Life Calculations Determine the Life Adjustment Factors a1, a2, a3:
a1, Life Adjustment Factor for Reliability
The most commonly used reliability level for bearing life calculations is 90%. This is referred to as, L10, or rating life, and is the life based upon 90% survival of a group of bearings at the specified load and speed. Should the application require a higher degree of reliability, the a1 life adjustment factors can be selected from Table 4.
a2, Life Adjustment Factor for Bearing Material In previous catalogs Fafnir used a Factor of 3 for material and processing for Fafnir Superior Steel. This factor has now been incorporated in the CE value and accordingly, the a2 factor for Fafnir superior steel now is 1. Factors for other materials are given in Table 5.
1 0.62 0.53 0.44 0.33 0.21
90 95 96 97 98 99
Life Adjustment Factor
For Material: a2
Fafnir Superior Steel (standard material) Vacuum Melted (VIM-VAR) 52100
*In certain applications this factor can exceed 4. Consult our Engineering Department.
a3, Life Adjustment Factor for Application
Conditions Many bearing users will find that they are able to calculate bearing life with acceptable accuracy using an Application Factor (a3) of 1. The a3 factor can be made up of any number of application factors based upon the degree of detail the user wishes to employ in analysis. Such factors as lubrication, alignment, mounting stiffness, and temperature can be considered. The factors are multiplied together to develop the final a3 factor.
The Engineering Department will assist in developing various application factors when requested by the user. The following may be used as a guide to determine the a3 factor based on lubrication considerations.
In order to obtain a3, it is necessary to compare the actual lubricant operating viscosity, , centistokes (mm2/sec.) to a Reference Viscosity, R, which is based on requirements determined by the application speed and bearing pitch diameter.
Determine the Reference Viscosity from Figure 1 (page E44) by entering the bearing pitch diameter, which is equal to the outer diameter plus the bore divided by 2 and the bearing speed.
In order to obtain the value of, a3, the Life Adjustment factor for Lubrication, from Fig. 2 by entering the value of,
/R. The value of, , the actual viscosity of the lubricant in
the bearing must be obtained from the lubricant manufactur- ers viscosity index specification for the temperature of the oil in the bearing at operating conditions. Where the operating temperature of the oil is unknown considerable care is necessary to estimate this temperature, since it depends on loading, speed, lubricant flow and heat transfer characteristics of the shaft and housing.
The factor, a3, is a multiplier of the bearing life, Ln, reflecting lubricant effectiveness in an adequately filtered lubrication system. The values of, a3, are a consequence of the direct contact between the bearing rolling elements and the bearing rings. Contaminants in the lubricant, exceeding lubricant film thickness, result in shorter lives than would be computed using the values of, a3. The use of the a3 factor is also based on the adequate supply of lubricant which will not deteriorate over the life of the bearing.
When bearings are grease lubricated determine the a3 factor using the specifications for the oil used in the grease, however, the maximum value of a3 should not exceed 1. One reason for this limitation is the question on grease maintenance over long periods which is out of control of the designer.
Bearing Life Under Varying Loads and Speeds In many applications, bearings are required to run at a number of different loads and speeds. If the different loads and speeds and the portions of time they are in effect are known, the life can be found from the following relation:
N o t e : p 1 + p 2 + p 3 +
+ pn = 1.0
+p L n nn
+ p 2 1 + p 3 + L n 2 L n3