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Precision-ground balls of hardened martensitic stainless (440C, 420) for corrosion-resistant bearings, or of austenitic 302, 304 and 316 for valves, pumps, sprayers and food equipment. They are graded for roundness and size accuracy by ISO 3290-1, from G3 (the most precise) to G1000.
Steel balls start as wire or rod cut into slugs and cold headed (or, for large sizes, hot forged) into rough spheres. The rough balls are filed and ground between grooved plates, heat treated if the grade is hardenable, then ground again and lapped to the final size and roundness. The finest grades are lapped to a mirror finish and sorted into size lots a fraction of a micrometre apart.
Two families of stainless are used. Martensitic 440C (about 17% Cr, 1% C) hardens to around 58–62 HRC and is the standard material for stainless rolling bearings, combining wear resistance with reasonable corrosion resistance; 420 is softer and cheaper. Austenitic 302, 304 and 316 cannot be hardened by heat treatment, so they are used where corrosion resistance, low magnetism or food contact matter more than hardness — check valves, sprayer and pump balls, aerosol mixing balls and cosmetic roll-on applicators.
Accuracy is set by the ball grade in ISO 3290-1 (and the equivalent JIS and DIN standards). The grade number is roughly the permitted deviation from a true sphere in millionths of an inch: a G10 ball is round and uniform in diameter to about 0.25 µm, a G100 ball to about 2.5 µm. Bearings use G3 to G28; valves, pumps and general mechanisms use G100 to G1000.
There is little ball manufacturing in the GCC; balls are imported, chiefly from China, Europe, Japan and India, by bearing, valve and pump makers and maintenance distributors. Regional demand comes from valve assembly and repair for oil, gas and water, from pumps and from food and packaging machinery.
Balls are ordered by diameter, material, grade (accuracy) and quantity; for hardenable materials the hardness range, and for valves sometimes the surface finish.
| Region | Standard | What it covers |
|---|---|---|
| International | ISO 3290-1 ISO |
Rolling bearings — balls — steel balls: grades, tolerances on diameter, form and surface roughness |
| Japan | JIS B1501 JSA |
Rolling bearings — steel balls |
| Germany | DIN 5401 DIN |
Balls for rolling bearings and general industrial use |
Grade equivalents and the standards bodies: Steel grades & standards
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