CRBAMBRY MECHANICS 295 



7rd2 

 Area of piston = —^' 



Example : 



P = 4o lbs. 



1 = 2 ft. 



a = 20 sq. inches. 



n = 400. 



40 X 2 X 20 X 400 = 640,000 

 640,000 -J- 33,000 = 19.4 = H. P. 



CALCULATING SIZE AND SPE:ED OF PULLEYS. 



In creameries where new shafting and new machinery 

 are being put up, it is important to know how to determine 

 the required speed of the shafting as well as the speed 

 and size of the pulleys. This calculation is not difficult 

 when we remember the following rule: 



The speed varies inversely zvith the diameter of the 

 pidley. Thus, with the same speed of the engine, the 

 speed of the main shaft becomes less as the diameter of 

 the pulley on that shaft is increased. 



It must be remembered, also, that in a creamery where 

 the churn and separators are run directly from the main 

 shaft, the speed of this shaft must be fixed at from 175 to 

 200 revolutions per minute in order to permit the use of 

 suitable sized pulleys. 



We usually speak of two kinds of pulleys: the drive 

 pulley and the driven pulley. Where the engine drives the 

 main shaft the pulley on the engine is called the drive 

 pulley and that on the main shaft the driven pulley. When 

 we refer to the main shaft driving the intermediate, then 

 the pulley on the main shaft becomes the driver and that 

 on the intermediate the driven pulley. 



In creameries there are two problems that present them- 

 selves with respect to pulleys : one is to find the speed of 



