The rule for proportioning the size of the rivets in punched holes, as laid down by Sir W. Fairbairn, is: For the diameter of the rivets, double the thickness of the plate under œ inch thick, the larger plate always being taken as the basis for their diameters; in plates above œ inch thick the diameter of the holes should be 1 1/2 times the thickness of the larger plate.
The holes are made slightly larger than the rivets, which expand when hot; and they are set out to different pitches, by which is meant the distance from centre to centre of the holes, the numeral used (as 4-pitch) denoting the number of diameters of the rivet from centre to centre.
The common pitches are 4 or 5 diameters, equivalent to from 4 to 6 inches, according to the size of the rivets. In the outside row no rivet should be placed nearer than its own diameter to the edge of the plate; and punched holes should be farther away from the edge of the plate than drilled ones, because they weaken the plate more.
Riveting, as before noted, is distinguished by the order and number of the rows at the joint required to transmit the whole strain from one plate to the other, through the medium of the rivets themselves. Lap joints require only one set to convey a strain from plate to plate; while fish joints require two sets, the one to transmit the strain from one plate to the fish-plate, and the other set to take it from the fish-plate to the other long plate. Thus, two single rows at a fish joint would be called a double row at a lap joint.
Fig. 429 illustrates quadruple zigzag riveting on a lapped joint, similar in section to fig. 427, while it would be called "double zigzag riveting" if the joint were fished similar to fig. 430; so that single, double, triple, and quadruple riveting differs for lap and fish joints, and others of similar character, and care must Always be taken to describe them properly, according to the nature of the joint Lap or lapped joints, as fig. 427, consist of two plates overlapping one another, to be riveted together.
Section Fig 427.
Jump joints, in riveting, are for compression when they are employed, which should be very seldom, as the strength of the connection depends more on the butting surfaces of the plates than the rivets; and, whether these surfaces be planed or not, the very act of riveting the joint has a tendency to twist them, so that the weight is not, as it should be, evenly distributed over the butting surfaces; and where the rivets have not been calculated to take the whole strain the joint fails.
Plan Fig. 428.
All riveted joints should be calculated and designed so that the rivets can, if necessary, take the whole strain successfully, whether it be of tension or compression, in the length of the girder, stanchion, or framing.
Plan Fig 429.
Double riveting, and other kinds, superior in the number of the rows, may be either chain or zigzag in form, as figs. 428 and 429 respectively; the "zigzag" being considered better than the "chain," because the holes, being alternate, do not take so much out of the continuity of the fibres of the plates. For example, in fig. 429, only one hole, as it were, is taken out of one continuous fibre; whereas if it were "chain" (fig. 428), two holes would be taken.
Section Fig. 430.
The strength of a solid plate being taken at 100 single riveted joints, by way of comparison, may be taken at 56, double at 70, and chain riveting at 85.