Hardware, Fasteners, Bolts, and Nuts

7-37 were firmly held together. Several methods are used to hold metal parts together; they include riveting, bolting, brazing, and welding. The process used must produce a union that is as strong as the parts that are joined. Identification Their specification number or trade name identifies most items of aircraft hardware. Threaded fasteners and rivets are identified by AN (Air Force-Navy) numbers, NAS (National Aircraft Standard) numbers, or MS (Military Standard) numbers. Quick-release fasteners are usually identified by factory trade names and size designations.

Threaded Fasteners

Various types of fastening devices allow quick dismantling or replacement of aircraft parts that must be taken apart and put back together at frequent intervals. Riveting or welding these parts each time they are serviced would soon weaken or ruin the joint. Furthermore, some joints require greater tensile strength and stiffness than rivets can provide. Bolts and screws are two types of fastening devices that give the required security of attachment and rigidity. Generally, bolts are used where great strength is required, and screws are used where strength is not the deciding factor. Bolts and screws are similar in many ways. They are both used for fastening or holding, and each has a head on one end and screw threads on the other. Regardless of these similarities, there are several distinct differences between the two types of fasteners. The threaded end of a bolt is always blunt while that of a screw may be either blunt or pointed.

The threaded end of a bolt usually has a nut screwed onto it to complete the assembly. The threaded end of a screw may fit into a female receptacle, or it may fit directly into the material being secured. A bolt has a short threaded section and a comparatively long grip length or unthreaded portion, whereas a screw has a longer threaded section and may have no clearly defined grip length. Turning the nut on the bolt generally tightens a bolt assembly; the head of the bolt may or may not be designed for turning. Turning its head always tightens a screw. When it becomes necessary to replace aircraft fasteners, a duplicate of the original fastener should be used if possible.

If duplicate fasteners are not available, extreme care and caution must be used in selecting substitutes. Classification of Threads Aircraft bolts, screws, and nuts are threaded in the American National Coarse (NC) thread series, the American National Fine (NF) thread series, the American Standard Unified Coarse (UNC) thread series, or the American Standard Unified Fine (UNF) thread series. There is one difference between the American National series and the American Standard Unified series that should be pointed out. In the 1-inch diameter size, the NF thread specifies 14 threads per inch (1-14 NF), while the UNF thread specifies 12 threads per inch (1-12 UNF). Both types of threads are designated by the number of times the incline (threads) rotates around a 1-inch length of a given diameter bolt or screw. For example, a 1/4-28 thread indicates that a 1⁄4-inch (4⁄16 inch) diameter bolt has 28 threads in 1 inch of its threaded length.

Class of fit also designates threads. The Class of a thread indicates the tolerance allowed in manufacturing: • Class 1 is a loose fit • Class 2 is a free fit • Class 3 is a medium fit • Class 4 is a close fit Aircraft bolts are almost always manufactured in the Class 3, medium fit. A Class 4 fit requires a wrench to turn the nut onto a bolt, whereas a Class 1 fit can easily be turned by hand. Generally, aircraft screws are manufactured with a Class 2 thread fit for ease of assembly. Bolts and nuts are also produced with right-hand and left-hand threads. A right-hand thread tightens when turned clockwise; a left-hand thread tightens when turned counterclockwise.

Aircraft Bolts

Aircraft bolts are fabricated from cadmium- or zinc-plated corrosion-resistant steel, un-plated corrosion-resistant steel, or anodized-aluminum alloys. Most bolts used in aircraft structures are either general purpose, AN bolts, NAS internal wrenching or close tolerance bolts, or MS bolts. In certain cases, aircraft manufacturers make bolts of different dimensions or greater strength than the standard types. Such bolts are made for a particular application, and it is of extreme importance to use like bolts in replacement. The letter “S” stamped on the head usually identifies special bolts. AN bolts come in three head styles: hex head, Clevis, and eyebolt. [Figure 7-19] NAS bolts are available in hex head, internal wrenching, and countersunk head styles. MS bolts come in hex head and internal wrenching styles.

