Spherical Roller Bearing Failure Analysis: Evidence and Diagnostic Checks

Why Bearings Fail — And Why “Replace It” Isn’t Enough

When a spherical roller bearing fails, replacing it without investigating the machine can leave the initiating condition unchanged. Preserve the evidence before cleaning or discarding components.

Systematic failure analysis answers three questions: What happened? Why did it happen? How do we prevent it from happening again?

Five Failure Modes to Investigate

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.

Possible causes: Rolling-contact fatigue, overload, contamination dents, lubrication-film breakdown, fit, or current leakage can produce different forms of surface distress. Appearance alone does not prove that the bearing reached its calculated rating life.

Checks: Compare the damage location and morphology with load history, lubricant condition, fits, alignment, material evidence, and the original L10h calculation assumptions.

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.

Checks: Inspect housing seals, purge paths, lubricant samples, handling practices, and nearby wear sources. A sealed bearing may be one option, but its speed, temperature, width, grease, and external sealing still need confirmation.

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.

Checks: Recalculate lubricant viscosity, quantity, and interval using the selected bearing and grease data. Inspect grease supply, purge paths, seals, temperature history, and compatibility before changing the interval or product.

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.

Possible causes: Shaft deflection, housing-bore misalignment, shoulder runout, mounting error, or axial displacement can shift the load pattern. The permissible misalignment is model- and arrangement-specific.

Checks: Measure shaft and housing geometry, shoulder runout, fit, axial freedom, and loaded deflection against the machine drawing and bearing data.

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.

Possible causes: Vibration, acceleration, roller skidding or skew, lubrication, mounting damage, excessive speed, misalignment, or a design mismatch can contribute to cage damage.

Checks: Confirm the complete suffix, cage construction, screen-duty designation where applicable, speed rating, lubrication, acceleration, and mounting history. Do not substitute cage types without model-specific approval.

How to Document a Bearing Failure

Before discarding a failed bearing, document it:

1. Photograph the bearing from multiple angles before cleaning
2. Photograph the raceways and rollers after cleaning
3. Record the operating conditions at the time of failure: speed, load, temperature, environment
4. Note the bearing’s service life in hours
5. Inspect the lubricant — color, consistency, presence of metal particles
6. Check the housing and shaft for damage, wear, or misalignment

This documentation supports a structured root-cause review and a more defensible replacement decision.