Utilize the following load calculations, in conjunction with the life modifiers listed below, to estimate a bearing's operational life. Please contact the Precision Division if you need assistance.

Both gears and bearings are typically sized, and life is calculated based on the duty cycle from what is known as the weighted average torque and speed that represents a single torque and speed which would produce the same life as would be achieved if the system operated at the full duty cycle of torque speeds and percent times.

Bearingcalculation formula PDF

More often than not, bearings whose loads are primarily radial are also subjected to some axial forces. When the magnitude of this axial component of the load is greater than a negligible value, it is convenient to interpret this combined radial and thrust load in terms of a simple radial load so that the basic life equation may be used. This simple radial load is referred to as the equivalent load. It is defined as the constant stationary radial load that, if applied to a rotating inner ring, would lead to the same life calculation as the actual conditions of load and rotation. For conventional bearing types other than those with filling notches, the equivalent radial load is given by the maximum of the two values where:

Equivalentbearingload formula

For a more conservative estimate of life than L10, multiply the above formula by a reliability modifier (a2). Values for a2 can be found in the Reliability Material Life Modifier table shown below. The table provides selected modifiers for calculating failure rates down to 1% (L1) for both 440C and 52100 alloys.

A bearing's effective life is often governed by the lubricant's life. This is usually the case for applications involving very light loads and/or very slow speeds. In such instances, the conventional fatigue life calculated will be unrealistically high. The lubricant's ability to provide sufficient film strength is affected by:

Specialized oils and greases which exhibit favorable performance characteristics over an extended period, are available.

For 440C, NHBB recommends reducing the load rating published for 52100 by 20%. This is a conservative approach to ensure that the bearing capacity is not exceeded under the most adverse conditions. This is incorporated in the reliability modifier (a2) as shown:

Normally, it would have been necessary to use a value of S (speed) in both the weighted average load calculation and the bearing life calculation, with the value of S being the same in both places.

In most gearbox applications, there is not one single torque and speed that the gearbox operates at but a series of torques and speeds and time intervals at each condition. These various conditions constitute data points on a duty cycle and together, they must be considered in their entirety.

Bearingload capacity chart

Loads are seldom applied ideally. Often, the conventional rating life has to be modified due to application abnormalities, intentional or unknown. The following conditions have the practical effect of modifying the ideal, theoretical rating life (L10):

Keep in mind that these are ABMA life formulas, or so-called catalog life values, and do not consider or use any adjustment factors, such as those used for lubrication, cleanliness, or other factors.

The rating life L10 of a group of apparently identical ball bearings is the life in millions of revolutions that 90% of the group will complete or exceed. For a single bearing, L10 also refers to the life associated with 90% reliability. The rated life (L10) can be computed from the formula:

In the following chart, the factor e represents the ratio of Fa /V Fr for which the two equations are equal. If the ratio of loads is such that Fa /V Fr < e, then formula (a) is used; if Fa /V Fr > e, then formula (b) is used.

How to calculate radial load on shaft

where L10 = bearing life in hours at 90 percent reliability, C = bearing dynamic rating, P = bearing load, and e = 3 for ball bearings and 10/3 for roller bearings.

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The main item to remember and consider is which type of bearing you are analyzing. Ball bearings use 3 and 1/3 as exponents in the equations and roller bearings (tapered, spherical, and cylindrical) use 10/3 and 3/10 for exponents.

Bearingaxial load calculation

For most cases, the L10 life obtained from the basic rating life formula described earlier will provide a satisfactory criteria for bearing performance. For particular applications, however, it may be desirable to consider life calculations for different reliabilities and special bearing properties and operating conditions.

How to calculate radial load onbearing

In calculations for miniature and instrument bearings whose outside diameter is .625 inches and below, the following values are sufficiently accurate: X=0.56, Y=2.10 and e=0.16. With the larger bearing series, consult the following table (see 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.

In many applications, bearings operate at different loads and speeds for extended periods of time. If the data for loads, speeds and their time periods are available, the following formula can be used to calculate life:

It would seem that this value of S would be important to the resulting life and that a weighted average speed would be required for this equation, but that is not the case. There is no requirement to calculate or use a weighted average speed in this formula, and since the speeds cancel each other out, any speed used would produce the same result. The differing speeds for the different points on the duty cycle are taken into account and reconciled in the weighted average radial load(force) calculation for P.

Dynamic loading (C) includes built-in preload, weight supported members, and the effect of any acceleration due to vibration or motion changes. The dynamic load rating (C) for a radial or angular contact ball bearing is a calculated, constant radial load that a group of apparently identical bearings will theoretically endure for a rating life of one million revolutions. The dynamic load rating is a reference value only. A base value rating life of one million revolutions has been chosen for ease of calculation. The dynamic load rating values (C) given in the product engineering tables, found on this website and in our catalogs, include the effects of race-to-ball conformity and are in accordance with ABMA standards #9 and #12.

The presence of a thin film of oil at the mutually contacting ball-to-raceway interface enhances the load capacity of a ball bearing.

Calculating bearing loadspdf

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.

For a duty cycle where there is variable torque (load), speed, and percent (fraction) of time, the weighted average bearing radial load or torque is:

SKFbearingload calculation

While it is difficult to provide the exact effect upon life under any of these conditions, NHBB can provide bearing life estimates based on empirical data to help you forecast bearing life for your application.

C = dynamic radial load rating (lb) The dynamic radial load (C) can be found on the product listing pages.Pr = dynamic equivalent radial load (lb) See calculating dynamic equivalent radial load below.

The static load rating (Co) given in the product listings is the radial load that a non-rotating bearing will support without damage. In evaluating static load conditions, any forces exerted during assembly and test must be considered along with vibration and impact loads sustained during handling, test, shipment and assembly.

So, either the L10 bearing life must be calculated only once as in the first example formula or calculated for each part of the duty cycle, as in the last formula.