TM 55-2925-236-40
d. If resurfacing the commutator (paragraph
Warning: Use solvent only in a well-venti-
lated area. Avoid inhaling solvent fumes. Do not
the allowable minimum of 2.800 inches. This di-
allow solvent to contact the skin as burns may
ameter is indicated by a step cut into the edge
occur.
of the commutator.
Note. Refer to paragraph b for specific clean-
ing procedures for armature assembly (39, figure 3-1)
between commutator bars and ground shows in-
and housing and stator (53).
dication of insulation breakdown.
(1) Ultrasonic cleaning equipment may be
used, if available. Consult the manufacturer of
Caution: When performing this test, keep
this equipment for proper cleaning solutions and
test probes outside brush path to avoid damag-
methods to be used.
ing contact surface.
(2) Dry parts with a clean, lint-free cloth
(item 3, table 2-3).
3-6. Resurfacing the Commutator
b. Armature Assembly and Housing Stator.
a. To remove oil or carbon film, mount arma-
Using a cleaning solvent specified in paragraph
ture assembly (39, figure 31) in a lathe and
take a single light cut across the face of the com-
assembly (39, figure 31 ) and housing and stator
mutator at a speed of approximately 600 surface
(53) thoroughly to remove any carbon and cop-
feet per minute. If the contact surface is scored
per particles imbedded between commutator bars
or pitted, take a series of light cuts at approxi-
and stator bars.
mately 200 surface feet per minute. A diamond-
(1) After cleaning, bake the armature as-
tipped cutting tool (figure 3-3) is recommended,
sembly and the housing and stator in an oven
The point of the cutting tool must be held on the
at 250 F (121 C) for two hours or at 200 F
centerline of the armature shaft (see figure 3-
(93 C) for three hours.
4). If a Carboloy-tipped cutting tool (figure 3-5)
(2) After baking, apply a light film of lu-
is used, the point of the cutting tool must be held
bricating oil (item 4, table 23 ) or corrosion pre-
0.031 in, below the centerline of the armature
ventive (item 5, table 2-3) to all ferrous metal
shaft. The minimum diameter to which the com-
surfaces to prevent rusting.
mutator may be turned down is 2.800 inches. This
Caution: Do not apply oil to the commu-
limit is indicated by a step cut into the commuta-
tator.
tor. If necessary to remove metal beyond this
point, replace the armature assembly.
b. After the commutator has been turned
3-4. Repair or Replacement
down, measure the depth of undercutting between
commutator bars. If the depth is less than 1/32
describe procedures for reparable parts only.
in., undercut to this depth and to a width of .030
a. General.
in,
(1) Replace all parts that are worn and
c. After undercutting, take a final light cut
damaged,
of not more than 0.001 in. across the face of the
(2) Replace split-type brushes (4, figure 3-
commutator to remove burrs, If a diamond-tipped
1) and ball bearings (36 and 44) at each over-
cutting tool is not available, make the final cut
haul.
with a freshly honed Carboloy-tipped tool. Cut-
(3) Replace all wiring having burned or
ting speed should be approximately 600 surface
cracked insulation, or broken or corroded termi-
feet per minute with dither type tool. Do not use
nals.
polishing abrasives. After the final cut, remove
burrs between commutator bars with a strip of
3-5. Armature Assembly
fiber.
Replace armature assembly (39, figure 3-1) If
d. Check that the commutator is concentric
any of the following conditions exist:
with the bearing surfaces of the armature shaft
within 0.005 in.. full indicator reading.
a. If the shaft diameter is less than 0.9843 in.
e. After resurfacing, clean the commutator to
b. If the commutator has badly burned bars,
remove all traces of oil, grease, and metal chips,
because such a condition is usually the result of
open-circuited armature coils.
as specified in paragraph 3-36.
f. Check the armature assembly for static and
c. If commutator bars are loose or out of align-
ment.
dynamic balance, using balancing machine, model
