Fatigue spalling, contamination damage, fretting, false brinelling and overheating: how to identify bearing failure modes from visual inspection and what to do about them.
When a bearing fails, the damaged parts tell you why. Most bearings that fail early do not fail because the bearing was bad. They fail because of how they were mounted, lubricated, sealed, loaded or stored. If you only replace the bearing and put the machine back to work, the same failure usually comes back.
This guide explains the five most common bearing failure modes, how to recognise each one by looking at the bearing, and what to change so it does not happen again. Use it as a practical first-pass check before sending anything for lab analysis.
Why Failure Analysis Matters
A failed bearing is evidence. Before you clean it, wash it or throw it away, look at it.
- Keep the failed bearing and its parts together, including the seals, cage and grease.
- Do not wash or wipe the bearing before inspection. Cleaning removes evidence such as debris, discoloured grease and contact marks.
- Photograph the bearing as it came out, then again after careful cleaning.
- Note the operating hours, load, speed, temperature, environment and any recent changes.
- Check the shaft, housing and mating parts too. They often show the real cause.
Fatigue Spalling
What it is: Spalling (also called flaking) is when small pieces of metal break away from the raceway or rolling elements, leaving pits or craters.
What it looks like: Rough, pitted patches on the raceway or rolling elements. Early on it may be a small crater. As the bearing keeps running, the damaged area spreads along the raceway. The bearing becomes noisy and vibration increases.
Why it happens: Every bearing eventually fails from rolling contact fatigue if nothing else damages it first. Repeated stress under the contact surface causes tiny cracks that grow until material breaks free. This is the normal end of a bearing’s life. It is only a problem when it happens much earlier than the expected life.
Common causes of early fatigue:
- Loads higher than the bearing was selected for
- Misalignment or shaft deflection
- Excessive preload or too little internal clearance
- Poor lubrication that allows metal contact
- Dents from contamination or poor handling that start cracks early
- Poor material quality or heat treatment
How to prevent it:
- Check the load and speed against the bearing’s rating and expected life, and select a larger or different bearing if needed.
- Check alignment and shaft and housing fits.
- Check clearance for the operating temperature.
- Use the correct lubricant and keep it clean.
- Buy from a manufacturer who controls material and heat treatment properly.
Contamination Damage
What it is: Damage caused by dirt, dust, metal particles, water or other foreign material getting into the bearing.
What it looks like: Small dents and indentations on the raceways and rolling elements. A dull, scratched or worn surface where abrasive particles have been wearing the metal. Rust, pitting or etched marks if water or corrosive fluids got in. Grease that looks gritty, dark or contains visible particles.
Why it happens: Hard particles that get rolled through the contact leave dents. Each dent concentrates stress and can start fatigue cracks, which shortens life. Abrasive particles also wear the surfaces and increase clearance. Water breaks down lubricant and causes corrosion.
Common causes:
- Worn or damaged seals
- Dirty assembly conditions
- Dirty grease or oil, or unclean grease guns and tools
- Poor housing sealing or breather arrangements
- Open bearings used where a sealed type is needed
How to prevent it:
- Choose the right seal or shield for the environment.
- Assemble in clean conditions and keep bearings in their packaging until fitting.
- Use clean lubricant and clean tools.
- Check and replace seals during service.
- Improve housing design and sealing if the environment is dirty or wet.
Fretting
What it is: Wear caused by tiny repeated movements between two surfaces that should be fixed together, such as the bearing ring and the shaft or housing.
What it looks like: Reddish-brown or dark rust-coloured powder or staining on the bore, the outside diameter or the faces of the rings, and on the shaft or housing seat. The surfaces may look worn, polished or pitted. The fit feels loose where it should be tight.
Why it happens: When a ring is not held firmly enough, vibration or load changes cause very small movements. These break down the surface and produce fine debris that looks like rust. The loose fit then gets worse over time.
Common causes:
- Fit between the ring and shaft or housing is too loose
- Housing or shaft out of tolerance or out of round
- Heavy vibration or load that changes direction
- Soft or worn mating parts
- Wrong tolerance on the shaft or housing for the load
How to prevent it:
- Use the correct fit for the load, speed and which ring rotates.
- Check the shaft and housing dimensions and roundness.
- Make sure the mating parts are stiff enough and in good condition.
- Support the ring properly and use correct locking or clamping.
- Where a looser fit is needed, consider anti-fretting measures recommended for your application.
False Brinelling
What it is: Damage that looks like small dents in the raceway, but is actually caused by wear from vibration while the bearing is not rotating.
What it looks like: Shallow, evenly spaced marks or depressions in the raceway, spaced at the same distance as the rolling elements. The marks often look bright, polished or reddish, and the edges are not as sharp as true dents. The rest of the raceway may look normal.
