Bearing **balls** are special highly spherical and smooth balls, most commonly used in ball bearings, but also used as components in things like freewheel mechanisms. The balls come in many different *grades*, as defined by the American Bearing Manufacturers Association (ABMA), which defines the precision of the balls. They are manufactured in machines designed specially for the job.

In 2008, the United States produced 5.778 billion bearing balls

Bearing balls are manufactured to a specific grade, which defines its geometric tolerances. The grades range from 2000 to 3, where the smaller the number the higher the precision. Grades are written "GXXXX", i.e. grade 100 would be "G100".and lack of defects, such as flats, pits, soft spots, and cuts. The surface smoothness is measured in two ways: surface roughness andwaviness.

Size refers to how tight the tolerances are on the size, as measured by two parallel plates in contact with the ball surface. The starting size is the *nominal ball diameter*, which is thenominal, or theoretical, ball diameter. The ball size is then determined by measuring the *ball diameter variation*, which is the difference between the largest and smallest diameter measurement. For a given lot there is a *lot diameter variation*, which is the difference between the mean diameter of the largest ball and the smallest ball of the lot.

Sphericity, or *deviation from spherical form*, refers to how much the ball deviates from a true spherical form (out of roundness). This is measured by rotating a ball against a linear transducer with a gauge force of less than 4 grams (0.14 oz). The resulting polar graph is then circumscribed with the smallest circle possible and the difference between this circumscribed circle and the nominal ball diameter is the variation.

Grade | Size range [in] | Sphericity [in] | Lot diameter variation [in] | Nominal ball diameter tolerance [in] | Maximum surface roughness (Ra) [μin] |
---|---|---|---|---|---|

3 | 0.006–2 | 0.000003 | 0.000003 | ±0.00003 | 0.5 |

5 | 0.006–6 | 0.000005 | 0.000005 | ±0.00005 | 0.8 |

10 | 0.006–10 | 0.00001 | 0.00001 | ±0.00005 | 1.0 |

25 | 0.006–10 | 0.000025 | 0.000025 | ±0.0001 | 2.0 |

50 | 0.006–10 | 0.00005 | 0.00005 | ±0.0002 | 3.0 |

100 | 0.006–10 | 0.0001 | 0.0001 | ±0.0005 | 5.0 |

200 | 0.006–10 | 0.0002 | 0.0002 | ±0.001 | 8.0 |

1000 | 0.006–10 | 0.001 | 0.001 | ±0.005 |

Grade | Sphericity [mm] | Lot diameter variation [mm] | Nominal ball diameter tolerance [mm] | Maximum surface roughness (Ra) [µm] |
---|---|---|---|---|

3 | 0.00008 | 0.00008 | ±0.0008 | 0.012 |

5 | 0.00013 | 0.00013 | ±0.0013 | 0.02 |

10 | 0.00025 | 0.00025 | ±0.0013 | 0.025 |

25 | 0.0006 | 0.0006 | ±0.0025 | 0.051 |

50 | 0.0012 | 0.0012 | ±0.0051 | 0.076 |

100 | 0.0025 | 0.0025 | ±0.0381 | 0.127 |

200 | 0.005 | 0.005 | ±0.025 | 0.203 |

1000 | 0.025 | 0.025 | ±0.127 |