Analysis of bearing failures: visual identification of spalling, brinelling, fretting and electrical erosion

1. Introduction

Rolling bearings are critical components of almost any industrial equipment in Ukrainian heavy industry, machinery manufacturing and energy sectors. According to the technical audit data of UNITEC-D GmbH, about 35% of unplanned stoppages of rotating equipment are related to premature failure of bearing units. Calculated bearing life according to standard ISO 281 ($L_{10h}$) is rarely achieved in practice due to violations of operating, mounting or lubrication conditions. The main task of a reliability engineer is not just to replace the failed component, but to accurately determine the root cause (Root Cause Analysis). Visual and microscopic analysis of rolling tracks and rolling elements allows to identify mechanisms of metal failure before a catastrophic equipment stoppage occurs.

2. Fundamental Principles

The operating mechanism of the rolling bearing is based on the transmission of radial and axial loads through elastic deformation of the contact surfaces of the rolling elements and the rings. The contact stresses according to Hertz in the contact zones reach 1500–2500 MPa for steels of grade 100Cr6 (ShX15 according to GOST / EN ISO 683-17). Under the action of cyclic loads, shear stresses arise in the subsurface layers of the metal. After $10^6 - 10^9$ cycles, this leads to accumulation of fatigue damage, formation of microcracks and further material flaking (spalling).

Violation of the conditions of surface separation by lubricating film (calculated viscosity of the lubricant below the minimum according to ISO/TR 28168) causes metallic contact, frictional wear, micro-welding and destruction of the crystalline lattice of the surface layer. Any deviation from the calculated operating parameters changes the stress-strain state of the node, leaving specific visual marker traces on the metal.

3. Technical Specifications and Standards

Classification of failures, methods of their measurement and evaluation are regulated by international and national standards harmonized in Ukraine:

• ISO 15243:2017 — Rolling Bearings. Dictionary, Terminology and Classification of Failures and Failures of Rolling Bearings. This standard is fundamental for defining the six main groups of failures.
• ISO 281:2007/Amd 2:2019 — Rolling Bearings. Dynamic Load Rating and Calculated Life.
• ISO 10816 / ISO 14819 — Vibration Condition Monitoring of Industrial Machinery.
• DSTU GOST 520:2014 — Rolling bearings. General technical specifications.
• DIN 51825 — Plastic Lubricants. Classification and Requirements.

4. Selection and Sizing of Bearings

Correct selection of the bearing according to the static ($C_0$) and dynamic ($C$) load capacity prevents overloading. The equivalent dynamic load $P$ is calculated by the formula:

P = X * F_r + Y * F_a

Where $F_r$ is the radial load, $F_a$ is the axial load, and $X$ and $Y$ are the radial and axial load factors respectively according to the manufacturer's catalog.

5. Best Practices for Installation and Commissioning

About 16% of premature failures occur due to errors during installation. Compliance with the installation schedule is mandatory:

• Посадка з натягом: Заборонено передавати зусилля запресовування через тіла кочення (уникнення брінеллінгу). Монтаж здійснюється виключно на внутрішнє кільце за допомогою індукційних нагрівачів (температура нагріву не вище +110°C згідно з рекомендаціями SKF/FAG) або гідравлічних гайок.
• Cleanliness: Installation should be carried out in conditions free from dust and moisture. Use of contaminated lubricant causes abrasive wear in the first hours of operation.
• Gap Check: The radial internal clearance after installation is checked with a feeler gauge or micrometer according to standards ISO 5753.

6. Failure Modes and Root Cause Analysis

According to ISO 15243, defects are classified by visual signs. Below is a detailed description of four critical types of failures that UNITEC-D engineers most often encounter on Ukrainian plants.

6.1. Spalling (Fatigue)

Definition: Material removal from the working surface of the ring or rolling body, followed by the formation of cavities (cracks).

Visual indicators: Sharp edges of cavities, rough bottom of the recess, matte gray color in the failure zone. Microcracks are observed nearby.

