. They should be used only once
because the tabs tend to break when bent a second time.
Special washer
The ball socket and seat washers, AC950 and AC955, are
special washers used where a bolt is installed at an angle to a surface, or
where perfect alignment with a surface is required. These washers are used
together. [Figure 5-30]The NAS143 and MS20002 washers are used for internal
wrenching bolts of the NAS144 through NAS158 series. This washer is either
plain or countersunk. Thecountersunk washer (designated as NAS143C andMS20002C)
is used to seat the bolt head shank radius,and the plain washer is used under
the nut.
Installation of bolt and nut
Importance to use applicable MM and
IPC in replacement. Ensure the washers are used under both the heads bolt and
nut. When possible a bolt head positioned on top or forward position, this
position to prevent the bolt from slipping out if the nut lost. Grip length
must be correct, should be equal the thickness of material to be bolted. If
slightly greater may be use the washer under the nut and bolt head. Very
importance to safe tied after installation.
AIRCRAFT SCREWS
Screws are the most commonly used threaded fastening devices
on aircraft. They differ from bolts inasmuch as they are generally made of
lower strength materials. They can be installed with a loose fitting thread,
and the head shapes are made to engage a screwdriver or wrench. Some screws
have a clearly defined grip or unthreaded portion while others are threaded
along their entire length. Several types of structural screws differ from the
stan-dard structural bolts only in head style. The material in them is the
same, and a definite grip length is pro-vided. The AN525 washer head screw and
the NAS220 through NAS227 series are such screws.Most common use threaded
fastening differ from bolt, they are generally made of lower strength
materials. Common used screws are classified in:
(1) structural screws, which have the same strength as equal
size bolts;
(2) machine screws, which include the majority of types used
for general repair; and
(3) self-tapping screws, which are used for attaching
lighter parts. A fourth group, drive screws, are not actually screws but nails.
They are driven into metal parts with a mallet or hammer and their heads are
not slotted or recessed.
a. Structural Screws
Structural screws are made of alloy steel, are properly heat
treated, and can be used as structural bolts. These screws are found in the
NAS204 through NAS235 and AN509 and AN525 series. They have a definite grip and
the same shear strength as a bolt of the same size. Shank tolerances are
similar to AN hex head bolts, and the threads are National Fine. Structural
screws are available with round, brazier, or countersunk heads. The recessed
head screws are driven by either a Phil-lips or a Reed & Prince
screwdriver.
The AN509 (100°) flathead screw is used in counter-sunk
holes where a flush surface is necessary.
The AN525 washer head structural screw is used where raised
heads are not objectionable. The washer head screw provides a large contact
area.
b. Machine Screws
Usually of the flathead (countersunk), roundhead or
washer-head types. These screws are general purpose screw and available in low
carbon steel, brass, corrosion resistant steel, and aluminum alloy.
Roundhead screws, AN515 and AN520, have either slotted or
recessed heads. The AN515 screw has coarse threads, and the AN520 has fine
threads.
Countersunk machine screws are listed as AN505 and AN510 for
82°, and AN507 for 100°. The AN505 and AN510 correspond to the AN515 and AN520
round-head in material and usage.
The fillister head screw, AN500 through AN503, is a general
purpose screw and is used as a capscrew in light mechanisms. This could include
attachments of cast aluminum parts such as gearbox cover plates. The AN500 and
AN501 screws are available in low carbon steel, corrosion-resistant steel, and
brass. The AN500 has coarse threads, while the AN501 has fine threads. They
have no clearly defined grip length. Screws larger than No. 6 have a hole
drilled through the head for safetying purposes.
c. Self-tapping Screws
Machine self-tapping screws are listed as AN504 and AN506.
The AN504 screw has a roundhead, and the AN506 is 82° countersunk. These screws
are used for attaching removable parts, such as nameplates, to castings and
parts in which the screw cuts its own threads.AN530 and AN531 self-tapping
sheet metal screws, such as the Parker-Kalon Z-type sheet metal screw, are
blunt on the end. They are used in the temporary attachment of metal for
riveting, and in the permanent assembly
of nonstructural assemblies.
Self-tapping screws should not be used to replace standard screws, nuts,
bolts, or rivets. Drive screws, AN535, correspond to the Parker-Kalon U-type.
They are plain head self-tapping screws used as capscrews for attaching
nameplates in castings and for sealing drain holes in corrosion proofing
tubu-lar structures. They are not intended to be removed after installation.
Identification and Coding for Screws
The coding system used to identify screws is similar to that
used for bolts. There are AN and NAS screws. NAS screws are structural screws.
Part numbers 510, 515, 550, and so on, catalog screws into classes, such as
roundhead, flathead, washer head, and so forth. Letters and digits indicate
their material composition, length, and thickness. Examples of AN and NAS code
numbers follow.
