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TABLE 29.-Of the TENSILE STRAIN on PUMP-RODS in Practice.

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less violent shocks from the motion of the water, the factor of safety 3, by col. 5 of Table 141 becomes 3

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= 18;

hence we have 47266182626 lbs. per square inch working load.

CHAPTER V.

ON THE SHEARING STRAIN.

(123.) "Single and Double Shear."-When two plates are connected by a rivet or pin, as at A in Fig. 6, and the rivet is severed by a tensile strain applied to the plates, we have a case of single-shearing, and it has been found that the strain is simply proportional to the area sheared, being independent of the form of the pin in cross-section, whether round, square, &c.

In Fig. 8, or at C in Fig. 6, we have two side plates and one central one: it is obvious that to shear the pin a double area has to be severed requiring double strain for the double shear.

Mr. E. Clark made direct experiments on the resistance of -inch rivet-iron to single and double shearing: he found that the

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tons per square inch. Double-shearing gave as the result of eight experiments :

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tons. The mean of the two = 23.12 tons per square inch: the direct tensile strength of the same iron was 24 tons, from which it appears that the shearing and tensile strains are practically equal to one another, and this is admitted as a general rule: it requires, however, some modification as applied to rivets in joints. It appears that in the process of riveting red-hot in the

usual way the metal is damaged and its strength reduced by the maltreatment experienced: thus by Table 1, Mr. Kirkaldy gives 26 tons per square inch as the mean tensile strength of rolled rivet-iron; but Mr. Fairbairn found it to be 22 tons only in single-riveted joints of boiler-plate (19); hence we have 22÷26·846, or say 85 per cent. realised; therefore 15 per cent. is lost by riveting hot.

With steel rivets the loss is very great, as shown by (42); the tensile strength of bar steel is 47.84 tons per square inch, but that of steel in riveted joints is 23.77 tons only: hence 23.7747·84 50, or 50 per cent. only, is realised, and 50 per cent. is lost by riveting hot.

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With treenails of English oak, commonly used in shipbuilding, the shearing strain across the grain by experiments at H.M. Dockyard was 4000 lbs. per square inch, and as by Table 79 the mean tensile strength of oak = 12,332 lbs., the ratio is 3.1 to 1.0.

(124.) "Rectangular Bars." - Experiments recorded by the Institute of Mechanical Engineers show that in shearing flat bars, the shearing strain is nearly the same whether the bar is flat or on edge; thus bars inch by 3 inches gave on the flat 22.3 and on edge 23 1 tons per square inch. Others 1 inch by 3 inches gave 23.1 and 22.7 tons per square inch respectively in these experiments the shear blades were parallel.

(125.) "Oblique Shearing."-When the blades are fixed at an angle so as to shear a plate obliquely, the strain is less than with parallel shearing to an extent which varies with the angle of the blade and the thickness of the plate. Say, for illustration, that Fig. 29 is a blade 12 inches wide, with four steps in it, each 3 inches wide, and B the plate to be sheared, the thickness of the plate and the height of the steps being inch. Now, it will be observed that the steps act one after the other, thus C will have done its work and passed through the plate before D begins to act, &c., hence the strain is 4th only of that due to a parallel blade 12 inches wide, but of course the travel is 4 times as much; therefore the mechanical power is the same in either case. The line E, F, G at a slope of 1 to 12 or 1 to 8 would

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evidently give the same result as a blade with steps, that is to say, with a 1-inch plate as in our case. But the slope would vary with the thickness; thus for 4-inch plate it might be 1 to 16; with 1-inch plate 1 to 4, &c.; the strain being then th of that with a parallel blade. For ordinary and general purposes a slope of 1 in 8 to 1 in 12 is commonly used.

"Strain for Punching."-An ordinary punch may be regarded as a circular cutter or shearing blade whose length is equal to the circumference: then by Table 1 the mean strength of plateiron is 48,454 lbs., or 21 63 tons per square inch: hence a punch 1 inch diameter with a plate 1 inch thick would require

TABLE 30. Of the STRAIN for PUNCHING RIVET-HOLES in
PLATE-IRON and STEEL.

Thick-
Dess.

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a force of 21.63 x 3.1416 68 tons, and we have for wroughtiron plates the rule:

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In which d = the diameter of punch, and t = the thickness of the plate, both in inches; Sp being the punching strain in tons: thus, for example, with 2-inch punch and 1-inch plate we obtain Sp × × 68 = 25.5 tons.

With steel plates the mean tensile and shearing strength by Table 1 = 85,977 lbs., or 38 38 tons per square inch; hence a punch 1 inch diameter with 1-inch plate requires 38.38 x 3.1416 120 tons, and we have for steel plates the rule :—

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Thus for say-inch punch and 4-inch steel plate the punching strain becomes Sp = × × 120 = 15 tons. Table 30 has been calculated by these rules.

DETRUSION.

(128.) This term has been applied to the shearing strength of timber in the direction of the fibres. Experiments have shown that, 1st, This is practically the same as the tensile strength perpendicular to the grain which is given at the end of Table 1; and, 2nd, That both are very small and very variable: with

Poplar

Oak

Larch Scotch Fir Memel

the mean resistance to detrusion with the grain and tensile strain across the grain is ::

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In practice, simple detrusion is easily avoided by bolts through the bar, hence the great weakness of some kinds of timber to that particular strain is a matter of small importance.

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