TRANSMISSION OF POWER 297 



nothing to do with the speed of either of these two. That is 

 to say, the ratio between the speeds of the first and last gear 

 is not changed by put- 

 ting any number of gears 

 between them. The con- 

 tinued product of the 

 revolutions of the first 

 driver and the teeth of all 

 the driving gears is equal FlG 154 .__ Train of Gears . 



to the continued product 



of the revolutions of the last follower and the teeth of all 

 the driven gears. The formula for this is RDd=rFf. This 

 principle is true for any number of driving and driven gears. 

 The position of a driver does not affect the speed of the last 

 follower. Thus, either driver in Fig. 154 can be placed at D 

 or at d. Either follower can go on at F or at / without 

 affecting the speed of the last follower. 



341. Gearing Terms. There are certain terms relating 

 to gears with which the mechanic should be familiar. Some 

 of the most important of these are explained below. (See 

 Fig. 155.) 



Spur. Spur originally meant a projection or tooth, but is 

 now used to distinguish spur gears from other varieties of 

 gears, such as bevel gears and worm gears. 



Pitch Circle. The pitch circle of a gear is the distance 

 around the teeth and is the same size as the friction rollers or 

 cylinders would be if no teeth were present: i.e., when two 

 spur gears roll together their pitch circles are considered 

 to be constantly in contact. 



Pitch Diameter. The pitch diameter of a gear is the 

 diameter of the pitch circle. 



