GE BC-375-E Preliminary Instructions page 16

transmitter back cover is removed and lead (43)
is resoldered.
d. Dynamotor Unit PE-73-C
(1). BRUSHES: The brushes can be removed
by unscrewing the slotted ,brush cap on each side of
the bearing bracket. It is recommended that each
brush be suitably marked to indicate which brush
holder it came .from and its relative position in that
brush holder in order that the brushes may later be
replaced in their original positions. THIS IS IM-
PORTANT. Blow out dust and clean all foreign
matter from each brush holder and brush and make
certain that the brushes slide freely in their brush
holders, dressing the brushes with fine sandpaper, or
a file, if necessary. Any brush which has worn down
to a length of less than 3 A in. (measured from contact
surface to near end of spring) should be replaced
with new one. In installing new brushes it may be
necessary to "sand-in " the brush in order to make
its contact surface fit the contour of the commu-
tator. The brush must slide freely in its holder.
"Sanding-in" may be accomplished by using a
strip of No. 00 sandpaper about 5 in. by 1 in. for
L.V. brushes and S in. by
N
in. for H.V. brushes.
Wrap the strip of sandpaper around the commutator
with the sand surface out. Insert the brushes in the
brush holders and replace the brush caps so that
the brushes are pressed tightly against the sandpaper.
Holding the ends of the sandpaper so as to stretch
it tightly against the commutator, rotate the armature
back and forth until the full width of the brush face
is making contact against the sandpaper as indicated
by the sanding marks or scratches on the contact
surface of the brush when it is withdrawn. Sand the
sides of the brush, if necessary, for a free fit in the
holder.
No sanding should be necessary to secure a good
brush fit with the original brushes if they are replaced
correctly. Never apply oil, grease or any other lubri-
cant to the brush, commutator or brush holder.
Under normal conditions of operation, it is estimated
that the useful life of brushes is 2000 hours for low-
voltage and 10,000 hours for high-voltage brushes.
(2). COMMUTATORS: Both commutators should
be wiped with a clean, Tintless cloth. Any scum appear-
ing on the low-voltage commutator should be removed
by moistening the cloth in carbon tetrachloride.
The normal black or dark brown polished surface
on the high-voltage commutator should not be
removed. The type of brush material used in the
low-voltage brushes of the dynamotor unit may,
under certain conditions of installation, cause a
discoloration of the low-voltage commutator. Under
these circumstances the commutator will have a
mottled appearance which is caused by the formation
of a very thin oxide film on the surface of the commu-
tator bars. This film is normal and is not injurious
to the commutator or brushes. Do not try to remove
the oxide film described above. A rough or pitted
commutator should be smoothed with No. 00 or finer
sandpaper. Never use emery cloth or a file. Under
normal conditions of operation the low-voltage and
high-voltage commutators should not require turning
down before 2000 hours and 10,000 hours of service
respectively. However, if the commutator bars have
worn down flush with the mica, the armature should
be removed from the dynamotor to turn down the
commutator face and undercut the mica between bars.
(3). BEARINGS: If the armature does not spin
freely when rotated by hand with the brushes re-
moved the following may be the cause:
(a) Dirt or other foreign matter in a bearing.
(b) Defective ball bearing (cracked race, chipped
or flattened ball) .
(c) Grease in bearing has become hard and
gummy due to oxidation during long periods
without actual use.
In any case, the armature should be removed from
the frame as described in paragraph 22d (4) and
the bearings thoroughly cleaned. If, after cleaning,
the outer race will not spin smoothly, it is probably
due to a cracked race or chipped or flattened ball,
and the defective bearing should be removed and
replaced with a new one. Always use a bearing puller
to remove a defective bearing and never hammer
or pry the bearing off since this may bend the shaft
and injure the commutator. A new bearing should be
pressed on the shaft until the inner race of the bearing
rests against the shoulder on the shaft. For this
operation always use an arbor press and a metal
cylinder or collar which bears only against the inner
race of the bearing. Care must be taken that no force
or stress is placed on the outer race of either bearing
at any time since this will usually result in a damaged
bearing.
(4). DISASSEMBLY: The following procedure
is recommended for disassembling the dynamotor
unit for cleaning and repacking the bearings:
(a) Remove the three "safety-wired " screws
on each end of the dynamotor unit and take off the
two end bells.
(b) Unscrew the slotted brush cap in each brush
holder and remove all four brushes. Take special
care to mark the position of the brushes in their
brush holders such that they may later be replaced
in their exact original positions.
(c) Unscrew the four slotted screws located
around the rim of the low-voltage bearing bracket
and pry the bracket loose from the frame. The
low-voltage end of the dynamotor unit can be.
identified by the long, small diameter commutator
and the large copper-graphite brushes.
(d) Detach the two field leads from the terminal
clamps on the low-voltage brush holders by un-
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