Locomotive Boiler Design.The design of locomotive boilers and engines is a very deep subjec-one requiring much thought and study. Limited space prevents going into a discussion of the reasons for the adoption of different designs. The following formulae for the calculation of thickness of plates, spacing of rivets, etc., are given. Some of these formulae, while being semi-empirical, are based on theoretical assumptions and represent modern practice in the design of parts mentioned. In figuring the thickness of the boiler shell, the following formula is given:

The following standard thicknesses of plates are used in locomotive boiler construction: Crown sheet, side sheet, and back fire-box sheet, 3/8 inch in thickness; for boiler pressures not exceeding 200 pounds, the boiler head, roof, sides, and dome, 1/2 inch thick, while for boilers with steam pressures between 200 and 240 pounds, these plates are 9/16 inch thick.

In designing the riveted joints, their strength must be considered from several different standpoints. It must be sufficiently strong to withstand the tensional stress on the metal contained in the plate between the rivets. The plates must be of such thickness as will safely carry the compressional stresses behind the rivets and the rivets must be placed in rows sufficiently far apart and far enough from the edge of the plate to insure against shearing or tearing out of the metal. In the formulae for the design of a riveted joint, the following notation will be used:

d= diameter of rivet hole in inches

p= pitch or distance in inches between center to center of rivets

t= thickness of plate in inches

h= distance in inches from edge of plate to center of first rivet hole

T= tensile strength of plate in pounds per square inch, usually taken as 55,000

S= shearing strength of rivets in pounds per square inch, usually taken as 55,000

R= shearing strength of plate in pounds per square inch, usually taken as 45,000 ponds per square inch

C= crushing strength of plate in pounds per square inch, usually taken as 50,000 pounds per square inch

f= factor of safety usually taken not less than 4-1/2

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