1. Introduction
The reliability of rotational equipment in Ukrainian heavy industry enterprises directly depends on the condition of rolling bearings. Operating conditions on metallurgical plants, mining and processing plants, and energy enterprises are characterized by high loads, contamination, and vibrations. Premature failure of bearings leads to emergency shutdowns and significant financial losses.
According to maintenance statistics, nearly 34% of premature failures of electric motors and reducers are caused by problems with bearings. The main goal of the reliability engineer is not just to replace the damaged component, but to perform an accurate root cause analysis (RCA — Root Cause Analysis) using visual and microscopic examination of rolling contact paths and rolling elements. Early detection of defects and compliance with maintenance schedules allow achieving an average time between failures (MTBF) over 50 000 hours for heavy industrial fans and pumps.
2. Fundamental Principles
Rolling bearing operation is based on the conversion of sliding friction force into rolling friction. The main stresses occur at the contact points of the rolling elements (balls or rollers) with the raceway of the rings. Contact stresses according to Hertzian contact stress in these areas are critical parameters and are determined by the formula:
p_max = (3 * F) / (2 * pi * a * b)
Where:F — radial or axial load,a, b — semi-axes of the Hertzian contact ellipse.
Under normal operating conditions, the maximum stresses are not concentrated on the surface of the metal, but at a depth of approximately 0.1–0.4 mm below it (depending on the bearing size and load). It is precisely in this zone of maximum shear stress that material microstructural changes (formation of dark wear structures) begin, which over time lead to the formation of fatigue cracks that propagate to the surface. Understanding this mechanism is key to differentiating surface defects (incorrectly mounted or externally damaged) from internal fatigue failures.
3. Technical Specifications and Standards
Failure analysis of bearings is regulated by international and national standards, which define terminology, measurement methods and acceptance criteria for the condition of components:
- ISO 15243:2017 — Rolling Bearings. Dictionary. Damage and Failure of Rolling Bearings. Classification of Defects According to Six Main Groups.
- ISO 281:2007 — Rolling Bearings. Dynamic Load Rating and Nominal Life.
- DSTU ISO 10816 / ISO 10816-3 — Vibration Monitoring of Machine Condition Based on Vibration Measurements on Unconsidered Components.
- DIN 51825 — Lubricants. Plastic lubricants K… for roller bearings and sliding bearings.
- CE / UkrSEPRO — Compliance Norms for Industrial Equipment and Components Supplied to the Ukrainian Market.
- Shaft fit control: Check the tolerances of the shafts (according to ISO h5, h6, j6) and housings (ISO H7, J7) using micrometers and plug gauges. Form deviations (roundness, taper) must not exceed 50% of the tolerance field.
- Heating methods: Open flame or soldering lamps must not be used. Use temperature-controlled induction heaters (not exceeding +110 °C for standard bearings with thermoset).
- Installation torque: Never transmit pressing force through the rolling elements. The force must be applied exclusively to that ring which is mounted with interference fit (press fit).
- Clean working area: Contamination with abrasive dust (especially on cement plants and GZK) during installation reduces bearing life by 70%.
- Vibration analysis (spectral analysis): Detection of characteristic frequencies of defects (BPFO — outer race frequency, BPFI — inner race frequency, BSF — rolling element frequency, FTF — separator frequency).
- Impulse Method (SPM / Spike Energy): Effective for detecting high-frequency spikes caused by the initial stage of erosion and lubricant film damage.
- Thermography: Bearing housing temperature monitoring. A sharp increase in temperature above +80 °C indicates problems with lubrication or overload.
- Oil Analysis (Lubricating Oil and Grease): Spectrometry and ferrography for detection of wear products (iron, copper particles) and assessment of the degree of degradation of the base oil.
- ISO 15243:2017. Rolling Bearings — Vocabulary — Damage and Failures.
- ISO 281:2007. Rolling Bearings — Dynamic Load Ratings and Rating Life.
- SKF General Catalogue. Bearing Damage and Failure Analysis Guidelines.
- FAG Industrial Bearing Maintenance Manual. Publication WL 82 210/2 EA.
- IEEE Std 841-2009. IEEE Standard for Petroleum and Chemical Industry — Premium Efficiency Severe Duty Squirrel Cage Induction Motors.