General Purpose Bolts The hex head aircraft bolt (AN-3 through AN-20) is an all-purpose structural bolt used for general applications involving tension or shear loads where a light drive fit is permissible (0.006-inch clearance for a 5⁄8-inch hole and 7-38 EC-801 (black) MIL-S-7502A Class B-2 EC-800 (red) EC-612 P (pink) MIL-P-20628 PR-1302HT (red) MIL-S-8784 PR-727 potting compound MIL-S-8516B HT-3 (grey–green) EC-776 (clear amber) MIL-S-4383B 12 parts of EC-807 to 100 parts of EC-801 Use as is 10 parts of PR-1302HT-A to 100 parts of PR-1302HT 12 parts of PR-727A to 100 parts of PR-727 Use as is 2–4 hours 8–12 hours Indefinite non-drying 2–4 hours 1½ hours minimum Solvent release, sets up in 2–4 hours 8–12 hours 6 months 6–9 months 6 months 6–9 months Indefinite in airtight containers 5 days at −20 °F after flash freeze at −65 °F Not applicable 5 days at −20 °F after flash freeze at −65 °F 5 days at −20 °F after flash freeze at −65 °F Not applicable Faying surfaces, fillet seals, and packing gaps Coating rivet Packing voids up to ¼" Sealing access door gaskets Potting electrical connections and bulkhead seals Sealing hot air ducts passing through bulkheads Top coating −65 °F to 200 °F −40 °F to 200 °F −65 °F to 200 °F −60 °F to 200 °F −65 °F to 200 °F EC-807 None PR-1302HT-A PR-727A None Sealant Base Accelerator (Catalyst) Mixing Ratio by Weight Application Life (Work) Storage (Shelf) Life After Mixing Storage (Shelf) Life Unmixed Temperature Range Application and Limitations other sizes in proportion).

Alloy-steel bolts smaller than No. 10-32 and aluminum- alloy bolts smaller than 1⁄4 inch in diameter are not used in primary structures. Aluminum-alloy bolts and nuts are not used where they are repeatedly removed for purposes of maintenance and inspection. Aluminum-alloy nuts may be used with cadmium-plated steel bolts loaded in shear on land airplanes, but are not used on seaplanes due to the increased possibility of dissimilar metal corrosion. The AN-73 drilled head bolt is like the standard hex bolt, but has a deeper head, which is drilled to receive wire for safetying. The AN-3 and the AN-73 series bolts are interchangeable, for all practical purposes, from the standpoint of tension and shear strengths.

Close Tolerance Bolts Close tolerance bolts are machined more accurately than the general-purpose bolt. Close tolerance bolts may be hex headed (AN-173 through AN-186) or have a 100° countersunk head (NAS-80 through NAS-86). They are used in applications where a tight drive fit is required. (The bolt moves into position only when struck with a 12- to 14-ounce hammer.) Internal Wrenching Bolts Internal wrenching bolts, (MS-20004 through MS-20024 or NAS-495) are fabricated from high-strength steel and are suitable for use in both tension and shear applications. When they are used in steel parts, the bolt hole must be slightly countersunk to seat the large corner radius of the shank at the head. In Dural material, a special heat-treated washer must be used to provide an adequate bearing surface for the head. The head of the internal wrenching bolt is recessed to allow the insertion of an internal wrench when installing or removing the bolt. Special high-strength nuts are used on these bolts. Replace an internal wrenching bolt with another internal wrenching bolt. Standard AN hex head bolts and washers cannot be substituted for them, as they do not have the required strength.

Identification and Coding Bolts are manufactured in many shapes and varieties. A clear-cut method of classification is difficult. The shape of the head, method of securing, material used in fabrication, or the expected usage can identify bolts. AN-type aircraft bolts can be identified by the code markings on the bolt heads. The markings generally denote the bolt manufacturer, the material used to make the bolt, and whether the bolt is a standard AN-type or a special purpose bolt. • AN st andard steel bolts are marked with either a raised 7-39 dash or asterisk or a single raised dash. • AN aluminum-alloy bolts are marked with two raised dashes to indicate corrosion-resistant steel.

