Ball Bearing Life

When radial ball bearings rotate, the inner and outer rings and balls are constantly loaded. This produces material fatigue and eventually bearing failure. The total number of revolutions reached or exceeded by 90% of a sufficiently large group of apparently identical bearings before a failure occurs is called the basic rating life.

Life of individual bearings varies considerably, even if they are of the same size, same material, same heat treatment and are under the same operating conditions.

You often see the load rating in the bearing parameter, there are static load ratings (Cor) and dynamic load ratings (Cr).

Dynamic load ratings are determined by bearing geometry, number and size of balls, bearing pitch diameter, and ring and ball material. This load rating is used in conjunction with the actual applied radial load to calculate bearing fatigue life.

Static load rating relates to limiting loads applied to non-rotating bearings. The static load rating depends on the maximum contact stress between the balls and either of the two raceways. It is affected by material, number and size of balls, raceway curvatures, raceway depths, and contact angles. It is also based on using clean, high quality bearing steel with typical hardness levels of 58-64 HRC for rings and 60-65 HRC for balls.

In all, for a bearing, a change in the pitch circle can impact the dynamic load rating, and a change in the ball diameter or ball quantity can impact both load ratings.

The basic dynamic load rating of a bearing with rotating inner ring and stationary outer ring is that load of constant magnitude and size which a sufficiently large group of apparently identical bearings can endure for a basic rating life of one million revolutions.

Life Formula

The equation for the basic rating life for dynamic loaded ball bearings is as follows:

L10= (Cr/P)3 X 106           (Revolution)

L10h = 16667 / n · (Cr/P)3         (hours)

Where:

L10 = Basic rating life

Cr = Basic dynamic load rating (N)

n = R.P.M. (revolutions per minute)

L10h = Basic rating life in operating hours

The above life formula is for general use. In cases where a reliability of over 90% is required and where influences apart from load and speed or operating frequency should be taken into account for the rating life, ISO 281, 1990 gives an extended life formula:

Lna= a1 x a2 x a3 x (Cr/P)3 x 106 (Revolution)

Where:

Lna = Adjusted rating life in millions with a reliability of (100-n)% (n= the reliability rate)

Cr = Basic dynamic load rating (N)

P = Equivalent dynamic load rating (N)

a1 = Factor for a reliability other than 90%

a2 = Factor for non-conventional materials

a3 = Factor for non-conventional operating conditions, in particular lubrication