4—Dynamic Load Ratings and Fatigue Life

Load ratings and fatigue life are crucial concepts in bearing reliability. A bearing’s basic dynamic load rating expresses the constant load that a rolling bearing with a stationary outer ring can support under inner ring rotation to one million revolutions with a certain level of reliability.
The outer and inner ring raceways and rolling elements repeatedly come under load as they rotate, causing material fatigue that eventually results in scale-like damage called flaking (spalling). Rolling fatigue life indicates the total number of revolutions before this damage appears and is often abbreviated as “bearing life” in technical contexts.

Bearing Life

In broad terms, bearing life expresses how long bearings can operate while satisfying required functions, meaning bearing life can be thought about in various ways. For example, a bearing may need to be replaced when noise, vibration, or torque rise or when bearing temperatures become too hot for an application. Bearing life can be estimated from experience or calculations. ​

Diagram of incalculable and calculable bearing life types. Sound and wear life requirements vary, so limits are often empirical but grease and rolling fatigue life can be determined through calculations. All types are part of the bearing life as defined for large machines, with sound, wear, and grease used in functional life for small machines.

Rolling Fatigue Life

As the bearing rotates under load, the raceways of the inner and outer rings and the rolling surfaces of the rolling elements are subjected to cyclic loading that causes internal stress. Eventually, material fatigue causes scale-like damage called flaking to appear on the raceways or rolling surfaces (mainly on the inner ring raceway). The total number of revolutions before this damage occurs is called the rolling fatigue life.

Diagram of rolling fatigue life in a roller bearing. With each pass of a roller within the bearing, the magnitude of stress in the raceway increases, with a large peak seen close to the surface just when the roller passes. Stress decreases when the roller has passed, creating a stress cycle that causes material fatigue.
Actual spalling (flaking) on the inner ring raceway surface of a bearing. The normally smooth surface appears rough around the entire circumference of the ring.

Example of inner ring flaking

Variation in Rolling Fatigue Life

Rolling fatigue life varies considerably, even when bearings with the same dimensions, structure, materials, heat treatment, and processing are operated under the same conditions. This is because material fatigue is inherently variable, as illustrated below. Therefore, variation in life is treated as a statistical phenomenon in the definition of basic rating life.

Bar chart and curve indicating the frequency of bearing failure over time. Rating (L10) life is determined when 10% of bearings fail, meaning that 90% of bearings can be expected to live past this metric, with decreasing variation over time. As a result, actual maximum life is much higher than rating life.

Basic Rating Life L10

The basic rating life (L10) refers to how long identical bearings can rotate under the same conditions until flaking occurs on 10% of them from rolling fatigue. It indicates life with a statistical reliability of 90% and may be given in hours or the number of revolutions.

Line chart of results from bearing life tests. The L10 basic rating life of a bearing is calculated for the point where 10% of bearings fail under pre-determined conditions. In this example. the L10 life was 11 hours.
  • The Weibull plot here visualizes damage (worsening reliability) over time using a linear approximation of life test results.
  • By plotting the results of reliability tests on a Weibull plot, we can confirm that bearing life follows a Weibull distribution*. The plot shows time on the X-axis with a log10 scale and cumulative failure (damaged bearings) on the Y-axis.

    *A method to describe a statistical distribution of probability, used often in reliability engineering to visualize a product’s strength over time.

Basic Dynamic Load Rating

Definition:
The constant load that a bearing can endure for a rating life of one million rotations under inner ring rotation with a stationary outer ring to a set reliability (typically in L10 terms for a 10% failure rate/90% reliability).

Animation comparing point contact with ball on left and line contact with roller on right.

Point contact
Suitable for light loads, low torque, and high speed

Linear contact
Suitable for heavy loads and low speeds

Basic Life Calculation Formula:

Ball bearings: L10 = (C/P)3

Roller bearings: L10 = (C/P)10/3

where

L10: Basic life rating (x106 revolutions)
C: Basic dynamic load rating (N)
P: Bearing load (dynamic equivalent load) (N)

For radial bearings, C is written Cr, For thrust bearings, C is written Ca
For radial bearings, P is written Pr, For thrust bearings, P is written Pa.
Note that characters may lack mathematical formatting (italics, subscripts, etc) in some descriptions.

 

Load and Life

As established above, bearing life reflects the relation between a bearing's basic dynamic load rating and load. However, for convenience, basic rating fatigue life is often expressed in hours and with the following relationship incorporating  rotational speed.

Equation for basic rating life in ball bearings.

Ball bearings

Equation for basic rating life in roller bearings

Roller bearings

where
L10: Basic life rating (x106 revolutions)
C: Basic dynamic load rating (N)
P: Bearing load (dynamic equivalent load) (N)
n: Bearing rotational speed

A guideline from this relationship holds that:

When load is halved:

life for ball bearings becomes 8x longer
and life for roller bearings becomes 10x times longer.