• Additional information, such as bolt diameter, bolt length, and grip length, may be obtained from the bolt part number. For example, in the bolt part number AN3DD5A, • The “AN” designates that it is an Air Force-Navy standard bolt. • The “3” indicates the diameter in sixteenths of an inch (3⁄16). • The “DD” indicates the material is 2024 aluminum alloy. • The letter “C” in place of the “DD” would indicate corrosion-resistant steel, and the absence of the letters would indicate cadmium-plated steel. • The “5” indicates the length in eighths of an inch (5⁄8). • the “A” indicates that the shank is undrilled. If the letter “H” preceded the “5” in addition to the “A” following it, the head would be drilled for safetying.

Close tolerance NAS bolts are marked with either a raised or recessed triangle. The material markings for NAS bolts are the same as for AN bolts, except that they may be either raised or recessed. Bolts inspected magnetically (Magnaflux) or by fluorescent means (Zyglo) are identified by means of colored lacquer or a head marking of a distinctive type. Special-Purpose Bolts Bolts designed for a particular application or use is classified as special-purpose bolts. Clevis bolts, eyebolts, Jo-bolts, and lockbolts are special-purpose bolts. Clevis Bolts The head of a Clevis bolt is round and is either slotted to receive a common screwdriver or recessed to receive a cross point screwdriver. This type of bolt is used only where shear loads occur and never in tension. It is often inserted as a mechanical pin in a control system.

Eyebolt The eyebolt is a special-purpose bolt used where external tension loads are to be applied. The eyebolt is designed for the attachment of devices, such as the fork of a turnbuckle, a Clevis, or a cable shackle. The threaded end may or may not be drilled for safetying. Jo-Bolt Jo-bolt is a trade name for an internally threaded three-piece rivet. The Jo-bolt consists of three parts: a threaded steel-alloy bolt, a threaded steel nut, and an expandable stainless steel sleeve. The parts are factory preassembled. As the Jo-bolt is installed, the bolt is turned while the nut is held. This causes the sleeve to expand over the end of the nut, forming the blind head and clamping against the work. When driving is complete, a portion of the bolt breaks off. The high shear and tensile strength of the Jo-bolt makes it suitable for use in cases of high stresses where some of the other blind fasteners would not be practical. Jo-bolts are often a part of the permanent structure of late model aircraft. They are used in areas that are not often subjected to replacement or servicing. (Because it is a three-part fastener, it should not be used where any part, in becoming loose, could be drawn into the engine air intake.) Other advantages of using Jo-bolts are their excellent resistance to vibration, weight saving, and fast installation by one person.

Presently, Jo-bolts are available in four diameters: • 200 series, approximately 3⁄16 inch in diameter • 260 series, approximately 1⁄4 inch in diameter • 312 series, approximately 5⁄16 inch in diameter • 375 series, approximately 3⁄8 inch in diameter. Jo-bolts are available in three head styles: F (flush), P (hex head), and FA (flush millable). Lockbolts Lockbolts are used to attach two materials permanently. They are lightweight and are equal in strength to standard bolts. Lockbolts are manufactured by several companies and conform to Military Standards, which specify the size of a lockbolt’s head in relation to the shank diameter, plus the alloy used in its construction. The only drawback to lockbolt installations is that they are not easily removable compared to nuts and bolts.

