Dimensioning the bearing arrangement
Dimensioning the bearing arrangement
The bearing size and execution re-quired for a given bearing arrange-ment are determined based on the following criteria:
● life
● static load carrying capacity
● axial load carrying capacity
● minimum load
● normal acceleration and cage load carrying capacity
● friction and cooling
● speed capability
● internal clearance and preload
● adjustment values for single row angular contact bearings.
In many cases bearing size is simply selected on the basis of the calculated life. The list above and the following comments serve to show that for reli-able performance of the bearing, a number of other criteria should be con-sidered in addition to the calculated bearing life.
Life calculation
The Lundberg and Palmgren theory of bearing fatigue life forms the basis for bearing life calculations. The life equa-tions derived from the theory are to be found in the SKF General Catalogue.
Their use for gearbox bearing calcula-tion will be discussed here.
Bearing life can be calculated with greater accuracy and reliability, the more accurately the operating condi-tions are known or can be determined.
To calculate the basic rating life L10h according to ISO it is only necessary to know the basic dynamic load rating of the bearing, the equivalent bearing load and the rotational speed. Import-ant influences such as lubricImport-ant film formation in the bearing and lubricant cleanliness are not considered in the L10hcalculation. In spite of this, if ex-perience of similar bearing arrange-ments is available and the other para-meters which affect bearing life, but which are not considered in the calcu-lation are reasonably constant, a basic rating life calculation may be sufficient to determine the appropriate bearing size.
Gearbox application L10h
(operating hours)
Machines and equipment infrequently used: 300 to 3 000 Household appliances
Agricultural machinery Medical equipment
Machines used for brief periods or intermittently: 3 000 to 10 000 Cranes
Lifts and elevators Construction machinery
Machines for daily (8 hour) use: 10 000 to 30 000
Machine tools
Woodworking machines Fans
Conveyor drives Centrifuges
Machines for 24-hour use: 30 000 to 50 000
Rolling mills Compressors Pumps Barges
The following parameters are con-sidered when calculating L10aah:
● dynamic load rating of the bearing,
● fatigue load limit of the bearing,
● equivalent dynamic bearing load,
● rotational speed,
● lubricant viscosity,
● operating temperature and cooling, and
● contamination and sealing.
Calculations according to the New Life Theory are particularly suitable for making parametric studies to deter-mine the influence of the different fac-tors. It should be noted that the various factors have a strong influence on each other, and such calculations are only meaningful when the operating conditions are exactly known. When bearing life calculations for the selection of bearing size are made, only those results obtained using one and the same method should be compared.
When determining a suitable life it is necessary to consider how the gear-box is to be used. The requisite basic rating life is dependent on the type
and size of the driven machine, on the length of service and on demands regarding operational reliability. If no experience is available then the guide-line values for the requisite basic rating life L10hgiven in Table can be used.
In similar applications, the drives of large machines are generally subject-ed to more arduous conditions than the drives of smaller machines be-cause of stronger shock loads and larger defomations. This should be taken into consideration when choosing the guide-line value from Table 4.
When bearing arrangements are in-tended for very slow rotational speeds and/or are to have a very short life, the requisite basic dynamic load rating of the bearing is very small. This can lead to an unsuitable bearing being chosen which will give inadequate static safety, or the formation of only an inadequate lubricant film, or to the overloading and consequent deforma-tion of the associated components. If, in addition to the requisite life, a mini-mum requisite value of the static safety factor s0is also to be considered, this
4
Guideline values for the requisite basic rating life L10hfor gearboxes for various appli-cations
4
Dimensioning the bearing arrangement
Table 4
should be based on the κ value (ratio of actual to required viscosity). The decision not only depends on the oper-ating speed therefore, but also on the viscosity at the operating temperature and on the mean bearing diameter.
Table contains recommendations as whether the bearing selection should be based on the requisite life or on the static safety, taking the value of κ into account. Thus
● whenκ > 0,5, the static safety factor s0should be checked in addition to the requisite life;
● whenκ ≤ 0,5 then the static safety factor s0must be considered;
5
● whenκ < 0,1 no life should be given;
the material will fatigue under condi-tions of small κ, but the operational reliability and service life will not depend on fatigue but on other fac-tors which are indirectly accounted for by the static safety factor s0.
Selection criteria
Guideline values for the static safety factor s0
Dimensioning the bearing arrangement
Bearing type Type of operation
Rotating, Rotating, Rotating at very slow Stationary statically brief shock speeds under load
loaded loads
nrel= 0 nrel> 0 κ < 0,1 κ = 0,1 to 0,5
Ball bearings 2 2 10 5 0,5
Roller bearings 3,5 3 10 5 1
Full complement – 3 20 10 1
cylindrical roller bearings
Viscosity Bearing selection based on
ratio fatigue life static safety factor
κover incl. L10h L10ah L10aah s0
0,1 − − − +
0,1 0,5 − o + +
0,5 1 + + + o
1 + + + o
Symbols + recommended – not appropriate o can also be used
Table 6 Table 5