How do I test an electric motor’s bearings?

Industrial motors use radial ball and roller bearings, angular-contact bearings, spherical and cylindrical roller bearings, and, in large machines, plain (sleeve) bearings. The assembly may include a fixed and a floating bearing, spring preload, grease or an oil bath lubrication, and insulation against bearing currents.

The testing method depends on the bearing type and the assembly design — rolling-element and plain bearings are diagnosed with different methods.

Testing is required:

• when noise, overheating or rising vibration appears;
• when motor current increases or rotation is unstable;
• after an emergency shutdown, jamming, or extended idle time;
• after mounting, a coupling replacement, alignment, or rotor or shaft repair;
• after water ingress or contamination;
• when the same bearing fails repeatedly;
• during scheduled maintenance and before commissioning.

The most common defects are fatigue spalling, pitting, scoring, corrosion, fretting, brinelling, electrical erosion, cage failure, insufficient or excess lubrication, an incorrect fit, and bearing currents.

Possible signs include humming, whistling, grinding, crackling, knocking, local overheating, vibration, axial or radial shaft movement, hard rotation, leaking or discolored grease, metal shavings, and repeated loosening of fasteners.

• a contact thermometer and a thermal camera;
• a vibrometer and a vibration analyzer;
• an ultrasonic instrument and a mechanic’s stethoscope;
• a tachometer, a dial indicator, feeler gauges, a micrometer;
• a grease-condition tester and a megohmmeter;
• an oscilloscope and a shaft-voltage meter;
• a borescope, a puller, an induction heater, and a balancing machine.

Temperature is measured on the bearing shield housing, near the outer ring, with an embedded sensor, or with a thermal camera, taking load, speed, lubricant type and past readings into account. What matters most is not a single figure but the trend — a fast rise, a difference between the two bearings, and instability are all suspicious.

Causes of heating include too little or too much grease, the wrong grease type, contamination, water, excessive fit interference, ring creep, misalignment, imbalance, excess axial load, bearing currents, and raceway damage.

Noise is assessed by ear, with a stethoscope, or with an ultrasonic instrument. A steady hum points to raceway wear or excess interference, a rustling sound to contamination or lack of grease, crackling to electrical discharges or debris, and periodic knocking to a local pit or a damaged rolling element.

Overall vibration is measured in the horizontal, vertical and axial directions at both bearings, recording RMS velocity, acceleration, displacement and the spectrum. Rising overall vibration can come from more than just the bearing — imbalance, misalignment, looseness or resonance can all be the cause.

Defects in the bearing elements create characteristic repeating impulses, and spectral analysis can reveal damage to the inner or outer race, the rolling elements, or the cage. At an early stage, overall vibration may still be low — this is where high-frequency envelope analysis helps, catching microscopic raceway defects, insufficient lubrication, or contamination sooner.

Ultrasonic monitoring can reveal friction, insufficient lubrication, early damage and electrical discharges before temperature or overall vibration rises noticeably, and it is also used to optimize the grease quantity during relubrication.

Axial play is checked with a dial indicator or a displacement sensor. Excess axial play can come from bearing wear, incorrect assembly, missing locating parts, or worn thrust surfaces; too little clearance is also dangerous, since it can block the shaft’s thermal expansion. Radial play is assessed the same way, and excess movement can mean internal bearing wear or a loose fit.

Before a manual check, fully disconnect the motor, lock out re-starting, and verify there is no voltage present. When turned by hand, the shaft should move smoothly, with no sudden change in resistance, grinding or jamming — but even a serious early defect may not be felt by hand, because of low speed, no load, or grease masking small defects. Manual turning is therefore no substitute for vibration diagnostics.

Check the amount of grease, its color, smell, consistency, presence of water, metal particles, dirt, and signs of overheating.

• dark or black grease — overheating, oxidation, wear or electrical discharges;
• light, emulsified grease — water ingress;
• hard or dry grease — aging or too long a service interval;
• excessively fluid grease — overheating or thickener breakdown.

A grease shortage shows up as whistling, high-frequency noise and rising temperature, while excess grease causes churning, extra resistance, leakage past the seals and possible contamination of the winding. Mixing incompatible greases can cause thinning, hardening or corrosion, so compatibility must be checked before switching grease types.

