288 



THE HORSE IN SERVICE 



Iron tires — Walk three miles per hour .require traction of 48 lb. per ton 

 Iron tires — Trot GVa miles per hour. . . .require traction of 59 lb. per ton 

 Iron tires — Fast trot 9.4 miles per hour .require traction of 77 lb. per ton 



Pneumatic tires — Walk require traction of 48 lb. per ton 



Pnemuatic tires — At other paces requii'e traction of 50 lb. per ton 



Bearings. — Rubbing friction between the axle and tlie box of 

 the hub with which it is in contact is controlled by lubrication 

 and special bearings of rollers, cones or balls, in place of the 

 simple friction bearing. 



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Fig. 155. — The cylindrical axle arm. 



Fig. 156. — The tapered axle arm. 



Axles are either cylindrical or tapered. The cylindrical 

 axle arm (Fig. 155) insures a truer bearing and easier imnning, 

 in case the road is level from side to side, the tire at right angles 

 to the face of the wheel and the arm itself horizontal. There are 

 many reasons for modifying these conditions prerequisite to the 

 best results from tlie cylindrical axle, however, which render the 

 tapered axle better adapted to common use. The tapered arm 



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Fig. 1571-^— Tapered axle arms pitched. 



Fig. 158. — An unpitched tapered axle. 



(Fig. 15G) is stronger for a given weight, since it is heaviest at 

 the point of greatest strain, the shoulder. With the tapered axle 

 it is a much simpler matter to maintain a good fit between axle 

 and box by the use of leather washers placed between the wheel 

 and the shoulder of the axle than in the case of the cylindrical 

 axle, in which no subsequent adjustment is possible. For tliis 

 reason the tapered axle is much more easily constructed. Axle 

 anus are so pitched (Fig. 157) as to keep the wheel running 



