The Ultimate Guide To Aviation Nas Screw Standards

aviation nas screw sizes and lengths

Screws are the most commonly used threaded fastening devices on aircraft. They are available in a variety of types, including structural screws, machine screws, self-tapping screws, and drive screws. The size and length of aviation screws vary depending on the specific application and are identified by a coding system that includes information on the head style, diameter, thread, and length. For example, the AN525 washer head screw is a commonly used structural screw, while the AN509 and AN510 series screws are often used for general repair. The length of aviation screws is typically measured in inches or eighths of an inch, with specific codes indicating the length of the screw.

Characteristics Values
Screw type AN525 washer head, AN526, NAS220 through NAS227 series
Screw head Slotted, Phillips, recessed
Screw material Alloy steel, low-carbon steel, brass, corrosion-resistant steel, aluminum alloy
Screw diameter 3/16 inch, 4/16 inch, 4/16 inch, etc.
Screw length 1 inch, 1-1/4 inch, 1-3/8 inch, etc.
Screw grip Definite grip, no clearly defined grip length
Screw thread Coarse, fine, National Fine
Screw head style Round, brazier, countersunk, flathead, fillister, truss
Washer type AN90-10L steel washer, No. 10 fiber washer

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Screw types: structural, machine, self-tapping, and drive screws

Screws are the most common type of threaded fastening device used in aircraft. They are typically made from lower-strength materials than bolts and can be installed with a loose-fitting thread. Screws differ from bolts in that their heads are designed to engage a screwdriver or wrench.

There are four main types of screws used in aviation: structural screws, machine screws, self-tapping screws, and drive screws.

Structural screws are similar in strength to bolts of equal size. They are made from alloy steel, 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. The AN525 washer head screw and the NAS220 through NAS227 series are examples of structural screws.

Machine screws are the most common type used for general repairs. They are typically made from low carbon steel, brass, corrosion-resistant steel, or aluminium alloy. Examples include the AN515 and AN520 roundhead screws, which have either slotted or recessed heads. The AN515 screw has coarse threads, while the AN520 has fine threads. The AN500 through AN503 fillister head screw is another type of machine screw, used as a capscrew in light mechanisms, such as the attachment of cast aluminium parts.

Self-tapping screws are used for attaching lighter parts. They tap their own mating threads in metallic and non-metallic materials, such as plastic. Some self-tapping screws, like the AN504 and AN506, are machine screws used for attaching removable parts, while others, like the AN530 and AN531, are used in the temporary attachment of metal for riveting. Self-tapping screws should not be used to replace standard screws, nuts, bolts, or rivets.

Finally, drive screws are not actually screws but nails. They are driven into metal parts with a mallet or hammer and are typically used as capscrews for attaching nameplates in castings and for sealing drain holes in corrosion-proof tubular structures. They are not intended to be removed after installation. The AN535 drive screw corresponds to the Parker-Kalon U-type.

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Screw heads: flat, round, fillister, truss, and flush

Screws are the most common type of threaded fastening devices used on aircraft. They are usually made of lower-strength materials than bolts and can be installed with a loose-fitting thread. The heads of screws are designed to engage a screwdriver or wrench, and they can have a clearly defined grip or be threaded along their entire length.

Flat Head

The flathead screw, also known as a countersunk flat head screw, is used to create a flat, smooth surface after installation. The standard angle for a flathead screw is 82 degrees, but they also come in 90-degree and 100-degree angles. The 100-degree flat head is used for thinner materials that require a flat head. Flat Undercut Heads are used for some short screw sizes, as they allow for more shallow countersinking than the standard 82-degree flat head.

Round Head

Roundhead screws have a rounded top surface and are available in slotted or recessed head styles. They are becoming less common but can offer an appearance alternative to other rounded-head screws. Round Washer Heads are similar to Modified Truss Heads and provide a larger bearing surface in situations where a rounded head is preferred.

Fillister Head

Fillister head screws have a smaller diameter and higher profile than round or pan heads, which allows for a deeper slot. They are similar to pan head screws but with a greater side height. The standard fillister head has a smaller diameter than the round head and is used for general applications and where there is a requirement for a smaller head diameter.

Truss Head

Truss heads, sometimes called mushroom heads, provide a larger bearing surface and a lower profile than pan or round heads. They are weaker than pan or round heads but are preferred in applications where minimal clearance exists above the head. The truss profile provides a trim, finished appearance. Modified Truss Heads provide an extra-large bearing surface with low profile/clearance.

Flush Head

Flush-head screws are used in applications where the protrusion of the fastener above the mating surface is unacceptable. They have a flat top surface and a cone-shaped bearing surface with a head angle of approximately 82 or 90 degrees.

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Screw materials: alloy steel, low carbon steel, brass, and aluminium alloy

Screws are the most common type of threaded fastening device used on aircraft. They are typically made from lower-strength materials than bolts and can be installed with a loose-fitting thread. Screws differ from bolts in that their head shapes are designed to engage a screwdriver or wrench, and some have a clearly defined grip or unthreaded portion.

When it comes to screw materials, there are several options available, including alloy steel, low carbon steel, brass, and aluminium alloy. Each of these materials has unique properties that make it suitable for specific applications in aviation.

