Undercutting Mica

If lamp lights. armature is grounded and must be

replaced.

(3) To test for grounded fields. place one

probe, of test lamp on field frame and other probe on

field terminal. If lamp lights. field coils are grounded and

must be replaced if ground cannot be located and

repaired.

h. Undercutting Mica. Carefully inspect the

commutator. If burned. rough. or out-of-round, it must

be cut down and the mica undercut.

(1) Place armature in a lathe and turn down

comnmutator until true. Make certain cut is not made on

commutator riser bars as solder will be removed.

weakening coil connections at this section. Remain

approximately 3/ 16 inch from riser bars cutting.

(2) Undercut mica between bars to a depth

not exceeding 0.030 inch. Undercut must be-full width of

mica and flat at bottom. (fig. 3-90.) After undercutting.

clean out slots to remove any dirt or copper dust.

(3) Sand commutator lightly with No. 00

sandpaper to remote an. burrs left from undercutting.

Figure 3-90. Undercutting mica.

(4) Check armature on a growler for short

circuits. Refer to subparagraph h. step (1).

i. Armature Shaft Removal.

Note. Pressure to remove or install shaft may exceed

20 tons.

(1) Place the drive end of the shaft in a heavy

duty pipe approximately 2 1 / 4 inches I.D. and at least

18 inches long. (fig. 3-91.) Caution: Make certain the

top end of the pipe makes complete contact with the

core from above shaft, but within a radius of the core

rivets.

(2) Place a second pipe of 1 1/4 inches I. D.,

1 1/2 inches maximum 0. D and about 4 inches long.

over commutator end against bearing shoulder on

shaft. Apply vertical pressure on pipe (commutator end)

and press shaft from armature.

j. Reassembly.

(1) Inspect all parts for signs of unusual wear,

broken insulation or loose connections. Replace any

damaged or worn parts.

(2) Reassembly of this unit is a reversal of the

disassembly

procedure.

with

the

following

recommendations.

(a) To replace shaft. place a 6 inch, 1

1/2 inch I. D 2 1/4 inch 0. D. pipe against commutator

sleeve with assembly vertical. Insert shaft as far as it will

Figure 3-91. Armature shaft pipes.

go. Using a 1 5 / 8 inch I. D. pipe against bearing

shoulder, apply exact vertical pressure until shaft

bottoms correctly against core support.

(b) Bearing retainer with seal must be

assembled on shaft before installing bearing.

3-100