Ball Bearing Load Ratings and Life Calculations

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General Notes About Bearing Life

Put simply, bearing life means how long you can expect your ball bearing to last under standard operating conditions. There will be a number of factors involved in the life of your bearing, including the amount of bearing load the ball bearing will be expected to handle. It’s important to know the bearing life of your ball bearings so that you can plan down the road as to when you will have to replacing your bearing. Bearing life is calculated in number of revolutions, so you will need to establish how much time a revolution takes and what percentage of time your application has the ball bearing in continuous revolution in order to determine your bearing life. The bearing life statistic is a measure of the amount of time in revolutions where 90% of the ball bearings can be expected to have survived. This does not mean every bearing will fail as soon as this number has been exceeded, of course. The median life for ball bearings, also referred to as the Mean Time Before Failure, or MTMF, is about five times the basic life number for the bearing. This means that at 5 times the Basic Life Rating revolutions you should expect about half of your ball bearings to have failed. It’s important to remember that there may be some variation in your individual results with a ball bearing, and factors such as proper lubrication, care and handling of the bearing, and stress on the bearing can result in very significant fluctuations in the life of the ball bearing. NMB Ball Bearings are used in a wide variety of applications. Many contain several application and environmental variables, all of which have an influence on bearing performance and life. Therefore it is extremely important to select the correct Bearing for each application in order to obtain the best possible results. These values are calculated according to JIS / ISO specifications.

Basic Rating Life (L10)

Defined as the life associated with 90 percent reliability.

According to ABMA Std. 9, for an individual bearing, or a group of apparently identical bearings operating under the same conditions, the life associated with 90% reliability, with contemporary, commonly used material and manufacturing quality, and under conventional operating conditions.

The calculation is based on JIS B 1518Life Calculation 1 

 

 

If the speed is constant, the life is usually expressed in hours.  The relationship between basic rating life and life hours is as follows:

Life Calculations 2

General Notes About Bearing Load

There are two types of bearing load to consider with a ball bearing: radial load, which represents loads perpendicular to the shaft, and axial, or thrust, load, which represents loads parallel to the shaft. A ball bearing can handle both of these kinds of loads, but different loads affect bearings in different ways, so multiple bearing rating calculations are required. The load bearing calculations are outlined by the JIS / ISO, which provides standards for not only the ball bearing but also for a wide variety of industrial activities requiring accurate measures. JIS / ISO measurements are widely accepted standards throughout the world. You will find all ball bearing standards under JIS B, as B is the classification regarding mechanical engineering, which is the classification the bearing falls under (other classifications include A for civil engineering and C for electrical engineering).
Use the load ratings to determine how many of each type of ball bearing you will need and which type of bearing will be appropriate to your needs, so that you can enjoy long, effective life for your bearing-using applications. Bearing information regarding various load ratings follows below.

Basic Dynamic Radial Load Rating (Cr)

Defined as the calculated, constant radial load that a group of apparently identical bearings will theoretically endure for a rating life of one million revolutions.  The calculation is explained in JIS B 1518. The Basic Dynamic Radial Load Ratings are for reference only.

Dynamic Equivalent Radial Load (Pr)

Bearings subjected to primarily dynamic radial loads are often also subject to some axial force. To interpret this combined radial and axial load it is convenient to consider a hypothetical load with a constant magnitude passing through the center of the bearing. This hypothetical load is referred to as the Dynamic Equivalent Radial Load and is calculated with the following equation:

Pr = XFr + YFa
X and Y are taken from the table below
Fr = Radial load (N)
Fa = Axial load (N)

 

Radial Axial Load Chart

Calculation Example

Calculations 1

Calculate Life Hours

Basic Static Radial Load Rating (Cor)

The load ratings shown were calculated in accordance with the ABMA standard. The ABMA has established the maximum acceptable stress level resulting from a pure radial load.  In a static condition, to be 4.2 GPa (609,000 psi).

Basic Static Load Rating (Cor) :
The static radial load rating (Cor) given on the product listing pages is the radial load which a non-rotating ball bearing will support without damage, and will continue to provide satisfactory performance and life.

The static radial load rating is dependent on the maximum contact stress between the balls and either of the two raceways.

Static Equivalent Radial Load (Por) :
For a stationary or slow rotating bearing, the theoretical static radial load that produces the same contact stress at the area of contact between the most heavily stressed ball and raceway, as the contact that occurs under the actual load conditions.

Using the calculations below, the larger of the two values should be used as the Static Equivalent Radial Load:

Por = XoFrYoFa
Por = Fr

Xo, Yo:  JIS B 1519

Coefficient of deep groove ball bearings:
X0=0.6; Y0=0.5
Fr= Radial Load (N)
Fa= Axial Load (N)

If you have any further questions regarding ball bearing loads or ball bearing life , please feel free to contact NMB at any time in order to get more specific information. Our highly qualified ball bearing engineers will be happy to answer any questions you may have promptly and completely.
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