Cause: Achievement of the calculated bearing life ($L_{10h}$), chronic overload, excessive interference fit, or insufficient lubricant viscosity (lubricant factor $\kappa < 1$).

6.2. True and False Brinelling (Brinelling)

True brinelling: Occurs under static loads exceeding the material's yield limit, or from impact loads during stopping. Visually: indentations on the raceway, which exactly repeat the pitch of the rolling elements. On the bottom of the indentations, signs of plastic deformation and metal extrusion are visible.

False brinelling (Fretting corrosion / False brinelling): Occurs due to micro-vibrations of non-operational equipment (for example, standby pumps or equipment during transportation by rail transport). Visually: cup-shaped defects with formation of oxidation products (reddish-brown iron oxide powder $ ext{Fe}_2 ext{O}_3$, "rusty sediment"). Metallic luster on the bottom of the cups is partially preserved, unlike true brinelling.

6.3. Fretting (Fretting / Axial Sliding)

Definition: Destruction of seating surfaces due to minimal relative micro-movements of parts under vibration.

Visual indicators: Rust-colored spots appear on the shaft or housing mounting surfaces, material wear (increased clearance), oxide layer damage.

Cause: Insufficient interference fit of the bearing on the shaft or housing, loosening of locking nuts, vibrational loads in non-operational state.

6.4. Electrical Erosion (Electrical Erosion / Stray Currents)

Definition: Passage of electric current through the internal elements of the bearing, accompanied by micro-arc discharges.

Visual indicators: Dull surface of the rolling paths, densely covered with micro-craters (diameter from 10 to 100 micrometers). In severe cases, a "fluting" or "embossing" effect (fluting) is observed — parallel dark grooves with a pitch corresponding to the passage of rolling elements. The metal in the crater areas shows signs of tempering (martensitic transformation in micro-volumes).

Cause: Stray currents from welding work on the aggregate frame, magnetic field asymmetry of the electric motor, motor operation from an inverter without use of insulated bearings (for example, with ceramic coating on the rings hybrid bearings) or brushes for potential elimination of the shaft.

7. Prediction of Technical Condition and Monitoring

To prevent sudden accidents on Ukrainian production sites, a complex set of diagnostic methods is applied:

• Вібраційний аналіз: Вимірювання спектру вібрації (FFT) та високочастотні методи (demodulation, shock pulse method - SPM). Поява гармонік частот дефекту зовнішнього кільця ($BPFO$), внутрішнього кільця ($BPFI$), тіл кочення ($BSF$) та сепаратора ($FTF$) свідчить про розвиток локальних дефектів.
• Oil Analysis (Ferrography): Determination of wear products concentration in oil (spectrometry), presence of metallic particles larger than 20 micrometers indicates active track damage.
• Thermometry: Bearing housing temperature monitoring. Exceeding the threshold of +75°C... +95°C (depending on the type of grease) indicates problems with lubrication or overload.

8. Comparative Defect Matrix of Bearings

9. Conclusion

Accurate identification of defects in rolling bearings is key to reducing maintenance costs of industrial equipment. Ignoring the first visual signs of electrical erosion, fretting, or fatigue spalling inevitably leads to catastrophic failure of units. Engineers from UNITEC-D GmbH provide supply of certified bearings (according to European quality standards), as well as expert support for component selection for Ukrainian industrial enterprises.

For selecting reliable bearing units, industrial lubricants, and mounting tools, refer to our electronic catalog: UNITEC-D E-Catalog.

10. References

1. ISO 15243:2017. Rolling bearings — Vocabulary, and classification of damage and failures in bearings.
2. ISO 281:2007. Rolling Bearings — Dynamic Load Ratings and Rating Life.
3. SKF General Catalogue. Bearing Damage and Failure Analysis Guidelines. Publication 10000 EN.
4. DIN 51825: Lubricating greases K — Classification and requirements.
5. GOST 520-2014 / DSTU GOST 520:2014. Rolling Bearings. General Technical Specifications.