34 CYLINDRICAL AXLE-ARM CH. Ill 



strain. A taper arm can be considerably reduced 

 at its outer end without weakening it and a smaller 



box and hub can be used ; 

 —7 for this reason, in light car- 



riages the taper arm (Fig. 



15) is generally adopted. 

 One disadvantage of it is, 

 that the tightness of the wheel depends upon the 

 thickness of the leather washer behind it ; if this is 

 too thin, the wheel ma)' be so shoved up on the 

 taper as to run too tight ; if it is too thick, the 

 wheel will be too loose and will not run true. 



With a cylindrical arm, a certain amount of end- 

 play can be given by using a thin washer while the 

 wheel will still run perfectly true. Some coachmen 

 think it well to have this end-play (to have the 

 wheel talk, as it is called), thereby avoiding some 

 sidewise shocks from small stones and ruts. A 

 slight end-motion undoubtedly helps to distribute 

 the oil, prevents the arm from wearing in grooves, 

 and is in accordance with the best modern practice 

 in railway and mill work. 



There is a parallel or cylindrical arm made, which, 



to a certain extent, fulfils 

 both conditions, by being ta- 

 pered in a curve at its back 



******* ~*\ 



\J 



> 



l6- end and parallel throughout 



most of its length (Fig. 16). The effects of taper 

 and of parallel arms will be further considered when 

 treating of Wheels. 



