296 



APPLIED SCIENCE 



diameter of the pitch circle and not the diameter outside of 

 the teeth, it is often hard to measure it exactly. For this 

 reason gears are usually classified according to the number of 

 teeth. As we can count the teeth we can get a more exact 

 answer when figuring their speeds than if we figured from 

 pitch circles. 



339. Ratio of Gears. Suppose we have two shafts, Dand F, as 

 shown in Fig. 153 and that we want to connect these shafts by gears 

 so that shaft D will make one revolution while shaft F makes two. 



In order to do this we 



"^v must place a gear on 



shaft D having twice 

 the number of teeth 

 of the gear on shaft F. 

 If we put a gear on D 

 with 24 teeth, the gear 

 on F will then have 12 

 teeth, or half as many, 

 and each time the gear 

 on D turns around 

 once the gear on F 

 will turn twice; that 

 is, the 24 teeth on 



gear D will have to turn gear F twice in order to mesh with 12 

 teeth on F. 



The relation of the speed of F to the speed of D is 2 to 1. This is 

 called the ratio of the gearing. We can now write the ratios be- 

 tween the speeds and the number of teeth in the form of a proportion 

 thus: 24 : 12 =2:1, that is, the number of teeth on gear D is to the 

 number of teeth on gear F as the speed of F is to the speed of D. 



340. Direction of Gears. The number of turns or revolu- 

 tions which a gear makes is always proportional to the number of 

 its teeth. It makes no difference how many gears there are 

 :n a train, the gears between the first and last gear have 



FIG. 153. Ratio of Gears. 



