nhbb :: hitech engineering :: load ratings & bearing life

load ratings & bearing life

The load rating calculations 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.

Basic Dynamic Load Rating

The Basic Load Rating (C) for a radial or angular contact ball bearing is a calculated constant radial load that a bearing with a stationary outer ring can theoretically endure for a rating life of 1,000,000 revolutions of the inner ring.

Static Load Ratings

A static load is a load acting on a non-rotating 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 STD-9 and STD-11.

Rating Life

The “rating life” L₁₀ of a group of apparently identical bearings is the life in millions of revolutions that 90 percent of the group will meet or exceed. For a single bearing, L₁₀ also refers to the life associated with 90 percent reliability. The life which 50 percent of the group of bearings will meet or exceed, “median life” or L₅₀, is usually not greater than five times the rating life.

The calculation of rating life involves many parameters and is based on historical test data. Estimated rating life can best be determined by referring to ANSI/ ABMA STD-9 and STD-11.

Reliability

Where 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).

Life Adjustment Factors for Reliability

Reliability (%)	Rating Life	Life Adjustment Factor on Conventional Rating Life
90	L₁₀	1.00
95	L₅	0.62
96	L4	0.53
97	L₃	0.44
98	L₂	0.33
99	L₁	0.21

Material Factors

Certain materials have 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 below:

Life Adjustment Factors for Material

Material	Factor
M50	10
52100 VIM/VAR	7
52100 CEVM	5
440C Modified	3
52100	1
440C	.8

Other Life Adjustments

The conventional rating life often has to be modified as a consequence of application abnormalities, whether they be intentional or unknown. Seldom are loads ideally applied. The following conditions all have the practical effect of modifying the ideal, theoretical rating life, L₁₀.

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

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