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Thickness in

Tensile Strain on the Metal: Tons per Square Inch.

Bursting Pressure in Tons per Square Inch.

TABLE 18.-Of the STRAINS in a 10-INCH CAST-IRON PIPE, with DIFFERENT THICKNESSES of METAL.

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metal seat and forms it own bed, giving a precise area, and thereby a certain pressure. Thus for 3 inch diameter, the area = · 11 inch, requiring for say 1 ton per square inch 2240 ×·11 = 246 lbs. strain, and with a leverage of say 20 to 1, we have 24620 12.3 lbs. weight on the lever per ton pressure. The knife-edges at A and B, also the key K must be of hardened steel, and in order to adjust the level of the lever and compensate for wear (which is a practical necessity) the upper edge of the key should be wedge-shaped, and at an angle adapted to its seat in the slotted recess prepared for it.

(84.) The actual load on the ram of a hydraulic press is not often known with accuracy, but in the presses used for raising the Conway and Britannia bridges we have more precise information. For the Conway tube, a ram 183 inches diameter, or 265 square inches area, was used at each end; the gross weight of the tube, &c., was about 1300 tons, or 650 tons at each end: hence we have 650 265 = 2.45 tons per square inch. The cylinder was 20 inches diameter internally, and 10 inches thick: hence R = 20, r = 10, and the rule (82) gives 2.45 x (2010) = 4.08 tons tensile strain per square

S

=

202-102

inch of metal; this being the maximum strain, or that at the inside of the cylinder (80).

(85.) With the Britannia tube, the two Conway presses were used at one end and a large one with 20-inch ram at the other. The gross weight of the tube, &c., was about 1640 tons, or 820 tons at each end: then the 20-inch ram being 314 square inches area, we have 820 ÷ 314 2.61 tons pressure per square inch. The cylinder was 22 inches internal diameter and 11 inches thick, hence R 11, and rule (82) gives

S =

2-61 x (22 +11)

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per square inch of metal.

=

=

22, and r =

= 4.35 tons maximum tensile strain

It is probable that this pressure, 2.61, and strain 4.35 tons per square inch, were very nearly the breaking weights, indeed one cylinder failed by the bottom blowing off, which would have led to most disastrous results, but for the wise precaution taken of blocking up the tube inch by inch as it was raised by the

press. It would appear from this, that the tensile strength of cast iron in great masses 10 and 11 inches thick is much below the normal strength, or that for small thicknesses, namely, 7 tons per square inch.

(86.) "Tensile Strength of Thick Cast Iron."-Experiments have shown (933) that the specific transverse strength of cast iron is not the same for castings of all sizes, but that large castings, or rather castings with great thicknesses, are specifically weaker than small ones, so that bars 1 inch, 2 inch, and 3 inch square have specific transverse strengths in the ratio 1.0, 0.7184, 0.6195, and may be found approximately by the rule (934) or R = 1÷t. How far the tensile strength of cast iron is affected by the thickness of the casting is not known experimentally, but admitting the same law as for transverse strength we obtain the ratios given by col. 7 of Table 18, from which we obtain col. 8, also col. 9 from col. 5: thus for 5 inches thick, we get 4.059 x 497 2.017 tons bursting pressure per square inch, &c.

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(87.) By repeated re-melting the tensile strength of cast iron may be greatly increased as shown in (5), where metal of the fourth melting (pig iron being the first) gave as much as 18.26 tons per square inch in small thicknesses; applying to this iron the ratio in col. 7 we obtain the tensile strengths in col. 10, and finally from col. 5 the bursting pressures in col. 11. Thus for 5 inches thick we have 4.059 × 18.267 ×·497 5.265 tous, &c. But we have seen (7) that there is great uncertainty in this method of increasing the strength of cast iron; the safest course where heavy pressures are required, is to test the iron selected by direct experiments on its tensile strength.

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(88.) Table 18 seems to show by cols. 9 and 11, that there is no sensible advantage from great thicknesses of metal, the pressure remaining practically constant with all thicknesses from 5 to 10 inches, &c. Of course this rests on the assumption that the tensile strength follows the ratio in col. 7, but as that is derived from limited experiments on transverse strength, where the thickness did not exceed 3 inches, the results are not absolutely reliable, but are the best we can give in the present state of our knowledge.

