SECT. VII.] 
STEAM ENGINES. 
245 
514. The strength of beam gudgeons may be determined by the rule, 
PD 2 = 854 a\ 1 
It reduces to, 
and the length should not be less than eight-tenths of the diameter. For low 
pressure steam, twice its force is 28 lbs. per circular, inch, or P = 28, and 
therefore in that case one-sixth of the diameter of the cylinder should be that of 
the gudgeon. For pins for connecting rods where the bearing is double, the 
stress is. reduced one-half, and 
a = 
D Ay/ P 
43 
Or in the case of low pressure steam, 
D 
515. The strength of shafts. The shafts are supposed to be supported so as 
to render the lateral stress as small as possible, then the resistance to twisting 
alone has to be considered, and as no part of the shaft should be less than the 
bearings or journals, therefore allowing onG-sixth for wear R D 2 P = 960 a 3 ; 2 
when the shaft revolves in the same time, the piston makes a double stroke, and 
if the radius R = n D, we have 
a 
for the diameter in inches. 
If it revolve N times while the piston makes a double stroke, then (art. 500.) 
we have, 
For wrought iron the divisor should be 1080 instead of 960. 
Example. What should be the diameter of a shaft of cast iron, when the crank 
arm is equal to the diameter of the cylinder, double the force of the steam in the 
boiler 28 lbs. per circular inch, the piston 30 inches in diameter, and one revolu 
tion of the shaft made in the same time as a double stroke ? In this case n and N 
are each = 1, and, 
a = D ( — ^ i = D (J?*L = 0’31 D, in inches, 
\960 / i960/ 
or a — 0’31 x 30 = 9‘3 inches. 
1 Essay on Strength of Iron, art. 139. 
2 Essay on Strength of Iron, art. 224. R being in this case in inches.