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.

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.

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.

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.