ISO 21940 balancing classes: understanding G0.4, G1, G2.5 and G6.3
ISO 21940-11 gives the industry a common language for specifying how precisely a rotating part must be balanced. Here are the main G grades used in machine tools, and how to pick the right one without over-specifying.
The principle behind the G quality grades
TLDR: A G grade represents a constant velocity — the product of the permissible specific unbalance and the maximum service angular velocity. It makes the requirement comparable across rotors of very different masses and speeds.
ISO 21940-11:2016, which replaced ISO 1940-1:2003 while keeping the substance of its principles and values, defines grades written as the letter G followed by a number: G0.4, G1.0, G2.5, G6.3, and so on up to G4000 for the most tolerant applications.
The principle is that this number represents a constant velocity, in millimetres per second: the product of the permissible residual specific unbalance and the maximum service angular velocity. This makes the requirement comparable across very different rotors — a "G6.3 pump" denotes exactly the same relative requirement in any workshop in the world, whatever its size or actual speed.
The main grades used in machine tools
TLDR: From G0.4 for very high precision grinding to G6.3 for general process machinery, by way of G1.0 for high precision and G2.5 for everyday spindles.
- G0.4 — ultra precision: very high precision grinding spindles, gyroscopes, microelectronics manufacturing components. The strictest level routinely met in industry.
- G1.0 — high precision: high-precision machine tool spindles, turbochargers, high-speed dental and medical spindles.
- G2.5 — the most common grade in production: standard electric motors, everyday machine tool spindles, turbines, machine tool drives.
- G6.3 — general process: pumps, fans, flywheels, two-pole motors. Adequate for many industrial applications, but generally not enough for a high-speed machining spindle.
Why higher speed demands finer balancing at the same G grade
Since the G grade denotes a constant velocity — the product of unbalance and rotational speed — a direct principle follows: at the same G grade, a higher speed imposes a smaller permissible residual unbalance in absolute terms, expressed in gram-millimetres.
Concretely, a grinding spindle at 40,000 rpm targeting G1.0 must be balanced to a far tighter physical precision than a large electric motor turning at 1,500 rpm targeting the same G1.0. The grade expresses a requirement relative to speed, not a fixed physical tolerance.
Do not over-specify: a common technical and commercial trap
TLDR: Routinely taking the next grade up "to be safe" costs money without a proportionate benefit — all the more so as dynamic cutting forces often exceed the residual centrifugal forces of an already correct balance.
The trap is common and well documented: picking the grade immediately above what is genuinely needed, out of caution — targeting G2.5 where G6.3 would technically do. It is often unjustified and adds significant cost with no measurable benefit.
A technical argument reinforces that caution: on a machine in real operation, dynamic cutting forces — those generated by the interrupted cuts typical of milling, in particular — are frequently larger in amplitude than the residual centrifugal forces of an unbalance already corrected to a standard grade. In other words, past a level of precision that is already sufficient, refining the balance further no longer improves process stability: the limit lies elsewhere — machine structure stiffness, tool clamping quality, cutting parameters.
The reasoning mirrors bearing precision classes: the right class is not the highest one, it is the one the application calls for.
Frequently asked questions
What G grade do you usually target for a standard machine tool spindle?
We match the grade to the spindle's original specification and actual use, generally in the G1.0 to G2.5 range for everyday spindles — without over-specifying.
Why does a grinding spindle need finer balancing?
Grinding applications often run at very high speed and aim for very fine surface finishes. That justifies a stricter grade, G0.4 or G1.0, than routine milling.
Is balancing to G0.4 always better than G2.5?
Technically more precise, yes. Always relevant, no. Over-specified balancing is a cost with no measurable benefit if the application does not genuinely require it.
How do I know which G grade suits my spindle?
It depends on service speed, rotor mass and the application — grinding, high-speed milling, general machining. We make that assessment during our diagnosis.
Does the balancing grade appear in the original documentation?
Sometimes, where the builder's documentation is available. Failing that, we work from the spindle's actual application and the sector's usual standards to settle on the right grade.
Balancing to the grade your application actually needs
We settle the right grade from service speed, rotor mass and real application — not from a precautionary reflex that would cost you without giving anything back.
Our teams are available from our sites in Beaurepaire (France) and Lussery-Villars (Switzerland).
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