Life and Load Ratings

Radial Ball Bearing Life

When radial ball bearings rotate, the inner and outer rings and rolling elements are constantly loaded. This produces material fatigue and eventually bearing failure. The total number of revolutions 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.

Statistically, the total number of revolutions reached or exceeded by 90% of a sufficiently large group of apparently identical bearings before the first evidence of material fatigue occurs is called the basic rating life.

Fatigue (L10) Life

The following formulas permit the calculation of the fatigue life of bearings subjected to radial loads or combined radial and axial loads. Some of the formulas include approximations. Contact  NPB® Bearings Engineering Department if the calculated life is borderline in comparison to the desired life or if the loading  conditions differ from those presented.

The Fatigue Life (synonymous with L10 and Rating Life) of the  bearings is the number of hours or revolutions reached by 90% of a group of bearings subjected to the same loads before the onset of fatigue. Conversely, 10% of the same group of bearings can be expected to show evidence of fatigue before the L10 Life is reached.

Bearing life is defined as the length of time (number of revolutions) until a specific failure occurs. Predicting the life of a ball bearing is a statistical calculation of the fatigue properties of the various bearing components. “Life” is the  number of hours that a percentage of similar bearings have survived under an essentially identical set of operating conditions and loads. Life can be affected by a number of factors including loads, speed, lubrication, fit, maintenance, temperature, contamination, and others. Because of the diverse variety of contributing factors, it is extremely difficult to predict life precisely. Because handling and contamination damage can dramatically reduce bearing life, bearings should be properly stored, mounted, dismounted, and inspected. Optimized performance and life is also contingent on appropriate lubrication and sufficient protection form foreign matter. Bearings should be stored in a cool, clean, low humidity environment free of dust, shocks and vibrations. Proper fitting, using specialized tools and techniques, will also help  maximize bearing life.

The Average Life of this same group of bearings will be approximately 5 times  the L10 Life.

Fatigue Life Calculation

LR = (C/P)3 LR is in millions of revolutions LH  = (C/P)3  X (16,667 / RPM). LH is in hours C = Basic  Dynamic Rating. Ratings for each bearing are included in this catalog P = Radial Equivalent Load Calculated as indicated below

Radial Ball Bearing Load Ratings

Manufacturers of ball bearings typically publish Load Ratings for each bearing they produce. The methods used to calculate ratings can vary from manufacturer to manufacturer. However, both ABMA and ISO have published standards related to load ratings.

  • ABMA Std. 9 – Load Ratings and Fatigue Life for Ball Bearings
  • ABMA Std. 12.1 and 12.2 – Instrument Ball Bearings
  • ISO 76 – Static Load Ratings
  • SO 281 – Dynamic Load Ratings and Rating Life

With regard to load ratings, one thing to remember is – static load ratings (Cor) and dynamic load ratings (Cr) are formulated on completely different premises and have no direct relationship to one another.

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.

The 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.

Based on the above, for a given 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. Changing all of these variables at the same time (depending on what the actual changes are) can result in the dynamic capacity moving in one direction and the static capacity moving in the opposite direction (when compared to the original configuration).

Radial Ball Bearings Basic 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.

Radial Ball Bearings Life Formula

The equation for the basic rating life for dynamically 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

P = Equivalent load (N)

Basic 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 rolling element 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

Equivalent Dynamic Bearing Load “P”

Load conditions on bearings are usually a combination of radial and axial loads, In order to establish the equivalent radial load with definite force and direction we use the following formula:

P=XFr+YFa (N)

Where:

Fr= Radial load (N)

Fa=Axial load (N)

X=Radial load factor

Y=Axial load factor

D=Ball diameter (mm)