FORM OF TEETH OR COGS 59 



act obliquely together, and thus a part of their force is lost, 

 and they continue scraping together with a large amount 

 of friction so long as they remain in contact. These 

 effects are avoided by giving to them the curved form, 

 represented by h. Here, instead of pressing each other 

 obliquely, they act at riglit angles, that is, not obliquely, 

 and instead of scraping, they roll over each other with 

 ease. These curves are as- ig. 6i. 



certained by mathemat- ^-'"'~~><'' '\~V'^^ ^^-^ 



ical calculation, which "7'~~^-^^/ ^j( V""' \ 



can not be here given ; ' /^\ _ _^^ / \ ' 



it may be enough to state -'""^^ "^B ^H- * " 



in contact shall always ,^ i ^*^^^^ 



work at right angles to j 



each other. For ordi- Mode of giving the best form to cogs. 



nary practical purposes, however, they may be made 

 as shown in the annexed figure (fig. 61), by striking 

 circles whose diameters shall embrace just three teeth. 

 The points of the teeth thus formed are removed, leaving 

 a blunt extremity, according to the figure. 



There are a few other rules that should always be ob- 

 served in constructing wheel- work, in order that the 

 wheels may run easily together, without jerking o;* rat- 

 tling, the most important of which are the following : 



1. The teeth must be of uniform size and distance from 

 each other, through the whole circumference of the wheel. 



2. Any tooth must begin to act at the same instant 

 that the preceding tooth ceases to touch its corresponding 

 tooth on the other wheel. 



3. There must be sufficient space between the teeth not 

 only to admit those of the other wheel, but to allow a cer- 

 tain degree of play, which should bo equal to at least one- 

 tenth of the thickness of the teeth. 



4. The pinions should not be very small, unless the 



