SECT, vii.] STEAM ENGINES. 245 



514. The strength of beam gudgeons may be determined by the rule, 

 P D 2 = 854 a". 1 

 It reduces to, 



30 



and the length should not be less than eight-tenths of the diameter. For low 

 pressure steam, twice its force is 28 Ibs. 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 



Or in the case of low pressure steam, 



D 

 a=: 8- 



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 one-sixth for wear R D z P = 960 a 3 ; * 

 when the shaft revolves in the same time, the piston makes a double stroke, and 

 if the radius R = n D, we have 



a = 



\960 

 for the diameter in inches. 



If it revolve N times while the piston makes a double stroke, then (art. 500.) 

 we have, 



* P 



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 Ibs. 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, 



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. 