The great uncertainty as to the important data connected with this subject, should lead to the adoption of large diameters permitting low values for the strain S and pressure p. For example, in the case of the Britannia press, say that the ram shall be 30 inches diameter = 707 square inches area, giving 820707 1.16 ton pressure per square inch. The cylinder might be 32 inches internal diameter, and say 5 inches thick, therefore R = 21 and r = 16; for strong iron of the fourth melting 5 inches thick S 9.08 tons by col. 10 of Table 18: 9.08 × (21 163)

=

then rule (81) becomes p

=

212 + 162

= 2.4 tons bursting pressure per square inch, or double the working pressure, 1.16 ton; thus leaving a fair margin for contingencies.

(89.) Another advantage of these proportions would be that the weight of the cylinder is reduced nearly to half, despite the increase in diameter; thus with the original sizes, 22 inches diameter 380 area, and 44 inches = 1520 area, hence 1520 380 1140 square inches, the area of the annulus. With the enlarged cylinder, 32 inches diameter = 804 area, and 42 = 1385 area, hence the annulus = 1385 804 = 581 square inches, or about half.

(90.) "Cylinders Hooped with Wrought Iron."-When large diameters are inadmissible and heavy pressures a necessity, the best course is to abandon dependence on the strength of cast iron altogether, and to rely on wrought-iron hoops shrunk hot, on a comparatively thin cast-iron shell, as in Fig. 25. In that case, the cast-iron cylinder may be regarded as a padding adding nothing to the strength of the combination, because when the hoops are shrunk on they exert a powerful compressive strain on the cylinder, which will be partially or perhaps wholly relieved when the internal pressure comes on: if wholly relieved the cylinder will be simply restored to its primitive state, being unstrained either way. It is rather difficult to calculate the strength under these conditions; if we take it as a wrought-iron cylinder 26 inches diameter, 5 inches thick, we have R = 18, r = 13, and the ultimate strength of wrought bar-iron or S being about 25 tons per square inch by Table 1,

25 × (182 - 13o)
182 + 132

= 7.86 tons per

But we have here taken the diameter, whereas it really acts

the rule (81) becomes p = square inch bursting pressure. pressure as acting on 26 inches on 22 inches only: hence 7.86 × 26 ÷ 22 = 9.3 tons per square inch. As applied to the Britannia tube, where the actual pressure due to the weight of the tube was 2.61 tons as shown by (85), we have 9.32.61 = 3.5 as the "factor of safety."

It is shown in (625) that whatever the initial strain on wrought iron may be, the permanent working load with a fixed length cannot exceed 8 or 9 tons per square inch: in our case it is 25 × 2.61÷9∙3 = 7 tons only.

(91.) Care must be taken that the longitudinal pressure does not blow the bottom out: the area of 26 = 531, and of 22 = 380, hence of the annulus 531 380 151 square inches, giving with 7 tons per square inch of metal, 151 x 7 = 1057 tons, or 1057380 2.78 tons per square inch bursting pressure, or about only of 9.3 tons the bursting pressure circumferentially. This difficulty is easily overcome by the construction shown by Fig. 25; the bottom of the cylinder, supported all over by the sole plate of the press, B, is entirely relieved of the bursting pressure.

(92.) "Wrought-iron Press-pipe."-Wrought-iron drawn tubes are commonly used for hydraulic-press work, &c., where the pressures are very heavy; the ordinary sizes are 1 inch diameter outside, inch bore, therefore inch thick. A series of experiments was made on pipes of this kind with a pressure of 3 tons per circular inch, or 37854 = 3.82 tons per square inch; the pressure was obtained by a steel plunger 1 inch diameter, loaded with 3 tons of direct weights; there was therefore no uncertainty as to the real pressure. The result was that nearly all the pipes tested in this way bore the strain satisfactorily, the faulty ones alone failing. The maximum strain on the metal by the rule (82), with Rinch, and r = inch, becomes 3.82 × (12 + 12) = 6.37 tons per square inch, which is

S=

12 - 12

about only of 25.7 tons, the mean tensile strength of wrought iron by Table 1, &c.

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