Specialists in the Industry
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 are probably the most important, and in general terms, all major ball standards are based on AFBMA 10. DIN 5401 used to be the exception but was brought in line with AFBMA 10 in 1993.
The specifications define balls in terms of a series of properties:
Basic Diameter – The nominal diameter of the ball, e.g. 10mm.
Ball Diameter Variation – The difference between the largest and smallest diameter measured for a single ball.
Ball Mean Diameter – The arithmetic mean of the maximum and minimum diameters measured for the ball.
Sphericity or deviation from spherical form.
Lot – Normally a manufacturing batch manufactured under uniform conditions.
Lot Mean Diameter – The arithmetic mean of the smallest and the largest mean ball diameters within the lot.
Lot Diameter Variation – The difference between the smallest and the largest mean ball diameters within the lot.
Gauge – The deviation of the lot mean diameter from the nominal diameter.
Grade – A number by which the properties of the ball are specified.
Previous versions of DIN 5401 used class rather than grade. 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, while important to the needs of bearing manufacture, have little relevance to other applications. For example:
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 the requirements of a specific application in terms of the properties required of 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 a ball, and by doing so considerably reduce cost.
Of all the engineering ceramics so far developed, silicon nitride offers by far the best combination of properties for most applications. Its hardness, combined with toughness, allows use in many situations where conventional ball materials fail, or have an unacceptably short life. Its increasing use for high-speed bearing applications has resulted in significant cost reduction, such that it can now provide a cost-effective solution for an ever-increasing range of problems.