An extensive range of fasteners and other hardware items used in aircraft construction and industrial applications. They are offered both under different brands of various manufacturers or as part of our own branded (AVIATIONEU NEW ERA) product lines. More information on specific fasteners can be found at our Web Platform for Supplies.
Although fasteners and other hardware find uses in many industrial sectors, this article focuses on their aviation applications. It aims to provide an overview on fasteners and other hardware commonly used in aircraft construction. These include but they are not limited to screws and bolts, nuts, rivets, washers, cotter pins and safety wire, turnlock fasteners, miscellaneous items such as o-rings, crush washers, etc., control cable hardware, fluid lines and fittings, electrical wiring and connectors and others listed in our Web Platform for Supplies.
Aircraft fasteners conform to different standards including Army-Navy (AN), predecessor to "Military Standard " (MS) and subsequent NAS (National Aerospace Standard) which followed (*). In addition, all fasteners are identified with a specification number and a series of letters and dashes identifying their size, type of material, etc. This system presents a relatively simple method of identifying and cataloging the thousands of hardware items, several of them coming both an AN number and an MS number that are used interchangeably to identify the exact same item. Cross reference tables have been created and are currently used. Such information are also included in respective product presentations at our Web Platform for Supplies for our offered product lines briefly discussed below.
Screws and Bolts
Screws are the most commonly used threaded fastening devices on aircraft. They differ from bolts 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 standard structural bolts only in head style. The material in them is the same, and a definite grip length is provided. The AN525 washer head screw and the NAS220 through NAS227 series are such screws. Commonly used aircraft screws are classified in three groups: (1) Structural screws, made of alloy steel, which have the same strength as equal size bolts; (2) Machine screws, usually of flathead (countersunk), roundhead, or washer head types. These screws are general purpose screws for general repair and are available in low carbon steel, brass, corrosion resistant steel, and aluminum alloy; and (3) Self-tapping screws, 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.
Bolts are used in aircraft construction in areas where high strength is needed. Where this strength is not necessary screws are substituted. Aircraft quality bolts are made from alloy steel, stainless or corrosion resistant steel, aluminum alloys and titanium. The first two are the most common in aviation applications. Aircraft bolts will always have a marking on their head varying according to the manufacturer.
Each bolt typically has a hexagon shaped head and a shank that fits into the hole. The shank is threaded on the end and the unthreaded portion of the bolt is termed the grip. The "grip" is the unthreaded portion of the shank. The diameter of a bolt is the width of the grip. Available with shank drilled for cotter pin or undrilled for stop nut application, and with or without drilled head for safety wire. Bolts with undrilled shank are specified by letter "A" after the dash number. The length of AN aircraft bolts is measured from under the head to the end of the shank. "C" stands for stainless steel. Threads are pressed into the steel, not cut, thus maintaining strength throughout the entire shank. The surface under the head of AN bolts is designed to be a bearing surface to reduce non-uniform stress as the bolt is torqued thus reducing premature fatigue.
The standard bolts used in aircraft construction are AN3 through AN20. AN Bolts are typically cadmium plated per specification QQ-P-416A, Type II, Class 3 for superior corrosion resistance as compared to commercial grade zinc plated bolts. Made of high-strength type 4037 or 8740 alloy steel, hex head aircraft bolts are centerless ground and threaded after heat treatment. Minimum tensile strength 125,000 psi. The size, material, etc. of a bolt is identified by an AN number. A breakdown of a typical AN bolt number AN4-8A follows:
AN: the bolt is manufactured according to Air Force-Navy specs.
4: identifies the diameter of the bolt shank in 1/16" increments
8: identifies the length of the shank in 1/8" increments
A: the shank of the bolt is undrilled (no letter here means a drilled shank)
NAS bolts have a higher tensile strength (usually about 160,000 psi) and can be identified by a cupped out head. Close tolerance bolts are machined more accurately than general purpose bolts and they are used in applications requiring a very tight fit. Close tolerance bolts can be either AN or NAS and typically have a head marking consisting of a raised or recessed triangle. Clevis bolts are manufactured with a slotted head and are used for control cable applications.
Aircraft nuts usually have no identification on them but they are made from the same material as bolts. Aircraft nuts are made of cadmium plated carbon steel, stainless steel, or anodized 2024T aluminum alloy and come in a variety of shapes and sizes. Due to the vibration of aircraft, nuts must have some form of a locking device to keep them in place. They can be divided into two general groups: non self-locking and self-locking nuts. The aircraft nuts commonly encountered are castle nuts, self-locking nuts, and plain nuts. Wing nuts and anchor nuts are also used.
