You could be asked to investigate an installed electric motor that is not operating properly, has a history of frequent bearing and insulation failures, or other malfunctions.

Motor difficulties are classified as either mechanical (for example, bearings or alignment) or electrical (examples include windings, brushes, or overspeeding).

Most mechanical problems will be bearing related and usually will be indicated by unusual noise, vibration, or heat. Electrical operating difficulties and inspection procedures will be discussed in another lesson.

Do NOT just assume the bearings are at fault. It is necessary to perform certain operational and static inspections when a motor mechanically malfunctions. Static inspections are normally held first to prevent further damage to the equipment.

If you follow the procedures in this lesson, you will usually be able to determine if the bearings are actually the cause of the problem or to locate and eliminate any other faulty conditions. In addition, unnecessary disassembly and replacement of properly operating bearings will be avoided.

Preparation for mechanical inspection and tests

Ensure the power supply is secured and tagged out by tended unit personnel.

Use a voltmeter to test incoming power leads in the controller to ensure power is secured. Try to determine if any maintenance was recently accomplished to the driven component. If so, then improper packing of the component or misalignment may be the problem.

Mechanical inspections and tests

You can refer to NAVSEA 0900-LP-060-2010, Volume 1, Chapters 2 and 4, for additional information regarding subject matter included in this lesson.

If possible, turn the shaft by hand at least three complete revolutions. Freedom of rotation is mandatory. The presence of high spots, binding, or drag will require disconnecting the driven unit from the motor to pinpoint which component is at fault. Binding in the motor is usually caused by a faulty bearing, a bent shaft, improperly assembled motor, or warped bearing retainers or end bells. If binding or drag is present after the coupling is disconnected, it will be necessary to lift the brushes to eliminate the possibility of excessive brush pressure or high commutator bars.

Inspect the holddown, end bell, and bearing retainer bolts for tightness. Tighten as necessary.

Inspect the end bells to ensure that they are mounted flush to the field housing. If they are improperly mounted, loosen and retighten all bolts to the proper torque. If binding is still present, it will be necessary to remove the motor end coupling half in preparation for dial indicator tests.

• Conduct a shaft radial runout test. Attach the magnetic base on the motor end bell, deck, or another suitable stationary surface. Mount the dial indicator on the tool post holder. Adjust the tool post holder to permit easy reading of the instrument face.

Bring the sensor button into contact with the surface to be measured. Tilt the adjustable sensing arm to wipe across the surface being measured. The button should contact the shaft as close to the end as possible.

Raise and lower the sensor button with the tool post thumbscrew to determine the full travel of the indicator. Leave the pointer at midtravel; zero the bezel. Slowly rotate the shaft. The instrument pointer should remain at zero if the shaft is straight.

Observe and record dial indicator reading. Runout should not exceed 0.002 inch total indicated runout unless otherwise specified on equipment drawings. Excessive runout readings will necessitate straightening the shaft.

• Attempt to move the shaft horizontally and vertically. Excessive movement is an indication of bearing wear.

• Measure face runout of the bearing inner rings (drive and back end) if the design permits. Remove the outer bearing cap. Mount a magnetic tool holder on the motor end bell. Mount the dial indicator on the tool post holder.

Bring the sensor button into contact with the bearing inner ring. Raise and lower the sensor button to determine the full travel of the indicator pointer. Leave the pointer at midtravel; zero the bezel.

Rotate the shaft with the sensor button against the bearing inner ring. Observe and record dial indicator readings. The maximum allowable face runout should be within 0.001 inch per inch diameter for general bearing applications and 0.0003 inch (maximum) for quiet (NT-3) bearings. Excessive bearing face runout will necessitate bearing replacement.

• Measure face runout of the bearing outer ring (drive end only) if the design permits. Use a C-clamp tool post holder to attach the dial indicator to the motor shaft. Attach the dial indicator to the tool post holder.

Bring the sensor button into contact with the bearing outer race face. Raise and lower the sensor button to determine the full travel of the indicator pointer. Leave the pointer at midtravel; zero the bezel.

Rotate the shaft with the dial indicator attached. Check the stiffness of the indicator setup at several positions. Since the gravity load may cause the indicator to shift its position, it is important that the indicator be held rigidly.

Observe and record dial indicator readings. Excessive bearing face runout will necessitate bearing replacement.

• Measure commutator radial runout. Remove inspection covers from the commutator end of the motor. If necessary, remove the brush or brushes to allow access to the commutator.

Attach the magnetic base on the motor end bell, deck, or another suitable stationary surface. Mount the dial indicator on the tool post holder. Adjust the tool post holder to permit easy reading of the instrument face.

Using a mushroom-shaped sensor button, contact the commutator near the bearing end. The reading must be taken on the brush contact area of the commutator. Raise and lower the sensor button to determine the full travel of the indicator pointer.

Leave the pointer at midtravel; zero the bezel. Rotate the armature with the sensor button against the commutator. Disregard the bump in the readings caused by the gap between commutator bars.

Observe and record dial indicator readings. Take readings at the center and riser end of the commutator. Record the readings. All readings must be taken on the brush contact area of the commutator. Runout should not exceed 0.003 inch total indicated runout unless otherwise specified on equipment drawings.

Excessive commutator runout will require that the commutator be trued. If possible, the armature should be removed and transported to a repair shop to be turned in a lathe.

• It will be necessary to conduct an operational test if all readings and observed conditions appear to be satisfactory.

Operational test

Preparation: Ideally, operational inspections should be conducted with the motor and driven unit disconnected or the load removed from the motor. Turn the shaft by hand at least three complete revolutions to ensure freedom of rotation.

Check ventilation openings for cleanliness. Clean as necessary. Reset the brushes on the commutator. Notify tended unit personnel to tag in and energize the power supply.

Test: Start the unit and listen for unusual noise. Cautiously touch the bearing housing periodically to check for excessive heating. Unusual noise or excessive heating of bearings can be caused by loose components; an improperly assembled motor; improper lubrication of bearings; a bent shaft; or faulty brushes, commutator, or bearings.

Observe brushes for chattering or sparking. If either condition is present, it may indicate excessive brush pressure, improperly seated brushes, brush rigging off neutral, or chemical contamination.

Place a metal rod, screwdriver, or mechanic's stethoscope on the bearing housing to listen for unusual noise emanating from the bearing. Only a purring sound should be heard.

Stop the unit and observe the coastdown. Short coastdown time and abrupt stops indicate binding. If necessary, mark the shaft to see if it stops in the same place each time. Listen for the unit passing through resonances that may pinpoint a faulty or loose component.

Maintenance criteria

Mechanical components: The motor will require disassembly if bearing replacement or shaft, end bell, or commutator maintenance is required.

Electrical components: Armatures with high, low, or uneven bars will require stoning or turning in a lathe. The method used will depend on the physical size of the armature and whether the motor design will permit installation of an external driving attachment to turn it at a controlled speed