TRAINING NOTES

6.5 FASTENERS
Locking Devices (LP – 16 to 20)
a. Tab and spring washers
b. Locking plates, split pins, pal nuts,
c. Wire locking,
d. Quick release fasteners,
e. Keys, circlips, and cotter pins.

 Installation of Nuts and Bolts

 Examine the markings on the head to determine whether a bolt is steel or aluminum alloy.
 It is of extreme importance to use like bolts in replacement.
 In every case, refer to the applicable maintenance instruction manual and illustrated parts
breakdown.
 Be sure that washers are used under the heads of both bolts and nuts unless their omission
is specified.
 A washer guards against mechanical damage to the material being bolted and prevents
corrosion of the structural members.
 An aluminum alloy washer may be used under the head and nut of a steel bolt securing
aluminum alloy or magnesium alloy members.
 Corrosion will attack the washer rather than the members.
 Steel washers should be used when joining steel members with steel bolts.
 Whenever possible, the bolt should be placed with the head on top or in the forward
position.
 This positioning helps prevent the bolt from slipping out if the nut is accidentally lost.
 Make sure that the bolt grip length is correct.
 Generally speaking, the grip length should equal the thickness of the material being bolted
together.
 Not more than one thread should bear on the material, and the shank should not protrude
too far through the nut.

 Application of Torque
 Torque is the amount of twisting force applied when you are tightening a nut.
 If torque values are specified in the appropriate manual, a torque wrench must be used.
 Regardless of whether torque values are specified or not, all nuts in a particular installation
must be tightened a like amount.
 This permits each bolt in a group to carry its share of the load.
 It is a good practice to use a torque wrench in all applications.

 Safetying of Nuts and Bolts

 It is very important that all nuts except the self-locking type be safetied after installation.
 This prevents nuts from loosening in flight because of vibration.
 Methods of safetying are discussed later in this chapter.

 Safety Methods:
 Safetying is a process of securing all aircraft bolts, nuts, capscrews, studs, and other
fasteners.
 Safetying prevents the fasteners from working loose due to vibration.
 Loose bolts, nuts, or screws can ruin engines or cause parts of the aircraft to drop off.
 To carry out an inspection on an aircraft, you must be familiar with the various methods of
safetying.
 Careless safetying is a sure road to disaster.
 Always use the proper method for safetying.
 Always safety a part you have just unsafe tied before going on to the next item of
inspection.

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  You should always inspect for proper safetying throughout the area in which you are
working.
 There are various methods of safetying aircraft parts.
 The most widely used methods are safety wire, cotter pins, lock washers, snap rings, and
special nuts.

1. Safety Wiring
 Safety wiring is the most positive and satisfactory method of safetying.
 It is a method of wiring together two or more units.
 Any tendency of one unit to loosen is counteracted by the tightening of the wire.

 Nuts, Bolts, and Screws

 Nuts, bolts, and screws are safety wired by the single-wire double-twist method.
 This method is the most common method of safety wiring.
 A single-wire may be used on small screws in close spaces, closed electrical systems, and
in places difficult to reach.
 The following steps required to install a standard double-twist safety wire for two bolts with
right-hand threads.
1. Step 1. Assemble the unit. Torque the bolts and carefully align the safety wire holes.
2. Step 2. Insert the proper size wire through the hole in the first bolt.
3. Step 3. Bend the left end of the wire clockwise around the bolt head and under the other
end of the wire.
4. Step 4. Pull the loop tight against the bolt head. Grasp both ends of the wire. Twist them in
a clockwise direction until the end of the braid is just short of the second bolt.
5. Step 5. Check to ensure that the loop is still tightly in place around the first bolt head. Grasp
the wire with pliers just beyond the end of the braid. While holding it taut, twist it in a
clockwise direction until the braid is stiff.
NOTE: The braid must be tight enough to resist friction or vibration wear, but should not be
over tightened.
6. Step 6. Insert the upper end of the safety wire through the hole in the second bolt. Pull the
braid until it is taut.
7. Step 7. Bring the other end of the wire counterclockwise around the bolt head and under
the protruding wire end.
8. Step 8. Tighten the loop and braid the wire ends in a counterclockwise direction. Grasp the
wire with the pliers just beyond the end of the braid and twist in a counterclockwise direction
until the braid is stiff. Make sure you keep the wire under tension.
9. Step 9. With a final twisting motion, bend the braid to the right and against the head of the
bolt.
10. Step 10. Cut the braid, being careful that between three and six full twists still remain. Avoid
sharp projecting ends.

