1. Introduction
In the aerospace and energy sectors, the reliability of rotating equipment is essential. Bearing failure results in significant production downtime costs and major safety risks. Correct bearing sizing is not limited to choosing a geometric dimension; it requires rigorous analysis of operating conditions, applied loads and environmental factors. This guide provides the technical foundations to guarantee a service life that meets operational requirements.
2. Fundamentals
The theory of bearing life is based on rolling contact fatigue (Hertzian). Under the effect of cyclic stresses, micro-cracks develop under the surface of the tracks and rolling elements, inevitably leading to spalling. Rated life $L_{10}$ is defined by the ISO 281 standard as the life that at least 90% of a large group of apparently identical bearings will achieve or exceed. This probabilistic model is the reference for maintenance and design engineers.
3. Technical specifications and standards
Compliance with international standards is essential to ensure interchangeability and reliability:
- ISO 281: Methods for calculating rated life and basic dynamic load.
- ISO 76: Calculation of the basic static load capacity ($C_0$).
- ISO 15: Overall dimensions of radial bearings.
- DIN 51825: Classification of lubricating greases according to temperature and load conditions.
4. Selection guide and sizing
The selection is made by comparing the dynamic equivalent load ($P$) to the basic dynamic load capacity ($C$). The basic formula is: $L_{10} = (C/P)^p$.
Where $p=3$ for ball bearings and $p=10/3$ for roller bearings.
| Parameter | Selection criteria | Impact on design |
|---|---|---|
| Dynamic Load ($C$) | Must be greater than $P$ | Determines the fatigue load capacity |
| Static Charge ($C_0$) | $S_0 = C_0 / P_0 \ge 2$ | Avoids permanent plastic deformation |
| Speed Limit | $n \le n_{limit}$ | Limited by warm-up and kinematics |
| Contamination factor | $e_c$ (ISO 281) | Critical influence on actual lifespan |
5. Installation and commissioning
More than 30% of early failures are due to incorrect assembly. Best practices include:
- Adjustments: Respect of shaft and housing tolerances (ISO 286). Too tight a fit reduces internal clearance and causes excessive heating.
- Cleanliness: Maintaining ISO 4406 cleanliness is critical during handling.
- Lubrication: Use of lubricants meeting the required viscosity at operating temperature ($v \ge v_1$).
6. Failure Modes and Cause Analysis
A systematic analysis makes it possible to identify the origin of the problems:
- Spalling (Fatigue): Indicator of reached nominal life or overload.
- Brinelling: Permanent deformations due to excessive static shocks or vibrations when stationary.
- False brinelling: Wear due to micro-movements (fretting) in the absence of rotation.
- Inadequate lubrication: Trace of blue coloring (heating) or abrasive wear.
7. Predictive maintenance and monitoring
The condition of the bearings must be continuously monitored. The main techniques are:
- Vibration analysis (ISO 10816): Detection of characteristic fault frequencies (inner ring, outer ring, rolling element).
- Oil analysis: Monitoring the concentration of metal particles (spectrometry) to detect abnormal wear.
- Thermography: Monitoring of abnormal temperature peaks (> 70-80°C continuously).
8. Bearing Type Comparison Matrix
| Bearing Type | Load capacity | Speed Capacity | Rigidity | Typical application |
|---|---|---|---|---|
| Ball bearing | Low | Very high | Moderate | Electric motors |
| Cylindrical roller bearing | High | High | High | Industrial gearboxes |
| Tapered roller bearing | Very high | Moderate | Very high | Hubs, gears |
| Spherical roller bearing | Very high | Low | Moderate | Wind turbines, conveyors |
9. Conclusion
Optimal bearing selection is a balance between load, speed, temperature and operating environment. By rigorously following the ISO 281 standard and applying predictive maintenance principles, engineers can significantly increase the lifespan of critical equipment. To consult our detailed technical specifications and access our complete range, visit our electronic catalog: UNITEC-D E-catalog.
10. References
- ISO 281:2007, Bearings - Basic dynamic loads and rated life.
- ISO 76:2006, Bearings - Basic static loads.
- Harris, T.A., Rolling Bearing Analysis, 5th Edition, CRC Press.
- DIN 51825, Lubricants - K greases - Classification and requirements.
- Technical maintenance manuals - Manufacturer standards (SKF, FAG, NTN).
