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 Ibs. 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 = a, and the breadth = one- 

 sixteenth of the depth. . 



Of wood with the same proportions, '78 D = a, 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 / = 



212 b a*, we have 



Pi _ PI 

 b ~ 2-25 S* x 212 "" 477 S* 



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 Ibs. per circular 

 inch, or P = 28 Ibs., 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 a z , or 

 D* P R = 3 x 212 b a*. If we consider the arms to be one-third of the breadth of 

 the wheel, and allow, as excess of strength, that which is added to give it lateral 



strength, then 



D'PR 2 _ ^/ P R 



212 fl2 ' 212T 



When R the radius = 1, and P = 28 = twice the force of the steam in the 



boiler, then 



D 



Q -^- 



as inserted along with the proportions of teeth in the table, art. 513. 



