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19. A steel rod is to carry a stress of 32,500 lbs. in tension or compression. Find its diameter when used in a bridge. as a hanger, when used in a building as a tie, and when used as a piston rod in a steam engine? Ans.: The ultimate. strength for steel in either tension or compression is 65,000 fbs. per sq. in. For bridge work where the loads are varying, use a factor of safety of six. In building work with practically steady work, use a factor of four. When the rod is subject to shocks, as in an engine, use a factor of safety of ten. The answers are: 2 ins.; 15% ins. ; 2 9/16 ins.

Fig. 6-Steel Flue and Anchor.

20. A steel bar 2X1⁄2 inches in section is ruptured under a tension of 64,000 lbs. What tension will rupture a 2X14 in. bar of the same material? Ans.: 160,000 pounds.

21. What tensile force is required to stretch a wrought iron bar 4X34 from 25'-0" to 25′ and 5/16 in.? Ans.:

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For wrought iron the Modulus of elasticity 25,000,000.
The elongation in 25 ft. is 5/16 in. The unit elongation

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or elongation per one inch = 5/16 300 ins. 1/960 in. and this is the unit strain.

Unit stress 25,000,000 X 1/960 26,042 lbs. per sq. in.
Area of cross section of bar = 4 X 34 = 3 sq. in.
Required tensile force = 3 X 26,042 = 78,126 lbs.

22. Find the compressive force which will shorten a block of cast iron 8 inches square from 4 ft. to 3 ft. 1134 ins. Ans. 5,000,000 lbs.

23. Find the unit stress which will stretch a steel bar one-tenth of one per cent. of its length. Ans.: 30,000 lbs. per sq. in.

24. What tensile force is required to stretch a steel bar 8 in. by 3/4 in. from 30 ft. to 30 ft. and 3% in.? Ans.: 187,500 pounds.

25. A steel bar 12 in. by 3/4 in. and 28 ft. long was stretched in length to 28 ft. and 1⁄2 in. Find the tensile force. Ans.: 401,600 lbs.

26. A steel block 6 in. by 6 in. and 4 ft. long was shortened under compression to 3 ft. 115% in. Find the unit strain, unit stress, total stress and per cent. shortening. Ans.: Unit strain .0078 in. per in. of length. Unit stress 232,000 lbs. Total stress = 843,000 lbs. Per cent. shortening, 78/100 of I per cent.

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27. A steel bar 10 ft. long at 32° F. is heated to 500° F. Find the change in length due to expansion.

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Ans. Elongation per foot per 1° F. = .0000065; elongation per 500° 32° or 468° F. = .0000065 X 468; elongation per io ft. .0000065 X 468 X 10.0304 ft. or nearly 3% in. 10 = 28. A vertical boiler flue 20 stories high is used to carry away furnace gases at a temperature of 500° F. Find the total expansion from 60° F. to 500° F. assuming each story 12 ft.

6 in. high. Ans.: 89/16 ins. approximately.

29. An outside steel flue 4 ft. diam. and 250 ft. high is used for gases varying in temperatur from 60° F. to 500° F. The flue is kept in place at each story by means of flat 2x3% in. steel straps. The straps are larger on one side by 1⁄2 in. Find the expansion across the diameter. (Fig. 6).

Length of circumference 3.14 X 48 in. = = 150.7 in. approximately.

Expansion of circumference per degree F. = .0000065.0009795 ins. or, say, .00098.

150.7 X

Expansion of circumference for (500-60)° = .00098 X 440°.431 ins.

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Length of new circumference at 500° 150.7 X .431 = 151.13 ins.

Length of new diameter = 48.13 ins.

The expansion in diameter is therefore about 1⁄2 in. and the 1⁄2 in. clearance is not necessary, the hoop being able to resist the effect of this expansion, especially that the hoop will also expand to a certain extent. Flue straps should be made tight against the face of the flue.

30. A steel column is so placed in the boiler room of a building that it cannot expand. The column is 18 ft. long. Find the additional unit compression caused in this column by a change of temperature from 60° F. to 160° F.

If the column was free to expand, the change in length for 100° F. or from 60° F. to 160° F. would be equal to 18 ft. X 100 X .0000065 X 12 in. = .1404 in.

The force required to stretch this column .1404 in. is equal to the additional compression caused by not allowing the column to expand.

Expansion per inch length

unit strain.

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Unit tensile stress causing this expansion equals 30,000,000 X unit strain = 19,500 lbs. per sq. in. This load comes

on the column in addition to its load at 60° F. In general, however, structures expand as a whole and this reduces the effects of expansion upon any one column.

31. What force is required to punch a 13/16 in. hole in a 3% in. steel plate? Also a 9/16 in. hole in a 5/16 in. steel plate?

Ans. Circumference of a 13/16 in. hole = 2.55 in. Area to be sheared by the edges of the punch when passing through a 3% in. plate =2.55 X 3%.96 sq. in. Shearing force 50,000 lbs. per sq. in. X 96 47,900 lbs. Similarly for a 9/16 in. hole in a 5% in. plate the shearing force will be 27,500 lbs.

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CHAPTER III.

The Manufacture of Iron and Steel.

CAST IRON.

Cast Iron was first made in England at the beginning of the fifteenth century.

Definition. Cast iron is a product of the blast furnace; it is iron which is not malleable and which is produced by a process involving fusion.

Manufacture. All iron is obtained from iron ores. The most common ores used for this purpose are:

Hematite or red iron ore, containing about 70% iron. Limonite or brown iron ore, containing about 60% iron. Magnetite or black iron ore, containing about 60% iron. In addition, the ores contain various impurities, like alumina, manganese, phosphorus, silica, sulphur, etc. Part of these impurities separate from the iron and go into the slag during the process of melting the ores in the blast fur

nace.

The product of the blast furnace is allowed to flow into channels dug in sand, where it cools off. The main channel is the "sow," while the branch channels are called the "pigs." Hence the name of pig iron. This pig iron is remelted in a cupola and poured into moulds forming castings.

Classifications. Various grades of cast iron may be classified as follows:

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