When the materials are of best quality, then there only remains to
rivet and stay the boiler. _Riveting_ is of two kinds, single and
double. Fig. 37 shows the method of single riveting, and Figs. 38 and
39 show the plan and cross-section of double riveted sheets.
[Illustration: Fig. 37.]
_Double Riveting_ consists in making the joints of boiler work with
two rows of rivets instead of one—nearly always, horizontal seams
are double riveted as well as domes where they join upon the boiler.
Usually all girth seams,—those running round the body of the boiler,
are single riveted. The size of the rivets is in proportion to the
diameter of the boiler, being 5/8, 3/4 and 7/8 as required in the
specification.
Rivet holes are made by punching or drilling, according to the material
in which they are made. In soft iron and mild steel they may safely be
punched, but in metal at all brittle the holes should be drilled.
[Illustration: Fig. 38.]
Rivets are driven by hand, by steam riveting machines or by an
improved pneumatic machine which holds the sheet together and strikes
a succession of light blows to form the head of the rivet while
hot. Rivets are made both of iron and steel, and there are certain
well-known brands of such excellent quality that they are almost
exclusively used in boiler work.
A place where skill is shown in boiler construction is in laying out
the rivet holes, with a templet, so that the sheets come exactly
together with the holes so nearly opposite that the dreaded drift pin
does not have to be used.
In these figures the letters P and p refer to the “pitch of the
rivets,” _i.e._, the part from centre to centre, and the dimensions
given at the sides indicate the amount of lap given in inches and
tenths of inches—the diameter of the rivet (1″) is also shown, and the
turned over portion of the shank of the rivet is shown by dotted lines.
[Illustration: 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.