Page 96 - Rollingbearings

B.3 Bearing size
Life modiication a large life modiication factor a SKF and a
factor, a SKF limited basic rating life L , the impact of
10
variations in operating conditions should
be evaluated in a sensitivity analysis.
The life modiication factor a SKF expands the • Area C is where the life modiication factor
scope of the basic rating life model, L , is less sensitive to changes.
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which depends purely on load and size, by Deviations from estimated load level,
taking the following important operational cleanliness factor and lubrication condi-
factors into account: tions (for example, from uncertainties in
temperature) will not substantially affect
• the fatigue load limit in relation to the act- the value of a SKF , which means the result-
ing bearing equivalent load (P /P) ing SKF rating life is more robust.
u
• the effect of the contamination level in the In the load level domain, area C has the
bearing (η ) ranges:
c
• the lubrication condition (viscosity ratio κ) – P ≤ P ≤ 0,5 C for ball bearings
u
– P ≤ P ≤ 0,33 C for roller bearings
u
This makes the resulting SKF rating life,
L 10m , more encompassing than L when Use the schematic a SKF graph to evaluate
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verifying bearing size selection: how changes to operational conditions
p would affect the life modiication factor. This
L nm = a a L = a a qCw can help you check whether a potential ben-
1 SKF 10 1 SKF ——
<Pz eit is worth the effort. For example, you can
see how:
A graph for estimating a SKF is shown in
diagram 8. The horizontal axis represents • improved cleanliness (better sealing, iltra-
the combined inluence of load and contami- tion and assembly conditions) increases
nation on fatigue. The viscosity ratio, κ, rep- the contamination factor η c
resents the lubrication conditions and their • cooling or using a lubricant with higher
inluence on fatigue. viscosity increases the viscosity ratio κ
Use diagram 8 to see how operating con- • choosing a larger bearing size increases
the ratio P /P (and the basic rating life L )
ditions affect the basic rating life: • using SKF Explorer bearings allows a 10
u
Bearing size • Area A is dominated by very high load more favourable scale on the horizontal
axis for the combined effect of the η times
and/or severe indentations.
c
The lubricating conditions in this
P /P
u
expected fatigue life, so a potential life
B.3 domain can only marginally improve the The following graphs show plots of the life
modiication factor a
improvement depends on what dominates
for the four bearing
SKF
the relationship between the contamina- types, as a function of η (P /P), for SKF
u
c
tion level and the load level P /P. To Explorer and SKF basic design bearings, and
u
achieve a greater SKF rating life, either the for different values of the viscosity ratio κ:
load must be reduced, or the cleanliness
must be improved, or both. • diagram 9, page 96: radial ball bearings
• Area B offers high life modiication factors, • diagram 10, page 97: radial roller
which is beneicial because a large life bearings
modiication value will convert a low basic • diagram 11, page 98: thrust ball bearings NOTE
rating life suficiently to produce a large • diagram 12, page 99: thrust roller The graphs in diagram 9, 10, 11 and 12
SKF rating life. bearings are plotted for values and safety factors
In this part of the graph, small devia- typically associated with fatigue load
tions from estimated load level, cleanli- limits for other mechanical components.
ness factor and lubrication conditions will Considering the simpliications inherent
greatly affect the life modiication factor. in the SKF rating life equation, even if
Small changes to lubricating conditions, the operating conditions are accurately
slightly higher loading and larger indenta- identiied, it is not meaningful to use
tion severity (for example, from mounting values of a SKF in excess of 50.
or transport damage) may result in a
change in a SKF from 50 to 5. This would
result in a 90% loss of SKF rating life. In
cases where the SKF rating life consists of

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