Bolted Joint

This is one which depends either entirely or partially upon one or more screw bolts and nuts for its efficacy and security. A screw bolt, which should be forged from best bar-iron (if not made of copper or gunmetal), usually consists of a headed rod or spindle varying from in. to 3 in. in diameter, but sometimes it is square in part. An ordinary sized bolt in carpentry has an effective diameter of 7/8 in. or thereabouts; but in deciding upon its stoutness the quality of the iron must be taken into account as well as the description of stress to be withstood, besides which the amount of weakness introduced by boring a large hole through timber must not be lost sight of. At one end of the spindle is a permanent knob called the head or bolt-head, which is, perhaps, all the stronger for being screwed and welded on. The head is preferably square or hexagonal in shape (though it is sometimes cone, rose, or button-shaped), and it equals in width two diameters of the spindle, and in thickness 7/8 of one in practice; but theoretically the part A B, Fig. 28, need only be equal to 1/3 the diameter since the section sheared equals A B in breadth by a length corresponding to the circumference of the spindle. The tail-end of the spindle has a screw thread cut upon it, the effective diameter of the bolt being the diameter of the spindle inside the thread ; and in order that the cutting of the thread should not weaken the bolt it is sometimes thickened out at the screw end, but as this entails an enlargement of the bolt-hole, the consequence of such a procedure must not be overlooked. The pitch of the screw, that is, the distance between two revolutions of the thread measured parallel to the axis, ought not to be greater than one-fifth of the effective diameter; but it is better for being less, and the projection or depth of the thread is generally about half the pitch. The nut consists of a small block of wrought iron, or other appropriate metal, of square or hexagonal form. It is tapped with a screw to fit the thread of the bolt, and in all cases the clean cutting and proper fitting of the nuts and screws should be well seen to. The thickness of the nut is usually not less than one diameter of the spindle, because the thread lessens considerably the shearing resistance, and its breadth across should equal two diameters. In some cases, instead of a nut, the bolt is tightened up and held by a key or cotter, for which a slot is made in the spindle. The cornbined weight of the head and nut may be approximately assumed as equalling a length corresponding to four diameters of the spindle when the heads are rose-shaped and the nuts square, to five diameters when both are hexagonal, and to six when both are square. In carpentry, washers or discs of metal twice the diameter of the head and half its thickness are interposed at each end of the spindle between the timber and the head or nut, to prevent either of the latter from bruising the fibres. Sometimes the heads when square are let into the timber, which enables the bolt to be screwed up without requiring the head to be at the same time grasped by a tool, but it is a doubtful expedient except on the score of sightliness. In some situations the bolt has to resist tension, and in others shearing, but in either the safe or working strength of a round wrought iron bolt of fair average quality can be found by multiplying the square of its effective diameter by 4 and taking the product as tons. Thus the safe stress that can be put upon a 1 1/8 in. bolt is 5 tons. The carpenter uses bolts and nuts in all kinds of heavy work, such as trusses, partitions, domes, cofferdams, spirelets, framed floors, gates, built ribs, curved and laminated ribs, fished and scarfed beams, flitch beams, trussed girders, bridges, centres, etc, etc. In those instances where the union of parts is entirely dependent upon their holding powers, bolts as used by the carpenter are notoriously unscientific in principle, for they are loosened by the shrinkage of timber, and have no grip upon the fibres like a screw thread. The bolt holes also are a source of weakness, and when racking sets in, the spindles inside and the heads, and nuts, and washers outside crush the fibres against which they bear. They are, however, easily applied, and useful for tightening up work after it has been raised, loaded, and has taken its bearing, but the same advantages are claimed for ordinary wood screws if of sufficient size and turned or driven with a spanner. Before fixing bolts in timber structures, they are sometimes, like other iron fastenings, heated to a blue heat (about 550° Fahr.), and then immediately immersed in raw linseed oil, and painted when dry. It is at all events advantageous that they should be protected from oxidation, and this is perhaps one of the best means to that end, though the immersion in oil is a troublesome and disliked process, and one that is not at all unlikely to be evaded.

Bolted Joint 27

Fig. 26.

Bolted Joint 28

Fig. 27.

Bolted Joint 29

Fig. 28.