434 ELECTRICAL MACHINERY 



of the paper being outside. (It is supposed that the 

 brushes have been adjusted for the proper tension of 

 about 1.4 Ibs. per sq.in. of contact surface.) Then with 

 the strip of sandpaper held tightly against the surface of 

 the commutator, it is worked back and forth, grinding 



FIG. 283. Sandpaper Held Improperly and Resultant Shape of 



Brush. 



the under surface of the brush to a shape that just fits the 

 commutator. 



Improper Fitting of Brushes. While the brush is being 

 ground it is extremely important that the sandpaper be so 

 held that it lies tightly against the commutator for at least 

 an inch or two both ahead and behind the brush, otherwise 



FIG. 284. Sandpaper Held Properly and Resultant Shape of Brush. 

 The sandpaper should be allowed to grind the brush only when 

 being pulled in the same direction as that in which the commuta- 

 tor ordinarily runs. 



the brush will be ground in such a fashion that only the 

 center part of it fits on the commutator, the toe and heel 

 having been cut away by the sandpaper. Fig. 283 shows 

 the sandpaper improperly held and the resultant shape of 

 the brush. The surface of such a brush actually touching 

 the commutator surface is very small and, when a load is 



