Bryant Lange, of Ashburton, asks :-

How are ball bearings made?

David Aitchison, a mechanical engineer at the University of Canterbury, responded.

The raw material used in the manufacture of balls in ball bearings is a specially formulated grade of steel wire. Production of the balls from the wire is a relatively simple process comprising of the following steps.

Wire is fed from a decoiler through a guillotine where it is cut into predefined lengths. The resulting pellets are then pressed into balls between two hemispherical cups (dies). The balls then progress through two filing operations. The first removes the ridge (burr), left around the periphery of the balls at the die join line, while the second produces a close spherical form. After filing, the balls are heat treated (hardened and tempered) in automatic furnaces which in turn necessitates one final grinding operation to bring all the balls back to the same size. To conclude the manufacturing process finish lapping (metal polishing) takes place to obtain the correct dimensions, roundness and surface finish.

After cleaning, the balls are most frequently assembled into ball bearings comprising a cage, balls, inner and outer ring. Despite all parts having being manufactured to a high level of accuracy, small variations in size from one part to the next inevitably occur. Rather than try to eliminate these process dependant variations, which would make the bearings extremely expensive, the balls and rings are measured, grouped and matched for size - so that the assembled units meet the original design requirements. In this way 'small' balls may be fitted into 'large' rings or conversely 'large' balls into 'small' rings. The assembled ball bearings will however all perform equally and appear the same to the naked eye.