90 



MECirA>JICS. 



Prictixrn-wheels. 



Ficf. 99. 



the friction of axles in certain cases. By this contrivance, 

 the axle, a (fig. 98), instead of revolving in a simple hole 

 or cavity, rests on or Letween the edges 

 of tvv^o other Avheels. As the axle re- 

 volves, the edges turn with it, and the 

 rubbing of surfaces is only at the axles 

 of these two wheels. If, therefore, these 

 axles be twenty times smaller than the wheels, the friction 

 will be only one-twentieth the amount Avithout tliem. 

 This contrivance lias 

 been strongly recom- 

 mended and con- 

 siderably used for 

 the cranks of grind- 

 stones (fig. 99), but 

 it was not found to 

 answer the intended 

 pui-pose so well as 

 was expected, for 

 the very plain reason '"' 

 that, in using a 

 grindstone, nearly all the friction is at the circumference, 

 or between the stone and the tool, v*^hich friction-wheels 

 could not, of course, remove. 



Grindstone on Friction-wheels. 



LUBKICATING SUBSTANCES. 



Lubricating substances, as oil, lard, and tallow, applied 

 to rubbing surfaces, greatly lesson the amount of friction, 

 partly by filling the minute cavities, and partly by sepa- 

 rating the surfaces. In ordinary cases, or where the 

 machinery is simple, those substances are best for this 

 purpose which keep their places best. Finely-powdered 

 black-lead, mixed with lard, is for this reason better for 

 greasing carriage wheels than some other applications. 

 Drying oils, as linseed, soon become stifl:' by drying, and 



