A screw is a threaded metal or plastic device for fastening things together or transmitting motion. It is one of the six simple machines, in effect being an inclined plane wound spirally around a cylinder or cone.
The spiral ridge around the cylinder or cone is called a thread. It has a uniform section whose shape depends on the work the screw is designed to do. A thread on the outside of a cylinder or cone is called an external, or male, thread. A mating thread on the inside of a hollow cylinder or cone is called an internal, or female, thread. On a cylinder, the thread is a straight or parallel thread. On a cone or part of a cone, the thread is a taper thread. The thread is a right-hand thread if, when viewed from the head, the screw turns in a clockwise direction to recede. The thread is a left-hand thread if it turns counterclockwise to recede. The top of a thread is called the crest. The bottom of the groove is called the root. The distance from one crest to the next is called the pitch. The axial movement during one complete rotation is called the lead. A single thread is a single helix from one end to the other. Its lead is equal to its pitch. Single threads are used for fastening and for measuring, as on the screw of a micrometer. Threads used for imparting motion may be multiple. A double thread comprises two distinct threads with the angle of the helix increased to allow room for the second thread. The threads start 180° apart. Triple threads start 120° apart. Quadruple threads start 90° apart for rapid advance of the screw. The lead of a double thread is twice as long as its pitch, and the lead of a triple thread is three times as long as its pitch.
Three types of threads are used for fastening: a V thread at a 60° angle with a sharp crest and root; a V thread at a 60° angle with crest cut off and root filled in (American standard); and a Whitworth thread with a 55° angle and rounded crest and root ( English standard). The United States, United Kingdom, and Canada agreed in 1948 on the American type as a standard thread. Three types of threads are used for transmitting motion: the square thread; the Acme thread, which has a 29° angle and squared-off crest and root; and the buttress thread, which has one face vertical and the other at a 45° angle.
Screws, bolts, and nuts depend on their threads for holding power. They make excellent fasteners, especially for parts that may be disassembled at some future time. Friction between the threads of a bolt and its nut tends to prevent unscrewing except when vibration is great. Bolts are generally distinguished from screws because they pass completely through the parts being joined and are fastened by a nut on the threaded end. A screw does not require a nut. Instead, it passes entirely through only one of the parts and seats directly in threads in the second part. A metal part in which a machine screw is seated can be drilled and tapped with mating threads. A wood screw (made of metal for use in wood) makes its own thread in the wood into which it is driven, as does a self-tapping machine screw used in sheet metal and relatively soft materials.
Screws and bolts are identified either by their application or the shapes of their heads. They include wood screws, machine screws, setscrews, cap screws, thumbscrews, lag screws or bolts, carriage bolts, stove bolts, eyebolts, and studs. They are made of iron, steel, copper, brass, bronze, aluminum alloy, or plastics such as nylon. Screvvheads commonly are countersunk and flat so as not to protrude from the surface of the part through which they are inserted. They generally are slotted so that they can be driven in place with a screwdriver. Bolt heads usually are square or hexagonal to provide gripping surfaces for a wrench. Nuts are perforated square or hexagonal metal blocks threaded internally to mate with the thread of the bolt and avoid binding. When pulled up with a wrench, they draw and hold the parts together. The various kinds of nuts include full nuts, cap nuts, machine-screw nuts, wing nuts, castellated nuts, and knurled nuts. They may be used with a washer (ring of metal), leather, or other material around the bolt to form a seat to protect the surface against which it is drawn or to lock it in place against loosening by vibration. Lock washers are split or toothed, with sharp, canted ends or corners that bite into the joining faces and prevent counterrotation.
Screw machines turn out billions of screws and bolts annually. Essentially they are fully automatic turret lathes with one or more spindles. The multiple-tool holder is pivoted so that each tool in turn is brought into action for turning and threading bar stock, forming the head, and cutting off. Special screws and bolts can be made in a screw-cutting lathe with a single-edge cutting tool, or by using chasers or comb tools controlled by master screws.
As part of a mechanical device, the screw is an efficient and extremely useful form of inclined plane. The mechanism consists of a frame, a screw, and a sliding member. One of the earliest is Archimedes' screw, which raises water from one level to another. Other screw machines include the screw conveyor for moving bulk materials through a trough; the household meat grinder, which forces meat through a cutter; and the carpenter's auger, which disposes of chips by conveying them up the thread.
The screw also is important in machine tools such as lathes and milling machines, where it produces a slow, feeding motion in the cutter or the work piece in clamps and vises. It closes the jaws to hold the work piece firmly. It also is useful in precise measurement and adjustment, as in micrometers, surveying instruments, and astronomers' telescopes; in propellers on airplanes and ships; and in jackscrews for raising and adjusting heavy loads. For instance, an automobile jack can lift and hold a ton or more by working a simple hand lever against a screw through a pawl and ratchet. See also inclined plane; simple machine.