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Bearing Rating Life: Theory and Calculation Formulas
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By: 创始人
- Aug 29, 2025
- 1 Visit
For modern high-quality bearings, there can be a significant discrepancy between their nominal or basic rating life (a standardized theoretical value) and their actual service life in specific applications. The actual lifespan of a bearing is not fixed; it is heavily influenced by a range of real-world factors, including lubrication quality, contamination levels, installation alignment, proper mounting practices, and environmental conditions (e.g., temperature, humidity). To address this gap and provide a more accurate life prediction method, international standards and industry practices have been revised to incorporate correction factors that account for these practical variables. Below is a detailed explanation of the bearing rating life theory, the revised calculation formulas, and key parameters involved.
1. The Need for Revised Life Calculation Standards
Traditional basic rating life calculations (e.g., the original ISO 281 standard) focused primarily on the bearing’s material fatigue resistance under ideal conditions, without fully accounting for non-ideal factors common in real operations. For example:
- Poor lubrication can accelerate wear and fatigue, shortening life far beyond theoretical estimates.
- Contamination (e.g., dust, metal shavings) can cause abrasive damage, leading to premature failure.
- Material fatigue limits may vary under different load or temperature conditions.
To address these limitations, ISO 281:1990/Amd 2:2000 (an amendment to the international standard for rolling bearing life calculation) introduced a revised life equation. This updated formula incorporates correction factors to adjust for three critical real-world variables:
- Lubrication condition
- Contamination degree
- Material fatigue limit
Additionally, the standard allows bearing manufacturers to recommend customized methods for calculating these correction factors based on specific operating conditions, ensuring greater accuracy for application-specific life predictions.
2. Key Concepts in Rating Life Calculation
Before diving into formulas, it is essential to clarify core terms that underpin bearing life theory:
| Term | Definition |
|---|---|
| Basic Rated Life (L₁₀) | A standardized theoretical life value, defined as the life that 90% of a group of identical bearings will achieve (or exceed) under a given constant load, assuming ideal conditions (proper lubrication, no contamination, correct installation). It is typically expressed in million revolutions (10⁶ revolutions). |
| Fatigue Load Limit (Pᵤ) | The maximum load below which a bearing can operate indefinitely without developing material fatigue (i.e., no fatigue-induced failure will occur, regardless of operating time). This parameter is specific to each bearing type and is provided in manufacturer product catalogs (e.g., SKF’s product listings). |
| Viscosity Ratio (ηᵥ) | A parameter used to evaluate lubrication quality, calculated as the ratio of the actual operating viscosity of the lubricant to the minimum required viscosity for the bearing. A viscosity ratio ≥ 1 indicates adequate lubrication; values < 1 signal insufficient lubrication, which accelerates wear. |
| Contamination Coefficient (η_c) | A factor that quantifies the impact of contamination on bearing life. It ranges from 1 (clean, ideal environment) to values < 1 (contaminated environments, e.g., dusty or wet conditions), where lower values indicate more severe contamination and greater life reduction. |
3. SKF Rating Life Formula (Aligned with ISO 281:1990/Amd 2:2000)
As a leading bearing manufacturer, SKF has developed a practical rating life formula that adheres to the revised ISO standard. This formula integrates correction factors to reflect real operating conditions, making it widely used in industry.
3.1 SKF Rated Life (in Million Revolutions: Lₙₘ)
The SKF rated life formula calculates the life of a bearing for a target reliability level (not just the standard 90%). Its general form is:
Lₙₘ = a₁ × a_SKF × L₁₀
And since the basic rated life (L₁₀) is calculated using the classic dynamic load formula:
L₁₀ = (C / P)ᵖ
L₁₀ = (C / P)ᵖ
The full SKF rated life formula can be expanded as:
Lₙₘ = a₁ × a_SKF × (C / P)ᵖ
Lₙₘ = a₁ × a_SKF × (C / P)ᵖ
3.2 SKF Rated Life (in Operating Hours: Lₙₘₕ)
For applications where bearing speed is constant, it is often more intuitive to express life in operating hours (instead of million revolutions). By converting revolutions to time, the formula becomes:
Lₙₘₕ = a₁ × a_SKF × (10⁶ / (60n)) × L₁₀
Substituting L₁₀ with (C / P)ᵖ, the expanded hourly life formula is:
Lₙₘₕ = a₁ × a_SKF × (10⁶ / (60n)) × (C / P)ᵖ
Lₙₘₕ = a₁ × a_SKF × (10⁶ / (60n)) × (C / P)ᵖ
3.3 Definition of Parameters
Each variable in the formulas above has a specific meaning and unit, as outlined below:
| Parameter | Description | Unit |
|---|---|---|
| Lₙₘ | SKF rated life (for a reliability of (100 - n₁)%) | Million revolutions (10⁶ rev) |
| Lₙₘₕ | SKF rated life (for a reliability of (100 - n₁)%) | Operating hours (h) |
| L₁₀ | Basic rated life (for 90% reliability) | Million revolutions (10⁶ rev) |
| a₁ | Life reliability factor | Dimensionless (varies with target reliability) |
| a_SKF | SKF life correction factor (accounts for lubrication, contamination, and fatigue limit) | Dimensionless (calculated based on ηᵥ, η_c, and Pᵤ) |
| C | Basic rated dynamic load (the maximum constant radial load a bearing can withstand for L₁₀ life) | Kilonewtons (kN) |
| P | Equivalent dynamic load (a combined load that converts actual radial/axial loads into a single equivalent radial load) | Kilonewtons (kN) |
| n | Bearing rotational speed | Revolutions per minute (r/min) |
| p | Life equation exponent (material-dependent): - p = 3 for ball bearings - p = 10/3 (≈3.333) for roller bearings |
Dimensionless |
| n₁ | Failure probability (e.g., n₁ = 5 means a 5% failure probability, corresponding to 95% reliability) | Percentage (%) |
4. Application-Specific Adjustments to Life Units
While million revolutions and operating hours are the most common units for expressing bearing life, some industries require alternative units to align with their operational metrics. For example:
- Road and rail vehicles: Axle bearing life is often measured in kilometers (km) traveled, as this directly correlates with vehicle usage (e.g., a truck’s axle bearing life may be specified as 500,000 km).
- Wind turbines: Bearing life may be expressed in operating cycles (e.g., number of start-stop cycles) or megawatt-hours (MWh) of energy production, depending on maintenance and performance tracking needs.
In such cases, the base formulas (Lₙₘ or Lₙₘₕ) can be converted to the desired unit using application-specific conversion factors (e.g., km = average speed × operating hours).