20
MODERN STEAM PRACTICE.
weakened to the extent of the sectional area punched out; andit is found also that the metal between the holes is deterioratedby the process of punching . 1 This deteriorating result was clearlydemonstrated by a series of experiments which took place someyears ago, and in which the strength of almost every descrip-tion of rivetted joints was determined by tearing each directlyasunder. The results obtained from these experiments were con-clusive as regards the relative strength of rivetted joints and thesolid plates. In two different kinds of joints, double and singlerivetted, the strength was found to be in the ratio of ioo for the solidplate, 70 was the strength of a double-rivetted joint after allowingfor the adhesion of the surfaces of the plates, and 56 was the strengthof a single-rivetted joint. These proportions of relative strength ofplates and joints may therefore in practice be safely taken as theStandard value in the construction of vessels required to be steamand water tight, and subjected to pressure varying from 10 lbs. to100 lbs. on the square inch.
The following is the rule for proportions as given by ProfessorRankine: 2 —•
“ Let r denote the radius of a thin hollow cylinder, such as theshell of a high-pressure boiler; t, the thickness of the shell; f thetenacity of the material in lbs. per square inch; p, the intensityof the pressure in lbs. per square inch required to burst the shell.This ought to be taken at SIX TIMES the effective working pressure,
then p = and the proper proportion of thickness to radius is given
by the formula^- = ^.”
“The following formula gives approximately the collapsingpressurep in lbs. on the square inch of a plate iron flue, whose length l,diameter d, and thickness t, are all expressed in the same units of
t' 2 '
measure: p = 9,672,000 -g-
“Tenacity of wrought-iron plates = 51,000 lbs. per square inch.
Tenacity of wrought-iron joints, double rivetted = 35,700 lbs, per square inch.
Tenacity of wrouglit-iron joints, single rivetted = 28,600 lbs. per square inch.”
In the construction of boilers exposed to severe internal pressure,it is desirable to adopt such forms, and so to dispose the material,as to apply the greatest strength in the direction of the greatest