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Life modification factor, aSKF

Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

The SKF life modification factor aSKF expands the scope of the basic rating life model, L10, which depends purely on load and size, by taking the following important operational factors into account:
  • the fatigue load limit in relation to the acting bearing equivalent load (Pu/P)
  • the effect of the contamination level in the bearing (ηc)
  • the lubrication condition (viscosity ratio κ)
This makes the resulting SKF rating life L10m more encompassing than L10 when verifying bearing size selection:

SKF rating life
Diagram 1 shows a graph for estimating aSKF. The horizontal axis represents the combined influence of load and contamination on fatigue. The viscosity ratio, κ, represents the lubrication conditions and their influence on fatigue.

Diagram 1 can be used to see how operating conditions affect the basic rating life:

  • Area A is dominated by very high load and/or severe indentations.
    The lubricating conditions in this domain can only marginally improve the expected fatigue life, so a potential life improvement depends on what dominates the relationship between the contamination level factor and the load level, Pu/P. To achieve a greater SKF rating life, either the load must be reduced, or the cleanliness must be improved, or both.

  • Area B offers high life modification factors, which is beneficial because a large life modification factor will convert a low basic rating life sufficiently to produce a large SKF rating life.
    In this part of the graph, small deviations from estimated load level, cleanliness factor and lubrication conditions will greatly affect the life modification factor. Small changes to lubricating conditions, slightly higher loading and larger indentation severity (for example, from mounting or transport damage) may result in a change in aSKF from 50 to 5. This would result in a 90% loss of SKF rating life. In cases where the SKF rating life consists of a large SKF life modification factor aSKF and a limited basic rating life L10, the impact of variations in operating conditions should be evaluated in a sensitivity analysis.

  • Area C is where the life modification factor is less sensitive to changes.
    Deviations from estimated load level, cleanliness factor and lubrication conditions (for example, from uncertainties in temperature) will not substantially affect the value of aSKF, which means the resulting SKF rating life is more robust.
    In the load level domain, area C has the ranges:
    • Pu ≤ P ≤ 0,5C for ball bearings
    • Pu ≤ P ≤ 0,33C for roller bearings

Use the schematic aSKF graph to evaluate how changes to operational conditions would affect the SKF life modification factor. This can help you check whether a potential benefit is worth the effort. For example, you can see how:

  • improved cleanliness (better sealing, filtration and assembly conditions) increases the contamination level factor ηc
  • cooling or using a lubricant with higher viscosity increases the viscosity ratio κ
  • choosing a larger bearing size increases the ratio Pu/P (and the basic rating life L10
  • using SKF Explorer bearings allows a more favorable scale on the horizontal axis for the combined effect of the ηc times Pu/P

The following graphs show plots of the life modification factor aSKF for the four bearing types, as a function of ηc(Pu/P), for SKF Explorer and SKF basic design bearings, and for different values of the viscosity ratio κ:

NOTE

The graphs are plotted for values and safety factors typically associated with fatigue load limits for other mechanical components. Considering the simplifications inherent in the SKF rating life equation, even if the operating conditions are accurately identified, it is not meaningful to use values of aSKF in excess of 50.
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