Electric Motor Bearing Noise: Causes and What to Do

Electric motor bearing noise comes from five groups of causes identified by SKF: metal-to-metal contact from poor lubrication, contamination, looseness, raceway damage and rubbing of adjacent parts. Whistling usually means too little lubricant; rumbling and knocking point to contamination or damage. Check lubrication and measure ultrasound first, then decide whether replacement is needed.

Which sound is normal and which is not

The SKF Bearing Installation and Maintenance Guide describes the sound as follows:

Sound	Meaning
Even "purring"	Normal operation
Whistling or screeching	Inadequate lubrication
Uneven rumbling or hammering	In most cases contaminants in the bearing or damage during mounting

Listening is useful as a first signal, not as a diagnosis. SKF notes that fatigue starts with a microcrack below the raceway surface that cannot be seen or heard on a running machine. By the time noise appears, the defect is already developing. How to check a bearing with instruments is covered in How to check a motor bearing without disassembly.

Five groups of noise causes

The SKF troubleshooting table lists these causes for the symptom "excessive noise":

Group	Cause	What to do (per SKF)
Metal-to-metal contact	Oil film too thin: temperature too high or speed very slow	Choose lubricant viscosity and NLGI grade for the operating conditions
	Insufficient lubricant: never lubricated, leakage through worn or improper seals	Check seals and oil level, replenish lubricant
	Rolling elements skidding: load below minimum or lubricant too stiff	Ensure minimum load, check grease consistency
Contamination	Solid particles enter the bearing and dent the raceways	Inspect seals, shorten the relubrication interval, supply smaller quantities of fresh grease more often
	Solids left in the housing from manufacturing or a previous failure	Clean the housing thoroughly before fitting a new bearing
	Liquid in the lubricant reduces its viscosity	Check seals, shorten the relubrication interval
Looseness	Inner ring turning on the shaft (undersized or worn shaft)	Restore the correct fit
	Outer ring turning in the housing (oversized or worn bore)	Restore the fit, eliminate unbalance
	Loose locknut on the shaft or adapter sleeve, bearing not clamped	Tighten and lock
	Too much radial or axial internal clearance	Keep rolling elements loaded, for example with springs
Surface damage	Dents from impact (including hammering during mounting)	Never pass mounting force through the rolling elements; use a mounting sleeve
	False brinelling from vibration while the machine stands idle	Rotate the shaft of standby equipment periodically
	Spalling from fatigue or from surface-initiated damage	Find the root cause: contamination, lubrication, impact, current
	Fluting from electric current passing through the bearing	Ground the equipment; if needed, an insulated or hybrid bearing
Rubbing	Misaligned housing seals rubbing, locknut tabs touching the seal, sleeve or spacers not clamped	Align seals, check fasteners

Lubrication: check it first

Two of the three metal-to-metal causes are about lubrication. SKF specifically notes that extremely stiff greases contribute to rolling element skidding, especially in very cold climates, and skidding generates excessive heat and noise. For unheated pump houses and outdoor sites in Kazakhstan this matters when choosing a grease for winter: see Choosing a bearing grease.

Too much grease is also harmful. Per SKF, excessive lubrication causes churning and elevated temperatures. Signs and consequences of over-greasing are covered in Bearing over-lubrication: consequences, and the link to heat in Why an electric motor bearing overheats.

To avoid guesswork, SDT recommends relying on ultrasound: both under- and over-lubrication increase friction and therefore the ultrasound level above the baseline. SDT treats a gain of more than 8 dB over baseline as a reason to investigate.

Damage: when grease will no longer help

If the noise is caused by dents, spalling or fluting of the raceways, grease will not remove it. Per SDT, a gain of 12–16 dB over the baseline indicates bearing degradation, and above 30 dB warns of potential catastrophic failure. Consistent, repetitive spikes in the time waveform are typical of a surface defect. At this stage a replacement is planned, and the cause is removed during the replacement; otherwise the new bearing will repeat the fate of the old one. How damage progresses is shown in 4 stages of bearing failure.

On electrical erosion, SKF writes that fluting is most commonly attributed to electric current passing through the bearing. The first measure is proper grounding. If that does not help, the options are an insulating sleeve in the housing bore, a bearing with an insulated outer ring, or a hybrid bearing with ceramic rolling elements.

Bearing or electrical

A motor can be noisy without any bearing defect. SKF notes that many electrical problems appear at twice line frequency: 100 Hz on a 50 Hz supply. Bearing defects produce other frequencies, linked to running speed and bearing geometry. The sources can be separated with a vibration spectrum (How to read an FFT spectrum) or with current and voltage analysis, which shows both mechanical and electrical faults (ESA, MCSA and vibration analysis).

What to do if a bearing is noisy

1. Find which bearing is noisy: drive end or non-drive end. Compare with a similar machine.
2. Judge the character of the sound: whistling, rumbling, knocking, constant or intermittent.
3. Measure ultrasound on both bearings and compare with the baseline.
4. If it looks like lubrication (whistling, a gain around 8 dB, inconsistent spikes), add grease in small amounts while watching ultrasound. Per SDT, the dB level decreases as grease is added; once it begins to climb slightly, there is enough grease.
5. If the signal does not decrease, lubrication is not the only cause. Check vibration: spectrum and enveloping. Look for bearing defect frequencies and twice line frequency.
6. Check fits and seals at the next stop: signs of ring creep, seal rubbing, loose nuts.
7. Plan a replacement if damage is confirmed. When replacing, clean the housing, mount without impact and check grounding.

What to use

The SDT340 measures ultrasound (20–50 kHz) and vibration (10 Hz – 10 kHz), has a built-in IR thermometer and tachometer, lets you listen to the bearing through headphones and records the signal for analysis. If the motor is inaccessible or mechanical causes need to be separated from electrical ones, the Artesis AMT Pro tester analyses current and voltage from the supply side; a test takes 7 minutes.

KEG TRK supplies SDT ultrasound detectors and Artesis motor testers in Kazakhstan. Price on request.

Sources

• SKF USA Inc. Bearing Installation and Maintenance Guide: "Test running", "Troubleshooting" and "Trouble conditions and their solutions" (codes 1–4, 12, 23, 27–42). https://cdn.skfmediahub.skf.com/api/public/0901d1968024f02a/pdf_preview_medium/0901d1968024f02a_pdf_preview_medium.pdf
• SKF. Spectrum Analysis: the key features of analyzing spectra (J. Mais), section 12 "Electrical problems". https://cdn.skfmediahub.skf.com/api/public/0901d1968024acef/pdf_preview_medium/0901d1968024acef_pdf_preview_medium.pdf
• SDT Ultrasound Solutions. Mechanical Condition Monitoring. https://sdtultrasound.com/applications/mechanical-condition-monitoring/
• SDT Ultrasound Solutions. Bearing Lubrication Monitoring. https://sdtultrasound.com/applications/bearing-lubrication-monitoring/
• Artesis. What is Condition Monitoring? A Technical Overview. https://artesis.com/condition-monitoring/
• KEG TRK product pages: SDT340, Artesis AMT Pro.