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, C_{O},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
L_{10}, 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 L_{10} also refers to the life associated
with 90% reliability. Median Life, L_{50}, 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:

L_{10} = (C/P)^{3} 
with 
L_{10} = rating life (10^{6}rev) 

C
= basic load rating (lb) 

P
= equivalent radial load (lb) 
To obtain the rating life in hours,use: 
L_{10
hrs.} = 16667/N * (C/P)^{3} 
with 
N
= speed (rpm) 
The Equivalent Radial Load is defined as: 
P
= XF_{r} + YF_{a} 
with 
F_{r} = radial load (lb) 

F_{a} = 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 L_{10} life.
F_{a} / nd^{2} 
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
