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AMBA Ball Bearing Definitions

Grades and Tolerances (ABMA STD-10)

Ball Grade: A specific combination of dimensional form and surface roughness tolerance. A ball grade is designated by a grade number.
Ball Diameter Variation: The difference between the largest and the smallest actual single diameter of one ball.
Lot: A definitive quantity of balls manufactured under conditions which are presumed uniform and which is considered and identified as an entirety.
Lot Diameter Variation: The difference between the mean diameter of the largest ball and that of the smallest ball in the lot.
Ball Diameter Tolerance: The maximum allowable deviation of any ball lot mean diameter from the nominal ball diameter.

Ball Diameter (ABMA STD-10)

Nominal Ball Diameter: The diameter value that is used for the purpose of general identification of a ball size, e.g., .125", 4mm, etc.
Ball Gage: The prescribed small amount by which the lot mean diameter should differ from nominal diameter, this amount being one of an established series of amounts. A ball gage, in combination with the ball grade and nominal ball diameter, should be considered as the most exact ball size specification to be used by a customer for ordering purposes.
Specific Diameter: The amount by which the lot mean diameter differs from the nominal diameter, accurate to the container marking increment for that grade. The specific diameter should be marked on the unit container.
Container Marking Increment: The standard unit steps in micrometers or in millionths of an inch, used to express the specific diameter.

Surface Qualities

Surface Roughness (Ra): Surface roughness is a numerical value of the mean roughness height or amplitude measured from the roughness centerline. It is approximately half of the average distance from peak to valley.
Sphericity (roundness): The radial distance of the maximum peak from the perfect circle plus the distance of the maximum valley from the perfect circle.
Waviness: The more widely spaced circumferential component of surface texture.

*American Bearing Manufacturers Association (ABMA); formerly the Anti Friction Bearing Manufactures Association (AFBMA)

**DIN 5401 and ISO 3290 are European originated precision ball specifications similar to the ABMA specifications referencing tolerance specifications.