SUMMARY The preceding rules are summarised in the following short table, in which the method has been carried out of indicating all such dimensions as are measured in inches by capitals, such as are measured in sixteenths of an inch by small letters, and all coefficients, &c., by black letters. C and C' B and B' = coefficients. permissible working pressure. percentage of joint, respectively of plate and of rivets. N = number of rivets included within one pitch of external F T and t P and P. = row. factor of safety. thickness of plate, measured respectively in inches and in sixteenths. pitches of rivets, or of stays, or of stay tubes. D = internal diameter of shells, or of plain tubes; effective diameters of rivets or of stays, or external diameter of furnaces in inches, measured at the plain cylindrical part or at the bottom of the corrugations. L' and L" length of plain cylindrical parts of furnaces, depth of combination chamber, or length of girders, measured respectively in feet and in inches. depth of girders measured in inches. sectional area of stays or of rivets in square inches. The coefficients for determining the working pressure of the shell depend on the material and on conditions of workmanship, the ultimate strength of the material being divided by the sum of 4-5 and one or both of the following additions: In these three cases 2 has to be added for double-ended boilers, but only for the central seam. 40,000 for wrought iron (across the grain), not tested. (with the grain) If tested, C is the lowest ultimate tensile strength in lbs. per sq. For superheaters use 30,000 and 22,400. inch. For steel of less than 32 tons the values of C are In no case should the working pressure exceed the values found by the following formulæ : For short lengths of flanged furnaces use the following formula: - These formulæ can only be used as long as L" 120 T-12. = The working pressures for various patent furnaces are found by the following formula: Purves's ribbed Fox's corrugated steel furnaces) W P = 14,000 . t For unequal pitches (see T. W. Trail, 1890, pp. 67, 199) apparently where p = diagonal pitch or diameter of largest inscribed circle. When used in the drawing office, it is strongly recommended that those parts of the following tables which are inapplicable to the particular works should be obliterated. TABLE FOR FINDING THE DIAMETERS OF RIVETS AND OF PITCH IN RIVETED JOINTS. A short table will be found on p. 321 which shows the smallest permissible percentage for any particular joint; the conditions are fully explained and illustrated. The following table contains the values of N. D÷T = number of rivets diameter of rivets thickness of steel plate. Having found the number in the table corresponding to the desired percentage, it has to be divided by the number of rivets and multiplied by the thickness of the plate. The result is the diameter of the rivet hole. This has to be multiplied by the corresponding value in the first column of this table, viz. pitch ÷ rivet diameter, and the product is the pitch of the rivet. For examples see pp. 326, 343. This is only true if the factor of safety is 45. For any other factor F the value N. D÷T has to be multiplied by 4.5 The pitch is limited by the formula P = 1 + C x T, and may not exceed 10". (See p. 362.) CC |