Electric motors fail mostly at their bearings, and most of those bearing failures are not mysterious. Surveys across industries consistently put rolling bearings at roughly 50 percent of all motor failures, and the mechanisms behind them are the same handful repeating plant after plant, on frames from a 6205 up to a 6310: shaft currents from drives, lubrication mistakes, misalignment and belt pull, moisture and contamination, and mounting damage. This field guide organises those mechanisms the way a technician meets them - by what the failed bearing looks like when it comes out, and what that appearance says about the repair and the next installation.
Reading the Motor Bearing as an Autopsy
The bearing is its own report. Read it before cleaning it, because the colour and the distribution of the marks carry as much information as the shape.
Fluting and frosting
Regular washboard ridges on the raceways are shaft-current damage, almost always from a variable-frequency drive, and a frosted or matte grey raceway is the earlier stage of the same mechanism.
Burnt or emulsified grease
Brown or black burnt grease with hardened patches is thermal - overload, over-speed, or the classic over-grease with a blocked relief. Rusty raceways with milky, emulsified grease are water ingress from washdown, condensation or a failed seal.
Dents, spalls and creep tracks
Dents spaced at ball pitch mean impact or press-mounting damage, while clean-edged spalling in a single load zone with healthy grease is honest fatigue or overload and the least alarming finding of all. Bright wear tracks on the outer-ring outside diameter with fretting debris point to ring creep from a loose housing fit.
| What you find on the part | Likely mechanism | Action at the machine |
|---|---|---|
| Fluting or frosted raceways | Shaft currents from a VFD | Insulated or hybrid bearing, shaft grounding, cable and earthing review |
| Burnt, carbonised grease | Overheat, over-grease or wrong grease | Correct the grease and the quantity; check load and cooling |
| Rust with emulsified grease | Water ingress | Seals, drains and storage or washdown practice |
| Fretting with ring creep | Loose housing fit | Restore seat tolerance and the correct fit class |
| Dents at ball pitch | Impact or poor mounting | Use presses and induction heating; stop hammering |
| Clean single-zone spalling | Fatigue or overload | Check load, alignment and belt tension |
Shaft Currents from Variable-Frequency Drives
The shift to variable-frequency drives turned electrical erosion from a large-machine curiosity into the most common premature killer of motor bearings. Inverter switching charges the shaft capacitively against the frame at shaft voltages of tens of volts, and at speeds of 1,500 to 3,000 rpm the voltage discharges through the bearing film thousands of times a second, cratering the raceway and carbonising the grease. The standard defence is layered: an insulated or hybrid bearing at the non-drive end to break a circulating loop, a shaft grounding ring where the duty is severe, and drive-side hygiene through shielded cable, correct earthing and common-mode filtering. A motor above roughly 100 kW on a modern drive should be evaluated for insulation as a matter of course. A motor repaired with fresh bearings but no electrical mitigation on a drive line will fail again on the same schedule - the bearing was only the messenger.
Over-Greasing, Under-Greasing and Heat
Motor bearings are sealed for life more often than not, and the field habit of adding grease for safety shortens more lives than it saves. Over-greasing churns the grease, raises temperature towards the 80 °C mark and beyond, and can blow past the seals and push grease into the windings, where the fault then arrives as an insulation failure rather than a bearing one. Under-lubrication shows up mostly in open or re-greaseable designs: stiff, dry grease channels, cage wear, and a rising high-frequency vibration before the temperature moves.
Sealed-for-life versus re-greaseable
A sealed bearing is filled once at the factory and should be left alone; added grease has nowhere to go except past the seal and towards the windings.
Quantity and interval
For re-greaseable motors, discipline means a measured quantity per shot - commonly 5 to 10 g of fresh grease - an interval set from speed and temperature, often every 2,000 to 4,000 running hours, and a genuine relief path so old grease can escape.
Misalignment, Belt Pull and Fit Errors
Misaligned couplings load bearings with forces they were never sized for, showing as edge loading and premature fatigue on one side of the raceway. Over-tensioned belts add a steady radial overload that no bearing calculation anticipated because the tension was set by feel. Wrong fits - an oversize shaft or an undersize housing bore - remove internal clearance and preheat the bearing to failure; a few tens of micrometres of extra interference on a 6205 is enough to close the running clearance completely. The repair-stage discipline that prevents repeats is unglamorous: laser alignment to within about 0.05 mm, belt tension checked with a gauge, bores measured rather than assumed, and bearings mounted with induction heaters instead of hammers or an open flame, which destroys the temper and the grease in one stroke.
| Motor bearing position | Typical duty and load | Points to watch |
|---|---|---|
| Drive end, inverter-fed | Locating position, combined load, highest shaft-current exposure | Insulation or hybrid set; clearance for the temperature; a closure that suits the environment |
| Non-drive end, inverter-fed | Floating position, radial load, the end usually insulated to break the loop | Insulation here is often the single most effective change |
| Belt-driven end | Heavy steady radial load from belt tension | Belt tension by gauge; radial capacity of the bearing |
| Vertical or flange mount | Axial load from rotor weight plus thrust | Axial capacity and a locating arrangement that carries it |
Failures That Begin in Storage
A surprising share of new bearings fails early because of their history before mounting. Vibration during transport or shelf storage causes false brinelling, the polished wear marks at ball pitch that look like dents. Humid storerooms etch raceways, and grease degrades with years and heat. Rotate stock, store boxes flat and dry, and treat any bearing that has sat on a vibrating shelf for 12 to 18 months as suspect - its clock started in the crate. Motors in storage should have their shafts turned periodically to move the grease and prevent raceway imprinting under a stationary load.
