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

Below the caution statements are recommendations for shaft and housing fits for a number of different applications and a table of recommended shoulder diameters for select NHBB part numbers.

L = Loose fit.T = Tight fit.d = Bearing bore as listed.D = Bearing OD as listed.+Bearings must be purchased with bore and OD coding. Example: To use SSR-2 bearing in a potentiometer, the shaft diameter should be .1250 –.0001 to .1250 –.0003 or .1249 to .1247. The housing should be .3750 +.0002 to .3750 –.0000 or .3752 to .3750.

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.

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.

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).

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.

Bearinghousing tolerance

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.

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:

When establishing shaft or housing sizes the effect of differential thermal expansion must be taken into account. If thermal gradients are present or dissimilar materials are used, room temperature fits must be adjusted accordingly. Approximate thermal coefficients for common materials are available from NHBB.

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.

Accurate shaft and housing fits are important for proper bearing installation and operation. In ordinary applications, miniature ball bearings are used with a light load, so a light interference fit to a narrow clearance fit is usually sufficient. Light cross section bearings, such as Torque Tube and Thinex bearings, are more sensitive to shaft and housing imperfections and require looser fits than heavier cross section bearings.

New Hampshire Ball Bearings, Inc. (NHBB) is a leading manufacturer of precision bearings and complex bearing assemblies for the global aerospace, defense, medical, and high technology markets. The company is a subsidiary of MinebeaMitsumi Inc. and an integral part of the global brand, MinebeaMitsumi Aerospace.

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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:

Interferencefit

The bearing will be subject to early failure in most applications because it may already be overloaded. This problem can be solved either by a higher radial play or looser fit.

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:

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).

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A bearing that is fitted too tightly or too loosely, or is damaged during assembly by excessive force or shock loading, may degrade the performance of your assembly. Reduce this possibility by following some general guidelines during the design of mating parts and by observing the following four cautions during the assembly process.

New Hampshire Ball Bearings, Inc. (NHBB) is a leading manufacturer of precision bearings and complex bearing assemblies for the global aerospace, defense, medical, and high technology markets. The company is a subsidiary of MinebeaMitsumi Inc. and an integral part of the global brand, MinebeaMitsumi Aerospace.

If the outer ring or inner ring face will be clamped or abutted against a shoulder, make sure the shoulder configuration provides a good mounting surface:

a. Vibration and/or shock-impact loadsb. Angular misalignmentc. High speedd. Operating at elevated temperaturese. Lubricant effects

When miniature and instrument bearings are interference fitted (either intentionally or as a result of thermal gradients) the bearing radial play is reduced by an amount equal to approximately 80% of the actual diametral interference fit. Thus:

Fittolerance

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 Fa/VFr for which the two equations are equal. If the ratio of loads is such that Fa/VFr ≤ e, then formula (a) is used; if Fa/VFr > e, then formula (b) is used.

L (cycles) = Cycles (x 1 million)C = Dynamic load ratingPr = Equivalent radial loada1 = Reliability adjustment factora2 = Material adjustment factorN = rpm

It is relatively easy to damage a miniature and instrument bearing during assembly simply by exceeding its load capacity. Adequate fixturing must be provided for handling and assembling precision bearings to ensure that:

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