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Miniature & small ball bearings

6. Load rating and rating life

If high quality bearings are made with designs and materials based on bearing industry standards, such as JIS, the load rating and rating life can be calculated based on the specifications defined in JIS and ISO.

Life of ball bearing

The required life of a ball bearing depends on the application and requirements of the equipment. This is because there are such diverse applications, and the definitions of life also vary. Therefore, life needs to be defined based on the application and requirements.
There are several different types of life definitions: rating life, noise life, lubricant life, functional life, etc.
The noise life is considered when bearings become noisier than the originally set level. The lubricant life is considered as when lubricants lose their function due to degradation. The performance life is considered when speed and runout go beyond the acceptable limits and no longer meets the application requirements.
This section provides an overview of the "rating life" and modified rating life of "single-row deep groove ball bearings" specified in JIS B 1518.
Rating life is the predicted life based on the basic dynamic radial load rating.
Modified life is the life rating in which 90% or greater reliability, fatigue limit load, lubricant contamination, and/or special operating conditions is achievable.

Basic dynamic radial load rating (Cr)

Defined as "constant static radial load such that the bearing theoretically endures the basic rating life of 1 million revolutions". The calculation method is shown in JIS B 1518. The basic dynamic radial load rating is described in the dimension table.

Dynamic equivalent radial load (Pr)

Defined as "a constant static radial load applied on the bearing in which similar life is achieved as under actual load conditions". Using the following formula and table, replace the combined radial load and axial load with static radial load.

The ratio of axial load



≦ e


> e

1 0 0.56 2.30

Z : Number of balls
Dw : Ball diameter (mm)
Note 1: X, Y and e values not shown in the table are obtained by linear interpolation.
Note 2: As the formula for single-row bearings is used, the variable for the number of rows is excluded from the formula for calculating the axial load ratio specified in JIS.

Basic rating life (L10)

Defined "rated life at 90% reliability under normal operating conditions." This is the "total rotational speed at which 90% of the bearings can rotate without causing material separation when the same group of bearings is operated under the same conditions."
It is calculated from the following formula based on JIS B 1518.

If the speed is constant, the life is usually expressed in hours. The relationship between basic rating life and life hours is as follows.

Modified rating life Lnm

If operating conditions are acceptable, a long life can be calculated with modified rating life as compared to the basic rating life. The life will be Reduced in poor operating conditions. JIS B 1518:2013 defines a modified rating life where the fluctuation and interaction of each factor that affects bearing life is taken into consideration. The modified rating life of the reliability n% is expressed in the following formula.

Reliability factor α1

The coefficients range from 90% to 99.95% reliability. See the table below.

Reliability (%) Lnm α1
90 L10m 1
95 L5m 0.64
96 L4m 0.55
97 L3m 0.47
98 L2m 0.37
99 L1m 0.25
99.2 L0.8m 0.22
99.4 L0.6m 0.19
99.6 L0.4m 0.16
99.8 L0.2m 0.12
99.9 L0.1m 0.093
99.92 L0.08m 0.087
99.94 L0.06m 0.08
99.95 L0.05m 0.077

Life correction factor αiso

It is a coefficient derived from fatigue limit load, contamination coefficient, viscosity ratio, etc.

Fatigue load limit cu

The load applied to the bearing that causes fatigue limit stress at the contact point of the maximum load of the track. (Fatigue limit stress is the maximum stress that does not cause fatigue in the bearing material)

Pollution coefficient ec

If the lubricant is contaminated with solid particles and the particles enter between the track and rolling elements, the track may become indented. In these indentations, the stress increases locally, leading to shortened life of the ball bearing. The pollution coefficient that takes into account this reduction in life. The following are excerpts from JIS B 1518:2013 as indicators.

Index of pollution coefficient ec
Pollution level ec
Extremely high cleanliness 1
High cleanliness 0.8~0.6
Standard cleanliness 0.6~0.5
Mild pollution 0.5~0.3
Normal pollution 0.3~0.1
Severe pollution 0.1~0
Extreme pollution 0

Viscosity ratio Κ

Indicates the separation of the rolling contact surface of the lubricant, and is the ratio of the kinematic viscosity of the oil at the actual operating temperature to the reference kinematic viscosity.
Note: There are various restrictions when calculating the modified rating life, and there are concerns about how appropriate it is for applications using miniature size bearings. It may be necessary to confirm complicated any application conditions that involve many factors such as the design, intended use and usage environment of the device. Therefore, be sure to consider the validity of the calculation result with sufficient evaluation and verification.
The basic static radial load rating and static equivalent radial load of rolling ball bearings are specified in JIS B 1519 "Calculation method of static load rating of rolling ball bearings."

Basic static radial load rating (Cor)

Defined as "the static load at which the calculated contact stress at the contact center between the ball receiving the maximum load and the track is 4,200 MPa." The permanent deformation of the ball and raceway as a result of this contact stress is approximately 0.0001 times the diameter of the ball. The basic static radial load rating is described in the dimension table.

Static equivalent radial load (Por)

Defined as "the static radial load that generates the same contact stress as that generated under actual load conditions at the center of the contact portion between the ball receiving the maximum load and the track."

Take the larger value obtained from the following equation.

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