Page 106 - Rollingbearings

B.3 Bearing size
Fatigue load limit, P η that would have general validity. However, Static load rating
u
c
guideline values in accordance with
The fatigue load limit P for a bearing is ISO 281 are listed in table 6. The basic static load rating C is deined in
u
0
deined as the load level below which metal To simplify calculation of the contamina- ISO 76 as the load that results in a certain
fatigue will not occur. For this to be valid, the tion factor η , use the SKF Bearing Calculator value of contact stress at the centre of con-
c
lubricant ilm must fully separate the rolling (skf.com/bearingcalculator). tact of the most heavily loaded rolling ele-
elements from the raceways and no inden- A more detailed method for estimating the ment/raceway. The contact stress values are:
tations, from contaminants or from damage contamination factor η is described in a
c
related to handling, may exist on the rolling separate paper (Method for estimating con- • 4 600 MPa for self-aligning ball bearings
surfaces. tamination factor, η , based on lubricant • 4 200 MPa for all other ball bearings
c
cleanliness, skf.com/go/17000-B3). • 4 000 MPa for all roller bearings
These stress values produce a total perma-
Contamination nent deformation of the rolling element and

factor, η Size selection raceway that is approximately 0,0001 of the
c
rolling element diameter. The loads are
based on static purely radial for radial bearings and axial,
The contamination factor, η , takes into centrically acting, for thrust bearings.
c
account how the level of solid particle con- load
tamination of the lubricant inluences the
calculated bearing fatigue life. The particles
cause indentations in the rolling surfaces of When any of the following conditions exist,
the bearing, and these indentations increase bearing size should be selected or veriied
the local contact stress, which reduces the based on the static load that the bearing can
expected fatigue life (ig. 3). accommodate, taking into account the pos-
sible effects of permanent deformation:
• η = 1 means perfectly clean conditions
c
without any indentations. • The bearing is not rotating and is sub-
• η → 0 means severely contaminated con- jected to continuous high load or intermit-
c
ditions resulting in pronounced tent peak loads.
indentations. • The bearing makes slow oscillating move-
Bearing size In the SKF rating life model, the contamina- • The bearing rotates and, in addition to the
ments under load.
normal fatigue life dimensioning operating
tion factor for a certain bearing acts as a
stress raiser, by reducing the bearing fatigue
loads, has to sustain temporary high peak
loads.
u
tamination factor η ).
B.3 load limit P (i.e. multiplying it by the con- • The bearing rotates under load at low
c
Comparing the reduced fatigue load limit
speed (n < 10 r/min) and is required to
to the actual bearing load, the fatigue resist- have only a limited life. In such a case, the
ance value (η P /P) takes both the relative rating life equations, for a given equivalent
c u
bearing load and the local stress ield into load P, would give such a low requisite
account (diagram 8, page 95). basic dynamic load rating C, that a bearing
selected on a fatigue life basis would be
• Clean conditions (large contamination seriously overloaded in service.
factor η ) and a bearing load lower than
c
the fatigue load limit results in a high In such conditions, the resulting deformation Fig. 3
resistance to fatigue. can include lattened areas on the rolling Example of stress ields
• Contaminated conditions and a bearing elements or indentations in the raceways.
load larger than the fatigue load limit The indentations may be irregularly spaced
results in a lower resistance to fatigue. around the raceway, or evenly spaced at
positions corresponding to the spacing of the
The stress-raising inluence of contamina- rolling elements. A stationary or slowly oscil-
tion on bearing fatigue depends on a num- lating bearing supporting a load great
ber of parameters, including: bearing size, enough to cause permanent deformation
relative lubricant condition, size and distri- will generate high levels of vibration and fric-
bution of solid contaminant particles and tion when subjected to continuous rotation.
types of contaminants (soft, hard, etc.). It is also possible that the internal clearance
Therefore, it is not meaningful to specify will increase or the character of the housing
precise values for the contamination factor and shaft its may be affected.

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