Life of Bearing


Definitions

Life:

For an individual rolling bearing, the number of revolutions which one of the bearing rings (or washers) makes in relation to the other rings (or washers) under the prevailing working conditions before the first evidence of fatigue develops in the material of one of the rings (or washers) or rolling elements.


Reliability:

For a group of apparently identical rolling bearings, operating under the same conditions, the percentage of the group that is expected to attain or exceed a specified life.


Basis for Calculation

Bearing life is defined as the length of time, or the number of revolutions, until a fatigue spell of a specific size develops. This spell size, regardless of the size of the bearing, is defined by an area of 0.01 inch2 (6 mm)2. This life depends on many different factors such as loading, speed, lubrication, fitting, setting, operating temperature, contamination, maintenance, plus many other environmental factors. Due to all these factors, the life of an individual bearing is impossible to predict precisely. Also, bearings that may appear to be identical can exhibit considerable life scatter when tested under identical conditions. Remember also that statistically the life of multiple rows will always be less then the life of any given row in the system.


L10 Life

L10 life is the life that 90 percent of a group of apparently identical bearings will complete or exceed before the area of spalling reaches the defined 0.01 inch2 (6 mm2 size criterion. If handled, mounted, maintained, lubricated and used in the right way, the life of your tapered roller bearing will normally reach and even exceed the calculated L10 life.


Bearings Industry

If a sample of apparently identical bearings is run under specific laboratory conditions, 90 percent of these bearings can be expected to exhibit lives greater than the rated life. Then, only 10 percent of the bearings tested would have lives less than this rated life.


Bearing Life Equation

As you will see it in the following, there is more than just one bearing life calculation method, but in all cases the bearing life equation is :


L10 = (C / P)10/3 × (B / n) × a


L10 in hours

C = radial rating of the bearing in lbf or N

P = radial load or dynamic equivalent radial load applied on the bearing in lbf or N. The calculation of P depends on the method (ISO) with combined axial and radial loading

B = factor dependent on the method ; B = 1.5 × 106 for the Timken method (3000 hours at 500 rev/min) and 106 /60 for the ISO method

a = life adjustment factor ; a = 1, when environmental conditions are not considered ;

n = rotational speed in rev/min.


This can be illustrated as follows :

  • Doubling load reduces life to one tenth. Reducing load by one half increases life by ten,

  • Doubling speed reduces life by one half. Reducing speed by one half doubles life.

In fact, the different life calculation methods applied (ISO 281) differ by the selection of the parameters used.


Bearing Ratings

Depending on the life calculation method used, the bearing ratings have to be selected accordingly. The Cr rating, based on one million revolutions, is used for the ISO method.


However, a direct comparison between ratings of various manufacturers can be misleading due to differences in rating philosophy, material, manufacturing and design. In order to make a true geometrical comparison between the ratings of different bearing suppliers, only the rating defined following the ISO 281 equation should be used. However, by doing this, you do not take into account the different steel qualities from one supplier to another.


ISO 281 Dynamic Radial Load Rating Cr

This bearing rating equation is published by the International Organization for Standardization (ISO) and AFBMA. These ratings are not published by any bearing manufacturers. However, they can be obtained by contacting our company.


The basic dynamic load rating is function of :

Cr = b m × fc × (i × Lwe × cos a)7/9 × Z3/4 × Dwe29/27

Cr = radial rating

bm = material constant (ISO 281 latest issue specifies a factor of 1.1)

fc = geometry dependent factor

i = number of bearing rows within the assembly

Lwe = effective roller contact length

a = bearing half-included outer race angle

Z = number of rollers per bearing row

Dwe = mean roller diameter