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281:1990 and ABMA Standards 9 and 11(1990) basic rating life of 1 million revolutions.
The simplest method of life calcula- tion is to use the ISO or ABMA equation for basic nonadjusted rating life, which is
. C .P
L10 = P ,
where L10 = basic rating life, millions of revolutions; C = basic dynamic load rating; P = dynamic bearing load; and p = exponent of the life equation.
For ball bearings, p = 3. For roller bearings, p = 10/3.
For bearings operating at constant
speed it may be more convenient to deal with a basic rating life expressed in operating hours using the equation
comments in parenthesis in the fol- lowing NEMA MG 1 and IEEE 841 standards.
NEMA MG 1
NEMA MG 1 defines maximum allowable radial overhung load for horizontal motors with antifriction ball bearings in NEMA MG 1 Table 14-1A.
■ The limits are given and should not be exceeded. Bearing and shaft failure constitute a safety hazard, and safeguards suitable to each application should be taken.
The figures in the NEMA table show limits for loads applied at the center of the N-W dimension and a reduction factor for loads applied at the end of the shaft. NEMA MG 1-
14.7.1 provides information on the mounting of sheaves (pulleys).
L10h =
1,000,000
60n ×
. C .P
,
P
■ In general, the closer pulleys, sheaves, sprockets, or gears are mounted to the bearing on the motor shaft, the greater the load on the closest bearing.
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where L10h = basic rating, operating hours; and n =
rotational speed, r/min.
ISO 281:1990/Amendment 2:2000 also makes pro- visions for bearing manufacturers to recommend a suit- able method for calculating the life modification factor to be applied to a bearing based on operating condi- tions. The ISO life modification factor aXYZ applies the concept of a fatigue load limit analogous to that used when calculating other machine components. Furthermore, the life modification factor aXYZ makes use of the lubrication conditions, a factor for contami- nation level to reflect the application’s operating con- ditions and performance level.
L10m = a1 aXYZ L10,
where L10m = adjusted rating life (at ∼10% reliability), millions of revolutions; and aXYZ = life modification factor.
Standards Defining Bearing Performance
To better clarify certain points, the authors have made
(The loading will be higher but the misalign- ment less from shaft deflection). This will give greater assurance of trouble-free service.
■ The center of the belt, or system of V-belts, should not be beyond the end of the motor shaft (for ade- quate support for the sheave).
■ The inner edge of the sheave or pulley rim should not be closer to the bearing than the shoulder on the shaft but should be as close to this point as possible.
■ The outer edge of a chain sprocket or gear should not extend beyond the end of the motor shaft.
■ NEMA MG 1-14.7.2 further discusses the mini- mum pitch diameter for drives other than a V-belt.