Gary Braun
Brehob Corp.
Indianapolis, Indiana
Technical Education Committee Member

When servicing DC motors, one of the many tests we do to determine the condition of the commutator is to check it for loose bars.

We check for loose bars by lightly tapping the face of the commutator with a very small hammer. Then we check for suspicious sounds and move­ment or vibration of the bars as they’re struck. A loose bar will have a dull thud while tight bars will have more of a crisp “peck.” You should not feel any movement of the bar with respect to adjacent bars. 

Correct underlying problems 
If the commutator is molded or riv­eted, it is not repairable and it should be replaced. However, V-ring commu­tators can be repaired. Before trying to tighten the loose bars, we must correct any underlying problems. A single loose bar may be caused by a dam­aged or burned mica V-ring, bad segment mica or a torn or dis­torted dovetail on the copper bar itself. 

Many loose bars may indicate bad mica V-rings or something worse. All of these problems must be cor­rected before attempt­ing to tighten the com­mutator.
If you are going to remove the V-ring from the commutator, it is also advisable to band the commutator with a glass band, un­der normal armature banding tension, to maintain closing ten­sion. 

Closing the commutator
Assuming we have corrected these prob­lems, or the commu­tator is simply loose, close the commutator as follows:

Place the entire ar­mature in the bake oven that has been heated to 350° F. Enough time must be allowed for the entire commutator and armature assembly to fully expand. This may take four hours or more. Take the arma­ture out of the oven and immediately tighten the tightening nut (see Figure 1). If the commutator is of bolted construction (see Figure 2), tighten all the bolts using a crisscross pattern (see Figure 3) using the ap­propriate torque value for the size and type bolt being used. A note of caution: over tightening may dam­age the V-rings or dovetails on the commutator bars. 

Bolts should be torqued to approxi­mately 50% of the published “bolted joint” values for the bolt size and grade, while the armature is still hot from the oven. If tightening is required at the prescribed values, the tightness should again be checked when the commutator cools to ambient tempera­ture.  See Table 1.  

The commutator should now be ready for string band, turn and un­dercut. 

For traction motor or similar high-stress applications, spin seasoning is required to stabilize the commutator and prevent shifting in service.
For more information on com­mutator maintenance and repair, see Section 4.8 in the EASA Technical Manual. Also, see Section 6 in EASA’s Fundamentals of DC Operation and Re­pair Tips manual.