Capacity and Fatigue Life of Ball Bearings

IntroductionSelection Guide Engineering DataBrochure

The BASIC DYNAMIC LOAD RATING, C, or "dynamic capacity," for a ball bearing is that calculated, constant radial load at which a group of apparently identical bearings with a stationary outer ring can statistically endure one million revolutions of the inner ring.

The BASIC STATIC LOAD RATING, CO,or "static capacity," is that uniformly distributed load,which produces a maximum theoretical contact stress on the most heavily loaded ball of 609,000 psi. At this contact stress a permanent deformation of ball and raceway occurs. This deformation is approximately 0.01% of the ball diameter in inches.

The RATING LIFE L10, is a statistical measure of the life which 90% of a group of apparently identical ball bearings will achieve or exceed. For a single bearing L10 also refers to the life associated with 90% reliability. Median Life, L50, is the life which 50% of the group of ball bearings will achieve or exceed. Median life is usually not greater than five times the rating life.

The relationship between rating life, load rating and load is:

L10 = (C/P)3
with L10 = rating life (106rev)
C = basic load rating (lb)
P = equivalent radial load (lb)

To obtain the rating life in hours,use:
L10 hrs. = 16667/N * (C/P)3
with N = speed (rpm)

The Equivalent Radial Load is defined as:
P = XFr + YFa
with Fr = radial load (lb)
Fa = axial load (lb)
X = see table below
Y = see table below

For radial contact ball bearings calculate P with X = 1.0 and Y=1.0. Then recalculate P with X = 0.56 and Y from the chart below. Use the larger value of P to determine L10 life.

Fa / nd2 Y
25 2.30
50 1.99
100 1.71
150 1.55
200 1.45
300 1.31
500 1.15
750 1.04
1000 1.00

with n = number of balls
        d = diameter of balls