ISO Balance Quality Grades
The grade is defined so that e × ω is constant, which is why the permissible eccentricity halves every time the speed doubles. The two speed columns show the same grade in µm (numerically identical to g·mm per kg of rotor) at two common speeds.
| Grade | G (mm/s) | e_per at 1500 rpm (µm) | e_per at 3000 rpm (µm) | Typical rotors |
|---|---|---|---|---|
| G0.4 | 0.4 | 2.5 | 1.3 | Precision grinder spindles, gyroscopes |
| G1 | 1.0 | 6.4 | 3.2 | Grinding-machine drives, small armatures |
| G2.5 | 2.5 | 15.9 | 8.0 | Gas and steam turbines, machine-tool drives, pumps |
| G6.3 | 6.3 | 40.1 | 20.1 | General machinery, fans, flywheels, pump impellers |
| G16 | 16.0 | 101.9 | 50.9 | Drive shafts, engine parts, agricultural machinery |
| G40 | 40.0 | 254.6 | 127.3 | Car wheels and rims, drive shafts |
| G100 | 100.0 | 636.6 | 318.3 | Reciprocating engine parts |
| G250 | 250.0 | 1591.5 | 795.8 | Rigidly mounted diesel crankshafts |
| G630 | 630.0 | 4010.7 | 2005.4 | Large marine diesel crankshafts, slow-speed |
Multiply the eccentricity by the rotor mass in kg to get the permissible unbalance in g·mm, or use the Rotor Balance tool, which also gives the bearing force that allowance produces. ISO 1940-1 was superseded by ISO 21940-11 in 2016; the grades themselves are unchanged. A rotor only counts as rigid below about 70% of its first critical speed.
A reference aid, not a substitute for engineering judgment. Verify against the governing standard.