Because the best ball bearings in the early 20th century were made by German manufacturers, ball bearings have usually been sized in millimeters, even those made in the United States–but the balls themselves were frequently sized in inches. The Anti-Friction Bearing Manufacturers Assn. adopted a classification of ball bearings according to their construction, with each type identified by a symbol (such as "BA"). Symbols that include the letter "I" identify types that are nominally sized in inches; of ten types, only two have an "I."
Embedded in the manufacturer's model number for most ball bearings is a three-digit code that gives its dimensions in millimeters. The first digit refers to the series. There are four principal series:
The table below shows the sizes up through a bore of 50 mm, but much bigger sizes are defined. In the medium series, for example, a 356 bearing takes a shaft with a diameter of 280 mm–more than eleven inches.
|Designation||Bore|| Series 100
(x = 1)
| Series 200
(x = 2)
| Series 300
(x = 3)
| Series 400
(x = 4)
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Last revised: 1 February 2010.