Quality Screws: Types, Tolerances and More
Quality screws are part and parcel of what makes aviation possible. aircraft. Of the 6 million parts in a Boeing 747, half are fasteners. Those joints on an aircraft that require frequent dismantling or replacement generally use threaded fasteners — bolts or screws.
Screw or Bolt?
Aircraft screws are especially common on light airplanes, and commonly outnumber other kinds of fasteners like bolts. Bolts and screws are similar. They both have a screwhead at one end and a head at the other. The threaded end of a bolt always goes into a nut, while the threaded end of a screw may attach directly to the material being fastened, to a nut, or to another female arrangement. The threaded end of a bolt is generally blunt, while the threaded end of a screw can be pointed or blunt. Bolts have much longer grip length (unthreaded section) than screws. Bolts assemblies are often designed to be tightened by turning the nut. Screws almost always are tightened by turning the head.
Quality Screws: Types
Among the many different types of quality screws, machine screws are the most utilitarian. They are manufactured out of common metals like aluminum, stainless steel, brass, and low carbon steel, and are used for general, non-structural applications throughout an aircraft. Machine screws use fine threads and coarse threads and can be treated to improve tolerances. The most common types of machine screws are truss-head, socket-head, pan-head, flush-head, round-head and fillister-head.
Structural screws are made of the same high quality material used for bolts. Generally, structural screws are made of steel and are heat treated. Quality screws can be used in place of structural bolts in some applications because they will have the same strength as a bolt of a similar size. Like bolts, structural screws are not threaded over their entire length.
Self-tapping screws are those that cut their own internal threads as they are turned into the hole. For that reason, they can only be used with relatively soft metals and materials. Self-tapping screws are always used for non-structural purposes, and can be divided into two general groups: machine self-tapping screws, which are used to attach removable parts, like nameplates; and sheet metal self-tapping screws, which are used for permanent assembly in nonstructural, blind applications.
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