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Common Cracking Phenomena and Their Causes in Bearing Rings

I. Common Cracking of Bearing Rings and Its Causes Bearing rings are one of the critical components of a bearing. During service, cracking or fracture of bearing rings is a frequently observed form of damage. Some bearings experience ring cracking even in the early stages of use, while others fail due to fatigue over time. Drawing on our understanding of bearing technology, this article examines the typical types of cracking that occur in bearing rings and analyzes the underlying causes.

I. Common Cracking of Bearing Rings and Their Causes

Bearing rings are one of the critical components of a bearing. During service, cracking or fracture of the bearing ring is a common form of failure. Some bearings develop cracks early in their life, while others fail due to fatigue over time. Drawing on our understanding of bearing technology, this article explores the typical types of cracking observed in bearing rings and analyzes the underlying causes.

1. Cracking of the outer ring of deep groove ball bearings and its causes

If the bearing outer ring fractures, the fracture surface is approximately perpendicular to the surface. The fracture initiates at the lower surface of the outer raceway on the right side of the figure and propagates rapidly toward the outer surface and to the left until complete failure. The fracture surface of the outer ring exhibits no obvious plastic deformation, displaying brittle fracture characteristics.

Upon analysis, it was found that during the heat‑treatment process, the protective atmosphere within the furnace consists of a mixture of various gases, including oxidizing, neutral, reducing, and carburizing gases. At elevated temperatures, the associated chemical reactions are highly complex; whether they involve decarburization or carburization, these processes can, under certain conditions, reach equilibrium and even proceed as reversible reactions, with the participation of free oxygen atoms.

Properly select and design heating parameters, including the heating medium, heating rate, heating temperature, and holding time; rigorously maintain furnace temperature uniformity, ensuring that temperature fluctuations remain within acceptable limits. By controlling the carbon potential inside the furnace, precisely regulate the carbon concentration and its gradient in bearing rings, thereby guaranteeing the quality of heat treatment and extending the service life of the rings.

2. Cracked Bearing Rings and Their Causes

If a bearing cracks during service, contact‑fatigue spalling on the bearing rings will lead to instability. Coupled with the material’s high hardness and significant brittleness, a sudden brittle fracture initiates at the localized spalled region—manifesting as relatively straight macroscopic cracks. The microscopic fracture surface is fairly planar, exhibiting cleavage‑like characteristics and propagating rapidly; moreover, the rapidly propagating zone occupies the majority of the fracture cross‑section.

Factors contributing to contact fatigue include the material’s microstructure, surface hardening processes, workpiece surface roughness, lubricants, and stress levels.

3. Cracking on the raceway surface of the bearing outer ring and its causes

After acid‑etching a fractured bearing outer ring, visual inspection reveals severe black burn marks on the raceway surface, along with transverse cracks arranged in parallel and roughly perpendicular to the grinding direction. Upon fracturing, a crescent‑shaped burn layer is observed at the fracture surface, which exhibits a fine, porcelain‑like texture; crack initiation occurs at the raceway and propagates rapidly inward until complete fracture.

The parallel cracks on the raceway surface of the bearing outer ring are typical grinding‑induced cracks, and their occurrence is primarily attributable to excessive material removal during grinding and unfavorable grinding process conditions. In addition, insufficient tempering of the bearing outer ring further increases its susceptibility to grinding‑related cracking.

This phenomenon can occur even with relatively low current intensities; over time, the annular pits evolve into wavy grooves. These wavy grooves are observed only on the contact surfaces of the roller and raceway, not on the balls, where only a darkening of the surface color is evident. The wavy grooves are evenly spaced, and the bottoms of the grooves on the raceways exhibit a darker hue.

4. Cracking of the Inner Raceway Surface of Bearings and Its Causes

After rough grinding of the inner ring raceway surface, magnetic particle inspection revealed numerous fine cracks on both sides of the raceway, particularly near the oil grooves. In some cases, several deeper cracks perpendicular to the grinding direction of the abrasive wheel, along with peeling, were observed on individual rings. Following wire EDM, entire sections of material detached from the raceway surface. Subsequent hot acid pickling disclosed cracks on both sides of the inner ring raceway; these cracks were predominantly network‑like, with some also appearing as straight cracks oriented perpendicular to the grinding direction.

To prevent the formation of grinding cracks, it is necessary to reduce the generation of grinding heat and accelerate its dissipation.

5. Cracking of the bearing gear ring and its causes

At low equivalent stress amplitudes and within the long‑life regime, the crack initiation life is primarily governed by the crack initiation threshold. Under the same equivalent stress amplitude, a higher crack initiation threshold corresponds to a longer crack initiation life; to achieve a high crack initiation threshold, it is essential to enhance the material’s yield strength. During prolonged service, the gear ring experiences oxidative corrosion on the inner wall of its boreholes, with varying degrees of corrosion depending on the specific hole and its location; at the bottoms of the corrosion pits, microcracks have even been observed.

Since the gear ring is subjected not only to service stresses but also to tangential tensile stresses, once a crack initiates, these tangential tensile stresses accelerate its propagation. Moreover, because the gear ring is rigidly mounted on the equipment and constrained on both sides, the amplitude of vibrational excitation is relatively small. Coupled with the lower stress levels, the crack propagates via a prolonged fatigue‑dominated mechanism, resulting in a large proportion of the fracture surface being occupied by the crack‑propagation zone.

Six Common Questions and Solutions in Rolling Bearing Installation

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