SKF ConCentra - Page 9

SKF life modification factor a
SKF
Values of a
SKF
can be obtained from
diagram 1 for different values of h
c
(P
u
/P)
and viscosity ratio k. Diagram 1 has been
drawn up for a safety factor commonly used
in fatigue life considerations. Values for a
SKF
can also be obtained by using the calculation
programs provided on line at www.skf.com/
bearings.
Contamination – the factor h
c
for contamination level
It is not possible to determine precise values
for h
c
that have general validity for SKF Con-
Centra ball bearing. However, since SKF
ConCentra ball bearing are protected by very
efficient seals, guideline values for the con-
tamination factor h
c
between 0,5 and 0,8
can be used to determine the a
SKF
factor.
Lubrication conditions
– the viscosity ratio k
For SKF ConCentra ball bearing, the viscosity
ratio k is calculated as the ratio between the
actual operating viscosity n to the rated
viscosity n
1
for adequate lubrication.
To determine the operating viscosity use
the kinematic base oil viscosity of 190 mm
2
/s
at 40 °C and 15 mm
2
/s at 100 °C for standard
SKF ConCentra ball bearing. For bearings
with an E2 prefix, contact the SKF
application engineering service.
If k > 4, use curve for k = 4
As the value of h
c
(P
u
/P) tends to zero, a
SKF
tends to 0,1 for all values of k
Diagram 1
Factor a
SKF
for radial ball bearing
0,005 0,01 0,02 0,05
0,05
0,1
0,1
0,2
0,2
0,5
0,5
1
1
2
2
5
10
20
50
5
h
c
P
P
u
––
k
= 4
2
0,8
0,5
0,15
0,1
0,2
0,6
0,4
0,3
1
a
SKF
Table 1
Values for life adjustment factor a
1
Reliability Failure probability SKF rating life Factor a
1
%
n %
L
nm
90 10 L
10m
1
95 5 L
5m
0,62
96 4 L
4m
0,53
97 3 L
3m
0,44
98 2 L
2m
0,33
99 1 L
1m
0,21
9
B