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  Torque all parts to the recommended values, and align holes before you attempt to proceed
with the safetying operation.
 Never over torque or loosen a torqued nut to align safety wire holes.

 Oil Caps, Drain Cocks, and Valves

 In the case of the oil cap, the wire is anchored to an adjacent fillister head screw.
 This system applies to any other unit that must be safety wired individually.
 Ordinarily, anchorage lips are conveniently located near these individual parts.
 When this provision is not made, the safety wire is fastened to some adjacent part of the
assembly.

 General Safety Wiring Rules

When you use the safety wire method of safetying, follow these general rules:
1. A pigtail of one-fourth to one-half inch (three to six twists) should be made at the end of the
wiring.
2. This pigtail must be bent back or under to prevent it from becoming a snag.
3. The safety wire must be new upon each application.
4. When you secure castellated nuts with safety wire, tighten the nut to the low side of the
selected torque range, unless otherwise specified. If necessary, continue tightening until a
slot aligns with the hole.
5. All safety wires must be tight after installation, but not under such tension that normal
handling or vibration will break the wire.
6. Apply the wire so that all pull exerted by the wire tends to tighten the nut.
7. Twists should be tight and even, and the wire between the nuts should be as taut as
possible without being over twisted.

 Thread Inserts –

Screw thread inserts (or wire inserts) are used:

a. In soft materials (Aluminum, Magnesium, Bronze), wood, plastics or composite materials to
allow frequently assembly and dis-assembly of components with minimum thread wear to
the component itself.
b. To increase the effective thread diameter and allow higher torques to be used.
c. In schemes to repair a damaged thread.

 Prior to installation the insert is shorter in length and larger- in diameter than when installed.
 In most cases the tools and inserts come in kit form with full instructions supplied.
 Good motor skills are required to fit/remove inserts successively.

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 In general the installation procedure is as follows:
1. Refer to manufacturer’s information supplied with the Insert kit
2. Drill correct size hole.
3. Tap the hole.
4. Gauge the tapped hole.
5. Insertion of the insert.
6. Removal of the insert tang.
7. Inspection and checking of work.

 Special Fasteners:-
1. Taper-Lok
 Taper-Loks are the strongest special fasteners used in aircraft
construction.
 The Taper-Lok exerts a force on the walls of the hole because
of its tapered shape.

2. Hi-Tigue
 The Hi-Tigue special fastener has a bead that encircles
the bottom of its shank.
 The bead preloads the hole it fills, resulting in increased
joint strength.
 At installation, the bead presses against the sidewall of the
hole, exerting radial force that strengthens the surrounding
area.
 Hi-Tigue fasteners are made of aluminum, titanium, and stainless steel alloys.
 The collars are composed of compatible metal alloys and come in two types: sealing and
non-sealing.
 Just like the Hi-Loks, they can be installed using an Allen wrench and a box-end wrench.

3. Captive Fasteners
 Captive fasteners are used for quick removal of engine nacelles, inspection panels, and
areas where fast and easy access is important.
 A captive fastener has the ability to turn in the body in which it is mounted, but which will
not drop out when it is unscrewed from the part it is holding.

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4. 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.

a. Dzus Fasteners
 The Dzus turnlock fastener consists of a stud, grommet, and
receptacle.
 The grommet is made of aluminum or aluminum alloy material.
 It acts as a holding device for the stud.
 Grommets can be fabricated from 1100 aluminum tubing.
 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.
 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.
 A quarter of a turn of the stud (clockwise) locks the fastener.
 The fastener may be unlocked only by turning the stud counterclockwise.
 A Dzus key or a specially ground screwdriver locks or unlocks the fastener.

b. Camloc Fasteners
 Camloc fasteners are made in a variety of styles and
designs.
 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.
 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 counter bored 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.

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c. Airloc Fasteners
 The Airloc fastener consists of three parts: a stud, a
cross pin, and a stud receptacle.
 The studs are manufactured from steel and
casehardened to prevent excessive wear.
 The stud hole is reamed 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
satisfactorily 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 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.
 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.

 Some fasteners cannot be classified as rivets, turnlocks, or threaded fasteners.