The lockbolt combines the features of a high-strength bolt and rivet, but it has advantages over both. The lockbolt is generally used in wing splice fittings, landing gear fittings, fuel cell fittings, longerons, beams, skin splice plates, and other major structural attachments. It is more easily and quickly installed than the conventional rivets or bolts and eliminates the use of lock washers, cotter pins, and special nuts. Like the rivet, the lockbolt requires a pneumatic hammer or “pull gun” for installation. When installed, it is rigidly and permanently locked in place. Three types of lockbolts are commonly used: the pull type, the stump type, and the blind type. [Figure 7-20] Pull Type Pull-type lockbolts are used mainly in aircraft primary and secondary structures. They are installed very rapidly and have approximately one-half the weight of equivalent AN steel 7-40 AIR ASSO CIATES A R E O N M S COO P ER A037 4 AN C SPEC W F O X S 63-59131 Magnetically inspected Special bolt Drilled head bolt Special bolt NAS close tolerance bolt Clevis bolt Reworked bolt Low strength material bolt Aluminum alloy (2024) bolt AN standard steel bolt AN standard steel bolt AN standard steel bolt Eyebolt Clevis bolt Countersunk head bolt Internal hex head bolt Standard head bolt Drilled hex head bolt AN standard steel bolt AN standard steel bolt AN standard steel bolt AN standard steel bolt AN standard steel bolt AN standard steel bolt (corrosion resistant) Orange-dyed magnetically inspected AN bolts and nuts. A special pneumatic “pull gun” is required to install this type of lockbolt. One person can accomplish installation since bucking is not required.

Stump Type Stump-type lockbolts, although they do not have the extended stem with pull grooves, are companion fasteners to pull-type lockbolts. They are used primarily where clearance does not permit installation of the pull-type lockbolt. A standard pneumatic riveting hammer (with a hammer set attached for swaging the collar into the pin locking grooves) and a bucking bar are tools necessary for the installation of stump- type lockbolts. 7-41 Pull type Stump type Blind type Blind Type Blind-type lockbolts come as complete units or assemblies. They have exceptional strength and sheet pull-together characteristics. Blind lockbolts are used where only one side of the work is accessible and, generally, where it is difficult to drive a conventional rivet. This type of lockbolt is installed in the same manner as the pull-type lockbolt.

Common Features Common features of the three types of lockbolts are the annular locking grooves on the pin and the locking collar, which is swaged into the pin’s lock grooves to lock the pin in tension. The pins of the pull- and blind-type lockbolts are extended for pull installation. The extension is provided with pulling grooves and a tension breakoff groove. Composition The pins of pull- and stump-type lockbolts are made of heat- treated alloy steel or high-strength aluminum alloy. Companion collars are made of aluminum alloy or mild steel. The blind lockbolt consists of a heat-treated alloy steel pin, blind sleeve and filler sleeve, mild steel collar, and carbon steel washer.

Substitution Alloy-steel lockbolts may be used to replace steel high-shear rivets, solid steel rivets, or AN bolts of the same diameter and head type. Aluminum-alloy lockbolts may be used to replace solid aluminum-alloy rivets of the same diameter and head type. Steel and aluminum-alloy lockbolts may also be used to replace steel and 2024T aluminum-alloy bolts, respectively, of the same diameter. Blind lockbolts may be used to replace solid aluminum-alloy rivets, stainless steel rivets, or all blind rivets of the same diameter. Numbering System The numbering systems for the various types of lockbolts are explained by the break-outs in Figure 7-21.

Grip Range To determine the bolt grip range required for any application, measure the thickness of the material with a hook scale inserted through the hole. Once this measurement is determined, select the correct grip range by referring to the charts provided by the rivet manufacturer. Examples of grip range charts are shown in Figures 7-22 and 7-23. When installed, the lockbolt collar should be swaged substantially throughout the complete length of the collar. The tolerance of the broken end of the pin relative to the top of the collar must be within the dimensions given in Figure 7-24. When removal of a lockbolt becomes necessary, remove the collar by splitting it axially with a sharp, cold chisel. Be careful not to break out or deform the hole. The use of a backup bar on the opposite side of the collar being split is recommended.

The pin may then be driven out with a drift punch.

Aircraft Nuts

Aircraft nuts are made in a variety of shapes and sizes. They are made of cadmium-plated carbon steel, stainless steel, or anodized 2024T aluminum alloy and may be obtained with either right- or left-hand threads. No identifying marking or lettering appears on nuts. Only the characteristic metallic luster or color of the aluminum, brass, or the insert can identify them when the nut is of the self-locking type. They can be further identified by their construction. Aircraft nuts can be divided into two general groups: non- self-locking and self-locking nuts. Non-self-locking nuts are those that must be safetied by external locking devices, such as cotter pins, safety wire, or locknuts. Self-locking nuts contain the locking feature as an integral part.

