RIVETING.
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.
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.
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.
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.
In [fig. 41] may be seen an example of zig-zag riveting.
Fig. 41.
Caulking.—By this is meant the closing of the edges of the seams of boilers or plates. In preparing the seams for caulking, the edges are first planed true inside and outside; and after the plates have been riveted together, the edges are caulked or closed by a blunt chisel about 1⁄4-inch thick at the edge, which should be struck with a 3 or 4-lb. hammer; sometimes one man doing the work alone and sometimes one holding the chisel and another striking.
Fullering a boiler plate is done by a round-nosed tool, while caulking is executed by a sharper instrument.
The thinnest plate which should be used in a boiler is one-fourth of an inch, on account of the almost impossibility of caulking the seams of thinner plates.
It is a rule well known to all practical boiler makers that the thinner the metal (compatible with due strength) the longer the life of the boiler under its varying stresses and the better the caulking will stand.
STEEL RIVETS.
Hitherto there has been some prejudice against steel rivets, and while this may have some foundation when iron plates are used, it is certainly baseless when steel plates are concerned. The United States government has clearly demonstrated this. All the ships of the new navy have steel boilers, riveted with steel rivets, and an examination of the character of the material prescribed and the severity of the tests to which it is subjected show that these steel-riveted steel boilers are probably the best boilers ever constructed.
United States Government Requirements for Boiler Rivets.
They are subjected to the most severe hammer tests, such as flattening out cold to a thickness of one-half the diameter, and flattening out hot to a thickness of one-third the diameter. In neither case must they show cracks or flaws.
Kind of Material.—Steel for boiler rivets must be made by either the open-hearth or Clapp-Griffith process, and must not show more than .035 of one per centum of phosphorus nor more than .04 of one per centum of sulphur, and must be of the best quality in other respects.
Each ton of rivets from the same heat or blow shall constitute a lot. Four specimens for tensile tests shall be cut from the bars from which the lot of rivets is made.
Tensile Tests.—The rivets for use in the longitudinal seams of boiler shells shall have from 58,000 to 67,000 pounds tensile strength, with an elongation of not less than 26 per centum; and all others shall have a tensile strength of from 50,000 to 58,000 pounds, with an elongation of not less than 30 per centum in eight (8) inches.
Hammer Test.—From each lot twelve (12) rivets are to be taken at random and submitted to the following tests:
Four (4) rivets to be flattened out cold under the hammer to a thickness of one-half the diameter without showing cracks or flaws.
Four (4) rivets to be flattened out hot under the hammer to a thickness of one-third the diameter without showing cracks or flaws—the heat to be the working heat when driven.
Four (4) rivets to be bent cold into the form of a hook with parallel sides, without showing cracks or flaws.
Surface Inspection.—Rivets must be true to form, free from scale, fins, seams and all other unsightly or injurious defects.
In view of the fact that the government is using many hundred tons of these rivets, shown by the records of the tests to be vastly superior to any iron rivet made, in all the essentials of a good rivet, it would seem that it would benefit the boiler maker, the purchaser of the boiler and also the maker of the rivet by adopting a standard steel rivet to be used in all steel boilers.