WAGONS, BUGGIES, AND SLEDS 



249 



357. Axle friction is the resistance of the wheel turn- 

 ing about its axle similar to the resistance of a journal 

 turning in its bearing, independent of the other elements 

 of draft. Axle friction is usually a small part of the total 

 draft. The power required to overcome it diminishes 

 as the ratio between the diameters of the wheel and axle 

 increases. Thus in Fig. 183 if R be the radius of the 

 wheel, r the radius of the axle, from the principle of the 

 wheel and axle 



Power : Axle friction : : r : R 

 p ower= AxlefncHpn 

 R/r 



In the standard farm wagon R/r has a value of from 1 1 

 to 20, or an average of about 15. 



Morin found in his experiments, which have been con- 

 sidered a standard for 

 years, that with cast-iron 

 axles in cast-iron bearings 

 lubricated with lard, oil of 

 olives or tallow gave a co- 

 efficient of friction of 0.07 

 to 0.08 when the lubrica- 

 tion was renewed in the 

 usual way. Assuming 0.08 

 to be the coefficient of fric- 

 tion and 15 to be the ratio 

 between wheel and axle 

 diameters, the force re- 

 quired per ton to overcome friction would be between 

 10 and ii pounds. Another authority* states that the 

 tractive power required to overcome axle friction in a 

 truck wagon which has medium-sized wheels and axles is 

 about 3J/2 to 4 l /2 pounds a ton. The use of ball and roller 



*I. O. Baker, "Roads and Pavements." 



FIG. 