Alloy steel is a type of steel that is alloyed with various elements, such as manganese, nickel, chromium, molybdenum, and others, to improve its mechanical properties. Alloy steels are commonly used for fasteners when strength is required. They also offer good cold-forming properties when annealed and can be heat-treated to enhance their strength, toughness, and shock resistance. Alloy steel is often used for structural screws in aviation, providing the same shear strength as bolts of the same size.

Low carbon steel, also known as mild steel, is a type of steel with a low carbon content, typically ranging from 0.06% to 0.22%. It exhibits good ductility, making it suitable for cold-forming processes. Low carbon steel is commonly used for machine screws in aviation, which are general-purpose screws used for general repair work. These screws are available in various head styles, including flathead, roundhead, and washer head types.

Brass is a non-magnetic alloy of copper and zinc, known for its corrosion resistance and electrical conductivity. Brass screws are commonly used in aviation for installing instruments and other applications where non-magnetic properties are required. They are available in different head styles, such as slotted or Phillips (recessed) heads.

Aluminium alloy, often designated as "DD" in bolt coding, is lightweight and corrosion-resistant. It is commonly used in aircraft structures and fasteners. Aluminium alloy screws offer good cold-forming characteristics and can be heat-treated to enhance their properties. They are used for both structural and non-structural applications in aviation, depending on the specific alloy and treatment.

In summary, the choice of screw material in aviation depends on the specific application and requirements. Alloy steel provides high strength and can be heat-treated, low carbon steel offers good ductility and is suitable for general-purpose screws, brass is non-magnetic and corrosion-resistant, and aluminium alloy is lightweight and corrosion-resistant, making it a versatile choice for fasteners.

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Screw coding: AN and NAS codes

AN and NAS codes are used to identify screws and bolts in aviation. The coding system is similar to that used for bolts, with specific letters and numbers indicating the type of screw, its size, and other characteristics.

AN Codes

AN stands for Army-Navy specification, a standardisation system that began in the early 1940s to standardise military items for World War II. While most AN specifications were cancelled in the 1950s, some have survived, including the AN3-AN20 bolt.

The diameter of an AN screw is denoted by the number following "AN". For example, AN3 indicates a 3/16-inch diameter. The length of the screw is then indicated by the number following a dash. For lengths up to 7/8 of an inch, this number indicates length in eighths of an inch. For longer screws, the first digit indicates the number of inches, and the second digit the number of eighths. So, for instance, AN3-12 indicates a screw with a 3/16-inch diameter and a length of 1 and 1/4 inches.

The material of an AN screw is also coded. No letter designation indicates cadmium-plated nickel alloy steel. A C designation means the screw is made of corrosion-resistant steel, while DD designates aluminium alloy.

NAS Codes

NAS stands for National Aerospace Standards, a system established in 1941 and handled by the Aerospace Industries Association, a group of aerospace companies. NAS codes cover a wide range of fasteners, including screws, nuts, and rivets, as well as other hardware and components.

NAS codes are structured differently from AN codes. The basic NAS number identifies the part, while suffix letters and dash numbers indicate different sizes, plating materials, drilling specifications, and so on. For example, NAS144 DH 22 indicates a screw with an internal wrenching head style, a diameter and thread of 1/4-28 bolt, a drilled head, and a length of 1 3/8 inches.

AN and NAS codes are essential for identifying the correct screws and bolts for aviation applications. Using the correct hardware is imperative, as non-aviation-grade hardware may not have the necessary strength or corrosion resistance. By understanding the coding system, it is possible to select screws and bolts with the appropriate dimensions and materials for specific applications.

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Screw sizes: diameter, length, and grip

When it comes to aviation NAS screw sizes and lengths, there are a few key components to consider: diameter, length, and grip.

Diameter

The diameter of a screw is a crucial factor in ensuring the stability and security of the joint it creates. It is important to select a screw with the appropriate diameter to match the size of the hole and the thickness of the material being joined.

Length

The length of a screw refers to the distance from the bottom of the head to the tip of the screw. Selecting the correct screw length is essential to ensure that the screw can effectively fasten the materials without being too long or too short.

Grip

The grip of a screw refers to the unthreaded portion of the shank, which is located below the head of the screw. The grip length can vary depending on the type of screw and its intended application. Some screws have a definite grip length, while others may have no clearly defined grip portion.

For example, structural screws, such as the AN525 washer head screw and the NAS220 through NAS227 series, have a definite grip length and are designed for high-strength applications. On the other hand, screws like the AN500 fillister head screw may not have a clearly defined grip length.

When choosing the right aviation NAS screws, it is important to consider the diameter, length, and grip to ensure they meet the specific requirements of your project.

In addition to these measurements, it is also important to select the appropriate material and finish for your screws. Aviation-grade screws are typically made from high-strength alloys, such as cadmium-plated nickel alloy steel or corrosion-resistant steel, to ensure they can withstand the rigorous demands of aerospace applications.

Frequently asked questions

AN and NAS refer to the coding system used to identify screws. AN screws are Air Force-Navy standard, while NAS screws are National Aircraft Standard.

Common sizes for AN525 screws are No. 8 and No. 10.

The different variables within an AN bolt are length, grip, diameter, drilled head, and shank.

AN bolts are industrial aircraft bolts, while NAS bolts are used for high-strength applications and consist of fine-threaded, dimple-headed hex bolts.

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