Load Ratings & Bearing Life

The load ratings for NHBB precision ball and cylindrical roller bearings are based on ANSI/ABMA Standards 9 and 11. These standards specify the accepted methods for calculating load ratings and fatigue life of ball and cylindrical roller bearings. Since a multitude of variables may affect these calculations, they should be used for baseline estimates only. Load ratings for your application’s specific operating conditions should be calculated before making a final bearing choice.

Below we define the terms basic dynamic load rating, static load rating, rating life, and reliability. We also provide the basic equations for calculating life and equivalent radial load, plus information about life adjustment, material factor, and other life adjustment factors.

Basic Dynamic Load Rating

The Basic Load Rating (C) for a radial or angular contact ball bearing is a calculated constant radial load which a bearing with a stationary outer ring can theoretically endure for a rating life of 1,000,000 revolutions of the inner ring. The ratings shown in this catalog are defined by ANSI/ABMA Standard 9 and Standard 11. The ratings for noncatalog bearings may be determined by referring to this standard.

Static Load Ratings

A static load is a load acting on a nonrotating bearing. Experience shows that a total permanent deformation of 0.0001 of the rolling element diameter, at the center of the most heavily loaded rolling element/raceway contact, can be tolerated in most bearing applications without the bearing operation being impaired. The basic static load rating is, therefore, that load which produces the above deformation. As with the dynamic load ratings, the static rating determinations can be found in ANSI/ABMA Standard 9 and Standard 11.

Rating Life

Bearing fatigue life is a baseline estimate of the number of revolutions or hours that a bearing will operate before failing. The principal factor at play is metal fatigue, so failure is defined by the presence of spalling or flaking on a bearing’s raceways. Since, in reality, identical bearings operating under identical conditions fail at unpredictable intervals, and since there is no way to predict the actual life of a specific bearing, the industry utilizes a statistical formula to calculate rating life. The calculations shown below involve many parameters and are based on historical test data.

Reliability — L₁₀

The standard value L₁₀ equals the total number of revolutions that 90% of a group of identical bearings will theoretically meet or exceed. For a single bearing, L₁₀ also refers to the life associated with 90% reliability. The life which 50% of the group of bearings will meet or exceed (median life, or L₅₀) is usually no greater than five times the rating life (refer to the table under Life Adjustment Factors).

Basic Equations

Ball Bearings
L (cycles) = (C/P_r)³ x a₁ x a₂

Roller Bearings
L (cycles) = (C/P_r)^10/3 x a₁ x a₂

Convert to Hours of Operation
L (hours) = 1,000,000/N x 60

L (cycles) = Cycles (x 1 million)
C = Dynamic load rating
P_r = Equivalent radial load
a₁ = Reliability adjustment factor
a₂ = Material adjustment factor
N = rpm

Calculating Equivalent Radial Load

More often than not, bearings with primarily radial loads are subject to some axial forces. When the magnitude of the axial component of the load is greater than a negligible value, it is helpful to translate the combined radial and axial load into a radial load so that the basic life equation may be used. This radial load, known as the equivalent radial load, is defined as that constant stationary radial load which, if applied to a rotating inner ring, would give the same life as that which the bearing will attain under the actual conditions of load and rotation. For conventional bearing types other than those with filling notches, the equivalent radial loads are given by the maximum of the two values where:

a) P_r = VF_r
b) P_r = XVF_r + YF_a

V is a rotation factor
X is a radial factor
Y is a thrust factor
F_r is the radial load
F_a is the axial load

Consult the table below for determining values X, Y and e. In all series, the rotational factor V is 1.0 for inner ring rotation and 1.2 for outer ring rotation with respect to load. The factor e (last column) represents the ratio of F_a/VF_r for which the two equations are equal. If the ratio of loads is such that F_a/VF_r ≤ e, then formula (a) is used; if F_a/VF_r > e, then formula (b) is used.

Factors X, V, and Y 

Additional nomenclature is as follows:
– Z is the number of balls
– D is the ball diameter in inches
Values of X, Y and e for load or contact angle other than shown are obtained by linear interpolation.

Life Adjustment Factors for Reliability

When a more conservative approach than conventional rating life (L₁₀) is desired, the ABMA offers a means for such estimates. The table below provides selected multipliers for calculating failure rates down to 1% (L₁).

Material Factors

Certain materials are proven to have greater fatigue life than others operating under identical conditions. The theoretical L10 dynamic life is based on air-melt steel and standard ABMA formulas. The life adjustment factors for materials frequently used are shown here:

Other Life Adjustments

The conventional rating life often has to be modified as a consequence of application abnormalities. The following conditions all have the practical effect of modifying the ideal theoretical rating life of L₁₀:

a. Vibration and/or shock-impact loads
b. Angular misalignment
c. High speed
d. Operating at elevated temperatures
e. Lubricant effects

NHBB can provide reliable bearing life estimates based on semiempirical data to assist in accurately forecasting bearing life.