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2 20 STRENGTH OF MATERIALS.
pounds per square inch, with 4 as factor of safety. The ulti-
mate tensile strength of the iron is, by experiments, found to be
20,000 pounds per square inch. ( See Table No. 25 ).
Solution
:
10-inch diameter = 5-inch radius.
1000 X 5
/ = 20000
—4
— — 1000
1000 X 5
5000 — 1000
5000
4000
t = IX inch.
Strength of Flat Cylinder Heads.
The American Machinist, in Question No 147, March 22,
1894, gives the following formula for flat circular heads firmly
fixed to the flange of the cylinder :
t
_ J2 X r2
X P
^ 3 X Si
t = Thickness of cylinder head in inches,
r = Radius of cylinder head in inches.
P = Pressure in pounds per square inch.
.Si = Allowable working stress in the material.
The allowable working stress may be taken as i to T^ of
the ultimate tensile strength and may, for cast iron, be from
1500 to 2500 and for wrought iron from 4000 to 6000. The
above formula was used to calculate the thickness of a cast iron
cylinder head of 30 inches diameter, to resist a pressure of 100
pounds per square inch. This formula is in that case con-
sidered to give sufficient thickness, so that no ribs or braces are
needed.
The above formula may also be used for wrought iron, by
selecting the proper value for Si. Assuming the tensile strength
of wrought iron to be 44,000 pounds, and allowing a factor of
safety of 8, the value of Si for wrought iron will be 5500.
Strength of Dished Cylinder Heads.
The American Machinist, in Question 183, April 12, 1894,
gives the following formula for dished circular heads, firmly
fixed to the flanges of the cylinder
:
t = P * (P 2
+ d2
)
4X SiXd
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