4. Guide for Selection and Sizing
Correct selection of bearing and related components is based on the calculation of equivalent load and nominal life according to standard ISO 281.. The table below shows the selection criteria and permissible parameters for industrial conditions.
| Parameter | Standard Conditions | Heavy conditions (Metallurgy/Mineral Processing) | High-temperature mode |
|---|---|---|---|
| Maximum temperature (T) | Up to +90 °C | Up to +120 °C | Up to +250 °C (stabilization S1/S2) |
| Viscosity of the oil (ISO VG at 40 °C) | ISO VG 46 - 68 | ISO VG 150 - 320 | Special High-Temperature Pastes/Oils |
| Maximum radial clearance | CN (Normal) | C3 or C4 | C4 or special clearance |
| Accuracy class (ISO 492) | P0 (Normal) | P6 or P5 | P5 |
| Calculated life (L10h) | 20 000 hours | 50 000 hours | 15 000 hours |
5. Best Practices for Installation and Commissioning
About 16% of premature failures are caused by errors during installation. Engineers in installation at Ukrainian enterprises must follow these rules:
6. Failure Modes and Root Cause Analysis (RCA)
According to ISO 15243, the main defects have clear visual and microscopic signs:
6.1. Spalling (Fatigue Flaking)
Visual indicators: Surface indentations and cavities on the raceways or rolling elements, resulting from material flaking under cyclic contact stresses.
Root cause: Fatigue life exhaustion (reaching the calculated life L10), or premature fatigue due to overload, insufficient lubricant viscosity (capsule coefficient kappa < 1) or contamination by particles.
6.2. Brinelling
Візуальні індикатори: Заглиблення на доріжках кочення, розташовані з кроком, що точно відповідає відстані між тілами кочення. Може бути справжнім (механічним) або фальшивим (вібраційним).
Коренева причина: Справжній брінелінг виникає від ударних навантажень або статичних перевантажень, що перевищують межу текучості матеріалу (наприклад, удари молотком під час монтажу). Фальшивий брінелінг (істинний вібраційний знос) виникає на нерухомому обладнанні під дією зовнішньої вібрації від сусідніх агрегатів, коли мастильна плівка руйнується.
6.3. Fretting Corrosion
Візуальні індикатори: Червоно-коричневі або чорні продукти зносу (оксид заліза) на посадочних поверхнях (внутрішній діаметр кільця та вал, або зовнішній діаметр та корпус), що супроводжуються локальними мікрократерами.
Коренева причина: Мікрозміщення між сполученими поверхнями (повзучість кільця) через занадто слабку посадку або наявність вібрацій у з'єднанні.
6.4. Electrical Erosion (Electrical Erosion)
Візуальні індикатори: Мікрократери (пітинги) густо покривають доріжки кочення та тіла кочення, часто утворюючи матову сіру поверхню або характерний «гофрований» рисунок (бриж). У тяжких випадках метал випаровується мікродозами.
Коренева причина: Проходження електричного струму через підшипник (часто від перетворювачів частоти (VFD) через ємнісні струми вала). Для запобігання цьому використовуються гібридні підшипники (керамічні кульки) або ізольовані підшипники (з оксидним покриттям кільця).
7. Predictive Technical Maintenance and Condition Monitoring
To prevent emergency failures at industrial sites, a comprehensive monitoring system is applied:
8. Comparative Matrix of Damage Types
| Type of defect | External appearance | Main cause | Prevention Method |
|---|---|---|---|
| Spalling | Cavities, metal peeling | Material failure, overload, poor lubrication | Calculation of life based on ISO 281, viscosity control of lubricant |
| True brinelling | Pitch of rolling element bearings | Static or dynamic overloads, impacts during installation | Compliance with installation rules, exclusion of impact loads |
| Fretting corrosion | Rust stains, red dust on the seating | Loose fit, micro-movements on the shaft | Tolerance adjustment (increasing interference) |
| Electrical Erosion | High density microcellular, corrugation (bridge) | Leakage currents, unfiltered currents from inverters | Use of hybrid or insulated bearings |
9. Conclusion
Detailed analysis of bearing failures is a fundamental practice for improving the efficiency of industrial production in Ukraine. A systematic approach to identifying defects — from detecting the first signs of vibration to microscopic surface analysis — allows avoiding repeated failures and optimizing the costs of technical maintenance.
UNITEC-D GmbH supplies a wide range of industrial components, certified according to CE and UkrSEPRO standards, including high-quality bearings, specialized lubricants and mounting tools.
Familiarize yourself with the full product range and select the necessary components for your equipment in our electronic catalog UNITEC-D.
