Bolted Joint

In connecting the parts of cast iron structures, bolts and nuts are indispensable, since riveting is out of the question, on account of the hammering required for the operation and the contraction of the rivets in cooling being too much for a metal of such brittleness and inflexibility to stand. The efficient manufacture of bolts, which is at all times of consequence, is, however, doubly important in wrought iron structures, where they are little used excepting when large enough to insure soundness. As a rule, bolts should be made from one piece, turned to exactly fit the holes drilled to receive them, and case-hardened, together with their nuts and washers, for superior work. To meet severe tensile strain, bolts are more equal and reliable with nuts at each end instead of the usual head and nut, and where very violent strains can be foreseen they should be of steel. It is essential that in every case the abstraction of strength caused by the bolt hole is compensated for by an equivalent increase of metal where necessary. In cast iron elements this is often accomplished by thickening the metal round about and a little beyond the region of the bolt hole. To the same end cast iron beams, etc, have flanches, and other pieces, lugs, or ears cast on them with the usual bolt holes, whilst where two parts cannot be directly joined together, intermediate pieces, called brackets or dogs, are cast with proper holes with the object of being bolted to both, and all additional holes that may be required in any of the parts must be drilled; but as the perforations weaken the metal, and as no additional strength can be imparted to the casting by increasing its mass, the utmost caution may, perhaps, be necessary before determining the points for the extra holes. In cast iron bridges the several segments are bolted together through plates set between the planed surfaces of contact to enable the cross-bracing, or stays, or cross girders to be riveted or otherwise attached to the main ribs. The application of bolts with round, square, or countersunk heads to fasten the parts of iron work used in building is a wide one. Cast iron chairs or shoes securing the feet of iron rafters are connected by bolts, as are likewise similar castings at times fixed to rafters for taking the purlins. Tie-rods, and king and queen rods are also bolted to other parts of the trusses which they brace, having sometimes jaws at their extremities through which pass the bolts to hold the parts they seize. In these situations the effective diameter of the bolts must be proportioned to resist transverse strain. Cast iron distance pieces are bolted between double bars of different section to obtain increased strength and stiffness when the parts are long, or timber is fiitched between their whole length instead. Side and central cast iron standards for supporting ventilators, raised skylights, etc, are bolted to the rafters. Corrugated iron is fastened to iron purlins with small galvanised bolts, and different lengths of eaves troughing, and ornamental guttering are rendered with red lead or putty at their lapping ends, after which the joint is secured with nuts and gutter bolts about in. in thickness. It will be observed that in various parts of smiths' work bolts are not screwed up tight, but allow a little play or springing motion, in which case the joints of which they form a part are movable and not fixed. If bolts and nuts are to be protected against rust by Barff's process the male and female screws must be cut larger by an estimated amount of increase.

Bolted Joint 76

Fig. 75.