Fig. 39.
No riveted boiler work can be considered fairly proportioned unless the strength of the plate between the rivets is fully equal to the strength of the rivets themselves. A margin (or net distance from outside of holes to edge of plate) equal to the diameter of the drilled hole has been found sufficient.
Rivets should be made of good charcoal iron or of a very soft mild steel, running between 50,000 and 60,000 pounds tensile strength and showing an elongation of not less than ninety per cent. in eight inches, and having the same chemical composition as specified for plates.
A long rivet, holding thick plates together, is rarely tight except immediately under the head. The heads are set and the centre cooled before the hole is properly filled. If it is a very long rivet there is a chance of the contraction fracturing the head of the rivet. In the Forth Bridge, which is made of very heavy plate girders, the rivets, first carefully fitted, were driven tight into the holes, the burr around the holes were removed, and the ends of the rivets heated to a sufficient degree to enable them to be closed over.
A simple mathematical deduction shows that a circle seam has just one-half the strain to carry as a longitudinal seam, under the same pressure and with the same thickness of metal, hence the custom of single riveting the former and double riveting the latter, or longwise seams.
Different Modes of Riveting.