Page 328 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Figure 11.11 Load-life relationship of bearings.

the constant applied load, P and the fatigue life expressed in millions of revolutions, L ,
10
canbestatedas

P L = const (11.11)
10
where
– load-life exponent, = 3.0 for ball bearings and = 10/3 for roller bearings.
Although a rated life is one million revolutions at basic dynamic load rating C,where
the basic dynamic load rating C is defined by manufacturers and published for each bear-
ing in bearing catalogues, bearing applications usually require lives different from that
listed in the catalogues. Therefore, from the relationship between load and life expressed
by Eq. (11.11), we have

P ⋅ L 10 = C ⋅ 1
The rated life of bearing in revolutions at load P is
( )
C
L 10 = (11.12)
P
For bearings operating at a constant speed, it may be more convenient to express rated
life in hours using the equation
10 6 ( )
C
L = (11.13)
10h
60n P
11.3.1.2 Modification of Life Prediction
When at operating temperature, load features are different from those reported in the
manufacturer’s catalogues, influence factors can be introduced to modify Eq. (11.13) to
predict the expected life of bearings.
Bearing ratings shown in manufacturers’ catalogues are usually for radial loads, except
in the special case of pure thrust bearings. When both radial and axial loads are exerted
on a bearing, an equivalent dynamic load is used to predict the life of bearings. The
equivalent dynamic load is an imaginary load that would produce the same rated life as
it will attain under the actual combined radial and axial loads. The following equation is
commonly used to calculate an equivalent dynamic load [12]
P = XR + YA (11.14)

X – dynamic radial load factor (see Table 11.1);
Y – dynamic axial load factor (see Table 11.1).