would only be equal to 51 per cent.; thus, 62.75 × 23 28 51.5. In the same manner the value of the rivet section (75) obtained in Example 2 would only be equal to 62 per cent. of the solid plate-thus, 75.5 x 23 = 62. 62. The plate and rivet sections of an iron joint, as stated elsewhere, will be of equal strength when the pitch is found by the αχη formula, + d = P, but to obtain the same results With plates inch thick, and rivets + d = P. a x n x 23 pitch for a double-rivetted steel joint would be 4417 × 2 × 23 •375 × 28 +75 2.69 inches, which, by the formula, would give 72 per cent. of plate section, and 87.6 per cent. of rivet 87.6 × 23 section; but 72, which is the value of the 28 = shearing strength of the rivets, as compared with the tensile strength of the solid plate. In double-rivetted joints the arrangement of the rivets may either be "zig-zag" as in Figs. 44 and 45, or "chain" as in Figs. 46 and 47. The former of these methods requires less lap than the latter, and the rivet pressure being consequently greater per square inch of surface, these joints, if well executed in other respects, should be sounder than those which are "chain" rivetted. On the other hand, the material with zig-zag rivetting is more liable to be injured under manipulation, particularly when the rivet holes are punched, and there is always the difficulty of getting the corner rivets to come in without departing from the general proportions of pitch and diameter. The “zig-zag” arrangement is more frequently employed than the "chain," but further experiment is necessary to determine which of the two methods is better adapted for equalising the strains and giving the strongest joint. The other forms of rivetted joints are treble-rivetted laps, and single and double butt joints. The former of these is rarely adopted in land boilers; but in lap-jointed marine and other boilers, where thick plates are used, the rivet area' required to bring up the strength of the joint renders treble rivetting necessary. Butt joints (Figs. 48 and 49) when properly proportioned and carefully executed, are stronger than lap joints; the rivets being in double shear, and as the rings of plating can be made truly cylindrical, they are free from the cross bending strains to which lap joints are subjected. This feature in butt jointing, as elsewhere explained, effectually prevents the grooving so frequently found at the edges of the longitudinal seams of lap-jointed locomotive boilers, and, next to sound welding, makes it the best method of forming the longitudinal joints of furnace tubes. Butt joints are more difficult to make, and being thus more liable to suffer from defective workmanship than the ordinary lap joint, they require to be very carefully supervised during construction. Butt joints fitted with single straps have little, if any, advantage over lap joints in point of strength, and as it is more difficult to make thoroughly sound work with the former, they are seldom employed in boiler construction. The circular seams of land boilers, as a rule, are single rivetted, there being no necessity, as will be seen, for double rivetting to enable them to resist with safety the stresses due to internal pressure. The springing at these joints, resulting from the inequalities of temperature found between the top and bottom of certain boilers (mostly of marine type), is frequently the cause of annoying leakages, to prevent which double and occasionally treble rivetting is resorted to, but it is only for such reasons that single rivetting is departed from. The strength at the circular seams of a plain cylindrical boiler to resist tearing asunder by force of internal pressure, is double that of the longitudinal seams, when the proportions of rivetting, &e, are equal, and in boilers which have tubes and stays extending between the end plates, the strains tending to tear asunder the circular seams are still further reduced. The pressure at which rupture of the circular seams would S XP, where S is the sectional area of the occur is equal to A plates, p the strength of the joint compared with the strength of the solid plate, and A the area of the end plate in square inches. The pressure at which rupture of the longitudinal seams would occur is equal to t× 2 × P. where t is the thickness d , of plate, p the strength of the joint, compared with the strength of the solid plate, and d the diameter of boiler in inches. Example. What are the pressures required to produce rupture through the circular seams and longitudinal seams of a boiler 5 feet (internal) diameter, composed of plates inch thick, the tensile strength of which is 40,000 lbs. per square inch, and the efficiency of the joints (single-rivetted) 56 per cent. of the solid plate? S, The sectional area of the plate is the difference between the area of the outer and inner circles of the boiler, which in this case are 5 feet 2 inch, and 5 feet respectively; thus, 28985 - 2827·4 = 711 square inches sectional area. p, The strength at the 40,000 × 56 joint is 22,400 lbs. per square inch. A, Area 100 = of the end plate 5 feet diameter = 28274 square inches. |