BEARINGS
Clean
off
any trace
of
impurities,
such
as grease,
carbon dust, oil, etc. Check
bushing
wear and
replace them
if
necessary,
considering
that
an
excessive radial play is extremely dangerous as
it
might
cause armature
friction
against the pole
shoes.
If necessary, replace brushes using new parts all
brushes
must
be replaced, even
if
only
one
among them
is
worn
or
defective. The replace-
ment however,
is
preceded by a brush
holder
check, regarding both mechanical features (out
of
shape) and
the
insulation,
or
lack
of
insula-
tion. Check the brush springs vs. the tabulated
data. Replace,
if
functionally
defective, any
of
the seals.
ARMATURE
Armature
must
undergo both mechanical
checks
and
electrical
testing. The
major
mechanical
check
concerns
the armature out-of-round, using
a dial
indicator
gauge. If the
eccentric
of
.0008"-.0012" (0.02-0.03 mm) is exceeded, (Fig.
C.lll/8). turn the
collector
down on the lathe, be-
ing careful to remove as
little
copper as
possible
to
eliminate
the trouble. If the bars are loose the
complete
armature
must
be replaced.
The max.
permissible
depth of
cut
is given in the
table of data. Once the
collector
is turned down,
undercut
the
mica
with
a tool (Fig.
0.11/7)
on a
depth
of
.020"
to
.040"
(0.5-1
mm) depending on
collector
size.
After
undercutting
the mica,
polish
the
collector
surface
with
0 grade sand
paper and then
blow
all residues
off
with
com-
pressed air.
The armature is
electrically
tested
to
make sure
that the armature
winding
and the
collector
are
insulated from the armature
housing
(counter-
ground test); no winding,
conductor
or
collector
FIG.
0.11/6
CHECKING COLLECTOR EXCEN·
TRICITY USING A DIAL INDICATOR GAUGE
(Out·of-round
not
to
exceed
.0008"·.0012"
(0.02·0.03 mm)
bar is shorted (short test); there is no electrical
discontinuity
throughout
the system.
In
case of
any
insulating
defects, shorts
or
interruptions,
the armature
must
be replaced.
Insulation
is
checked by inserting the armature
in an a.c.
circuit,
and a lamp connected in series.
Any ground
(which
closes the
circuit)
is shown
by
the
test
lamp
(L, Fig.
0.11/8).
FIG. D.ll/7 CHECKING THE MICA
OF
THE COL·
LECTOR
A. Grooving tool.
• • •
.
---~
-
FIG. D.ll/8 CHECKING THE INSULATION
TO
GROUND OF THE ARMATURE WINDING WITH
A TEST LAMP
(If any
grounding
occurs
the
test
lamp L
lights)
'
Shorts
are
found
by means
of
the armature
tester
(Fig,.
0.1119).
The armature is placed on the tester inserted in
the
a.c.
circuit,
then
it
is
slowly
rotated around
its
axis
while
a steel blade
is
kept in contact over
the
armature housing. If there
is
a short, the
blade vibrates as
it
gets
over the recess housing
of
the
shorted
coil.
The same armature
testing
equipment (Fig.
0.11111)
is
employed
to
check
the
windings
for
broken-off
conductors,
using the double-contact
tester.
This
test
is
done
by
touching
two
consecutive
collector
bars
with
the
two
contacts
of
the
tester
(which are
connected
to
an ammeter) and
Electrical System •
202
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