What a ball grade is
ISO 3290 is the international standard for balls for rolling bearings and general industrial use. It classifies balls into grades with a letter G followed by a number: the lower the number, the tighter the tolerance. The grade describes four things together: how far a ball may deviate from a perfect sphere, how much the diameter may vary inside one ball, how much diameters may vary inside a lot, and how smooth the surface must be.
A grade is not a material statement. Balls in the same grade may be chromium bearing steel, stainless 440C or ceramic, so the order must name both. Material properties are covered in GCr15 and 52100 bearing steel. This note concerns the geometry and what it means.
Reading the numbers
For the roundness limit, the grade number is, in effect, a scale factor: the maximum deviation from spherical form is the grade number multiplied by about 0.025 micrometres, as listed in ISO 3290-1. Treat the table below as a reading aid and confirm exact values against the standard or the supplier datasheet.
| Grade | Max deviation from spherical form (reference) | Typical use |
|---|---|---|
| G3 to G5 | About 0.08 to 0.13 micrometres | Instrument, gyroscope and special precision bearings |
| G10 | About 0.25 micrometres | Precision and high-speed ball bearings, spindles |
| G16 to G20 | About 0.4 to 0.5 micrometres | Quiet motor and instrument bearings, linear guides |
| G28 | About 0.7 micrometres | Standard-grade ball bearings for general industry |
| G40 | About 1.0 micrometres | Lower-speed bearings, ball screws in light duty |
| G100 | About 2.5 micrometres | Non-precision uses: sliders, casters, light mechanisms |
The same standard also covers grade steps below and above this range, from G3 up to G200. Series in the middle are also used. Which grade a given bearing precision class requires is set by bearing makers, not by the buyer of loose balls, so for complete bearings it is more useful to specify the bearing precision class and let the plant choose the matching ball grade.
Grade, bearing class and noise
Ball roundness feeds directly into vibration. A less round ball produces a periodic disturbance as it rolls, which appears as noise and vibration at the ball pass frequency. Raceway finish does the same, which is why quiet bearings use both smoother balls and smoother raceways. Standard deep groove ball bearings in normal tolerance usually use balls around G28 or tighter; motor and appliance bearings tuned for low noise use higher-grade balls and a stricter vibration group.
Precision classes P5 and P4, as defined in ISO 492, and high-speed bearings for spindles typically use G10 or better, and hybrid ceramic balls are made to similar grades. The sections of vibration grades and tolerance and clearance explain how the pieces fit together. The key point is that grade, class and noise group need to be consistent with each other. A very high-grade ball in a rough ring pays for precision that the bearing never uses.
Sizes, lots and sorting
Balls are produced in lots made from the same steel heat, forged or headed, flashed to remove the equator band, ground and lapped, then hardened and finally finished to a common diameter. Within a lot, balls are measured and sorted. Diameter is expressed as a nominal size plus the lot deviation, and bearing plants often request balls in graded sub-lots so that all balls in one bearing have nearly the same diameter. This matters because one ball larger than the rest carries more than its share of load and produces an uneven running torque.
Nominal sizes follow both metric and inch series. Millimetre sizes follow the common preferred series and inch balls follow fractional sizes. For repair kits and catalogue programs, confirm whether the order is in metric or inch, because a ball that is close in size but not equal can cause problems in a bearing or a linear mechanism.
Ordering balls from partner suppliers
Ball production in China is concentrated in a few areas, and the Dong'e area in Shandong, near Liaocheng, is known for steel ball supply. Industry sources describe a supply structure in which ball makers sell to bearing plants, distributors and other industries. Quantities for loose balls are normally counted in the tens of thousands, and the typical reference is on the MOQ and lead time page.
A complete ball order states:
- Material: chromium bearing steel, stainless 440C, ceramic or other, with the standard.
- Nominal diameter in millimetres or inches.
- Grade under ISO 3290, for example G10 or G28.
- Hardness range, if it is not covered by the plant default.
- Packing: weight or count per bag, oiling or VCI protection, and label requirements.
Balls are heavy, rust easily and are easily mixed. Good packaging keeps lots separate and labelled with the lot number. Documentation to ask for includes the lot record and a statement of the measured grade parameters on a sample. The testing and inspection page lists what the desk can request on a program basis.
How grade is checked
Ball grade is verified in the measuring room, not on the shop floor. Diameter and lot variation are measured with precision comparators or air gauges, roundness with a dedicated roundness instrument, and surface roughness with a stylus or optical instrument. These measurements need controlled temperature, because steel expands enough with a few degrees of warming to disturb results at the micrometre level.
For a buyer, the practical consequence is that a grade claim has value only when it is backed by records from the producing plant and can be re-measured on a sample. If you are buying a significant lot, agree the sample size and the parameters in advance, and keep a retained sample for later comparison. A mismatch between the grade on the label and the grade in the box is a common cause of disputes, and it is far easier to settle when the measurement method was written down before shipment.
When a higher grade is not needed
Higher grade means higher cost, and it only helps where the application can use it. A conveyor roller, a farm-equipment housing or a slow pivot gains nothing from G10 balls. A machine tool spindle, a gyroscope or a quiet motor does. Match the grade to the speed, the noise requirement and the rest of the bearing. When unsure, send the application and operating conditions through the quote form, and the desk will propose a matched ball grade and bearing class from partner plants.