Self-locking nuts contain the locking feature as an integral part. The most common method of locking is derived from a fiber insert. This insert has a smaller diameter than the nut itself so that when a bolt enters the nut it taps into the fiber insert producing a locking action. The designation of these nuts is AN365 and AN364 with a cross-reference to MS number. An AN365 is also termed MS20365 with the AN364 being MS20364. Both of these nuts are available in stainless steel versions. The AN364 is a shear nut not to be used in tension. An all metal locking nut is used forward of the firewall and in other high temperature areas. In place of a fiber insert, the threads of a metal locking nut narrow slightly at one end to provide more friction. An AN363 is an example of this type of nut. It is capable of withstanding temperatures to 550-deg. F.
The dash number following a self-locking nut defines the thread size. Self-locking nuts are very popular and easy to use. They should be used on undrilled bolts. They may be used on drilled bolts provided that the hole for burrs would not damage the fiber. Self-locking nuts should not be used on a bolt that is connecting a moving part such as a clevis bolt used in a control cable application.
Plain nuts (or else non self-locking nuts) must be safetied by external locking devices, such as cotter pins used in castle nuts, fiber inserts, lock washers, and safety wire. They are not widely used in most aircraft. AN315 is the designation used for a plain hex nut. These nuts are also manufactured with a right hand thread and a left hand thread. The check nut used to hold a plain nut in place is an AN316. If a lockwasher is used a plain washer must be under the lockwasher to prevent damage to the surface.
There are a number of other aircraft nuts available. Wing nuts (AN350) are commonly used on battery connections or hose clamps where proper tightness can be obtained by hand. Anchor nuts are widely used in areas where it is difficult to access a nut. Tinnerman nuts, instrument mounting nuts, pal nuts, cap nuts, etc. are all examples of other types that are used.
The main purpose of a washer in aircraft installation is to provide a shim when needed, act as a smooth load bearing surface, and to adjust the position of castle nuts in relation to the drilled hole in a bolt. Plain washers are also used under a lockwasher to prevent damage to a surface. Different types of washers are commonly used such as AN960 and AN970. AN960 washers are the most common. They are manufactured in a regular thickness and a thinner thickness (one half the thickness of regular). The dash number following the AN960 indicates the size bolt for which they are used. The system is different from others encountered. For example, an AN960-616 is used with a 3/8" bolt. "L" after the dash number, indicates that it is a thin or "light" washer. An AN960C would be - a stainless steel washer. AN970 washers, with a wide flat surface area used mainly for wood applications to protect the wood, have a different dash number system.
Other types of washers include split ring, sometimes internal tooth and even external tooth, nylon washers and finishing washers that usually have a countersunk head.
Cotter Pins & Safety Wires
Cotter pins are used for safetying bolts, screws, nuts and other pins. Those often used with castle nuts are cadmium plated cotter pins (AN380) and stainless steel (AN381) also known with their MS number (MS24665). The dash numbers indicate diameter and length of the pin. As an example, AN380-2-2 would be a cadmium plated pin 1/16" in diameter and 1/2" long. Charts showing the various sizes versus the reference number are available.
Safety wire widely used with most used sizes in diameter being .020, .032 and .041 or small variations thereof. The material is usually stainless steel or brass. The easiest method of installation is acquired by using safety wire pliers to twist the wire. The wire is installed so that if the nut or bolt begins to loosen it will increase the tension on the wire. Safety wire should not be overtwisted to avoid weakening.
There are a number of types of rivets, designed to meet different cost, accessibility, and strength requirements. Solid shank rivets, relatively low-cost, permanently installed fasteners used as structural parts of aircraft applications where reliability and safety count. Such rivets are made in their majority from aluminum alloy and come with rounded (universal) or 100° countersunk heads Other rivet types include a) High-strength structural steel rivets and semi-tubular rivets (also known as tubular rivets) similar to solid rivets, except they have a partial hole (opposite the head) at the tip and b) Blind rivets available in flat head, countersunk head, and modified flush head which are used mainly when access to the joint is available from only one side.
Clamps, like other fasteners are used to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure. Different types of clamps are used in aviation industry including AN735 loop-type bonding clamps, AN737 hose clamps, AN741 tube clamps, AN742 loop-type support clamps etc. meeting Aeronautical Standard requirements.
(*) Common Aerospace/Military Fastener Standards
Army-Navy Standards (AN)
Predecessor to Mil-Spec (MS), AN fasteners are the industry standard for aircraft, indicating that these fasteners are made to military specifications to stand up to the toughest jobs without faltering.
Military Standards (Mil-Spec, MS)
A United States Defense Standard is used to help achieve standardization objectives by the U.S. Department of Defense. Also used by other non-Defense government organizations, technical organizations, and industry.
National Aerospace Standards (NAS & NA)
“NAS” stands for National Aerospace Standard. The NA prefix is for Metric standards. NAS standards include Part Standards for high strength, precision fasteners.
Aerospace Industries Association Standards (NASM and NAM)
Aerospace Industries Association (AIA), in coordination with the US Department of Defense, has converted a number of MIL-spec's to NAS. These now carry a “NASM” (for inch based) or “NAM” (for Metric based) prefix. The standard continues to be procured to the MS part number.