Rotor Balance Grade Calculator (ISO 1940 / ISO 21940: Permissible Unbalance)
The residual unbalance a rotor may keep for a chosen balance quality grade G, at its operating speed and mass — in g·mm, per plane, as an eccentricity, and as the force on the bearings.
Results
What this tool does
ISO balance grades state that the product of the permissible eccentricity and the angular speed is a constant: G 6.3 means e × ω = 6.3 mm/s. So the faster the rotor turns, the smaller the unbalance it may keep — doubling the speed halves the permissible eccentricity. For a 25 kg rotor at 3000 rpm and grade G 6.3 that is about 20 µm, or roughly 500 g·mm. The grade comes from the type of machine; the examples shown are typical, and the machine maker or the standard decides which applies. A rotor with two correction planes is usually allowed half in each.
Formula
e_per = G ÷ ω · U_per = e_per × m · F = U ω² = m G ω (G in mm/s, ω = 2π n ÷ 60)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
gr | Balance quality grade G | — |
rp | Operating speed | rpm |
mm | Mass of the rotor | kg |
np | Correction planes | — |
U | Permissible residual unbalance (total) | g·mm |
UP | Permissible unbalance in each plane | g·mm |
E | Permissible eccentricity of the centre of mass | µm |
W | Angular speed | rad/s |
F | Force on the bearings from that unbalance | N |
Worked example
- Balance quality grade G6.3
- Operating speed3000 rpm
- Mass of the rotor25 kg
- Correction planes1
- Permissible residual unbalance (total)501.3 g·mm
- Permissible unbalance in each plane501.3 g·mm
- Permissible eccentricity of the centre of mass20.05 µm
- Angular speed314.2 rad/s
- Force on the bearings from that unbalance49.5 N
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.