Chapter 10 — SAFETY; TEST EQUIPMENT; TEST METHODS 



to slide up and down In the holders so that they 

 may foUow irregularities in the surface of the 

 commutator. A flexible bralded-copper conductor, 

 commonly called a pigtail, comiects each brush 

 to the external circuit. A spring forces each 

 brush to bear on the comi-nutator with from 1 1/2 

 to 10 pounds of pressure for every square inch of 

 brush surface riding on the commutator. These 

 springs ordinarily are mounted so that brush 

 pressure Is adjustable. 



Brushes should be inspected periodically to see 

 that proper spring tension is applied, that the 

 brashes ride free in the brush holders, that the 

 brushes are of adequate length, and that they 

 make proper m.ixlmimi contact with tlie commuta- 

 tor. 



If brushes become worn to an extent where 

 length Is reduced considerably, the ability of tha 

 spring to exert proper pressure is affected. Fail- 

 ure of the brush to ride free in the holder has the 

 same result. When these conditions occur, brush 

 replacement is necessary. 



To replace a brush, first secure all power to 

 the generator, wait until the shaft has stopped, 

 remove the defective brash, then place the new 

 brush in the holder. Never attempt to replace a 



brush on a generator while the armature still Is 

 turning. Residual magnetism may cause a genera- 

 tor output, even though all power is secured to the 

 un.'t. If the brush does not slide freely in the 

 holder, sand its sides with a medium-coarse 

 grade of sandpaper until the sides miike a close 

 but free fit. 



After the brush is fitted in the holder, fit 

 it to the commutator by sanding it with a medium- 

 coarse grade of sandpaper. Sand in the direction 

 of commutator rotation by placing the sandpaper 

 strip between the brush contact surface and the 

 commutator, with the rough side of the sandpaper 

 toward the brush. 



Apply pressure to the brush and pull the sand- 

 paper through the contact area. Repeat this 

 process until the brush face makes a good fit with 

 the commutator curvature. 



After all brushes are replaced in this manner, 

 blow the carbon dust away from the commutator 

 and toward the outside of the generator. 



After replacement of brashes and cleaning, 

 power may be restored and the generator turned 

 on. Watch the brushes for proper operation and 

 possible excessive spariung. If sparking is exces- 

 sive, some additional fitting may be required. 



175 



