292 APPLIED SCIENCE 



same size as on the rollers in Fig. 146. Teeth were simply 

 added to gear A and corresponding notches cut in gear B. 

 These two gears did roll together without slipping, but as the 

 teeth were short their points soon wore off. To overcome 

 such troubles, the teeth on gear B were made twice as long 



' CIRCLES' 



FIG. 146. FIG. 147. FIG. 148. 



and the corresponding grooves or recesses in gear A were 

 cut twice as deep, as in Fig. 147. Today this is the de- 

 sign of the teeth. Gears are made by casting a blank wheel 

 and then cutting the teeth in the gear according to the above 

 design. 



Note that the inside dotted circles in Fig. 148 are exactly 

 the same size as the rollers in Fig. 146 and are called pitch 

 circles. When the gears turn together they simply roll to- 

 gether on the dotted circles just as they would do if they had 

 no teeth. 



Two gears represent the mechanical principle of the wheel 

 and axle. The large gear represents the wheel and the small 

 gear the axle. The most important part of gearing is the 

 relative movements of the gears and the ratios between their 

 diameters, their teeth, and their speeds. 



When a small gear drives a larger one, the latter will 

 make fewer revolutions in a minute. Just the reverse is 

 true if a large gear drives a smaller one; i.e., the smaller 

 one will make fewer revolutions in a minute. The rate at 

 which a gear revolves is always proportional to the number 

 of its teeth. 



