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.
Ball Load Rating
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.
Dynamic Load Rating Cr
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.
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)
L10 = Basic rating life
Cr = Basic dynamic load rating (N)
n = R.P.M. (revolutions per minute)
L10h = Basic rating life in operating hours
P = Equivalent load (N)
Adjusted Life Formula
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)
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
Static Load Rating Cor
The Basic Static Load Rating applies to bearings where rotating motion does not occur or occurs only infrequently. The Basic Load Ratings and calculation methods are based on methods described in ISO 281 and in ISO Recommendations NR.76, taking into account the current level of bearing technology.
Excessive static load causes brinelling at the contact point between the balls and raceway. As a standard of permissible static load, the basic load rating Cor for radial bearings is specified as follows:
- Maximum contact pressure at the contact point between rolling element and bearing ring to be 4200 MPa and total permanent deformation of the bearing of approximately 1/10,000th of the rolling element’s diameter.
- Basic static load rating for stainless steel is 80% of that for standard bearing steel