Bearing Selection Criteria: Loads, Speed ​​Limits and Lifespan Calculation (ISO 281)

1. Introduction: The Maintenance Engineering Challenge

In the aerospace and energy industries, equipment reliability is critical. Each component contributes to overall performance and operational safety. Bearings, fundamental mechanical elements, support loads and reduce friction in rotating mechanisms. Their incorrect selection can lead to unplanned downtime, high maintenance costs and, in extreme cases, catastrophic failures. A bearing is a strategic investment, not just a spare part. Understanding selection criteria is essential for the sustainability and efficiency of production systems.

Load, rotational speed and operating environment dictate bearing selection. Rigorous analysis, based on engineering principles and established standards such as ISO 281, is imperative. At UNITEC-D GmbH, we provide certified bearings and precise technical advice to guarantee the performance of industrial installations.

2. Fundamentals of Bearings

2.1. Types and Geometries

Bearings are mainly classified into ball bearings and roller bearings. Each type has distinct load and speed characteristics:

• Ball bearings: Ideal for moderate radial loads and high speeds. The point contact of the balls allows minimal friction.
• Roller bearings: Designed for heavy radial loads and, in some cases, for axial loads. The linear (or quasi-linear) contact of the rollers distributes the load better.

Subcategories include angular contact ball bearings (for combined loads), tapered roller bearings (high radial and axial loads, separable), spherical ball or roller bearings (misalignment tolerance), and thrust bearings (pure axial loads).

2.2. Loads and Constraints

Bearings are subjected to three main types of loads:

• Radial Load: Acts perpendicular to the axis of rotation.
• Axial (or Thrust) Load: Acts parallel to the axis of rotation.
• Combined Load: A combination of radial and axial loads.

The contact stress, particularly the Hertz stress at the point of contact between the rolling element and the raceways, is a critical factor. It determines the potential for surface fatigue, the main mode of bearing failure. A thorough understanding of load transmission mechanisms is essential for selecting a bearing capable of withstanding these stresses without premature failure.

3. Technical Specifications and Standards

Standardization guarantees the interchangeability, quality and performance of bearings. International standards, particularly those of the International Organization for Standardization (ISO), are essential references.

3.1. Nominal Static and Dynamic Loads

• Nominal Static Load (C₀): Defines the maximum load that a bearing can support in static conditions without undergoing measurable permanent plastic deformation. Complies with the ISO 76:2006 standard. A permanent deformation of 0.0001 times the diameter of the rolling element is the criterion generally adopted.
• Rated Dynamic Load (C): Represents the constant load (pure radial for radial bearings, pure axial for thrust bearings) that a group of identical bearings can withstand for one million revolutions (L₁₀ = 10⁶) before 10% of them show signs of surface fatigue. Defined by the ISO 281:2007 standard.

3.2. Nominal Lifespan according to ISO 281

The nominal lifespan (L₁₀) is an essential criterion. The ISO 281:2007 standard specifies the calculation method. The basic formula is:

L₁₀ = (C / P)ᵖ

Where:

• L₁₀: Rated life in millions of revolutions.
• C: Basic dynamic nominal load (kN).
• P: Equivalent dynamic load (kN).
• p: Lifetime exponent (p=3 for ball bearings, p=10/3 for roller bearings).

For specific applications, the ISO 281 also introduces modified rated life (L₁₀ₘ) which takes into account factors such as reliability, material properties, lubrication conditions and contamination conditions.

3.3. Speed Limits

Limit speeds are crucial to avoid excessive heating and deterioration of the lubricant. They depend on:

• Bearing Type and Size: Ball bearings generally support higher speeds.
• Cage type: Polymer or machined brass cages allow higher speeds than pressed steel cages.
• Lubrication: Oil lubrication allows higher speeds than grease.
• Precision: High precision bearings (NF EN ISO 1132-1 tolerances) allow higher speeds.

There is a reference limit speed (theoretical speed) and an operating limit speed (practical speed, taking into account real load and lubrication conditions).

4. Selection and Sizing Guide

The selection of a bearing is an iterative process that integrates several technical and environmental parameters.

4.1. Selection Parameters

• Load: The magnitude, direction and character of the load (constant, variable, shock) are determining factors.
• Speed: The maximum rotational speed must be compatible with the limits of the bearing and lubricant.
• Required life: Determined by application (e.g. 20,000 hours for an energy critical pump, 5,000 hours for an auxiliary motor).
• Rigidity: Essential for applications requiring high precision.
• Precision: Complies with ISO tolerance classes (for example, P0, P6, P5, P4, P2 according to NF E 22-003).
• Operating temperature: Standard bearings typically withstand up to 120°C. Special versions are available for extreme temperatures (-60°C to +350°C).
• Lubrication: Choice between grease or oil, depending on speed, temperature and environment.
• Environment:Presence of dust, humidity, chemicals, vibrations, shocks.
• Misalignment: The ability of the bearing to tolerate misalignment of the shaft or bearing.

4.2. Bearing Selection Decision Matrix

The table below presents a structured approach for initial selection, based on key application requirements. This matrix is ​​a starting point and must be supplemented by precise calculations.

