The service life of a gearbox is determined
by many factors. Bearings, for
example, play a major role since they
contribute an important function while
also interacting with the shafts, casing
and oil. Without a doubt, the sizing of
the bearings is of great importance in
gearbox reliability. For over 50 years,
bearing dynamic carrying capacity has
been used to determine a suitable size
needed to deliver a sufficient fatigue life.
But despite the existence today of advanced
calculation methods, they do not fully predict service life. Producers of
high-quality bearings have introduced
better ways to express (quantify) improved
performance, but only in terms
of increased dynamic carrying capacity.
This article will cover the following:
? Sizing of bearings based on
dynamic carrying capacity and
how this relates to service life
? How the design of the interface
between bearing and shafts should be
adapted to modern shaft materials
? How the design of the interface
between bearing and gearbox casing
influences service life of the gearbox
? Influence of modern electric
motor speed controls on
bearing-type selection
Sizing of rolling element bearings
based on dynamic carrying capacity. For
modern high-quality bearings, the classic
basic rating life can deviate significantly
from the actual service life in a
given application. Generally speaking,
service life in a particular application
depends not only on load in relation to
bearing size, but also on a variety of influencing
factors, including lubrication,
the degree of contamination, misalignment,
proper installation and environmental
conditions.
The first method accepted by ISO
for determining a suitable bearing size
is the classic Lundberg and Palmgren
equation L10 =(C/P)
P
, making it possible
to determine a suitable dynamic
C-value (which in turn defines the bearing
size) needed to satisfy a need for a
fatigue life L10.

The influence of material properties
and lubricant film thickness was introduced
in the 1960s, represented by the
a23 factor (Fig. 1).
(1)
L10 =a23 (C/P)P
[a23 is a function of k and
the material]
As an attempt to take some of those
factors into account when determining
a suitable bearing size, the DIN
ISO281:1990/AMD2:2000 contains a
modification factor aSLF to the basic rating
life L10 =(C/P)
P
. The method makes
provision for bearing manufacturers to
recommend a calculation methodology
for this life modification factor to be
applied to a bearing based on operating
conditions. Some life modification factors
apply the concept of a fatigue load
limit Pu analogous to that used when dimensioning
other machine components.
Furthermore, the life modification factor
makes use of the lubrication conditions
and a factor ηc for contamination level to
reflect the application’s operating conditions.