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In technical terms, a screw is a helix or inclined plane wrapped around a cylindrical or conical body. In industrial applications, a screw typically refers to a threaded fastener with a conical shank and a specialized head; threaded cylindrical shanks are instead referred to as bolts. Whatever the definition, screws are used to connect or, occasionally, move objects.

Interestingly, the screw was rarely used as a metal fastener before the 1400s, when it was used to hold together pieces of metal, often armor. Screwdrivers, in fact, did not appear until the middle of the 18th century. Screws work through an interesting mechanical process. They use the basic principles of a wedge and inclined plane. As a screw is inserted into an object, it creates a corresponding inclined plane. As it is turned, the relatively small amount of force exerted by the operator is converted to a greater force exerted at a smaller point—the tip of the screw helix. The rotational force, or torque, generated by the person turning the screw is converted to a linear force concentrated at a much smaller area. Screws provide a much firmer hold than a nail. The preload, or stretch introduced to the screw as it is tightened, determines the strength of the connection. Provided the screw’s yield tensile strength and the strength of the threads of the material are not overburdened, the screw will not loosen. Otherwise, the joint will be compromised, and may break, loosen or become stripped.

Screws come in a variety of materials, including nylon, stainless steel, titanium, aluminum, brass and copper—even ceramic. The major types of screw driving shapes are slotted, Phillips, hex, square, one way and torx. These shapes simply determine the tools needed to tighten (or loosen) the screw. Head shapes include truss, socket, flat, round and dome, among others. The thread of a screw also plays an important part in its function. The pitch of the thread, its depth and the radius at its top and bottom all factor into its performance, as does the angle of the thread form. In the mid-1800s these angles began to be standardized, the Americans using a 60-degree thread form and the British using one at 55 degrees. Today, these attributes vary between uses. Machine screws, for instance, have finer threads than wood screw, which have deep threads. Other screws are designed to have sharp threads to cut through sheet metal and plastic. Tool sets are also an obvious issue when choosing screws, as different heads and driving shapes necessitate different drivers or driver attachments.