TRANSMISSION OF POWER 



293 



FIG. 149. Spur Gear. 



As there are driver and driven pulleys, so there are driver 

 and driven gears. The driver gear and the driven gear 

 may be distinguished by the following characteristics: The 

 teeth of the former are bright or 

 worn on the front side that is, the 

 side which faces in the direction of 

 the motion of the gear; the teeth 

 of the latter are worn on the side 

 opposite from the direction of 

 motion. 



Since gears are simply pulleys 

 with teeth on them, the principles underlying pulleys apply 

 to gears. When the teeth of two gears interlock they are 

 said to mesh. 



336. Types of Gears. Of the different types of gears in 

 use the principal ones are the spur (Fig. 149) , the bevel (Fig. 



150), and the worm (Fig. 151) 

 gears. Spur gears are wheels 

 with the teeth or cogs arranged 

 round the outer or inner sur- 

 faces of the rim, in the direction 

 of radii from the center, and their 

 action may be regarded as that of 

 two cylinders, rolling one upon the 

 other. Bevel gears are wheels the 

 teeth of which are placed upon 

 the outer periphery (circumfer- 

 ence) in a direction converging 

 to the apex of a circle and their 

 action is similar to that of two cones rolling upon each other. 

 When two bevel wheels of the same diameter work together 



FIG. 150. Bevel Gear. 



