Radial Ball Bearings: life ratings

The basic rating life in millions of revolutions (L10) is determined in accordance with, the basic rating life in operating hours (L10h) is determined in accordance with the equation.

Rating life in revolutions

Rating life in operating hours

L10	106	The basic rating life in millions of revolutions, that is reached or exceeded by 90% of a sufficiently large number of apparently identical bearings before the first indications of material fatigue appear
L10h	h	The basic rating life in operating hours, that is reached or exceeded by 90% of a sufficiently large number of apparently identical bearings before the first indications of material fatigue appear
C	N	Basic dynamic load rating, see product tables
P	N	Equivalent dynamic bearing load
p	–	Life exponent; for roller bearings: p = 10/3 for ball bearings: p = 3
n	min-1	Operating speed (nominal speed)

Radial ball bearings are a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. Their primary function is to support radial and axial loads, allowing for smooth rotation of parts. These bearings can handle both radial and axial loads and are typically used in applications where the load is primarily radial.

In this article, we discuss generally accepted industry standards and  principles concerning life calculations for radial ball bearings.

What Is Rating Life On Radial Ball Bearings?

The concept of rating life in radial ball bearings refers to the estimated operational lifespan under specified conditions. This life rating is a statistical prediction based on factors such as load capacity, lubrication, and environmental conditions.  The rating life of radial ball bearings, often referred to as the L10 life, is defined with a precise statistical approach.

The L10 life indicates the number of revolutions at which 90% of a group of identical bearings will still be operational without experiencing fatigue failure. This is calculated under assumed constant load and rotational speed conditions, and it’s derived from the basic dynamic load rating, which is a mechanical limit of the bearing’s load-carrying capacity. The calculation of bearing life typically adheres to ISO or ABMA standards, incorporating factors like dynamic load capacity and applied load.

Factors Affecting the Life Ratings of Radial Ball Bearings

Fatigue theory as a principle

The basis of the rating life calculation is the  Lundberg and Palmgren’s fatigue theory which always gives a final rating life. With bearing manufacturing improvements a further model was developed to correct for the higher performance capabilities from current bearings.

The expanded calculation of the adjusted rating life takes account of the following 

• the bearing load
• the fatigue limit of the material
• the extent to which the surfaces are separated by the lubricant
• the cleanliness in the lubrication gap
• additives in the lubricant
• the internal load distribution and frictional conditions in the bearing

Dimensioning of rolling bearings

Engineers can select rolling bearings based on these factors

• rating life
• load carrying capacity
• operational reliability

Dynamic load carrying capacity and life

Basic dynamic load ratings

The dynamic load carrying capacity is described in terms of the basic dynamic load ratings. The basic dynamic load ratings are based on DIN ISO 281.

The basic dynamic load ratings for rolling bearings are matched to empirically proven performance standards published in previous FAG and INA catalogues.

The fatigue behavior of the material determines the dynamic load carrying capacity of the rolling bearing.

Dynamic load carrying capacity

The dynamic load carrying capacity is described in terms of the basic dynamic load rating and the basic rating life.

Factors influencing the fatigue life

The fatigue life is dependent on:

• the load
• the operating speed
• the statistical probability of the first appearance of failure

Basic dynamic load rating C

The basic dynamic load rating C applies to rotating rolling bearings. It is:

• a constant radial load Cr for radial bearings
• a constant, concentrically acting axial load Ca for axial bearings

The basic dynamic load rating C is that load of constant magnitude and direction which a sufficiently large number of apparently identical bearings can endure for a basic rating life of one million revolutions.

Calculation of the rating life

Calculation methods

The methods for calculating the rating life are:

• basic rating life L10 and L10h to ISO 281
• expanded adjusted rating life Lnm to ISO 281 

Basic rating life

L10 or L10h

The basic rating life in millions of revolutions (L10) is determined in accordance with the basic rating life in operating hours (L10h) is determined in accordance with the equation.

Rating life in revolutions

Rating life in operating hours

L10	106	The basic rating life in millions of revolutions, that is reached or exceeded by 90% of a sufficiently large number of apparently identical bearings before the first indications of material fatigue appear
L10h	h	The basic rating life in operating hours, that is reached or exceeded by 90% of a sufficiently large number of apparently identical bearings before the first indications of material fatigue appear
C	N	Basic dynamic load rating, see product tables
P	N	Equivalent dynamic bearing load
p	–	Life exponent; for roller bearings: p = 10/3 for ball bearings: p = 3
n	min-1	Operating speed (nominal speed)

The life ratings of radial ball bearings are determined by  factors:

• Dynamic Load Rating (C): The primary parameter defining bearing capacity under dynamic loading conditions. It represents the load that a group of apparently identical bearings can endure for a nominal rating life of 1 million revolutions.

• Equivalent Dynamic Load (P): A hypothetical load that, if applied constantly, would result in the same life as the actual varying loads. This takes into account both radial and axial loads in their respective proportions.

• Operational Speed: Higher RPMs elevate operational temperatures, influencing lubricant effectiveness and material fatigue rates.

• Lubrication Quality : Adequate lubrication minimizes metal-to-metal contact, reducing wear. The choice between grease and oil lubrication depends on speed, temperature, and load conditions.

• Environmental Conditions: Exposure to contaminants like dust and moisture or extreme temperatures directly impacts bearing life. Bearings in hostile environments require robust sealing and material selection.

• Installation and Alignment Precision: Misalignment leads to uneven load distribution, increasing stress on the bearing elements and reducing life expectancy.

• Material and Manufacturing Quality: Metallurgical quality and manufacturing precision determine the bearing’s internal stress-handling capacity, directly impacting its fatigue life.

Understanding these factors allows for precise calculation and optimization of bearing rating life.

For more read our comprehensive tutorial – Life calculations for bearings

Bearing Life Ratings Calculating: Methods

L10 Life Formula
This formula calculates the life at which 90% of a group of bearings will not have failed due to classical fatigue. It’s a function of the bearing’s dynamic load rating and the actual load applied, using a formula L10 = (C/P)^3, where C is the dynamic load rating and P is the dynamic equivalent load.

Modified Life Rating
This method refines the L10 calculation by incorporating factors like lubrication quality, contamination levels, and load spectra. It adjusts the basic rating life based on these real-world operational conditions, providing a more tailored life estimate.

ISO 281 Standard
ISO 281 provides a sophisticated approach to calculating bearing life, taking into account not only the dynamic load rating but also the material fatigue limit. It includes more variables and offers a comprehensive understanding of expected bearing life under specified conditions.

Computer Modeling
Advanced computational methods use detailed simulations to predict bearing life. These models can incorporate complex variables like varying load conditions, material properties, lubrication regimes, and environmental factors, offering a highly detailed and specific prediction of bearing life.

Tips For Maximizing Bearing Rating Life

Improving the rating life of bearings in an engineering context requires a systematic approach. It is crucial to have a proper lubrication strategy, which includes selecting appropriate lubricants and maintaining optimal lubrication regimes. Implementing stringent maintenance protocols, such as regular bearing inspections, is essential for early detection of potential issues. To mitigate undue mechanical stress, precision in installation and accurate alignment and fit are fundamental. It is also vital to adhere to specified load parameters and monitor operational temperatures to prevent lubricant degradation and material fatigue.

These measures help maximize the lifespan of bearings.
For specialized assistance in selecting and procuring Radial Ball Bearings, PIB Sales is your go-to distributor. With a range of options and expert advice, they are equipped to provide the solutions you require.

Don’t hesitate to contact PIB Sales for professional support in meeting your bearing requirements.