A cylindrical fastener with an integral head on one end and an external screw thread on the other end designed to be inserted through holes in assembled parts and to mate with an internally threaded block, called a nut, which is turned to tighten or loosen the bolt. Tensioning the fastener by turning the nut differentiates a bolt from a screw, which is tightened by turning its head. See also: Bolted joint; Nut (engineering)
Bolts are generally manufactured from metals, but bolts made of other materials, such as nylon, are commercially available. The properties of some bolting metals are modified by heat treatment and other means to increase yield strength. Bolt heads have various shapes to suit different applications (Fig. 1). Hexagon-headed bolts (hex bolts) are available in two head widths for the same body diameter—regular hex bolts and heavy hex bolts, which have wider heads. The heavy-series bolts are normally supplied with heavy nuts that are wider and thicker than regular nuts.
Modern bolt-forming processes require little machine finishing. The American National Standard requires none of the surfaces of square, hex, or heavy hex bolts to be machined except for the threads, providing that the other dimensions satisfy the Standard. Such bolts are commonly termed unfinished or rough. For the finished bolts, only the undersides of the heads need to be finished; the bodies need not be machined if the forming process results in body dimensions that satisfy the Standard. Finishing of all bolt surfaces is required only for special applications. Ferrous bolts are obtainable with corrosion protection coatings such as zinc, cadmium, and organics, which are deposited on the bolts after machining.
Fits and special bolts
The holes in the assembled parts through which the bolts are inserted are usually slightly larger in diameter than the bolt body, resulting in a clearance fit. For some applications the holes are made slightly smaller than the bolt body in order to obtain an interference fit; then it is necessary to force the bolt through the holes. Bolts for force fits usually have a body diameter larger than the major diameter of the thread in order to avoid damaging the threads during installation. Two examples of such special bolts are turned bolts and ribbed bolts (Fig. 2). All the surfaces of the turned bolt are normally finished, with the body finished smooth. Ribbed interference-body bolts are used in structural work because the hole size is not as critical as for turned bolts. The ribs of high-strength ribbed bolts are serrated. See also: Limits and fits
Bolts are made in a wide range of tensile strengths. Standard specifications such as those of the American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), and the International Standards Organization (ISO) identify bolts by strength, grade, or property classes.
The following information is necessary to describe a bolt: external screw thread size, thread series, class of fit, and hand of thread; length; bolt head style; reference to standard specifications or material; and coating. (The bolt length is measured from the underside of the head to the tip of the thread, except for countersunk head and other special bolts.)
Straight-thread screws are sometimes substituted for bolts. The common “stove bolt” fastener is not a bolt but a slotted round-head machine screw, with a square nut. Screws are also substituted for bolts because of the wider variety of heads available on screws, some of which are designed for tightening with small portable power tools.
Various types of bolts (Figs. 1 and 2) may be used for automobile, machinery, appliance, farm implement, and structural connections. For example, low-carbon-steel unfinished bolts with hex heads (ASTM A307, Grade A) are used in machinery, and with square heads for structural steel connections. Heat-treated medium-carbon-steel finished hex-head bolts (ASTM A449) are high-strength bolts used for connections in structures as well as machinery. However, there are two kinds of high-strength bolts made specifically for structural steel connections (ASTM A325 and ASTM A490), both kinds are heavy hex structural bolts (dimensions differ slightly from those of heavy hex screws). Other kinds of bolts are medium-carbon-steel or atmospheric-corrosion-resistant-alloy-steel, quenched and tempered bolts (A325); and alloy-steel or atmospheric-corrosion-resistant-alloy-steel, quenched and tempered bolts (A490). Both the A325 and A490 bolts are designed to be tightened by torquing either the head or the nut. A hardened washer is often specified to be placed under the part being turned. See also: Screw fastener; Washer