Figure 8-7. Undercutting mica.

TM 10-3930-631-34

(a) Use a milli-voltmeter and touch voltmeter

(31.7 mm) inside diameter by 4.0 inches (101.6 mm)

probe to adjacent bars as shown.

long over commutator end against bearing shoulder on

(b) The meter will not show a reading unless the

shaft.

coil is open. If coils are open replace armature.

(6) Inspect the commutator. If commutator is

pressure on short pipe at commutator end and press

burned, rough or out of round it must be resurfaced and

shaft from armature. Pressure to remove or install shaft

undercut.

may exceed 20 tons (18120 kg).

(a) Place armature in a lathe and turn

(d) To install new shaft, place a piece of pipe

commutator down until true.

Do not damage

1.5 inches (38.1 mm) inside diameter, 2.25 inches (57.1

commutator riser bars at rear of commutator when

mm) outside diameter against commutator end. Insert

resurfacing.

shaft as far as it will go. Place a piece of pipe 1.625

(b) Undercut mica between bars to a depth not

inches (41.2 mm) inside diameter by 3.0 inches (76.2

to exceed 0.030 inch (0.752 mm). Undercut must be full

mm) long against bearing shoulder on shaft and apply

width of mica and flat at the bottom. Undercut mica as

vertical pressure to press shaft into armature. Shaft

shown in figure 8-7.

must bottom against core support.

e. Assembly.

(1) Heat fan (8, fig. 8-3) in an oven to 200

degrees F (93 degrees C). Position key (9) on shaft and

press fan into place. Install retaining ring (7) to secure

fan.

(2) Position pole shoes (30) and shims (31) in

housing. Install field windings (32) and connect stud

assemblies (10 and 11) to frame and windings. Install

pole shoes screws (29).

(3) Connect a test lamp between the field

terminal and the housing as shown in figure 8-8.

Figure 8-7. Undercutting mica.

any dirt and copper dust.

(d) Lightly rub the commutator with sandpaper

to remove any burs left from undercutting.

Figure 8-8. Checking field winding for grounds.

(7) Inspect armature shaft for wear and

damage. If shaft requires replacement proceed as

follows:

(a) Place drive end of shaft in a heavy duty pipe

2.25 inches (57.1 mm) inside diameter by 18 inches

(457.2 mm) long. Pipe must be in complete contact with

core above shaft but within the radius of the core rivets.

(b) Place another piece of pipe 1.5 inches

8-5