Bolts and Screws.-The width across the flats of the bolt-head, is the same as for nuts given in table 108, page 285. The width across the flats is approximately equal to 1 times the diameter of the bolt added to The angle of the triangular thread is 55°, the height of the triangle of thread is reduced one-third by rounding one-sixth off the top, and one-sixth off the bottom of thread. Depth of thread the pitch multiplied by '64. To find the diameter at the bottom of the thread, multiply the pitch by 1.28, and subtract the product from the outside diameter. = For screws with square threads, the number of threads per inch is one-half of the number for triangular threads, and the depth of thread is of the pitch, or equal to the space between the threads. Table 90.-WHITWORTH'S STANDARD GAUGES FOR WATCH AND INSTRUMENT MAKERS, WITH SCREW-Threads for THE VARIOUS SIZES, 1881. Conducting Power of Metals for Electricity at 32° Fahr.: 92 65 Lead . 8.3 19 The conductivity diminishes as the temperature increases above 32° F. SECTION VI. STRENGTH AND WEIGHT OF MATERIALS; WORKSHOP DATA, &c. Strength of Wrought-Iron.-The tensile strength of wrought-iron is about four times as great as that of cast-iron; good wrought-iron should be capable of standing the following tensile strains before breaking, in tons per square inch of section. Lowmoor or " Best Yorkshire iron" plates along the fibre Ordinary good angle iron Ordinary good boiler-plates along the fibre Ordinary good boiler-plates across the fibre Ordinary good ship-plates along the fibre. The strength of wrought-iron to resist a crushing or compressive strain is about half that of its tensile resistance, or say 12 tons, and its working strength in compression free from flexure is one-quarter that amount, or 3 tons per square inch of section. Testing Wrought-Iron.-Good wrought-iron has a fine close-grained fracture of silvery grey colour; inferior quality has a coarse granular fracture similar to that of cast-iron. The elongation under tensile strain is a test of the toughness of wrought iron; the ultimate elongation after fracture of Lowmoor iron plates is about 13 per cent., and of ordinary good iron boiler-plates 7 per cent., and of ordinary good ship-plates 5 per cent., of their original length when torn along the fibre. Lowmoor or best Yorkshire iron plates under inch thick, should bend double when cold without fracture; and from inch to 1 inch thick, should bend double when hot, both lengthways and across the fibre without fracture. The tests for ordinary wrought-iron boiler and ship-plates are the same as those used by the Admiralty, which are given below. Admiralty Tests for Wrought-Iron Boiler-Plates.-All boilerplates (with the exception of Lowmoor and Bowling iron, which are not tested) must be capable of standing the following test : Tensile strain per square inch lengthways, 21 tons: crossways, 18 tons. Forge-Test, Hot.-Plates to admit of being bent hot, without fracture, to the following angles. Lengthways of the grain, 125°; across, 100°. Forge-Test, Cold.-Plates to admit of being bent cold, without fracture, to the following angles. Admiralty Test for Ship-Plates.-Plate iron, first-class BB.; tensile strain per square inch, lengthways, 22 tons; crossways, 18 tons. Forge-Test, Hot.-All plates of the first-class, I inch thick and under should be of such ductility as to admit of bending hot, without fracture, to the following angles. Lengthways of the grain, 125°; across, 90°. Forge-Test, Cold. All plates of the first-class, should admit of bending cold, without fracture, to the angles given in the above table. Plate-Iron Second-class B., tensile strain per square inch lengthways 20 tons; crossways, 17 tons. Forge-Test, Hot.-All plates of the second class, 1 inch thick and under, should be of such ductility as to admit of bending hot, without fracture, to the following angles. Lengthways of the grain, 90°; across, 60°. Forge-Test, Cold. All plates of the second class should admit of bending cold, without fracture, to the following angles. Thickness of Plate, in Inches and under and and, and 1 and 1 and 1 Inch. 75° Steel Boiler-Plates are generally made of the best quality of mildsteel, their tensile strength is about one-third greater than that of Lowmoor iron, and they should stand the following test before breaking. Tensile strain per square inch both lengthways and crossways, 28 to 30 tons. All steel plates 1 inch thick and under, to admit of being bent double when hot, without fracture, and to admit of bending cold, without fracture, to the angles given in the table below. The safe working tensile strength, is one-fourth the breaking strain, or from 7 to 7 tons per square inch of section. |