Page 282 - Tribology in Machine Design

Rolling-contact bearings 267

Figure 7.21

experimental investigation is undertaken as, for instance, in the case of very
low or very high speeds or particularly demanding conditions. In this
section only a brief outline of the ISO design procedures is given. If
required, the reader is referred to the ISO Draft International Standard
281-Part 1 (1975) for further details.
An adjusted rating life L is given as

or

where a^ is the life-correction factor for reliability, a 2 is the life-correction
factor for material and a 3 is the life-correction factor for operating
conditions.
The reliability factor has been used in life estimation procedures for a
number of years as a separate calculation when other than 90 per cent
reliability was required. The ISO procedure uses «i in the context of
material and environmental factors. Therefore, when L na = L 10, 0i = l,
which means the life of the bearing with 90 per cent probability of survival
and 10 per cent probability of failure.
Factors accounting for the operating conditions and material are very
specific conceptually but dependent in practice. The material factor takes
account of the improvements made in bearing steels since the time when the
original ISO life equation was set up. The operating condition factor refers
to the lubrication conditions of the bearing which are expressed in terms of
the ratio of minimum film thickness to composite surface roughness. In this
way the conditions under which the bearing operates and their effect on the
bearing's life are described. In effect, it is an elastohydrodynamic lubri-
cation factor with a number of silent assumptions such as; that operating
temperatures are not excessive, that cleanliness conditions are such as
would normally apply in a properly sealed bearing and that there is no
serious misalignment. Both factors, however, are, to a certain extent,
interdependent variables which means that it is not possible to compensate
for poor operating conditions merely by using an improved material or vice