 Included in this category are snap rings, turnbuckles, taper pins, flat head pins, and flexible
connector/clamps.

 Pins
 The three main types of pins used in aircraft structures are
the taper pin, flathead pin, and cotter pin.
 Pins are used in shear applications and for safetying.
 Roll pins are finding increasing uses in aircraft construction.

1. Taper Pins
 Plain and threaded taper pins (AN385 and AN386) are used
in joints which carry shear loads and where absence of play is
essential.
 The plain taper pin is drilled and usually safetied with wire.
 The threaded taper pin is used with a taper pin washer
(AN975) and shear nut (safetied with a cotter pin or safety
clip) or self-locking nut.
2. Flathead Pin
 Commonly called a clevis pin, the flathead pin
(MS20392) is used with tie rod terminals and in
secondary controls which are not subject to
continuous operation.
 The flat head pin should be secured with a cotter
pin.
 The pin is customarily installed with the head up so
that if the cotter pin fails or works out, the pin will remain in place.

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3. Cotter Pins or Split Pin
 The AN380 cadmium plated, low carbon steel cotter pin is used for
safetying bolts, screws, nuts, other pins, and in various applications
where such safetying is necessary.
 The AN381 corrosion resistant steel cotter pin is used in locations
where nonmagnetic material is required, or in locations where
resistance to corrosion is desired.

4. Roll Pins or Spring Pin

 The roll pin is a pressed fit pin with chamfered
ends.
 It is tubular in shape and is slotted the full length of
the tube.
 The pin is inserted with hand tools and is
compressed as it is driven into place.
 Pressure exerted by the roll pin against the hole walls keeps it
in place, until deliberately removed with a drift punch or pin
punch.

5. Dowel pins
 Dowel pins are the most accurate, machined to tight tolerances
and pressed or free fitting into reamed holes.

Flexible Connectors/Clamps
 When installing a hose between two duct sections, the gap between the duct ends should
be one-eighth inch minimum to three-fourths inch maximum.
 When you install the clamps on the connection, the clamp should be one-fourth inch
minimum from the end of the connector.
 Misalignment between the ducting ends should not exceed one-eighth inch maximum.
 Marman type clamps, commonly used in ducting systems, should be tightened to the torque
value indicated on the coupling.
 Use the torque value as specified on the clamp or in the applicable maintenance instruction
manual.

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 Aircraft Washers
Aircraft washers used in airframe repair are plain, lock, or special type washers.

1. Plain Washers
 Plain washers both the AN960 and AN970 are used under hex nuts.
 They provide a smooth bearing surface and act as a shim in obtaining correct grip length for
a bolt and nut assembly.
 They are used to adjust the position of castellated nuts in respect to drilled cotter pin holes
in bolts.
 Use plain washers under lockwashers to prevent damage to the surface material.
 Aluminum and aluminum alloy washers may be used under bolt heads or nuts on aluminum
alloy or magnesium structures where corrosion caused by dissimilar metals is a factor.
 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 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.
 The AN970 steel washer provides a greater bearing area than the AN960 washer and is
used on wooden structures under both the head and the nut of a bolt to prevent crushing
the surface.

2. Lockwashers
 Lockwashers, both the AN935 and AN936, are used with machine screws or bolts where
the self-locking or castellated-type nut is not appropriate.
 The spring action of the washer (AN935) provides enough friction to prevent loosening of
the nut from vibration.
 Lockwashers should never be used under the following conditions:
 With fasteners to primary or secondary structures
 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.

3. 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 internal 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.

4. Special Washers
 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.
 The NAS143 and MS20002 washers are used for internal wrenching bolts of the NAS144
through NAS158 series.
 This washer is either plain or countersunk.

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  The countersunk washer (designated as NAS143C and MS20002C) is used to seat the bolt
head shank radius, and the plain washer is used under the nut.

5. Spring Washers
 These consist of a single coil of square section spring with sharp corners, or a double coil of
flat spring.
 They are placed under the nut and compressed on tightening, thus tending to prevent the
nut from slackening.
 The sharp corners type has the advantage of biting into the metal and nut, which increases
its locking properties.

 Spring washers may be used again, if still springy and retaining their sharp corners.

