If aircraft hardware is special, then there must be a standard against which it should be measured and manufactured. That standard was actually developed prior to World War 1, but became more definitive during the war. Each branch of the military originally had its own standard for hardware. As time went on these standards were consolidated and thus the term AN which means Air Force-Navy (some prefer the older term Army-Navy). Later the standards were termed MS which means Military Standard and NAS which means National Aerospace Standards.
Screws are the most commonly used threaded fastening devices on an aircraft. These aircraft screws differ from bolts in that they are not as strong, but are commonly used. When being installed, a loose fitting thread is used and the head shapes are made to engage a screwdriver or wrench.
Several types of structural screws differ from the standard structural bolts only in head style. The material in them is the same however and a definite grip length is provided. The AN525 washer head screw and the NAS220 through NAS227 series are such screws.
The most commonly used aircraft screws are classified in three groups:
- Structural screws which have the same strength as equal size bolts
- Machine screws, which include the majority of types used for general repair
- Selftapping screws, which are used for attaching lighter parts
Structural aircraft 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.
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.
Machine screws are usually flatheads, roundheads or washer head types. These screws are general purpose screws and are 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.
Drive screws are plain head self-tapping screws used as cap-screws for attaching nameplates in castings and for sealing drain holes in corrosion proofing tubular structures. They are permanent, hammer driven fasteners for joining together plastics and metals. These screws require a pre-drilled hole for installation and are not intended to be removed after the installation is complete.
If you have any questions about our aircraft screws or any of our other products, please call us today at (631) 287-6321.