L10 also refers to the life
associated with 90% reliability. As presently determined, the life which 50 percent of the
group of ball bearings will complete or exceed (“median life”. L50 is
usually not greater than five times the RATING LIFE.”
“Basic Load Rating. The “basic
load rating”, C, for a radial or angular contact ball bearing is that calculated,
constant, radial load which a group of apparently identical bearings with stationary outer
ring can theoretically endure for a RATING LIFE of one million revolutions of the inner
ring. For a thrust ball bearing it is that calculated, constant, centric, thrust load
which a group of apparently identical bearings can theoretically endure for a RATING LIFE
of one million revolutions of one of the bearing washers. The basic load rating is a
reference value only, the base value of one million revolu. tions RATING LIFE having been
chosen for ease of calculation. Since applied loading as great as the basic load rating
tends to cause local plastic deformation of the rolling surfaces, It is not anticipated
that such heavy loading would normally be applied.”
(b) Determination of Basic Load Rating.
Based on the preceding definitions the
standard lists the equations required for the determination of the basic load rating:
“Calculation of Basic Load Rating. The
magnitude of the basic load rating, C, for radial and angular contact ball bearings with
balls not larger than 25.4 mm (1 inch) in diameter is:
where |
C = fC (i cos a)0.7 Z2/3 D1.8”
(27) C = basic load rating, lbs. i = number of rows of balls in the bearing a = nominal contact angle, degrees (“nominal angle between the line of action of the ball load and a plane perpendicular to the bearing axis”) Z = “the number of balls per row in a radial or angular contact ball bearing or the number of balls in a single row, single direction, thrust ball bearing” D = ball diameter, in. fc = “a factor which depends on the geometry of the bearing components, the accuracy to which the various bearing parts are made and the material” |
“Values of fc are obtained
from the appropriate column of Table” 2.
“Calculation of RATING LIFE. The magnitude of the RATING LIFE, L10, in
millions of revolutions, for a ball bearing application is
L10 = (C/P)3”
(28)
(c) Illustration
Consider an ABEC 3 single row, radial ball bearing having 10 balls of 1/16” diameter,
0.330” inner race diameter
and 0.452* outer race diameter in a single shield configuration.
a = 0º (radial bearing)
Z = 10 (number of balls)
D = 1/16" (ball diameter)
and dm = 1/2 (0.330 + 0.452") = 0.391" (pitch diameter of ball races).
This gives (D cos a) / dm = 0.16. With this value we turn to Table 2, third column and find fc
= 4530. Substituting these
values into Equation (27) we obtain
C = 143 lbs.
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