6. Taper Pin Washers

 Both the plain and threaded taper pins are used in aircraft structures to make a joint that is
designed to carry shear loads.
 This type of pin does not allow any loose motion or play.
 The AN385 plain taper pin is forced into a hole that has been reamed with a Morse
standard taper pin reamer and is held in place by friction.
 It can be safetied by passing safety wire around the shaft and through a hole drilled in its
large end.
 AN 386 taper pin is similar to the AN 385 except that its small end is threaded to accept
either a self-locking shear nut (AN 364) or a shear castle nut (AN320).

 Locking plates
 These are thin metal plates, fitted round the nut after it has been tightened, and retained
against rotation by a small setscrew.
 The hole in the plate is usually twelve sided to allow for close adjustment.
 Sometimes they are made double ended to lock two nuts.
 Locking plates may be used again if they are a good fit on the nut.
 The set-screw securing locking plate is locked with a spring washer.

 Pal Nut
 Pal nut is a plain thin nut which is used for locking another nut which has no provision of
locking. Palnuts

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  Palnut is a registered trademark of Trans Technology and is the most
common name used for these stamped sheet metal check-nuts.
 They work in a very similar manner as a jam or checknut but their
construction allows them to distort under load and return to their original
shape in the threaded area because they are made form a spring like
steel.
 They are also lighter than standard jam nuts.
 Be advised they are not a substitute for a positive locking device such as a castle nut and
cotter pin or a corner drilled nut for safety wire.
 The Palnuts we stock are commercial versions of MS27151 series.
 The main difference is that the MS parts are Cadmium Plated and the Commercial ones we
stock are Zinc Plated.

 Circlips (Snap Ring)

 Circlips, (snap ring, retaining ring). These clips prevent axial movement on a shaft, but
normally with some play.
 A snap ring is a ring of metal, either round or flat in cross section that is tempered to have
spring like action.
 This spring like action holds the snap ring firmly seated in a groove.
 The internal types fit in a groove inside a cylinder.
 A special type of pliers is made to install each type of snap ring.
 Snap rings may be reused as long as they retain their shape and spring like action.
1. External and internal circlips in the standard design.
 The external types are designed to fit in a groove around the outside of a shaft or cylinder.
2. E clip (can be pushed on from the side of the shaft without the need to stretch over the end
of the shaft). Less secure.
3. Push-on clip. No groove required.

 Typical uses of circlips include:

 On a shaft or external diameter to provide axial restraint for a component;
 Inside a cylindrical bore, holding a bearing or other component into its housing;
 Holding short shafts or pins axially in position;
 Holding a bearing axially on its shaft;
 Carrying axial load from the helical gear.

 Keys and Keyways

 These are used where rotary power is to be transmitted from (or to) a shaft (or hub) and a
drive-wheel.
 The key is a solid piece of metal of rectangular or square cross-section, fitted into a
matched recess which is formed between the shaft and the drive-wheel.
 Several different types are available.

1. Tapered Keys
 These are made with a standard taper in thickness.
 The following types of taper keys are in common use:
a. Hollow Saddle Key - One side of this key is curved to suit the radius of the shaft when
driven into position; its taper provides a friction grip between hub and shaft that is capable
of taking a moderate load only.

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 b. Flat Saddle Key - This form of taper key is rectangular or square in cross-section and it
bears on a flat formed on the shaft.

2. Plain Taper and Gib-Headed Keys –

 These forms of taper key fit into keyways which are formed partly in the shaft and partly in
the hub.
 They are capable of transmitting greater power than either of the saddle types.
 The gib-headed key provides for easier removal.

3. Feather Keys
 Keys of this type are used in circumstances where it is required to allow axial movement
between shaft and wheel -- for example, a feather key might be used if it is necessary for a
pulley or gearwheel to move along a shaft while still being driven.
 The hub keyway is cut to allow for side and top clearance round the key, so permitting a
sliding fit to the key in the keyway.

4. Woodruff Key (half-moon keys)

 This key is made in the form of a segment of a parallel--sided disc.
 It fits into a keyway of similar shape, which is formed partly in the shaft and partly in the
wheel.
 The cavity in the shaft conforms closely to the rounded portion of the key, while an axial
groove, of uniform rectangular cross-section, is cut in the whet o a depth which permits a
push fit between hub and key.
 Woodruff keys may be fitted to parallel or tapered shafts

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Signature of the Instructor Signature of Training Manager

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