BOLT, an O.Eng. word (cf. Ger. Bolz, an arrow) , for a "quarrel" or cross-bow shaft, or the pin which fastened a door. ' From the swift flight of an arrow comes the verb "to bolt," as applied to a horse, to bolting food, etc., and such expressions as "bolt upright"; also the American use of "bolt" for refusing to support a candidate nominated by one's own party. From the sense of "fastening together" is derived the use of the word "bolt" as a definite length (in a roll) of a fabric (4oft. of canvas).
From another "bolt" or "boult," to sift (O.Fr. buleter, from the Med.Lat. buretare or buletare), come such expressions as in Shakespeare's Winter's Tale, "The fann'd snow, That's bolted by the northern blasts twice o'er." Bolt in Engineering.—In engineering a bolt is a pin or bar which serves to lock or unite parts, at the same time permitting of easy disconnection, as distinct from a rivet. Bolts occur in locks, safes and strong-room doors, being sometimes movable, sometimes fixed, and form a portion of the mechanism of a rifle, certain automatic pistols and other articles. A soldering-iron is called a bolt. Bolts for purposes of union of wood and metal ob jects are used in many millions per year, and a big industry is engaged in the production, with machines for forging, turning, screwing and finishing, as well as making the nuts. Black bolts are those screwed, but otherwise left as forged, bright bolts are finished all over, while for extra strength high-tensile steel is em ployed, and case-hardening is applied when wear must be defeated, as against frequent applications of the spanner, or if the bolt acts as a pivot. The smallest bolts are those for accurate scale models, about v in. diameter; the largest hold the heads and bases of armour-plate forging presses together, those on a Davy Brothers 12,000-ton press being 29in. diameter by 3 5f t. long and each weighing 38 tons. An ingenious method is adopted to tighten some big bolts which hold the frames of hydraulic rivetters and shearing machines together, instead of relying on mechanical means alone. The bolts are heated before the final assembly and the nuts run down. On cooling, each bolt shrinks in length, giving an immense tightening effect, fit to withstand the enormous pres sures, from 12,000 to 14,000 tons.
Most industries have their special bolts, but the diagram shows main types for general purposes in woodwork, engineering struc tures and machines, building, agricultural machinery, stoves, etc. A has square head and nut, B hexagon, for easier application of the wrench in confined places. Round heads are often used in stead of hexagon, as for connecting-rods. C is the cup-head or coach-bolt, also made with conical head, and required in wood or metal. The countersink or tyre, D, leaves a flush surface, a frequent necessity. The cheese or deck bolt, E, has its round head sunk into wood for a similar reason, F, the fang-bolt, has a large nut with spikes to sink into wood, whilst the bolt head is turned round. For thin metal work, shutes, galvanized iron and stoves, G and H are employed, the one being a mushroom slot in one member to be united. 0 is a stud-bolt, permanently screwed in by the tail, while P, the collar-bolt, is a form of stud secured through a plate by the lower nut. The hook-bolt, Q, mainly for suspension purposes, may be double, forming a U-bolt. For attachment to the sides of timber and other work, the strap bolt, R, has two or three holes. The term rag, or lewis-bolt, applies to S, grouting or lead being poured in the hole to secure the head, for attachment of iron work walls or foundations. T is also a lewis-bolt, inserted in a slot cut with dovetail side, and the packing-strip added, so that tightening the nut locks the whole. An expansion bolt (to go in walls) has a springy casing with spikes, that expands by a taper when the head is screwed home. The cotter or anchor-bolt U also serves chiefly for foundations, a large iron plate lying above the slot into which a cotter is inserted. The stay-bolt is really a rivet, screwed into boilers to unite the fire box and shell, and then rivetted over permanently.