SKF ConCentra - Selection of a bearing unit size

Selection of a bearing unit size
The size of an SKF ConCentra ball bearing
unit is initially selected on the basis of the
loads within the application, the bearing
load carrying ability, its calculated life and
reliability. The load carrying ability is ex-
pressed by the basic dynamic and basic
static load ratings. The basic dynamic load
ratings are determined in accordance with
the methods described by ISO 281:2007
while the basic static load ratings are in ac-
cordance with ISO 76:2006.
The simpliest method to calculate bearing
life is to use the ISO 281:2007 equation for
basic rating life, which is
q
C
w

3
L
10
= ––
<
P
z
For ball bearings rotating at a constant
speed, it may be more convenient to deal
with basic rating life expressed in operating
hours, using the equation
1 000 000
q
C
w
3
L
10h
= ––––––––– —
60 n
<
P
z
The life-extending improvements embodied
in SKF ConCentra ball bearing units can best
be understood by using the SKF rating life
equation
q
C
w
3
L
nm
= a
1
a
SKF
—
<
P
z
or
1 000 000
q
C
w
3
L
nmh
= a
1
a
SKF
–––––––––– —
60 n
<
P
z
where
L
nm
= SKF rating life (at 100 – n%
reliability)
1)
, millions of revolutions
L
nmh
= SKF rating life (at 100 – n%
reliability)
1)
, operating hours
L
10
= basic rating life (at 90% reliability),
millions of revolutions
L
10h
= basic rating life (at 90% reliability),
operating hours
a
1
= life adjustment factor for reliability
(† table 1)
a
SKF
= SKF life modification factor
(† diagram 1)
C = basic dynamic load rating, kN
P = equivalent dynamic bearing load, kN
(† page 10)
n = rotational speed, r/min
This calculation method, which is in accord-
ance with ISO 281:2007 is better able to
predict bearing service life. The a
SKF
factor
represents a very complex relationship be-
tween various influencing factors: the fa-
tigue load limit P
u
by the ratio P
u
/P, the con-
tamination level by the factor h
c
and the
lubrication conditions by the
viscosity ratio k.
More detailed information about
selection of a bearing size•
the fatigue load limit • P
u
the contamination level • h
c
determining the viscosity ratio • k
allocating a value for the SKF life modifi-•
cation factor a
SKF
can be found on line at www.skf.com/bearings
or the “SKF Interactive Engineering
Catalogue”, available on CD-ROM or online
at www.skf.com.
Values for the fatigue load limit P
u
are
provided in the product tables.
1)
The factor n represents the failure probability,
i.e. the difference between the requisite reli-
ability and 100 %
8