Non-Self-Locking Nuts Most of the familiar types of nuts, including the plain nut, the castle nut, the castellated shear nut, the plain hex nut, the light hex nut, and the plain check nut are the non-self-locking type. [Figure 7-25] 7-42 LC Lockbolt collar C Material C = 24ST aluminum alloy (green color). Use with heat-treated alloy lockbolts only. F = 61ST aluminum alloy (plain color). Use with 75ST aluminum alloy lockbolts only. R = mild steel (cadmium plated). Use with heat-treated alloy steel lockbolts for high temperature applications only. Diameter of a pin in 32nds of an inch C Lockbolt collar LC C C ALSF Head type ASCT509 = close tolerance AN-509 C-sink head ALSF = flathead type ALS509 = standard AN-509 C-sink head ALS426 = standard AN-426 C-sink head Pin materials E = 75S-T6 aluminum alloy T = heat-treated alloy steel Body diameter in 32nds of an inch Grip length in 16ths of an inch E 8 Stump-type lockbolt ALSF E 8 8 ALPP H T Head type ACT509 = close tolerance AN-509 C-sink head ALPP = pan head ALPB = brazier head ALP509 = standard AN-509 C-sink head ALP426 = standard AN-426 C-sink head Class fit H = hole filling (interference fit) N = non-hole filling (clearance fit) Pin Materials E = 75S-T6 aluminum alloy T = heat-treated alloy steel Body diameter in 32nds of an inch Grip length in 16ths of an inch 8 Pull-type lockbolt ALPP H T 8 8 BLBlind lockbolt Diameter in 32nds of an inch Grip length in 16ths of an inch, ± 1⁄32 inch 8 4 Blind-type lockbolt BL 8 4 The castle nut, AN310, is used with drilled shank AN hex head bolts, Clevis bolts, eyebolts, drilled head bolts, or studs.

It is rugged and can withstand large tensional loads. Slots (called castellations) in the nut are designed to accommodate a cotter pin or lock wire for safety. The castellated shear nut, AN320, is designed for use with devices, such as drilled Clevis bolts and threaded taper pins, which are normally subjected to shearing stress only. Like the castle nut, it is castellated for safetying. Note, however, that the nut is not as deep or as strong as the castle nut; also, that the castellations are not as deep as those in the castle nut. The plain hex nut, AN315 and AN335 (fine and coarse thread), is of rugged construction. This makes it suitable for carrying large tensional loads. However, since it requires an auxiliary locking device, such as a check nut or lock washer, its use on aircraft structures is somewhat limited.

The light hex nut, AN340 and AN345 (fine and coarse thread), is a much lighter nut than the plain hex nut and must be locked by an auxiliary device. It is used for miscellaneous light tension requirements. The plain check nut, AN316, is employed as a locking device for plain nuts, set screws, threaded rod ends, and other devices. The wing nut, AN350, is intended for use where the desired tightness can be obtained by hand and where the assembly is frequently removed. Self-Locking Nuts As their name implies, self-locking nuts need no auxiliary means of safetying but have a safetying feature included as an integral part of their construction. Many types of self- locking nuts have been designed and their use has become quite widespread. Common applications are: • At tachment of antifriction bearings and control pulleys • Attachment of accessories, anchor nuts around inspection holes, and small tank installation openings • Attachment of rocker box covers and exhaust stacks Self-locking nuts are acceptable for use on certificated aircraft subject to the restrictions of the manufacturer. Self-locking nuts are used on aircraft to provide tight connections that do not shake loose under severe vibration. Do not use self-locking nuts at joints, which subject either the nut or bolt to rotation.

