58 



MECHANICS. 



Fig. 60. 



or of bands (fig. 59). In ordinary practice, however, com- 

 bined wheels are made use of to multiply motion instead 

 of to diminish 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 prevent slip- 

 ping. Bands are chiefly useful where motion is to be 

 transmitted to a distance ; as, for example, from a horse- 

 power without a bam to a thrashing-machine within it. 



Liability -/f sliding is some- 

 times useful, 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 separately and inserted into the wheel, as with 

 wooden wheels. Pinions are the small wheels, or, more 

 properly, teeth set on axles. 



yarm of cogs a, badly formed , 

 b, iwoper foiin. 



FORM OF TEETH OR COGS. 



The form of the teeth has a great influence on the 

 amount of friction among wheel-work. Badly fonned 

 teeth are represented by the wheel-work at a, in the an- 

 nexed figure, consisting of square projecting pins. When 

 these teeth first come into contact with each other, they 



