Page 90 - Rollingbearings

B.3 Bearing size
Size selection Bearing rating life accurate and so a statistical approach is
needed to determine bearing size.
based on rating For estimating the expected bearing life, you The basic rating life, L , is the fatigue life
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can either use basic rating life, L , or SKF that 90% of a suficiently large group of
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life rating life, L 10m . apparently identical bearings, operating
If you have experience with the operating under identical operating conditions, can be
conditions related to lubrication and con- expected to attain or exceed.
For applications where bearings are running tamination, and know that the conditions To determine a relevant bearing size using
in typical operating conditions – i e normal you are working with do not have a dramatic the deinition given here, compare the calcu-
speeds, good lubrication conditions and not effect on the life of your bearings, use the lated rating life against the service life
highly or peak loaded – determine the basic rating life calculation, otherwise SKF expectations of the bearing application,
appropriate bearing size based on the recommends using the SKF rating life. using experience from previous dimension-
required bearing life, taking into account the ing where available. Otherwise, use the
possible effects of rolling contact fatigue (RCF) guidelines regarding speciication life of vari-
This subsection describes the bearing Bearing life deinition ous bearing applications provided in
rating life equations and the factors that table 1 and table 2.
must be determined to make the evaluation: Bearing life is deined as the number of Because of the statistical spread of bearing
revolutions (or the number of operating fatigue life, an observed time to failure for an
• Bearing rating life – the basis for bearing hours) at a given speed that the bearing is individual bearing can be evaluated in relation
rating life, showing how to calculate basic capable of enduring before the irst sign of to its rated life, only if the failure probability
rating life, L , and SKF rating life, L 10m metal fatigue (spalling) occurs on a rolling of that particular bearing is determined in
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• Basic dynamic load rating, C, page 91 element or the raceway of the inner or outer relation to the general population of bear-
• Equivalent dynamic bearing load, P, ring. ings running under similar conditions.
page 91 Tests on seemingly identical bearings, Numerous investigations on bearing fail-
• Life modiication factor, a SKF , page 94 under identical operating conditions, result ure, in a variety of applications, have con-
• Lubrication condition – the viscosity ratio, in a large variation in the number of cycles, irmed that design guidelines based on 90%
κ, page 102 or time, needed to cause metal fatigue. reliability, and use of dynamic safety factors,
• Fatigue load limit, P , page 104 Therefore, bearing life estimates based on lead to robust bearing solutions in which
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• Contamination factor, η , page 104 rolling contact fatigue (RCF) are insuficiently typical fatigue failures are avoided.
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Table 1
Bearing size Machine type Speciication life
Guideline values of speciication life for different machine types

Operating hours

Household machines, agricultural machines, instruments, technical equipment for medical use 300 ... 3 000
B.3 Machines used for short periods or intermittently: electric hand tools, lifting tackle in workshops, construction 3000 ... 8 000

equipment and machines
Machines used for short periods or intermittently where high operational reliability is required: lifts (elevators), 8 000 ... 12 000
cranes for packaged goods or slings of drums, etc.
Machines for use 8 hours a day, but not always fully utilized: gear drives for general purposes, electric motors 10 000 ... 25 000
for industrial use, rotary crushers
Machines for use 8 hours a day and fully utilized: machine tools, woodworking machines, machines for the 20 000 ... 30 000
engineering industry, cranes for bulk materials, ventilator fans, conveyor belts, printing equipment, separators
and centrifuges
Machines for continuous 24-hour use: rolling mill gear units, medium-sized electrical machinery, compres- 40 000 ... 50 000
sors, mine hoists, pumps, textile machinery
Wind energy machinery, this includes main shaft, yaw, pitching gearbox, generator bearings 30 000 ... 100 000
Water works machinery, rotary furnaces, cable stranding machines, propulsion machinery for ocean-going 60 000 ... 100 000
vessels
Large electric machines, power generation plant, mine pumps, mine ventilator fans, tunnel shaft bearings for 100 000 ... 200 000
ocean-going vessels

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