Bearing Fatigue Failure: How Rolling Contact Stress Leads to Cracks and Spalling

Fatigue failure remains the predominant cause of premature bearing damage, responsible for over 60% of failures in industrial applications. Rolling element bearings—comprising an inner ring, outer ring, rolling elements (balls or rollers), and a cage—operate under cyclic loading, with rolling elements continuously transmitting forces between the rings.

Due to the small contact area between rolling elements and raceways, the resulting Hertzian contact stress is extremely high, especially under high-speed or heavy-load conditions. This concentrated stress environment leads to stress fatigue, manifesting as surface pitting, cracks, and eventually spalling.

What Is Stress Fatigue?

Stress fatigue refers to localized structural damage caused by repeated cyclic loading below the material’s ultimate tensile strength. While the bulk of the bearing remains elastically deformed, microscopic zones experience plastic deformation over time, eventually initiating failure. The process typically unfolds in three progressive stages:

1. Microcrack Initiation

• Occurs at subsurface levels (0.1–0.3 mm below the raceway surface).

• Caused by cyclic stress concentrations at microstructural imperfections.

2. Crack Propagation

• Cracks gradually grow along paths of maximum shear stress.

• Influenced by material defects and operational loading cycles.

3. Final Fracture

• Surface damage becomes visible as spalling or pitting.

• Once cracks reach a critical size, material detaches from the surface.

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Fatigue Considerations for Heavy-Duty Electric Vehicles

In large goods vehicles (LGVs) and heavy goods vehicles (HGVs)—especially electric variants—fatigue resistance is even more critical due to:

• Wider RPM Range: Electric motors operate across broader speed bands than combustion engines, increasing cyclic loading frequencies.

• Higher Torque Output: Heavier torque transmission requires bearings with enhanced fatigue strength.

• Battery Weight Impact: The added mass of traction batteries raises stress on drivetrain components, particularly wheel and motor bearings.

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Key Contributors to Stress Fatigue

√ Alternating Loads

Bearings in dynamic systems are constantly exposed to varying radial, axial, and bending loads. As rolling elements rotate, contact stress shifts cyclically, creating high stress concentrations over time.

√ Material Defects

Inclusions, micro-cracks, and voids within the bearing material can act as stress concentrators, accelerating fatigue initiation.

√ Poor Lubrication

Insufficient or degraded lubrication increases friction and heat, reducing fatigue strength and accelerating wear.

√ Improper Installation

Misalignment, incorrect fits, or over-tightening during installation can introduce unexpected stress, compromising bearing performance.

Understanding and mitigating stress fatigue is essential for ensuring long service life in demanding applications—especially electric heavy-duty vehicles. While advances in materials and simulation technology have enhanced fatigue resistance, proper bearing selection, installation, and maintenance are still key to performance and reliability.

Collaborating with experienced bearing manufacturers can provide optimized solutions tailored to your specific application. If your project demands high-performance, fatigue-resistant bearings, our team is here to assist with technical support and product recommendations.

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