Page 91 - Rollingbearings

Size selection based on rating life
Basic rating life SKF rating life If the speed is constant, the life can be
expressed in operating hours, using
If you consider only the load and speed, you For modern high-quality bearings, the calcu-
can use the basic rating life, L . lated basic rating life can deviate signiicantly q 10 w
10
6
The basic rating life of a bearing in from the actual service life in a given appli- L nmh = —— L nm
n z
< 06
accordance with ISO 281 is cation. Service life in a particular application

depends not only on load and bearing size, where
q w p
C
L 10 = — but also on a variety of inluencing factors L nm = SKF rating life (at 100 – n 1) %
< z
P
reliability) [millions of revolutions]
including lubrication, degree of contamination,
proper mounting and other environmental L nmh = SKF rating life (at 100 – n 1) %
conditions. reliability) [operating hours]
If the speed is constant, it is often preferable ISO 281 uses a modiied life factor to L = basic rating life (at 90% reliability)
10
to calculate the life expressed in operating supplement the basic rating life. The life [millions of revolutions]
hours using modiication factor a SKF applies the same a 1 = life adjustment factor for reliability
concept of a fatigue load limit P (Fatigue (table 3, page 90, values in accord-
u
6
10
L = —— L load limit, P , page 104) as used in ISO 281. ance with ISO 281)
u
10h 60 n 10 Values of P are listed in the product tables. a SKF = life modiication factor
u
Just as in ISO 281, to relect three of the C = basic dynamic load rating [kN]
where important operating conditions, the life P = equivalent dynamic bearing load [kN]
L = basic rating life (at 90% reliability) modiication factor a SKF takes the lubrication n = rotational speed [r/min]
10
[millions of revolutions] conditions (Lubrication condition – the vis- p = exponent of the life equation
L 10h = basic rating life (at 90% reliability) cosity ratio, κ, page 102), the load level in = 3 for ball bearings
[operating hours] relation to the bearing fatigue load limit, and = 10 /3 for roller bearings
C = basic dynamic load rating [kN] a factor η for the contamination level
c
P = equivalent dynamic bearing load [kN] (Contamination factor, η , page 104) into For 90% reliability:
c
n = rotational speed [r/min] consideration using L nm = SKF rating life (at 100 – n 1) % reliability)
p = exponent of the life equation [million revolutions]
= 3 for ball bearings qCw p
L nm = a a L = a a
= 10 /3 for roller bearings 1 SKF 10 1 SKF —— Becomes:
<Pz

L 10m = SKF rating life [million revolutions]
Since the life adjustment factor a is related
1
to fatigue, it is less relevant for load levels P
below the fatigue load limit P . Dimension- Bearing size
u
ing with life adjustment factors relecting
very high reliability (such as 99%) will result
in large bearings for given loads. In these
cases, the bearing load must be checked B.3
against the minimum load requirement for
the bearing. Calculating minimum load is
Table 2 described in Requisite minimum load,
Guideline values of speciication life for axlebox bearings and units for railway vehicles page 106.
Commonly used conversion factors for
Type of vehicle Speciication life bearing life in units other than million revo-
Million kilometres
lutions are provided in table 4, page 91.
Freight wagons to UIC speciication based on continuously acting 0,8
maximum axle load
Mass transit vehicles: suburban trains, underground carriages, 1,5
light rail and tramway vehicles
Main line passenger coaches 3
Main line diesel and electric multiple units 3 ... 4
Main line diesel and electric locomotives 3 ... 5

1) The factor n represents the failure probability,
which is the difference between the requisite
reliability and 100%.

89