A special case - the adjustment factor a23

In previous SKF catalogues the basic rating life  was adjusted using the combined factor a23 for material and lubrication. This factor was introduced by SKF in 1975. In ISO 281:1990/Amd 2:2000 reference is made to this type of life adjustment as a special case of the more general life modification factor aSKF. The a23 adjustment implies a specific value of the "contamination-load ratio"  ηc(Pu/P)23 used in the diagrams for the SKF life modification factor aSKF. Because the factor a23 is only viscosity ratio κ dependent, an a23 scale is superimposed on the κ curves of diagram 1, diagram 2, diagram 3 and diagram 4 for the factor aSKF at the point where ηc(Pu/P) = [ηc(Pu/P)]23. The factor ηc for contamination level thus becomes:

ηc = [ηc(Pu/P)]23/(Pu/P)
The location of the point where ηc(Pu/P) = [ηc(Pu/P)]23 is marked by a dotted line and the values are listed in table for SKF standard as well as SKF Explorer bearings. For instance, for standard radial ball bearings the corresponding ηc is

ηc = 0,05/(Pu/P)
At that location of the "contamination-load ratio" [ηc(Pu/P)]23 = 0.05 in diagram 5, aSKF = a23 and a23 can be read directly from the aSKF axis using the κ curves. The life can then be calculated with the simplified equation:

Lnm = a1 a23 L10


Lnm=SKF rating life (at 100-n% reliability), millions of revolutions
L10=basic rating life (at 90% reliability), millions of revolutions
a1=life adjustment factor for reliability (table)
a23=life factor for lubrication conditions, when ηc(Pu/P) = [ηc(Pu/P)]23, diagram 6, diagram 7, diagram 8 and diagram 9
Using the adjustment factor a23 implies in practice a stress condition characterized by a value of ηc(Pu/P) = [ηc(Pu/P)]23. If the actual ηc(Pu/P) of the bearing is lower or higher than the [ηc(Pu/P)]23 value, there will be an over or under estimation of the life performance. In other words applications with heavy loads and increased contamination or light loads and improved cleanliness are not well represented by the adjustment factor a23. For standard bearings operating at a load ratio C/P of about 5 the contamination level for aSKF = a23 will require an ηc factor of about 0,4 to 0,5. If the actual cleanliness of the application is lower than the normal level the use of the a23 adjustment leads to an overestimation of the life performance of the bearing. Therefore SKF recommends using only the aSKF method to improve reliability in the selection of the bearing size.
The correspondence between a23 and aSKF is useful if it is required to convert applications that were traditionally designed using the adjustment factor a23 to the use of the more general aSKF adjustment factor. Indeed many applications that have a satisfactory record of operation, initially calculated using the adjustment factor a23, can be easily converted to an equivalent factor aSKF.
In practice this implies the adoption of a contamination factor ηc of the application based on the "contamination-load ratios" [ηc(Pu/P)]23 listed in table. The factor ηc derived in this way represents a simple approximation of the actual factor ηc. This first estimation of factor ηc can be further improved using oil cleanliness ratings as described in the section Factor ηc for contamination level.
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