AN501B-416-7
AN = Air Force-Navy standard
501 = fillister head, fine thread
B = brass
416 = 4⁄16-inch diameter
7 = 7⁄16-inch length
The letter “D” in place of the “B” would indicate that the
material is 2017-T aluminum alloy. The letter “C” would designate corrosion
resistant steel. An “A” placed before the material code letter would indicate
that the head is drilled for safetying.
NAS144DH-22
NAS = National Aircraft Standard
144 = head style; diameter and thread—1/4-28 bolt, internal wrenching
DH = drilled head
22 = screw length in 16ths of an inch—13⁄8 inches long
The basic NAS number identifies the part. The suffix letters
and dash numbers separate different sizes, plating material, drilling
specifications, and so forth. The dash numbers and suffix letters do not have
standard meanings. It is necessary to refer to a specific NAS page in the
Standards book for the legend.
Repair Internal Threads
Damage threads on bolts or nut can be seen only require
replacement defective parts. If internal thread are damaged;
• The part may be
replaces or the threads repaired or
• Replaced.
Two methods repairing are replacement bushings or
heli-coils.
Turnlock Fasteners
Turnlock fasteners are used to secure inspection plates, doors,
and other removable panels on aircraft. Turnlock fasteners are also referred to
by such terms as quick opening, quick action, and stressed panel fasteners. The
most desirable feature of these fasteners is that they permit quick and easy
removal of access panels for inspection and servicing purposes.Turnlock
fasteners are manufactured and supplied by a number of manufacturers under
various trade names. Some of the most commonly used are the Dzus, Camloc, and
Airloc.
Dzus Fasteners
The Dzus turnlock fastener consists of a stud, grommet, and
receptacle. Figure 5-58 illustrates an installed Dzus fastener and the various
parts.
The grommet is made of aluminum or aluminum alloy material.
It acts as a holding device for the stud. Grom-mets can be fabricated from 1100
aluminum tubing, if none are available from normal sources.
The spring is made of steel, cadmium plated to prevent corrosion.
The spring supplies the force that locks or secures the stud in place when two
assemblies are joined.
The studs are fabricated from steel and are cadmium plated.
They are available in three head styles: wing, flush, and oval. Body diameter,
length, and head type may be identified or determined by the markings found on
the head of the stud. [Figure 5-59] The diameter is always measured in
sixteenths of an inch. Stud length is measured in hundredths of an inch and is
the distance from the head of the stud to the bottom of the spring hole.
Camloc Fasteners
Camloc fasteners are made in a variety of styles and
designs. Included among the most commonly used are the 2600, 2700, 40S51, and
4002 series in the regular line, and the stressed panel fastener in the heavy
duty line. The latter is used in stressed panels which carry structural loads.
The Camloc fastener is used to secure aircraft cowlings and
fairings. It consists of three parts: a stud assembly, a grommet, and a
receptacle. Two types of receptacles are available: rigid and floating. [Figure
5-60]
The stud and grommet are installed in the removable portion;
the receptacle is riveted to the structure of the aircraft. The stud and
grommet are installed in either a plain, dimpled, countersunk, or counterbored
hole, depending upon the location and thickness of the material involved.
A quarter turn (clockwise) of the stud locks the fastener.
The fastener can be unlocked only by turning the stud counterclockwise.
Airloc Fasteners
The Airloc fastener consists of three parts: a stud, a cross
pin, and a stud receptacle. [Figure 5-61] The studs are manufactured from steel
and casehardened to prevent excessive wear. The stud hole is re
amed for a press fit of the cross pin.
amed for a press fit of the cross pin.
The total amount of material thickness to be secured
with the Airloc fastener
must be known
before the correct length of stud
can be selected for installation.
The total thickness of material that each stud will
sat-isfactorily lock together is stamped on the head of the stud in thousandths
of an inch (0.040, 0.070, 0.190, and so forth). Studs are manufactured in three
head styles: flush, oval, and wing.
The cross pin [Figure
5-61] is manufactured from chrome-vanadium steel and heat treated to provide maximum
strength, wear, and holding power. It should never be used the second time;
once removed from the stud, replace it with a new pin.
Receptacles for Airloc fasteners are manufactured in two
types: rigid and floating. Sizes are classified by number—No. 2, No. 5, and No.
7. They are also clas-sified by the center-to-center distance between the rivet
holes of the receptacle: No. 2 is 3⁄4 inch; No. 5 is 1 inch; and No. 7 is 13⁄8
inch. Receptacles are fabricated from high-carbon, heat-treated steel. An upper
wing assures ejection of the stud when unlocked and enables the cross pin to be
held in a locked position between the upper wing, cam, stop, and wing detent,
regardless of the tension to which the receptacle is subjected.
