b) Brushes
-
..
.
Put the brushes in their respective brush
holder and measure the spring tension with
a spring balancer. Tension should be more
than 3.52 lbs (1.6 kg).
2) Length of Brushes
Standard brush length .is 0.630 in. (16
mm).
If any of them are worn out by more than
0.157 in. (4 mm), it should be replaced with
a
new one.
After replacing, check that the new brush
moves smoothly in the brush holder, and
that the end of the brush makes even con-
tact with the commutator surface. If the
contact is uneven, correct
it.
3) Brush Holder Continuity Test
,--.
Carry out
a
continuitytest between the in-
sulated brush holder and the ground.
There should be no continuity.
c) Armature
1)
Measuring Shaft Straightness
Place the shaft upon an end supporting
stand, and measure the straightness of the
shaft.
If
it
is
distorted by more than 0.0031 in.
(0.08 mm),
it
should be corrected.
2)
Commutator
If the surface
is
burnt or pitted,
it
should
be polished using
#500-+600 sandpaper.
If
it
is out-of-round by more than 0.0078 in.
(0.2 mm),
it
should be turned down on
a
lathe.
/-.
3)
Depth of Mica between Commutator
Bars
The amount of under-cut of the insulation
(mica between commutator bar segments)
should be between 0.0197-0.0315 in.
(0.5-
0.8 mm). Be sure to bevel the edges of the
bar segments.
0.0197 -0.0315
in.
(0.5
-
0.8mm)
I
Bevel
wment
I
4) Ground Test
Check conductivity between the commuta-
tor and the
&aft
If conductivity
is
indi-
cated, there
is
a
short, so replace.
. .
5)
Insulation Test
Shorts in the armature coil are checked
with
a
"growler tester." A "growler tester"
is
an iron piece applied to the armature
core and the armature while the latter
is
rotated. If the iron piece starts vibrating
or "growling!',
it
is
an indication that there
is
a
short, and that replacement
is
necessary.
d) Field Coil
1)
Continuity Test
Check the continuity between the terminals
of the field coil. If there
is
no continuity,
it
is
an indication that there
is
a
break in
the winding, so
it
should be replaced.
2)
.
Ground Test
Check the continuity between either of the
terminals of the field coil and the yoke.
If there
is
continuity,
it
means that the
field coil
is
grounded somewhere and
is
therefore defective.
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