They may be used with antifriction bearings and control pulleys, provided the inner race of the bearing is clamped to the supporting structure by the nut and bolt. Plates must be attached to the structure in a positive manner to eliminate rotation or misalignment when tightening the bolts or screws. 7-43 Grip Number Grip Range Grip Number Grip Range Minimum MaximumMinimum Maximum 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 0.031 0.094 0.156 0.219 0.281 0.344 0.406 0.469 0.531 0.594 0.656 0.718 0.781 0.843 0.906 0.968 1.031 0.094 0.156 0.219 0.281 0.344 0.406 0.469 0.531 0.594 0.656 0.718 0.781 0.843 0.906 0.968 1.031 1.094 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 1.094 1.156 1.219 1.281 1.344 1.406 1.469 1.531 1.594 1.656 1.718 1.781 1.843 1.906 1.968 2.031 1.156 1.219 1.281 1.344 1.406 1.469 1.531 1.594 1.656 1.718 1.781 1.843 1.906 1.968 2.031 2.094 ¼" Diameter Grip Number Grip Range Grip Number Grip Range 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 0.031 0.094 0.156 0.219 0.281 0.344 0.406 0.469 0.531 0.594 0.656 0.718 0.781 0.843 0.906 0.968 1.031 1.094 1.156 1.219 1.281 1.344 1.406 1.469 1.531 0.094 0.156 0.219 0.281 0.344 0.406 0.469 0.531 0.594 0.656 0.718 0.781 0.843 0.906 0.968 1.031 1.094 1.156 1.219 1.281 1.343 1.406 1.460 0.156 0.219 0.281 0.344 0.406 0.469 0.531 0.594 0.656 0.718 0.781 0.843 0.906 0.968 1.031 1.094 1.156 1.219 1.281 1.343 1.406 1.469 1.531 0.094 0.156 0.219 0.281 0.344 0.406 0.469 0.531 0.594 0.656 0.718 0.781 0.843 0.906 0.968 1.031 1.094 1.156 1.219 1.281 1.343 1.406 1.469 1.531 1.594 Minimum MaximumMinimum Maximum Pin diameter Tolerance Below Above 0.079 to 0.032 0.079 to 0.050 0.079 to 0.060 The two general types of self-locking nuts currently in use are the all-metal type and the fiber lock type. For the sake of simplicity, only three typical kinds of self-locking nuts are considered in this handbook: the Boots self-locking and the stainless steel self-locking nuts, representing the all-metal types; and the elastic stop nut, representing the fiber insert type.

Boots Self-Locking Nut The Boots self-locking nut is of one piece, all-metal construction designed to hold tight despite severe vibration. Note in Figure 7-26 that it has two sections and is essentially two nuts in one: a locking nut and a load-carrying nut. The two sections are connected with a spring, which is an integral part of the nut. The spring keeps the locking and load-carrying sections such a distance apart that the two sets of threads are out of phase or spaced so that a bolt, which has been screwed through the load-carrying section, must push the locking section outward against the force of the spring to engage the threads of the locking section properly.

The spring, through the medium of the locking section, exerts a constant locking force on the bolt in the same direction as a force that would tighten the nut. In this nut, the load-carrying section has the thread strength of a standard nut of comparable size, while the locking section presses against the threads of the bolt and locks the nut firmly in position. Only a wrench applied to the nut loosens it. The nut can be removed and reused without impairing its efficiency. Boots self-locking nuts are made with three different spring styles and in various shapes and sizes. The wing type that is the most common ranges in size for No. 6 up to 1⁄4 inch, the Rol-top ranges from 1⁄4 inch to 1⁄6 inch, and the bellows type ranges in size from No. 8 up to 3⁄8 inch. Wing-type nuts are made of anodized aluminum alloy, cadmium-plated carbon 5/16" Diameter 3/16 ¼ 5/16 3/8 7-44 Top view Profile view Top view Profile view AN310 AN320 AN315 AN335 AN340 AN345 AN316 AN350 Top view Profile view Top view Profile view AN310 AN320 AN315 AN335 AN340 AN345 AN316 AN350 steel, or stainless steel. The Rol-top nut is cadmium-plated steel, and the bellows type is made of aluminum alloy only.

