Making ball bearings a precise business
Rolling-element bearings range in size from those so small that a microscope is needed for inspection to those large enough for a person to stand inside.
One of the big manufacturers alone lists more than 26000 different bearing combinations, varying in mass from 15 grams, three teaspoons of water, to nine tons, the mass of nine VW Citi Golfs.
Bearing making is an extremely precise business, demanding a lot of know-how and advanced manufacturing techniques.
Before the arrival of the global economy in the early 1990s brought outsourcing in its wake, it was commonly believed that a country's bearing production was a reliable indication of its industrial capabilities.
Such a strategic product are bearings that Allied bombing raids in World War II often targeted German ball bearing factories. If the production of bearings could be disrupted, virtually all output of tanks and aeroplanes, among others, would grind to a halt. Britain itself faced the problem at the beginning of the war that it couldn't make the special bearings needed for aircraft engines, and it resorted to desperate measures to obtain them from Sweden by boat, across German-patrolled waters.
The special steel used for bearings usually contains 1percent carbon, 1,2percent chromium, 0,25percent nickel and 0,25percent molybdenum, to provide the best compromise between hardness and toughness after the correct heat treatment. Hardness and toughness are two different things for an engineer - the glass in an ordinary window pane is hard but not tough, for instance.
Most important, however, is that bearing steel must be ultra-clean in that there must be no little bits of foreign matter included in the steel when it solidifies from its molten state.
During the steelmaking process the liquid steel must therefore be purged of all such inclusions. An intriguing question is how they get the balls in ball bearings so perfectly round. And very close to perfectly round they have to be, in addition to being exceptionally smooth.
Surprisingly, the balls start out as thick steel wire or steel rod. High-speed machines chop off the right length of wire, which is then smashed, "cold forged" is the technical term, between two hollow hemispherical dies. The result is a ball that looks like the planet Saturn with a ring around its middle called "flash". In the first machining process the balls are made to roll between two contra-rotating cast iron discs with rough grooves in their faces where the flash is removed. Then follows heat treatment and tempering. Thereafter the balls are put in grinding machines, similar in principle to the flash remover but less brutal, which grind them down very close to their finished size and almost perfectly round shape.
Finally the balls go to lapping machines where fine abrasive slurry is used to polish them for several hours to get a mirror finish.