TRANSMISSION OF POWER 



39 



others it is required. In the case of transmission of 

 power by belting it is absolutely necessary. 



58. Coefficient of friction is the 'ratio between the force 

 tending to bring two surfaces into close contact and the 

 force required to slide the surfaces over each other. This 

 force is always greater at the moment sliding begins. 

 Hence it is said that friction of rest is greater than sliding 

 friction. 



The following table of coefficients of friction is given 

 to show the effect of lubrication (Enc. Brit.) : 



Wood on wood, dry 0.25 to 0.5 



soaped 0.2 



Metals on oak, dry , 0.25 to 0.6 



wet 0.24 to 0.26 



soaped 0.2 



' metal, dry 0.15 to 0.2 



wet 0.3 



Smooth surfaces occasionally lubricated.... 0.07 to 0.08 



thoroughly .... 0.03 to 0.036 



FIG. 25 ROLLER BEARINGS AS APPLIED TO A MOWER 



-59. Rolling friction. When a body is rolled over a 

 sr-rface a certain amount of resistance is offered. This 

 resistance is termed rolling friction. Rolling friction is 

 due to a slight compression or indentation of the surfaces 

 under the load, hence is much less with hard surfaces 