. Stainless Steel Self-Locking Nut The stainless steel self-locking nut may be spun on and off by hand as its locking action takes places only when the nut is seated against a solid surface and tightened. The nut consists of two parts: a case with a beveled locking shoulder and key and a thread insert with a locking shoulder and slotted keyway. Until the nut is tightened, it spins on the bolt easily, because the threaded insert is the proper size for the bolt. However, when the nut is seated against a solid surface and tightened, the locking shoulder of the insert is pulled downward and wedged against the locking shoulder of the case. This action compresses the threaded insert and causes it to clench the bolt tightly. The cross-sectional view in Figure 7-27 shows how the key of the case fits into the slotted keyway of the insert so that when the case is turned, the threaded insert is turned with it. Note that the slot is wider than the key. This permits the slot to be narrowed and the insert to be compressed when the nut is tightened.

Elastic Stop Nut The elastic stop nut is a standard nut with the height increased to accommodate a fiber locking collar. This fiber collar is very tough and durable and is unaffected by immersion in hot or cold water or ordinary solvents, such as ether, carbon tetrachloride, oils, and gasoline. It will not damage bolt threads or plating. As shown in Figure 7-28 , the fiber locking collar is not threaded, and its inside diameter is smaller than the largest diameter of the threaded portion or the outside diameter of a corresponding bolt. When the nut is screwed onto a bolt, it acts as an ordinary nut until the bolt reaches the fiber collar.

When the bolt is screwed into the fiber collar, however, friction (or drag) causes the fiber to be pushed upward. This creates a heavy downward pressure on the load carrying part and automatically throws the load carrying sides of the nut and bolt threads into positive contact. After the bolt has been forced all the way through the fiber collar, the downward pressure remains constant. This pressure locks and holds the nut securely in place even under severe vibration. Nearly all elastic stop nuts are steel or aluminum alloy. However, such nuts are available in practically any kind of metal. Aluminum-alloy elastic stop nuts are supplied with an anodized finish. Steel nuts are cadmium plated.

Normally, elastic stop nuts can be used many times with complete safety and without detriment to their locking efficiency. When reusing elastic stop nuts, be sure the fiber has not lost its locking friction or become brittle. If a nut can be turned with the fingers, replace it. After the nut has been tightened, make sure the rounded or 7-45 Boots aircraft nut Flexloc nut Fiber locknut Elastic stop nut Elastic anchor nut Tightened nut Untightened nut Nut case Threaded nut core Locking shoulder Keyway chamfered end of the bolts, studs, or screws extends at least the full round or chamfer through the nut. Flat end bolts, studs, or screws should extend at least 1⁄32 inch through the nut. Bolts of 5⁄16-inch diameter and over with cotter pin holes may be used with self-locking nuts, but only if free from burrs around the holes. Bolts with damaged threads and rough ends are not acceptable. Do not tap the fiber locking insert. The self-locking action of the elastic stop nut is the result of having the bolt threads impress themselves into the untapped fiber.

Do not install elastic stop nuts in places where the temperature is higher than 250 °F, because the effectiveness of the self- locking action is reduced beyond this point. Self-locking nuts may be used on aircraft engines and accessories when the engine manufacturer specifies their use. Self-locking nut bases are made in several forms and materials for riveting and welding to aircraft structure or parts. [Figure 7-29] Certain applications require the installation of self-locking nuts in channels, an arrangement that permits the attachment of many nuts with only a few rivets. These channels are track-like bases with regularly spaced nuts, which are either removable or non-removable.

The removable type carries a floating nut that can be snapped in or out of the channel, thus making possible the easy removal of damaged nuts. Nuts, such as the clinch-type and spline-type, depend on friction for their anchorage and are not acceptable for use in aircraft structures. Sheet Spring Nuts Sheet spring nuts, such as speed nuts, are used with standard and sheet metal self-tapping screws in non-structural locations. They find various uses in supporting line clamps, conduit clamps, electrical equipment, access doors, and the like and are available in several types. Speed nuts are made from spring steel and are arched prior to tightening. This arched spring lock prevents the screw from working loose.

