Ball Bearing Sizes
Authority: ISO 15 Metric Bearing Numbering Convention
Metric deep-groove ball bearings follow a standardized numbering system: a 608 bearing has an 8mm bore, while 6000-series bearings like 6001 and 6002 have 12mm and 15mm bores respectively. The full bearing number also fixes outer diameter and width, letting engineers cross-reference parts across manufacturers without measuring.
| Bearing Number | Bore | Outer Diameter | Width |
|---|---|---|---|
| 608 | 8 mm | 22 mm | 7 mm |
| 6001 | 12 mm | 28 mm | 8 mm |
| 6002 | 15 mm | 32 mm | 9 mm |
| 6202 | 15 mm | 35 mm | 11 mm |
History & Origin
The ball bearing's basic principle -- using rolling spheres to reduce friction between a rotating part and its housing -- was patented in 1794 by Welsh inventor Philip Vaughan, who designed an axle assembly with iron balls running in a groove. Early bearings were limited by materials, though: cast iron balls were brittle and prone to cracking under load. It took the Bessemer process for cheap, reliable steel, combined with the bicycle boom of the late 19th century, to make ball bearings practical at scale -- a Parisian bicycle mechanic named Jules-Pierre Suriray patented a steel ball-bearing design in 1869, and a bicycle fitted with his bearings won an international race that same year.
The standardized numbering system used today, where a bearing's part number directly encodes its bore, outer diameter, and width, emerged from industrial cooperation during World War I, when US bearing manufacturers organized to streamline wartime production. That effort led to the bearing associations whose numbering conventions -- later folded into ISO standards -- still let engineers cross-reference a bearing from one manufacturer against an equivalent part from another just by matching the number.