SECT. VII.] 
STEAM ENGINES. 
243 
For wrought iron use 240 instead of 212, and for wood 64. 
509. Example 1. Beams. An engine beam is 3 times the diameter of the 
cylinder, from the centre to the point where the piston rod acts on it, the force of 
the steam in the boiler is 14 lbs. per circular inch, its double is 28, and the 
diameter of the piston is 24 inches. In this case, 
or a — 19*4 inches, and the mean breadth is 1*21 inches, and the breadth at top 
and bottom 2*16 inches. 
If of wrought iron with the same proportions, '78 D = #, and the breadth = one- 
sixteenth of the depth. 
Of wood with the same proportions, *78 D = ¿7, but the breadth = one-fourth of 
the depth. 
510. Cranks. A crank should embrace a shaft, so that its depth at the shaft 
should be 1*5 times the diameter of the shaft; hence, if S D be the diameter of 
the shaft, the depth of the crank must be 1*5 S D, but since (art. 508.) D 2 P l = 
212 b ar, we have 
P l 
b = 2-25 S 2 x 212 = 477 S 2 
Example 2. A crank shaft is equal in diameter to *31 times the diameter of 
the cylinder, and the force of the steam in the boiler being 14 lbs. per circular 
inch, or P = 28 lbs., required the breadth of the crank at the shaft, its radius 
being 2*5 feet. In this case, 
and as / = 2*5 feet, it is '6 x 2*5 = 1*5 inches, and the depth is 1'5 x '31 x 30 
= 14 inches. 
511. Wheel arms. The arms of wheels may be considered in respect to 
strength only; and if the rim be of equal strength, then a wheel should have six 
arms in all cases, when it is of sufficient magnitude to require its strength to be 
found by rule. With this condition we have 2 D 2 P R = 212 x 6 b or, or 
D 2 P R = 3 x 212 b a 1 . If we consider the arms to be one-third of the breadth oi 
the wheel, and allow, as excess of strength, that which is added to give it lateral 
strength, then 
When R the radius = 1, and P = 28 = twice the force of the steam in the 
boiler, then 
D 
a = 
s/lb 
as inserted along with the proportions of teeth in the table, art. 513.