5. Good Installation and Commissioning Practices

Proper installation is as critical as selection. Even the most efficient bearing will fail prematurely if it is incorrectly mounted.

5.1. Preparation and Tools

• Cleanliness: The mounting environment must be free of dust and contaminants. A 50 µm grain of sand can significantly reduce service life.
• Tools: Use specific assembly tools (chucks, sockets, hydraulic presses, induction heaters) to avoid damaging the raceways or cages. Never strike directly on the rolling element or cage.
• Tolerances: Check the fit tolerances of the shaft and housing (for example, interference fit for the rotating ring). The ISO 286-1 and ISO 286-2 standards define the tolerance systems.

5.2. Mounting and Misalignment

Assembly must be carried out precisely. For interference fits, heating (induction or oven) of the inner ring is preferred. The recommended temperature difference is often 80°C to 100°C above ambient temperature.

Excessive shaft or bearing misalignment is a common cause of failure. Use laser alignment tools to ensure that the couplings are aligned to within 0.05 mm (typical tolerance for high speeds). Spherical bearings tolerate up to 3° of static misalignment, but the reduction in service life remains a factor to consider.

5.3. Initial Lubrication and Break-in

Apply the appropriate lubricant (grease or oil) during assembly. Follow the type, quantity and method of application specified by the manufacturer. For a grease, the initial filling is often 30% to 50% of the free volume of the bearing. An initial break-in at reduced load and progressive speed is recommended to allow the lubricant to stabilize and reduce residual stresses.

6. Failure Modes and Root Cause Analysis

Identifying the failure mode is crucial to correcting the cause and preventing recurrence. Here are common faults and their visual indicators:

• Rolling Fatigue (Spalling): Appears in the form of small cracks and tearing of material on the raceways or rolling elements. Main cause: excessive contact stresses, lifespan exceeded, lack of lubrication.
• Abrasive wear: Matte and scratched surfaces. Cause: contamination by hard particles (dust, abrasives). Improve lubricant sealing and filtration.
• Adhesive wear (Grip/Scuffing): Damage caused by metal-to-metal contact due to insufficient lubricating film. Appears as rub marks on surfaces. Cause: inadequate lubrication, speed too low, excessive load.
• Corrosion: Rusty spots or pitting. Cause: ingress of moisture or corrosive substances. Improve sealing or use stainless steel bearings.
• Brinelling (False Brinelling): Imprints or marks on the raceways, without tearing of material. Cause: Excessive static loads (Brinelling) or small oscillations under load (False Brinelling) without complete rotation, resulting in contact corrosion.
• Lubricant degradation: Discoloration, thickening or liquefaction of the lubricant. Cause: excessive temperature, contamination, lubricant life exceeded.
• Electrical erosion: Craters or grooves on raceways and rolling elements. Cause: passage of electric current through the bearing. Use insulated bearings or effective grounding systems, complying with the EN 60034-17 standard for electric motors.

7. Predictive Maintenance and Condition Monitoring

Predictive maintenance makes it possible to anticipate failures and optimize interventions, thus reducing costs and increasing equipment availability.

• Vibration analysis (ISO 10816): Most common technique. Bearing defects generate characteristic frequencies that can be detected and analyzed. An RMS level above 4.5 mm/s (depending on machine class) may indicate a problem.
• Acoustic analysis (acoustic emission): Detects the first signs of deterioration, often before vibration analysis. Applicable for low or variable speeds.
• Temperature monitoring: Abnormal heating indicates excessive friction, insufficient lubrication or overload. Use of temperature sensors or thermographic cameras. An increase of 10°C above the reference temperature can be a warning sign.
• Oil Analysis: For oil-lubricated systems, analysis of wear particles (ferrography) and lubricant properties provides information on the condition of the bearing and system.
• Visual inspection: Look for signs of wear, corrosion or damage to seals.

Implementing these techniques in a proactive maintenance program, in compliance with NF X 60-000 directives, improves operational resilience.

8. Bearing Type Comparison Matrix

This table compares the main characteristics of bearings commonly used in industrial applications, helping to narrow down the selection.

9. Conclusion

Bearing selection and maintenance are not simple steps, but engineering disciplines that directly influence the performance and profitability of industrial equipment. By relying on standards such as ISO 281 for life calculation and considering specific loads, speeds and environments, engineers can make informed choices that ensure reliable and safe operation. Particular attention to installation practices and the implementation of predictive maintenance, in accordance with mechanical engineering standards, is also decisive. UNITEC-D is your partner for certified bearings and expert technical support. Explore our full range of solutions in our online catalog: https://www.unitecd.com/e-catalog/

10. References

• ISO 281:2007, Bearings – Dynamic load ratings and rated lifespans.
• ISO 15:2017, Bearings – Radial bearings – Main dimensions – General plan.
• ISO 76:2006, Bearings – Static rated loads.
• ISO 10816-1:1995, Mechanical vibrations – Assessment of machine vibrations by measurement on non-rotating parts – Part 1: General guidelines.
• SKF, Bearings Manual, General Edition.