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Steel balls are manufactured to a number of international standards produced to meet the needs of bearing manufacturers. Of these standards, AFBMA 10, ISO 3970, and DIN 5401 are probably the most common.
ISO and DIN are well-known organisations, AFBMA, the "Anti Friction Bearing Manufacturers Association", is probably less well known, but in relation to ball specifications is probably the most important. In general terms, all major ball standards are based on AFBMA 10. DIN 5401 used to be the exception, but was brought into line with AFBMA 10 in 1993.
o The Nominal Diameter of the Ball e.g. 10mm
Ball Diameter Variation
o The Difference between the Largest and Smallest Diameter, Measured for a Single Ball
Ball Mean Diameter
o The Arithmetic Mean of the Maximum and Minimum Diameters Measured for the Ball
Sphericity or Deviation from Spherical Form
o Normally a Manufacturing Batch Manufactured under Uniform Conditions
Lot Mean Diameter
o The Arithmetic Mean of the Smallest and the Largest Mean Ball Diameters within the Lot
Lot Diameter Variation
o The Difference between the Smallest and the Largest Mean Ball Diameters within the Lot
o The Deviation of the Lot Mean Diameter from the Nominal Diameter
o A Number by Which the Properties of the Ball Are Specified
In general terms these classes can be represented by current grades as follows:-
Class I - Grade 10
Class II - Grade 20
Class III - Grade 40
Class IV - Grade 100
Class V - Grade 500
Class VI - Grade 600
Class VII - Grade 700
However, in all cases it is important to confirm the suitability of the new specification for the specific application in which it is to be used.
All of the ball specifications discussed have been developed to cover the requirements of the bearing industry. As a result some aspects of the specifications whilst important to the needs of bearing manufacture have little relevance to other applications.
A high precision angular contact bearing may contain 40 or more balls. If the bearing is to function correctly with minimum run-out, it is essential that variation in ball size is minimised. For this reason the maximum lot diameter variation is specified as twice the allowable spherical error 0.5µm for grade 10. When the same ball is used singly in a check valve this tight control of size within a lot is unnecessary.
For this reason it is helpful to consider the requirements of a specific application in terms of the properties required from the ball, and then look for the highest grade that will satisfy them. This approach can avoid the specification of an unnecessarily high grade of ball and by doing so considerably reduce cost.