e. In addition to the main shunt field, an auxiliary
c. Brush thickness is very important and the
clearance of a new brush in the holder should be
field and a series field are mounted on the main field
approximately 0.004 to 0.010 of an inch.
If the
poles.
clearance is much less than the minimum the carbon
f. The auxiliary field is energized from a battery
will tend to stick in the box and if over 0.025 of an inch
source, to provide separate excitation for the generator,
the brush will rattle wear away its sides and chip or
thus assuring more rapid and positive buildup of
break. The width of a brush is not so important as it
generator voltage and improving the voltage regulation
may have as much as one-sixteenth of an inch
for application to this particular class of service.
clearance in the box without causing trouble.
g. The series field is supplied to be available for
d. Brush pressure on the commutator is
starting service in those cases where the generator is
maintained by means of flat clock-type springs which
used as a motor when starting the engine.
exert pressure on fingers which rest on top of the
brushes. Brush pressure can be measured by attaching
67. Cleaning
a small spring balance to each pressure finger directly
over the brush and pulling straight up in line with the
generator free from dust by occasionally blowing it out
brush travel until the finger is just lifted clear of the
with dry compressed air or a hand bellows.
brush.
If high air pressure is used do not hold the nozzle of the
e. If the brush pressure is incorrect, remove the
air hose too close to the windings as the pressure may
brushholder and adjust as follows:
damage the insulation tape.
(1) Insert a piece of one-eighth of an inch
b. Wipe as much as possible of grease, dirt, or oil
diameter drill rod in one of the holes in the
from the windings with a dry cloth. Cleanse with rags
tension barrel for the spring requiring
moistened with solvent. Dry the windings thoroughly
adjustment and relieve the tension on the
with clean, dry cloths. Parts which are inaccessible to
cotter pin.
the hand, may be cleaned with a swab moistened with a
(2) Remove the cotter pin, increase or
solvent.
decrease the tension as required, and
replace the cotter pin.
68. Bearings
f. Keep the under side of the brushholder carbon
a. The generator is equipped with a single bearing
box within one-eighth of an inch of the commutator to
at the commutator end. Refer to paragraphs 37 through
reduce chances of brush breakage. Adjustment can be
made by loosening the clamping block bolt and moving
b. At periods ranging from one to three years, take
the brushholder to the correct position. A fibre sheet of
the outer bearing cap off and remove the old grease
the proper thickness makes a handy gauge.
from the bearing and grease chamber by a wash
consisting of equal parts of kerosene and denatured
70. Commutator
alcohol.
Remove all caked or hardened grease.
a. A commutator which is taking on a polish and
Remove the cleansing liquid by washing with a hot, light
shows no signs of wear, does not require an} attention.
oil before repacking the bearing with new grease.
b. If the commutator is roughened, badly worn.
or burned, remove the armature and place in a lathe,
69. Brushholder and Brushes
then turn, grind or stone to give a uniform surface, after
a. The brushholder and brushes can be inspected
which the mica must be re-grooved with special saws.
by removing the commutator cover. Inspect at least
once a month to be sure that the presence of grit or dirt
c. The mica insulation between the copper
has not caused any of the brushes to stick in the
segments is grooved or undercut initially to a depth of
brushholders and that the proper spring tension is
3/64-inch; as the commutator wears, or if it is turned or
maintained at the value specified in paragraph 65.
ground, the grooves should be maintained since high
mica will spoil the brush fit and
b. Discard brushes when they have worn in length
to such an extent that the brush pressure finger will
come within one-eighth of an inch of the end of its travel
by the time of the next inspection.
27