These nuts should be used only where originally used in the fabrication of the aircraft. Internal & External Wrenching Nuts Two commercial types of high-strength internal or external wrenching nuts are available; they are the internal and external wrenching elastic stop nut and the Unbrako internal and external wrenching nut. Both are of the self-locking type, are heat-treated, and can carry high-strength bolt tension loads. Identification & Coding Part numbers designate the type of nut. The common types and their respective part numbers are: • Plain, AN315 and AN335 • Castle, AN310 • Plain check, AN316 • Light hex, AN340 and AN345 • Castellated shear, AN320 The patented self-locking types are assigned part numbers ranging from MS20363 through MS20367. The Boots, the Flexloc, the fiber locknut, the elastic stop nut, and the self- locking nut belong to this group. Part number AN350 is assigned to the wing nut.

Letters and digits following the part number indicate such 7-46 NutFiber collar items as material, size, threads per inch, and whether the thread is right or left hand. The letter “B” following the part number indicates the nut material to be brass, a “D” indicates 2017-T aluminum alloy, a “DD” indicates 2024-T aluminum alloy, a “C” indicates stainless steel, and a dash in place of a letter indicates cadmium-plated carbon steel. The digit (or two digits) following the dash or the material code letter is the dash number of the nut, and it indicates the size of the shank and threads per inch of the bolt on which the nut fits. The dash number corresponds to the first figure appearing in the part number coding of general purpose bolts.

A dash and the number 3, for example, indicate that the nut fits an AN3 bolt (10-32); a dash and the number 4 means it fits an AN4 bolt (1⁄4-28); a dash and the number 5, an AN5 bolt (5⁄16-24); and so on. The code numbers for self-locking nuts end in three or four digit numbers. The last two digits refer to threads per inch, and the one or two preceding digits stand for the nut size in 16ths of an inch. Some other common nuts and their code numbers are: Code Number AN310D5R: AN310 = aircraft castle nut D = 2024-T aluminum alloy 5 = 5⁄16 inch diameter R = right-hand thread (usually 24 threads per inch) Code Number AN320-10: AN320 = aircraft castellated shear nut, cadmium-plated carbon steel 10 = 5⁄8 inch diameter, 18 threads per inch (this nut is usually right-hand thread) Code Number AN350B1032: AN350 = aircraft wing nut B = brass 10 = number 10 bolt 32 = threads per inch

Aircraft Washers

Aircraft washers used in airframe repair are either plain, lock, or special type washers. Plain Washers Plain washers, both the AN960 and AN970, are used under hex nuts. [Figure 7-30] 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 lock washers 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 is 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 resists 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. Lock Washers Lock washers, 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. [Figure 7-30] Lock washers should never be used under the following conditions: 7-47 Boots aircraft channel assembly Elastic stopnut channel assembly • 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 Shake-Proof Lock Washers Shake-proof lock washers 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 lock washer 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 that fits a keyed bolt.

Shake-proof lock washers 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 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. [Figure 7-30] The NAS143 and MS20002 washers are used for internal wrenching bolts of the NAS144 through NAS158 series. This washer is either plain or countersunk. 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.

Installation of Nuts, Washers, & Bolts

Bolt & Hole Sizes Slight clearances in bolt holes are permissible wherever bolts are used in tension and are not subject to reversal of load. A few of the applications in which clearance of holes may be permitted are in pulley brackets, conduit boxes, lining trim, and miscellaneous supports and brackets. Bolt holes are to be normal to the surface involved to provide full bearing surface for the bolt head and nut and must not be oversized or elongated. A bolt in such a hole carries none of its shear load until parts have yielded or deformed enough to allow the bearing surface of the oversized hole to contact the bolt. In this respect, remember that bolts do not become swaged to fill up the holes, as do rivets.

In cases of oversized or elongated holes in critical members, obtain advice from the aircraft or engine manufacturer before drilling or reaming the hole to take the next larger bolt. Usually such factors as edge distance, clearance, or load