- Store flat and dry. Boxes on end let the rolling elements rest against one point of the raceway for months.
- Rotate the stock. First in, first out keeps grease fresh and prevents a bearing sitting for years before it is fitted.
- Isolate from vibration. Keep shelves away from presses, screens and other running machines that impose false brinelling.
- Turn stored motors. Rotating a shaft a few turns every few weeks redistributes grease and relieves stationary contact stress.
Making the Repair Hold
Every teardown is a chance to fix the cause, not just the part. The repair should be run step by step so nothing is skipped under time pressure.
- Photograph the bearings before cleaning and read the evidence against the tables above.
- Let the evidence choose the mitigation - electrical, lubrication, mechanical or storage.
- Fit the correct suffix for the duty: insulated or hybrid where drives are present, the right clearance for the temperature, and the correct seal set for the environment.
- Record what was found so the next failure at the same position compares against a documented baseline.
- Verify the electrical fix by checking insulation resistance and, where fitted, grounding-ring continuity before the motor returns to service.
Repeated Failures at One Position
Treat repeated failures at the same motor position as a system question rather than a parts question. If the third bearing in two years shows the same signature, the machine is sending one message very consistently: either the duty changed, the mitigation was never actually fitted, or the failure mode was misread the first time. A short written finding at each teardown - what the raceways looked like, what the grease looked like, what changed since the last repair - costs a few minutes and turns a cycle of repeat failures into a closed investigation. Read each removed part against the ISO 15243 failure-mode families and compute the expected life with the ISO 281 method if a replacement keeps arriving early. Bearings, sizes and electrical-duty variants for motor positions are filterable in the bearing selector, and the lubrication intervals and mounting procedures behind this guide are detailed in the technical library. For the electrical mechanism in depth see the insulated bearing guide and the electric-drive bearing notes, while the mounting discipline that prevents most installation damage is set out in the mounting and dismounting guide. Drive-end and non-drive-end positions most often use deep groove ball bearings or, in larger frames, cylindrical roller bearings, and the electrical-duty variants are widely available from makers such as SKF and NSK.
The arithmetic of a repeat is worth writing down. If a bearing that should deliver 20,000 hours at the stated duty is failing at 2,000, then the load is roughly a factor of 10 higher than assumed, the lubrication has failed, or the installation is damaging the part - and the teardown evidence separates those three candidates. A short calculation against ISO 281 turns a vague sense that a position keeps failing into a stated factor, and a stated factor is far easier to design out. That single habit, applied consistently across a motor fleet, is what moves a plant from replacing bearings on a schedule to understanding why each one failed.
- Most motor bearing failures are readable. Roughly half of motor failures are bearing failures, and most have a visible root cause on the part.
- Fluting and frosting mean shaft currents. The fix is electrical mitigation, not only a new bearing.
- Over-greasing kills sealed motor bearings. Leave sealed parts alone, and re-grease only re-greaseable designs, by quantity and interval.
- Alignment, belt tension and fits are bearing-life decisions made at installation, not by the bearing supplier.
- Storage starts or prevents failures before the bearing is ever mounted.
Frequently asked questions
What does fluting on a motor bearing tell me to fix?
Fluting points to shaft currents, almost always from a variable-frequency drive. The correct response is electrical: fit an insulated or hybrid bearing, add a shaft grounding ring where the duty is severe, and review the shielded cable, earthing and common-mode filtering. Replacing the bearing alone repeats the failure.
How do I stop a motor bearing failing repeatedly in the same position?
Treat the repeat as a system question. Confirm whether the duty changed, whether the intended mitigation was actually fitted, and whether the failure mode was read correctly. Record a short written finding at each teardown - raceways, grease, and what changed - so the third failure is compared against a documented baseline.
Is over-greasing worse than under-greasing a motor bearing?
On a sealed-for-life motor bearing, yes. Adding grease has nowhere to go, so it churns, raises temperature, can blow past the seal and reach the windings. Under-lubrication mainly affects open or re-greaseable designs. Match the grease quantity and interval to the bearing and the relief path, not to habit.
How does belt tension shorten motor bearing life?
An over-tensioned belt adds a steady radial load that the bearing selection never accounted for, because the tension was set by feel. The result is higher contact stress, more heat and earlier fatigue. Setting tension with a gauge rather than by hand is one of the cheapest reliability improvements available.
Can a new bearing fail because of how it was stored?
Yes. Vibration during transport or shelf storage causes false brinelling - polished wear marks at ball pitch that look like dents - and humid storerooms etch raceways while grease ages. Store boxes flat and dry, rotate stock, and turn stored motor shafts periodically to avoid raceway imprinting.
What should I photograph before cleaning a failed bearing?
Photograph each surface in place, before wiping anything, because colour, position and the distribution of the marks are the evidence. Include the raceways, the rolling elements, the cage and the grease. Read the images against the failure-mode families, then write one line linking the appearance to a correctable cause.