Ball Bearing Load Ratings and Life Calculations

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 Specs as follows:

Basic Life Rating

The Basic Life Rating (L10) is defined in specification JIS B1518 "Dynamic load ratings and rating life for rolling bearings" as follows:

The Basic Life Rating is the life obtained with 90% reliability, when an individual bearing or an identical group of bearings are manufactured with common materials, common manufacturing processes and quality, and operate under the same conventional conditions. L10 Life is the accumulated rotation where 90% of survive without material flaking when they are operated under fixed conditions, of a population of bearings.

The calculation formula for the Basic Life Rating is the following.
  Formula for Basic Life Rating in revolutions Basic Life Rating in revolutions : Basic Life Rating in millions of revolutions
Basic Dynamic Load Rating : Basic Dynamic Load Rating
Equivalent Dynamic Radial Load Factor : Equivalent Dynamic Radial Load Factor

There is a relationship between the Basic Life Rating (revolutions) and Basic Life (time).

  formula of relation between Basic Life Rating (revolution) and Basic Life (time) Rotation Speed : Rotation Speed (min-1)
hours : Time (hours)

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, the Japanese Industrial Standards system, which provides standards for not only the ball bearing but also for a wide variety of industrial activities requiring accurate measures. JIS 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 Load Rating (Cr)

The method for calculating the Basic Dynamic Load Rating can be found in JIS B1518 and is based on an endurance test of 1,000,000 revolutions.

Dynamic Equivalent Radial Load Factor (Pr)

The Dynamic Equivalent Radial Load Factor is defined as "the direction and magnitude to the bearing, which is able to obtain the same life under the actual load and rotation conditions".
From the calculation formula and the table below, the axial and the radial loads are replaced by the Dynamic Equivalent Radial Load Factor (Pr).

   

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

 
Axial Load Ratio Load Ratio Load Ratio e
Units X Y X Y
N {kgf}
Axial Load Ratio 1 0 0.56 2.30
1.99
1.71
1.55
1.45
1.31
1.15
1.04
1.00
0.19
0.22
0.26
0.28
0.30
0.34
0.38
0.42
0.44
0.172
0.345
0.689
1.03
1.38
2.07
3.45
5.17
6.89
{ 0.0175 }
{ 0.0352 }
{ 0.0703 }
{ 0.105 }
{ 0.143 }
{ 0.211 }
{ 0.352 }
{ 0.527 }
{ 0.703 }

i : No. of rows
Z : No. of balls
Dw : Ball Diameter (mm)
The values for X andY that are not in the above table shall be calculated by linear interpolation.

Basic Static Load Rating (Cor)

The formula for the Basic Static Load Rating and the Static Equivalent Radial Load Rating of ball bearings is defined in specification JIS B1519 as follows:

Basic Static Load Rating (Cor) :
The Basic Static Load Rating is the amount of static radial load that will cause a total permanent deformation (ball and raceway) on the most heavily stressed ball/raceway contact area (the center) that equals to 0.0001 of the ball diameter under a stress level of 4200 MPa.

Static Equivalent Radial Load (Por) :
The Static Equivalent Radial Load is a static radial load that would cause the same total permanent deformation on the most heavily stressed ball/raceway contact as the actual load.

The largest value obtained from the following two formulas will be used.

P0r = X0Fr + Y0Fa
P0r = Fr

X0 and Y0 are defined in specification JIS B1519 Table 2 (the coefficient values of X0 and Y0                  of radial ball bearing)
X0=0.6; Y0=0.5
Fr= Radial Load (N or kgf)
Fa= Axial Load (N or kgf)

If you have any further questions regarding ball bearing loads or ball bearing life and are not able to find your answer on the NMB site, please feel free to contact NMB Tech at any time in order to get more specific information. Our highly qualified expert ball bearing engineers will be happy to answer any questions you may have promptly and completely.




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