Why it happens: When a bearing stands still but vibrates, such as during transport, storage next to a running machine or on standby equipment, the rolling elements rock slightly in the same spot. This pushes grease out of the contact and wears the surface through metal-to-metal contact. Later, when the machine starts, the bearing is noisy and may fail early.
Common causes:
- Vibration during shipping of machines with fitted bearings
- Standby machines next to other running machinery
- Machines idle for long periods with vibration from outside
- Very small oscillating movements that never complete a rotation
How to prevent it:
- Protect machines during transport by locking the shaft or rotor and using proper packing.
- Isolate standby equipment from vibration where possible.
- Rotate idle equipment periodically.
- Consider a grease designed for oscillating or vibrating applications.
- For applications with constant small oscillation, discuss the duty with your bearing supplier.
False brinelling is often confused with true brinelling, which is a permanent dent from a shock or overload, usually with sharp-edged marks. The cause and fix are different, so look closely.
Overheating
What it is: Damage caused by the bearing running hotter than it was designed for.
What it looks like: Discolouration of the rings and rolling elements, from straw or brown to blue or black. Grease that is burnt, hard, dry or carbonised, or oil that has darkened and degraded. Softened steel and loss of hardness in severe cases. Seals or cages that are deformed or melted. Smeared or welded surfaces where metal contact has occurred.
Why it happens: Heat reduces the lubricant’s ability to protect, reduces the clearance inside the bearing, and can soften the steel. Once clearance is lost, the bearing runs hotter still, and failure can happen quickly.
Common causes:
- Too much or too little grease
- Wrong lubricant for the speed and temperature
- Too little clearance, or too much preload
- Excess speed or load for the bearing
- Heat from nearby parts such as motors or engines
- Poor ventilation or cooling
- Seals that create too much friction
How to prevent it:
- Choose a lubricant rated for the temperature and speed, and fill to the recommended amount.
- Check internal clearance and pick a larger group such as C3 for hot applications.
- Make sure the bearing is rated for the speed and load.
- Improve cooling and keep heat sources away from the bearing.
- Monitor temperature in service to catch problems early.
Other Failure Causes to Check
Poor lubrication: Not enough lubricant, the wrong type or old lubricant leads to wear, smearing and overheating.
Misalignment: Shows as a wear track that is off-centre or uneven around the raceway, often with early fatigue.
Incorrect mounting: Hammering a bearing or pressing it on through the wrong ring can leave dents and cracks.
Electrical damage: Current passing through a bearing, common with motors and drives, creates small pits and a washboard pattern called fluting.
Corrosion: Rust from moisture, condensation or poor storage damages the surfaces and starts fatigue.
How to Inspect a Failed Bearing
- Record the application: load, speed, temperature, environment, operating hours and recent changes.
- Photograph the bearing as removed, including grease, seals and the surrounding parts.
- Check the grease or oil for colour, texture, particles and water.
- Inspect the raceways and rolling elements for the type and position of the damage.
- Look at the wear pattern on the raceway. Its position and shape show how the bearing was loaded.
- Check the bore, outside diameter and faces for signs of fretting or ring movement.
- Inspect the shaft and housing for wear, damage and out-of-tolerance dimensions.
- Decide the most likely cause, fix it, and only then fit the new bearing.
If the cause is unclear or the failure is repeated, send the bearing and application details to your supplier for analysis.
Frequently Asked Questions
What is the most common cause of bearing failure?
Poor lubrication and contamination are the most common causes of early bearing failure, along with incorrect mounting and fits. Fatigue is the normal end of life for a bearing that has run correctly.
What is the difference between false brinelling and true brinelling?
True brinelling is a permanent dent caused by shock or overload. False brinelling is wear caused by small vibration movements while the bearing is not rotating, and it leaves marks spaced like the rolling elements.
How can I tell if a bearing failed from overheating?
Look for blue, brown or black discolouration of the metal, burnt or hardened grease, and damaged seals or cages. In severe cases the steel may be softened.
What does fretting look like on a bearing?
Fretting usually shows as reddish-brown, rust-like powder or staining on the bore or outside diameter of the ring and on the shaft or housing seat, which means the ring is moving where it should be fixed.
Can a failed bearing be reused?
No. A bearing with spalling, dents, fretting, overheating damage or any raceway damage should be replaced. Fix the cause of the failure before fitting a new one.
Need Help With Bearing Failures?
If you have bearings that fail early or you want to check a specification before the next batch, send us the failed bearing details, photos, drawing or application information. We will help you work out the likely cause and recommend the right bearing, fit and lubrication for the job.