SKF - Rolling Bearings

Selecting bearing size
Calculation examples
Example 1: Basic rating life and SKF rating life
An SKF Explorer 6309 deep groove ball bearing
is to operate at 3 000 r/min under a constant
radial load F r = 10 kN. Oil lubrication is to be
used, the oil has an actual kinematic viscosity
n = 20 mm 2 /s at normal operating temperature.
The desired reliability is 90% and it is
assumed that the operating conditions are
very clean. What are the basic and SKF rating
lives?
a) The basic rating life for 90% reliability is
q C w 3
L 10 = —
< P z
From the product table for bearing 6309, C =
55,3 kN. Since the load is purely radial, P = F r =
10 kN († Equivalent dynamic bearing load,
page 85).
q 55,3 w 3
L 10 = –—–
< 10 z
= 169 million revolutions
or in operating hours, using
10 6
L 10h = —–– L 10
60 n
1 000 000
L 10h = ————– ¥ 169
60 ¥ 3 000
= 940 operating hours
b) The SKF rating life for 90% reliability is
L 10m = a 1 a SKF L 10
• As a reliability of 90% is required, the L 10m
life is to be calculated and a 1 = 1 († table 1,
page 65).
• From the product table for bearing 6309,
d m = 0,5 (d + D) = 0,5 (45 + 100) = 72,5 mm
• From diagram 5 († page 72), the rated oil
viscosity at operating temperature for
a speed of 3 000 r/min, n 1 = 8,15 mm 2 /s.
Therefore, k = n/n 1 = 20/8,15 = 2,45
• From the product table P u = 1,34 kN and
P u /P = 1,34/10 = 0,134. As the conditions
are very clean, h c = 0,8 († table 4, page 74)
and h c (P u /P) = 0,107. With k = 2,45 and
using the SKF Explorer scale in diagram 1
(† page 66), the value of a SKF = 8 is
obtained. Then, according to the SKF rating
life equation
L 10m = 1 ¥ 8 ¥ 169
= 1 352 million revolutions
or in operating hours using
10 6
L 10mh = —–– L 10m
60 n
1 000 000
L 10mh = ————– ¥ 1 352
60 ¥ 3 000
= 7 512 operating hours
Example 2: Check contamination conditions
An existing application has to be reviewed.
An SKF Explorer 6309-2RS1 deep groove
ball bearing with integral seals and standard
grease fill is working under the same conditions
as described in example 1 (k = 2,45). The
con tam in ation conditions of this application
have to be checked to determine if it is possible
to use a more cost-effective bearing for a
min imum requisite life of 3 000 hours of
operation.
• Considering grease lubrication and integral
seals, the level of contamination can be
characterized as high cleanliness and from
table 4 († page 74), h c = 0,8.
With P u /P = 0,134, h c (P u /P) = 0,107, using
the SKF Explorer scale in diagram 1
(† page 66) and k = 2,45, a SKF = 8.
L 10mh = 8 ¥ 940 = 7 520 operating hours
90