After removal, inspect the bearing before cleaning it, noting the grease distribution, contamination, signs of overheating and ring position. Then check the inner and outer ring, the raceways, the rolling elements, the cage and the seals.

Raceways may show pitting, spalling, scoring, dents, corrosion, or traces of electrical discharges. Fatigue spalling — small pits and surface flaking — points to long service or overload and calls for replacement. Brinelling is dents from an impact during mounting or transport; false brinelling comes from small oscillating movements of a stationary bearing during storage or transport.

Electrical erosion happens when current passes through the shaft, the lubricant film and the rolling elements, creating a micro-arc at the point of breakdown. Signs include a dull gray surface, small craters, transverse grooves with a characteristic "washboard" pattern, dark grease and high-frequency noise.

Common causes include a variable frequency drive supply, common-mode voltage, magnetic asymmetry, incorrect grounding, or damaged bearing insulation. Checking involves measuring shaft voltage, an oscilloscope, checking the grounding device, and inspecting the raceways — an ordinary multimeter often misses short high-frequency pulses.

Check the diameter, ovality, taper and roughness of the fit on the shaft and in the end shield. Signs of a loose fit include a shiny surface, brown or reddish dust, marks of movement, and unstable vibration. Excessive interference reduces internal clearance and raises temperature, while insufficient interference causes ring creep, fretting and heating.

For an interference fit, the bearing is often heated with an induction heater, which heats the ring evenly, allows temperature control, and demagnetizes it afterward. After mounting, check ease of rotation, axial play, absence of jamming, noise, vibration, temperature and any grease leakage.

1. 01Gather the history: noise, temperature, vibration, the last relubrication date, load and prior repairs.
2. 02Inspect the assembly while running — leaks, noise, temperature, fasteners and seals.
3. 03Measure vibration in three directions at both bearings.
4. 04Analyze the spectrum — rotation frequency, harmonics, bearing frequencies and the envelope.
5. 05Check ultrasound, especially if a lack of grease or an early defect is suspected.
6. 06Stop the motor and check manual rotation and clearances.
7. 07If possible, disconnect the driven equipment to separate motor noise from a coupling or pump.
8. 08Remove the bearing, preserving its orientation, and record the grease condition.
9. 09Inspect the raceways, rolling elements, cage and rings, and measure the fits.
10. 10Identify the root cause instead of stopping at "the bearing is worn".
11. 11Replace or restore the assembly, accounting for fits, lubrication, alignment and current protection.
12. 12Run acceptance tests — noise, temperature, vibration and operation under load.

A replacement is required for raceway spalling, cracked rings, damaged rolling elements, significant corrosion, electrical-erosion grooves, severe overheating, excessive play, or repeated knocking.

• adding grease without a diagnosis — overheating can come from excess grease, interference, or raceway damage;
• judging condition from temperature alone or noise alone;
• using impact mounting that drives the force through the rolling elements;
• fitting a new bearing onto a worn seat;
• mixing greases without checking compatibility, or overheating the bearing;
• reusing a removed bearing without assessing it;
• replacing a bearing without finding the cause, especially on a repeat failure.

• use the correct bearing type and stick to the specified fits and recommended grease;
• do not exceed the grease quantity and keep to the relubrication intervals;
• keep seals, alignment, balancing and belt tension under control;
• track temperature and vibration trends;
• protect against bearing currents and store motors properly, turning the shafts periodically.

• Why do bearings get hot?
• Why is the motor noisy?
• Why does the motor vibrate heavily?
• How do I test the rotor?
• How do I measure shaft run-out?
• How is rotor balancing done?
• What are bearing currents?

EPR (Elektropromremont) performs comprehensive diagnostics, repair and testing of bearing assemblies in industrial electric machines.

The scope of work includes:

• vibration diagnostics, spectral analysis and envelope analysis;
• thermal and ultrasonic diagnostics;
• checking axial and radial clearance;
• bearing inspection and fit measurement;
• restoring seats, repairing shafts and end shields;
• bearing replacement with induction-heated mounting;
• diagnosing bearing currents;
• dynamic rotor balancing and laser alignment.