Where L is Breaking Weight in tons, and C the circumference of the rope in inches, working strength at high speeds one-tenth of this. The ultimate strength of sheet copper appears to vary from 14 to 17 tons per square inch, the elastic limit being reached at about one-half of this. When annealed the ultimate strength is reduced to about 15 tons, with an elastic limit of about 5 tons. At temperatures of about 400° F. there is a loss of strength of about 20 per cent. The strength of brazed joints appears to be about 75 per cent. of that of the metal. Solid drawn copper reaches about 18 tons, the elastic limit being reached about 16 tons. CORRUGATED FLUES. See pp. 70 and 474. From recent experiments on steel corrugated flues by Mr. Parker, of Lloyd's, the following formulæ are proposed for strength : Ultimate crushing strength in lbs. per sq. in. where t is the thickness of plate, and d the mean diameter of furnace. Working strength in lbs. per sq. in. = 1000 × (T − 2) D , where T is the thickness of plate in sixteenths of an inch, and D the greatest diameter of the furnace in inches. With the latter rule the margin of safety appears to be fully 5. The experiments were carried out with a flue having the corrugation 1 inch deep and 6 inches apart. The steel plate showed a tenacity of 22.7 tons per square inch, with an elongation of 35 per cent. in a length of 10 inches. The Board of Trade gives the following rule for the strength of these flues: D = outside diameter in inches, measured at the bottom of the corrugations when the furnace is of the corrugated or suspension type, or over the plain parts when it is of the ribbed and grooved description." STRENGTH OF SHAFTS FOR SCREW STEAMERS. See p. 79. Where d= diameter of crank shaft in inches. D2 = square of diameter of low-pressure cylinder in inches. or the sum of the squares, if there are more than r= one. the ratio of areas of low- to high-pressure cylinder. P = absolute pressure, or boiler pressure + 15. f, has the following values:-170 for two cranks set at right angles, and 180 for three cranks set at 120° apart. The diameter of the intermediate shafting 95 d. PHOSPHOR BRONZE. See p. 67. This metal, an alloy of copper, tin, and phosphorus, is found to possess a high tensile strength and great endurance as regards wear, and is being used principally for bearings and machinery. The various strengths of this metal appear to vary greatly according to the proportions of materials used, hence different alloys are employed for different classes of work; the range of ultimate tensile strength being from 22,000 lbs. to 57,000 lbs. per square inch, whilst the ultimate compressive strength varies from about 21,500 lbs. to 45,000 lbs. per square inch. It also appears that wire made of this metal possesses the quality of high tensile strength. MANGANESE BRONZE. See p. 67. The strength and freedom from corrosion of manganese bronze constitute it an excellent material for screw propellers, as blades of lighter section can be used than are required for steel. The transverse strength appears to be about double that of gunmetal and steel, and its tensile strength varies from 24 to 40 tons per square inch. COMBUSTION OF COAL. See p. 285. The rate of combustion in marine boilers at sea with ordinary bituminous coal with natural draught is about 15 lbs. per square foot of grate. With fan draught it is from 25 to 28 lbs. The fans in some cases force the air into the furnaces, in others they suck the air through; the former is known as forced draught, and the latter as induced draught. CORROSION IN BOILERS. See p. 467. Various methods have been tried to prevent corrosion and pitting in boilers. Zinc has been used successfully to prevent galvanic action; whilst lime and Portland cement have been introduced to protect the surfaces. When the water is pure and of a mossy character little corrosion takes place. One matter that requires special attention is the arrangement of the feed apparatus: the heating of the feed water and absence of air in it are of primary importance. ADDENDUM TO ARTICLE 317, p. 464. SAFETY VALVE Areas, as REQUIRED BY THE Board of Trade. |