Figure 3-10.--Measuring brush tension.

dc generator brush holder and brush-rigging

have spring-enclosed shunts and are worn to 40

assembly.

percent or less of the original length of the

brush (not including the brush head, which fits

Brushes are manufactured in different grades to

into one end of the spring).

meet the requirements of the varied types of service.

The properties of resistance, ampere-carrying

Where adjustable brush springs are of the positive

capacity, coefficient of friction, and hardness of the

gradient (torsion, tension, or compression) type,

brush are determined by the maximum allowable

adjust them as the brushes wear to keep the brush

speed and load of the machine.

pressure approximately constant. Springs of the

coiled-band, constant-pressure type, and certain

CORRECT BRUSH TYPE.-- The correct grade

springs of the positive-gradient type are not adjustable

of brush and correct brush adjustment are necessary to

except by changing springs. Adjust pressure following

avoid commutation trouble. For good commutation,

the manufacturer's technical manual. Pressures as low

use the grade of brush shown on the drawing or in the

as 1 1/2 psi of contact area may be specified for large

technical manual applicable to the machine, except

machines and as high as 8 psi of contact area may be

where NAVSEA instructions issued after the date of

specified for small machines. Where technical

the drawing or technical manual (such as the

manuals are not available, a pressure of 2 to 2 1/2 psi

instruction for brushes to be used in electrical

of contact area is recommended for integral

propulsion and magnetic minesweeping equipment)

horsepower and integral kilowatt machines. About

state otherwise. In such cases, follow the NAVSEA

twice that pressure is recommended for fractional

instructions. In the case of propulsion and magnetic

horsepower and fractional kilowatt machines. To

minesweeping equipment, only one grade of each of

measure the pressure of brushes operating in box-type

two different brush manufacturers is permitted for any

brush holders, insert one end of a strip of paper

machine. This restriction on brush interchangeability

between the brush and commutator; use a small brush

is based on the vital nature of the machines involved

tension gauge (such as the 0- to 5-pound indicating

and on the impracticability of factory testing these

scale) to exert a pull on the brush in the direction of

machines while operating them with several

brush holder axis, as shown in figure 3-10. Note the

manufacturers' grades, which have been qualified

reading of the gauge when the pull is just sufficient to

under any one of the six military grades. Never mix

release the strip of paper so that it can be pulled out

different types or grades of brushes from the same

manufacturer. Rapid brush wear could result from a

mismatch of brushes.

CARE OF BRUSHES.-- All brush shunts should

be securely connected to the brushes and the brush

holders. Brushes should move freely in their holders,

but they should not be loose enough to vibrate in the

holder. Before replacing a worn brush with a new one,

clean all dirt and other foreign material from the brush

holder.

Replace with new brushes, all brushes that

are worn or chipped so they will not move

properly in their holders;

have damaged shunts, shunt connections, or

hammer clips;

have riveted connections or hammer clips and

are worn to within one-eighth inch of the me-

tallic part;

have tamped connections without hammer clips

and are worn to one-half or less of the original

length of the brush; or

Figure 3-10.--Measuring brush tension.

3-18