PRINCIPLES OF MACHINES 



473 



transferred to reciprocat- 

 ing by a pitman rod, as 

 in the windmill and pump 

 or mowing machine , where 

 the sickle runs back and 

 forth, although the power 

 is generated from the cir- 

 cular motion of the main 

 wheels. 



Ratchet drive. The 

 ratchet drive (Fig. 365) 

 gives an intermittent 

 motion. Teeth are cut 

 on either the interior or 

 the exterior rim of a wheel, 

 and pawls engage these 

 teeth and cause motion 

 when turned in one di- 

 rection, but disengage 

 and prevent motion-when 

 turned in the opposite 

 direction. 



Cams. A cam (Fig. 366) 

 is a timing device used for 

 transmitting intermittent 

 motion. Cams are set on 

 a revolving shaft, and the 

 rims, to a greater or less 

 extent, are eccentric. As 

 the shaft revolves, the 

 cam presses on the "fol- 

 lower," causing motion. 

 The follower is pressed 

 against the cam by grav- 

 ity or the action of a 

 spring. 



31 



After International Harvester Co. 



Fig. 365. Ratchet driving device 



Cam 

 Shaft 



Fig. 366. A cam with follower and guide 



♦ 



