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Whereas fatigue life depends on the wear of a bearing based on
its use over a period of time, load ratings depend on the amount
of force that can be exerted on a bearing before permanent
deformation becomes evident. Types of load ratings include:
The dynamic load rating is the constant stationary radial load in
which 90% of apparently identical bearings from a sufficiently
large sample will achieve a nominal life of 1 million revolutions.
The static load rating is the constant stationary load in which a
total permanent deformation of 0.0001 times the ball diameter is
exhibited at the most heavily stressed point of contact between
the ball and race. For most applications, a measure of .0001 is
tolerated without any adverse implications on the expected
bearing performance and overall bearing life.
Equivalent load calculations are designed to calculate the actual
loads and life of a bearing under operating conditions. By
inserting actual (or anticipated) operating data into equivalent
load calculations, it is possible to estimate a reasonable
assumption of expected bearing life.
In applications where high smoothness or low torque is especially
required, it is important to select a bearing that is well within its
load capabilities in order to prevent deformation which could
compromise bearing performance.
Calculating equivalent loads may be achieved with the help of
the following equation:
Table 1: Values of X and Y for Calculating Load Ratings.
Calculating the bearing rating life is an expression of the
relationship between the bearing capacity, loading and expected
fatigue life. The bearing rating life (L 10 ) is defined as the life in
millions of revolutions that 90% of a group of apparently identical
bearings will complete or exceed. For a single bearing, it also
denotes the number of revolutions that a bearing will complete
with 90% probability. Bearing rating life can be expressed
through the formula:
The Dynamic Load Ratings (Cr) are provided in the Bearing
Series Tables found in the catalogue and will depend upon several
variables including the material used in the bearing, the contact
angle and the configuration of the internal geometry.
To view a sample calculation, please
click here.
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