Table 41.-APPROXIMATE WEIGHT OF ONE FOOT IN LENGTH OF WroughtIRON LAP-WELDED TUBES.

The Tensile Strength of Good Wrought-Iron Boiler-Plates when the strain is applied in the direction in which they are rolled, or along the grain, is 21 tons per square inch of section; when the strain is applied across the grain it is only 18 tons. The tensile strength of Lowmoor and Best Yorkshire-iron plates is 24 tons per square inch of section lengthways of the grain, and 22 tons across the grain. The tensile strength of mild steel boiler-plates averages from 28 to 30 tons per square inch.

Riveted Joints are liable to fracture in 4 different ways: (1.) The rivets may be shorn off-the force required to shear a rivet being the shearing strength of the rivet multiplied by the area of the rivet. The strength of rivet-iron to resist shearing is about that of the plate to resist tearing, or 21 tons per square inch of section. The strength of the rivets in a joint, may be found by multiplying the area in square inches of one rivet by the number of rivets, and multiplying the product by 47,000 for ordinary iron rivets, and by 53,760 for Lowmoor Iron rivets.

(2.) The plate may tear along the line of rivet-holes as shown at A B, Fig. 135, that is, between the rivet-holes. The strength of the plate between the rivet-holes is impaired by punching to the extent of 20 per cent.; and the strength to resist fracture between the rivet-holes is found thus:- first find the area of plate between two rivet-holes, which is found by subtracting the diameter of the rivet from the pitch of the rivets in inches, and multiply the remainder by the thickness of the plate in inches, giving the area in square inches between two rivet-holes. Multiply this by 38,700 for wrought-iron plates, when the rivet-holes are punched and by 44,000 when the rivet-holes are drilled. The answer will be the strength

of metal left between two rivet-holes.

(3.) The plate may crush in front of the rivet as shown at Fig. 136. The

AB

Fig. 135.

Fig. 136.

Fig. 137.

Figs. 135-137.-Fractures of Riveted-Joints.

resistance offered by a plate to the crushing strain of a rivet, is one and three-quarter times the amount of the tensile strength of that plate, or say 37 tons. The area which resists the crushing strain, is found by multiplying the diameter of the rivet by the thickness of plate in inches; and the strength of the wrought-iron plate between the rivet-hole and the edge of the plate is found thus :-multiply the diameter of the rivet by the thickness of plate in inches, and multiply the product by 82,800.

(4.) The plate may break across in front of the rivet as shown at Fig. 137. and the strength opposed to resist this transverse fracture may be found thus Multiply the square of the distance between the rivet-hole and the edge of the plate, by the thickness of the plate, then divide the product by the diameter of rivet, and multiply the quotient by 48. The answer will be in tons, which multiplied by 2240 gives the strength in lbs.

Example of the above rules.-Required the strength of the riveted joint of two wrought-iron plates, each 41 inches wide x inch thick, fastened with 20 rivets inch diameter x 2 inch pitch; in punched holes inch from edge of plate.

(1.) 4417 X 20 X 47,000 Rivets shearing off

415,198 lbs.

(2.) 2—75 = 125 × 437 X *} Tearing between rivet-holes. 422,600 „

38,700 x 20

Ν

(3.) '437 × 75 × 82,800 × Crushing in front of rivet} 541,500 lbs.

20.

(4.) 75 × 437

holes

75

48 × 2240 X 20

Srivet holes

The strength of the solid plate will equal its sectional
area × 47,000, or 41 inches wide x 437 thick x
47,000 lbs. tensile strength.

The Weakest Part of the above Seam is the resistance offered to the rivets shearing off. The strength to resist tearing between the rivet-holes,

is also very small compared with the strength of the solid plate, the strength of the joint with punched holes and single riveted, being only equal to one-half that of the solid plate. The efficiency of the joint is the ratio. of its strength to that of the solid plate, which Mr. Fairbairn found to be for single-riveting 56 per cent., and for double-riveting 70 per cent. of the strength of solid plate.

Fig. 138 shows a single-riveted lap-joint; Fig. 139 a butt joint with single

Fig. 142.

Fig. 143.

Fig. 144.

Figs. 142-144.-Methods of Strengthening Furnace-Tubes.

covering strip; Fig. 140, a butt-joint with double covering strip; Fig. 141, a double riveted joint with zigzag rivets; Fig. 142, Adamson's flanged seam for furnace-tubes: Fig. 143, Expansion hoop for furnace-tubes; Fig. 144, Angle-iron hoop, or strengthening ring for furnace-tubes.

The Pitch of Rivets for boilers, varies considerably in practice. The proportions of riveted joints given in Table 103, page 280, give good results.

Bursting Pressure of Cylindrical Steam Boilers.-To find the strength to resist-in a line parallel to its axis-the internal bursting pressure of a cylindrical boiler shell. Rule: Multiply twice the thickness of the plate in inches, by one of the following constant numbers, and divide the product by the diameter of the boiler shell in inches, and the quotient will be the bursting pressure in lbs. per square inch.

26,000 constant number for single-riveted joint of wrought-iron. double-riveted joint of wrought-iron. single-riveted joint of steel.

32,500 40,500 50,62;

"

Table 42 has been calculated by these rules. It gives the bursting pressure in pounds per square inch of lap-jointed wrought-iron cylindrical poiler shells, of from 2 feet to 9 feet diameter, of various thickness of plates, both single and double-riveted.

Bursting pressure of Spherical Shells. To find the bursting pressure in lbs. per square inch, of a wrought-iron spherical shell, take double the bursting pressure of a cylindrical shell, of the same radius and thickness.

To find the Collapsing Pressure in lbs. per Square Inch of boiler tubes, or flues of wrought-iron, of perfectly circular shape, or not more than the thickness of plate from the true circle. Rule: Multiply the square of the thickness of the plate in 32nd parts of an inch, by the constant number 800, and divide that product, by the product of the length in feet, multiplied by the diameter of the tube in inches. In calculating elliptical tubes, the diameter of a circle, equal to the largest circle of curvature of the tube should be used in the above rule, for finding the collapsing pressure.

Table 43 has been calculated by this rule.* It gives the collapsing pressure in pounds per square inch of wrought-iron cylindrical boiler flue tubes, of from 12 inches to 42 inches diameter, of various thickness and length.

Factor of Safety for New Cylindrical Steam Boilers, which have been tested by hydraulic pressure to twice the working pressure. When the quality of the materials and workmanship is known to be first-class, a factor of safety of 6 may be used, but when this condition is not complied with, the following additions should be made to the factor of safety 6, viz., 25 if the holes are not good and fair in the circumferential seams; 5 if the seams are not properly crossed; 5 if the holes are not good and fair in the longitudinal seams; 10 if the longitudinal seams are single-riveted; and 20 when the quality of the materials and workmanship is doubtful.

For a more accurate rule see the Author's work, "Steam-Boiler Construction," published by Crosby Lockwood & Son, London.

Table 42.-BURSTING PRESSURE IN LBS. PER SQUARE INCH OF LAP JOINTED WROUGHT-IRON CYLINDRICAL BOILer-Shells.

510 637

956

892 1114

361 451 421

526

481

295 369

443

553 492 615 541

677 590 738

412 376 471

588 518

NOTE.-Factor of Safety.-The Working pressure should never exceed one-sixth of the Bursting pressure.