Standard Metal Balls: Size, Grade and Application: Number of Metal Balls per Kilogram

Number of Metal Balls per Kilogram

The metric equivalent of the previous table, using densities in grams per cubic centimeter.

Material Densities (Grams per Cubic Centimeter)

Table 7: Balls per Kilogram by Size and Material

For sizes above 17 mm diameter, use this formula:

$$\text{No. balls per kilogram} = \frac{1{,}910{,}000}{(\text{nom. dia., mm})^3 \times (\text{material density, g/cm}^3)}$$

Note: For all sizes above 17 mm, use the formula provided above. The relationship is purely geometric—ball count scales inversely with the cube of the diameter and linearly with density.

the practitioner's Transformation: From Failure to Standard Operating Procedure

Six months after the rejected shipment, the practitioner had rewritten his company's incoming inspection protocol. The new procedure wasn't complex—it was specific.

Every purchase order now included all five specification elements: quantity, material, nominal diameter, grade, and ball gage. Incoming inspection verified the Specific Diameter on every container against the PO. Hardness testing on spherical surfaces automatically applied the curvature correction from Table 5.

The result? Zero ball-related rejections in the following eighteen months. Not because the suppliers suddenly got better—because the practitioner's team finally spoke the same language as the Standard.

What You Can Apply Right Now

The insight that changed everything for the practitioner was this: the AFBMA grading system is not arbitrary. Every number in every table represents a carefully calibrated relationship between dimensional precision, surface quality, and manufacturing capability. Once you understand the system's logic, specification becomes straightforward instead of intimidating.

Quick-Reference: Grade Selection Decision Guide

When choosing a ball grade, match your application's precision requirements to the tightest tolerance that matters for your design. Here's a practical framework:

Universal Formulas for Weight-to-Count Conversion

These two formulas work for any ball material and any size—even those not listed in the standard tables. Memorize them or keep them bookmarked.

Imperial (Inch) System

$$N_{lb} = \frac{1.91}{D^3 \times \rho_{in}}$$

Where:

• $N_{lb}$ = number of balls per pound
• $D$ = nominal diameter in inches
• $\rho_{in}$ = material density in lb/in³

Metric System

$$N_{kg} = \frac{1{,}910{,}000}{D^3 \times \rho_{cm}}$$

Where:

• $N_{kg}$ = number of balls per kilogram
• $D$ = nominal diameter in mm
• $\rho_{cm}$ = material density in g/cm³

These formulas derive from the volume of a sphere ($V = \frac{\pi}{6}D^3$) combined with the density-to-mass relationship. The constants 1.91 and 1,910,000 are pre-computed for the respective unit systems.

Your Next Step

Pull up the last ball specification you wrote or approved. Check it against the five required ordering elements: quantity, material, nominal diameter, grade, and ball gage. If any element is missing—especially the grade or ball gage—you now know exactly what to add, and why it matters.

The difference between a bearing that lasts ten years and one that fails in ten months often comes down to whether someone specified Grade 24 when the application needed Grade 10. That someone doesn't have to be you.

This guide is based on the American National Standard ANSI/AFBMA Std 10-1989 for standard metal balls. For complete details on material requirements, quality specifications, quality assurance provisions, methods of hardness testing, and minimum case depths for carbon steel balls, reference should be made directly to the Standard.