Load Rating and Bearing Life (continued) __________________________________________________________________
|
The L10 preload life of a
bearing is normally adjusted as follows, according to real-world
operating conditions:
|
= |
a1a2 L10 |
where: |
Lna = adjusted bearing fatigue
life |
a1 = Life correction factor for material
and desired statistical reliability, the values for a1 being given below:
Values
for Life Correction Factor a1
MATERIAL |
STATISTICAL RELIABLITY
(%)
|
90 |
95 |
96 |
97 |
98 |
99 |
52100 |
1.00 |
.62 |
.53 |
.44 |
.33 |
.21 |
440C |
.50 |
.31 |
.27 |
.22 |
.17 |
.11 |
DR/ES1 |
.8 |
.49 |
.42 |
.35 |
.27 |
.17 |
a2 = Life correction factor for
operating conditions. It has been empirically determined to have a
value between 2.0 under ideal operating conditions and 0.2 where one
or more of the following conditions may apply:
- The lubricant film thickness between the
raceways and the rolling elements is low
- Bearing raceways are excessively
misaligned
- Oscillatory motion
- Rotation speed of the bearing elements is
very low
- Contamination or water is present in the
bearing
Since there are many factors that determine
observed bearing life, the above figures should be used
conservatively.
|
MAXIMUM STATIC LOAD Co |
When subjected to an excessive load or
shock, rolling bearings may incur a permanent damage to the
raceways, called brinelling. This occurs when the elastic limit of
the raceway material is exceeded and will permanently effect the
smooth rotation of the bearing.
The maximum static load rating of a bearing
(Co) is defined as that load which will produce a contact
stress of 609,000 lbf /in2 (428 kgf/mm2)
between the rolling element (ball) and the raceway. The values of
Co are listed in the bearing tables for each individual
bearing. Exceeding this value will damage the
bearing.
|
EQUIVALENT DYNAMIC LOAD |
In most cases, the actual load applied
to a bearing is a combination of fluctuating radial and axial loads
which cannot be directly used for fatigue life calculations.
Instead, the Equivalent Load (P) is used. This represents the
hypothetical load of constant magnitude, passing through the center
of the bearing, that would give the actual observed bearing life
when used in the fatigue life formula. It is an estimate, dependent
on the initial ball contact angle αo and
the static capacity of the bearing Co, as well as the
actual load applied (F).
P = Pr +
Pa |
where: |
Pr ,
Pa are equivalent radial and axial equivalent
loads, calculated using the nomographs on the next
pages. |
|
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