 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.
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 C
r 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 C
r
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 =
bm × 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 | |
|
