242 
OF THE PARTS OF 
[sect. VII. 
inch, and the diameter of the cylinder 11 inches, the diameter of a piston rod of 
wrought iron should be, 
n .. x yjg = 2-34 inches. 
45 
If the rod be of steel, then the diameter should be, 
11 x = 1-46 inches. 
72 
Example 3. The force of the steam being 16 lbs. per circular inch, and the 
diameter of the piston 24 inches, the diameter of a cast iron connecting rod should 
not be less than, 
24 ^y 3 ' 2 = 3-23 inches. 
42 
The middle is commonly expanded into a form of greater lateral strength ; and 
in all cases should be of larger diameter than the ends, in the proportion of about 
one-tenth. 
507. For air pump rods, the pressure of the atmosphere and the diameter of 
the pump must be taken, instead of the force of the steam and the diameter of the 
cylinder. Parallel motion rods should be three-sevenths of the diameter of the 
piston rod, except in the case of that for steam boat engines, when there is lateral 
stress. Connecting rods for giving motion from the cross-head to beams, or to 
cranks, should be seven-tenths of the diameter of the piston rod. 
508. Of the strength of arms of beams, cranks, &c. It may be assumed as 
a principle, that a beam of uniform thickness should not be of less thickness than one- 
sixteenth of its depth, otherwise it is liable to overturn ; besides, in cast iron it is 
not safe to trust the strength of a casting, which is not a sixteenth part of its depth 
in thickness. Now for the case, when the velocity is the same as that of the piston 
D 2 P l = 212 b a 2 , 1 and when 16 b = a, and 12 / = n D, it becomes for cast iron 
That is, when D = the diameter of the piston in inches, and cl = the depth of the 
beam in inches, the breadth one-sixteenth of that depth, n the number of times the 
diameter is contained in the length from the centre of motion to the point where 
the force is applied, and P double the force of the steam in the boiler, in lbs. per 
circular inch. The depth at the end should be half the depth at the centre of 
motion, and the breadth uniform; and an access of strength may be given by 
forming the section, so as to increase the thickness at the edges to one-ninth of the 
depth, or till the parts between be reduced to the thickness of one-sixteenth of their 
width. 
1 Practical Essay on Strength of Iron, art. 116.