ISO 21940 · rotor balancing
G-Spec Balancing Calculator
ISO 21940 balance quality grades for rotating machinery: permissible residual unbalance Uper from G grade, rotor mass and service speed; reverse-check an achieved grade; and compute the centrifugal force a residual unbalance generates.
Permissible residual unbalanceAchieved grade from measured unbalanceCentrifugal force from unbalance
Permissible residual unbalance
eper = 9549 × G / nAchieved grade from measured unbalance
G = (U/m) × n / 9549Centrifugal force from unbalance
F = U × Ω² × 10⁻⁶ISO 21940 balance quality grades
| Balance quality grade (ISO 21940) | Typical applications |
|---|---|
| G 4000 | Crankshaft drives of rigid slow marine diesel engines |
| G 1600 | Crankshaft drives of rigid large two-stroke engines |
| G 630 | Crankshaft drives of rigid large four-stroke engines |
| G 250 | Crankshaft drives of fast six-cylinder diesel engines |
| G 100 | Fast diesel engine crankshafts, six or more cylinders |
| G 40 | Car wheels, wheel rims, drive shafts, propeller shafts |
| G 16 | Drive shafts with special requirements, parts of crushing and agricultural machinery |
| G 6.3 | Process machinery parts, centrifuge drums, electric motor rotors, machine tool parts |
| G 2.5 | Turbochargers, machine tool drives, precision electric motor rotors, turbine rotors |
| G 1 | Tape recorder and phonograph drives, grinding machine drives |
| G 0.4 | Spindles, disks and armatures of precision grinders, gyroscopes |
Formulas used
Permissible eccentricity: eper (µm) = 9549 × G / n — also written G = eper × Ω / 1000 with Ω = 2πn/60
Permissible unbalance: Uper (g·mm) = eper × rotor mass (kg)
Force: F (N) = U (g·mm) × Ω² × 10⁻⁶
Frequently asked questions
What does balance grade G 6.3 mean?
G is the product of permissible residual specific unbalance (eper, in g·mm/kg = µm) and angular velocity — numerically the eccentricity velocity in mm/s. G 6.3 covers general machine parts, fans, flywheels and electric motor rotors.
How do I find permissible residual unbalance for a grinding wheel?
G 6.3 (or G 2.5 for precision spindles): Uper = 9549 × G × mass / RPM. A 2 kg wheel at 6000 RPM graded G 6.3 allows Uper ≈ 20 g·mm.
Does permissible unbalance change with speed?
Yes — inversely. The same rotor allowed 50 g·mm at 3000 RPM is allowed only 25 g·mm at 6000 RPM for the same G grade, and centrifugal force rises with the square of speed.