5.0 SIZING BALL BEARINGS
(a) Basic Definitions
In the course of many years of experience with bait bearings and
extensive testing, it has been found that the prediction of the load capacity of a ball
bearing is a statistical event related to the fatigue life of the bearing. This makes the
sizing of ball bearings more difficult than that of many other machine elements.
A basic phenomenon in ball bearings is that
ball bearing life has been found to be inversely proportional to the cube of the bearing
load. This means that when the load is doubled the life expectancy of the bearing is
reduced by a factor of eight. This phenomenon has been studied extensively and has led to
the adoption of an industry-wide national standard for rating ball bearings pioneered by
the AFBMA (Anti-Friction Bearing Manufacturers Association, 1235 Jefferson Davis Highway,
Arlington, Virginia, 22202). The following represents a summary of the load rating of ball
bearings less than one inch in diameter, according to ANSI-AFBME Standard 9, 1978:
“Load Rating and Fatigue Life for Ball Bearings” — reprinted with the
permission of the American National Standards Institute Inc., 1430 Broadway, New York,
N.Y., 10018:
"Life Criterion. Even if ball
bearings are properly mounted, adequately lubricated, protected from foreign matter, and
are not subjected to extreme operating conditions, they can ultimately fatigue. Under
ideal conditions, the repeated stresses developed in the contact areas between the balls
and the raceways eventually can result in fatigue of the material which manifests itself
as spelling of the load carrying surfaces. In most applications the fatigue life is the
maximum useful life of a bearing. This fatigue Is the criterion of life used as the basis
for the first part of this standard.”
The material in the standard which follows
assumes bearings having non-truncated contact area, hardened good quality steel as the
bearing material, adequate lubrication, proper ring support and alignment, nominal
internal clearances, and adequate groove radii. In addition certain high-speed effects
such as ball centrifugal forces and gyroscopic moments are not considered. We now continue
with the standard.
"Life.
The "life” of an individual ball bearing is the number of revolutions (or hours
at some given constant speed) which the bearing runs before the first evidence of fatigue
develops in the material of either ring (or washer) or of any of the rolling
elements.”
"Rating life. The “RATING
LIFE”, L10, of a group of apparently identical ball bearings is the life n
millions of revolutions that 90 per cent of the group will complete or exceed. For a
single bearing,
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