Ball and Roller Bearings Machinery's Handbook, 31st Edition
2493
Table 34. Values of X and Y for Computing Equivalent Thrust Load P a for Thrust Roller Bearings
Single Direction Bearings
Double Direction Bearings
F
F
F
r a 2
r a #
r a 2
e
e
e
F
F
F
Bearing Type
X
Y
X
Y
X
Y
Self-Aligning Tapered Thrust Roller Bearings a α ≠ 0
1
0.67
1
tan α
1.5 tan α
tan α
a For α = 90°, F
r = 0 and Y = 1.
e = 1.5 tan α Life Adjustment Factors.— In certain applications of ball or roller bearings it is desir- able to specify life for a reliability other than 90 percent. In other cases the bearings may be fabricated from special bearing steels such as vacuum-degassed and vacuum-melted steels, and improved processing techniques. Finally, application conditions may indicate other than normal lubrication, load distribution, or temperature. For such conditions a series of life adjustment factors may be applied to the fatigue life formula. This is fully explained in AFBMA and American National Standard “Load Ratings and Fatigue Life for Ball Bearings” ANSI/AFBMA Std 9-1990 and AFBMA and American National Stan- dard “Load Ratings and Fatigue Life for Roller Bearings” ANSI/AFBMA Std 11-1990. In addition to consulting these standards it may be advantageous to also obtain information from the bearing manufacturer. Life Adjustment Factor for Reliability: For certain applications, it is desirable to specify life for a reliability greater than 90 percent which is the basis of the Rating Life. To determine the bearing life of ball or roller bearings for reliability greater than 90 percent, the Rating Life must be adjusted by a factor a 1 such that L n = a 1 L 10 . For a reli- ability of 95 percent, designated as L 5 , the life adjustment factor a 1 is 0.62; for 96 percent, L 4 , a 1 is 0.53; for 97 percent, L 3 , a 1 is 0.44; for 98 percent, L 2 , a 1 is 0.33; and for 99 percent, L 1 , a 1 is 0.21. Life Adjustment Factor for Material: For certain types of ball or roller bearings which in- corporate improved materials and processing, the Rating Life can be adjusted by a factor a 2 such that L 10 ′ = a 2 L 10 . Factor a 2 depends upon steel analysis, metallurgical processes, forming methods, heat treatment, and manufacturing methods in general. Ball and roller bearings fabricated from consumable vacuum remelted steels and certain other special analysis steels, have demonstrated extraordinarily long endurance. These steels are of exceptionally high quality, and bearings fabricated from these are usually considered special manufacture. Generally, a 2 values for such steels can be obtained from the bearing manufacturer. However, all of the specified limitations and qualifications for the applica tion of the Rating Life formulas still apply. Life Adjustment Factor for Application Condition: Application conditions which affect ball or roller bearing life include: 1) lubrication; 2) load distribution (including effects of clearance, misalignment, housing and shaft stiffness, type of loading, and thermal gradi ents); and 3) temperature. Items 2 and 3 require special analytical and experimental techniques, therefore the user should consult the bearing manufacturer for evaluations and recommendations. Operating conditions where the factor a 3 might be less than 1 include: a) exceptionally low values of Nd m (rpm times pitch diameter, in mm); e.g., Nd m < 10,000; b) lubricant vis cosity at less than 70 SSU for ball bearings and 100 SSU for roller bearings at operating temperature; and c) excessively high operating temperatures.
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