# 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

where

 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