Bolts & Screws
Bolts and screws are a group of devices used to hold objects together or to fasten one object to another. A bolt or screw consists of a metal rod with a spiral ridge, called a screw thread, running continuously around the rod. Most bolts and screws have a head, or a piece of widened diameter, at one end of the rod.
Screw threads are manufactured in several standard sizes and shapes. The United States, for example, has traditionally used a system called the American standard, and Great Britain has used the Whitworth standard. Several metric screw-thread systems are used in continental Europe and other areas. In 1948 the United States, Great Britain, and Canada adopted a common screw-thread standard, called the unified thread standard. This new standard is now in wide use and is expected eventually to replace the older standards.
Most bolts are used in combination with nuts. A nut is a small block of metal with a hole through its center. The inside surface of the hole has screw threads that fit the threads of the bolt. When a nut and bolt are used to join parts, the bolt is inserted through holes in the parts being joined, and a nut is screwed onto the end of the bolt. As the nut is tightened, the parts to be joined are drawn together between the nut and the head of the bolt.
Bolts and nuts are made in many shapes and sizes. Machine bolts are used on automobiles, airplanes, and other machines where a close fit is required. These bolts usually have square, hexagonal (six-sided), or circular flatheads, and the nuts are usually square or hexagonal. Stove bolts, used on electrical equipment and household appliances, have flat or rounded heads. Carriage bolts have roundheads and are threaded only at the end. A short square section just below the head, when placed in a square hole, keeps the bolt from turning when a nut is screwed on the end of the, bolt. Carriage bolts are used in woodwork and in fastening castings and forgings. Eyebolts, with heads shaped like upright loops, are used to support cables, chains, and metal rods. The expansion bolt, used to fasten objects to masonry, is fitted with a long split nut. The nut is placed in a hole in the wall, and when the bolt is screwed into it, its two pieces are forced apart to provide a firm anchor. Toggle bolts, used to anchor objects onto a hollow wall, have a nut that looks like a pair of folded wings. The wings spring open when placed inside the wall and thus prevent the bolt from being pulled out.
Screws resemble bolts, but they usually are not used with nuts. Many screws have a slotted head into which a screwdriver can be fitted. A machine screw is usually inserted through a hole in one of the parts to be joined and then screwed into a threaded hole on the other part. A nut is therefore not needed. A wood screw is usually driven into an unthreaded hole whose diameter is slightly less than that of the screw.
Like bolts, screws come in many forms. Cap screws, used in machine tools and for assembling automotive and aeronautical parts, usually have hexagonal or flat heads. Machine screws are similar to cap screws but are usually smaller. Setscrews prevent sliding motion between two parts, such as a pulley and its shaft. Self-tapping screws are made so that they can cut threads into unthreaded holes.