80 



MECHANICS, 



it, familiar instances of which occur in the grist-mill 

 and thrashing-machine. 



In connecting a system of wheels, the cord or strap 

 may be used where great force is not required, the 

 friction round the circumference being sufficient to pre- 

 vent slipping. Bands are chiefly useful where motion 

 is to be transmitted to a distance ; as, for example, 

 from a horse-power without a barn to a thrashing-ma- 

 chine within it. Liability of sliding is sometimes use- 

 ful, by preventing the machinery from breaking when 

 a sudden obstruction occurs. Where the force is great, 

 the necessary tension or tightness of the cord produces 

 too great a friction at the axle. In such cases, cogs or 

 teeth must be resorted to. 



The term teeth is usually applied when they are 

 formed of the same piece as the wheel, as in the case 

 of cast-iron wheels. Cogs are teeth formed separate- 

 ly and inserted into the wheel, as with wooden wheels. 

 Pinions are the small wheels, or, more properly, teeth 

 set on axles. 



Fig. 60. 



Form of cogs — a, bad/i/ 

 formed; b, proper Jo: 



FORM OF TEETH OR COGS. 



The form of the teeth 

 has a great influence on 

 the amount of friction 

 among wheel- work. Bad- 

 ly-formed teeth are repre- 

 sented by the wheel- work 

 at a, in the annexed fig- 

 ure, consisting of square 

 projecting pins. Wlien 

 these teeth first come into 



