Why Bearings Fail — And Why “Replace It” Isn’t Enough
When a spherical roller bearing fails, the immediate response is often to order a replacement and swap it in. But without understanding why the original bearing failed, the replacement will fail the same way — often faster, because the root cause was never addressed.
Systematic failure analysis answers three questions: What happened? Why did it happen? How do we prevent it from happening again?
The Five Most Common Failure Modes
1. Fatigue Spalling (Subsurface-Initiated)
Appearance: Flaking or pitting of the raceway surface, typically beginning below the surface and propagating outward. The spalled area has a characteristic “pitted” appearance with sharp edges.
Root cause: The bearing has reached the end of its calculated fatigue life (L10). The subsurface shear stress exceeds the material’s endurance limit after a predictable number of stress cycles.
Prevention: This is normal end-of-life. Verify that the actual service life matches the calculated L10h life. If failure occurs significantly before the calculated life, investigate load conditions — the actual load may exceed the design assumption.
2. Contamination-Induced Failure
Appearance: Fine abrasion marks on raceways and rollers. The running surfaces appear frosted or matte rather than polished. Under magnification, tiny indentations from hard particles are visible.
Root cause: Abrasive particles — dust, scale, sand, metal chips — enter the bearing through damaged or inadequate seals. The particles are crushed between rolling elements and raceways, creating stress concentrations that initiate spalling.
Prevention: Upgrade to sealed SB series bearings. Improve housing seal design. Implement cleaner handling procedures during installation. In heavily contaminated environments like mining and steel mills, sealed bearings are not optional — they are essential.
3. Lubrication Failure
Appearance: The bearing surfaces appear blued or discolored from heat. The grease is blackened, hardened, or has leaked out entirely. Raceways show smearing or galling rather than clean fatigue spalling.
Root cause: Inadequate lubrication — either insufficient quantity, incorrect viscosity, contamination of the lubricant, or excessively long relubrication intervals. The oil film breaks down, allowing metal-to-metal contact.
Prevention: Calculate correct relubrication intervals using the bearing manufacturer’s formula. Use the correct grease for the operating temperature and speed. For applications where regular relubrication is difficult, consider sealed SB series bearings with factory-filled grease.
4. Misalignment
Appearance: The wear pattern on the raceway is asymmetric — heavier on one side. The rollers show polished bands or wear marks that are not centered. The cage may show uneven wear on the prongs.
Root cause: The bearing inner and outer rings are not aligned. This can result from shaft deflection under load, housing bore misalignment, or incorrect mounting. Spherical roller bearings accommodate some misalignment (typically 1–2.5°), but excessive misalignment concentrates the load on a portion of the roller length.
Prevention: Check shaft and housing alignment during installation. Use self-aligning housing designs where possible. Verify that the shaft diameter and housing bore are within tolerance before mounting.
5. Cage Fracture
Appearance: The cage is broken, typically at a weld point (stamped steel cage) or at a prong root (machined brass cage). Loose cage fragments may be found in the bearing or housing.
Root cause: For stamped steel cages, vibration-induced fatigue at the weld points — common in vibrating screen applications. For brass cages, excessive speed beyond the cage’s design limit, or inadequate lubrication causing cage-roller seizure.
Prevention: Use machined brass cages (CA type) for vibrating screen and high-vibration applications. Never use stamped steel cage bearings in vibrating screens — they will fail prematurely. Verify that the operating speed is within the bearing’s limiting speed.
How to Document a Bearing Failure
Before discarding a failed bearing, document it:
- Photograph the bearing from multiple angles before cleaning
- Photograph the raceways and rollers after cleaning
- Record the operating conditions at the time of failure: speed, load, temperature, environment
- Note the bearing’s service life in hours
- Inspect the lubricant — color, consistency, presence of metal particles
- Check the housing and shaft for damage, wear, or misalignment
This documentation enables root cause analysis and prevents the same failure from recurring with the replacement bearing.