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ISO 21940-11 Balance Tolerance Calculator

Calculates the permissible residual unbalance for rigid rotors using the balance quality grade approach of ISO 21940-11 (formerly ISO 1940-1). The result can be used as the acceptance criterion after balancing.

Permissible residual unbalance (Uper) —g·mm
Permissible specific unbalance (eper)
— µm
Permissible mass at radius
— g

Typical applications

G4000Crankshaft drives of rigidly mounted slow marine diesel engines (uneven number of cylinders)
G1600Crankshaft drives of rigidly mounted large two-cycle engines
G630Crankshaft drives of rigidly mounted large four-cycle engines; elastically mounted marine diesels
G250Crankshaft drives of rigidly mounted fast four-cylinder diesel engines
G100Complete engines for cars, trucks and locomotives
G40Car wheels, wheel rims and drive shafts
G16Drive shafts with special requirements; crushing machine parts; agricultural machinery parts
G6.3Process plant machines, fans, pumps, flywheels, centrifuge drums, paper machinery rolls, medium and large electric armatures
G2.5Gas and steam turbines, turbo-generators, turbo-compressors, machine tool drives, small electric armatures
G1Grinding machine drives, small armatures with special requirements
G0.4Spindles, discs and armatures of precision grinders; gyroscopes

These tools are intended for preliminary assessment and training. Decisions on critical equipment should be based on on-site measurement and expert evaluation.

How to use

  1. Select the grade suited to the equipment (usually G6.3 for fans and pumps, G2.5 for precision equipment).

  2. Enter the rotor mass and maximum service speed.

  3. Optionally enter the correction radius to see the permissible weight at that radius.

Theory & formulas

The grade G (mm/s) is the product of specific unbalance and angular velocity: G = eper · ω.

Permissible residual unbalance: Uper = 1000 · G · m / ω [g·mm], with ω = 2π·n / 60 [rad/s] and m [kg]. Practical form: Uper ≈ 9549 · G · m / n.

For symmetric rotors balanced in two planes, Uper is split equally. For asymmetric rotors the split depends on the centre of mass position relative to the bearings (ISO 21940-11, clause 7).

Frequently asked questions

Which balance grade is used for fans?

Industrial fans are typically balanced to G6.3; precision or high-speed fans may use G2.5. Fan-specific categories and grades are also defined in ISO 14694.

What is the difference between ISO 1940 and ISO 21940-11?

ISO 21940-11 is the current version that replaced ISO 1940-1. The grades and basic calculation method are unchanged; terminology and some explanations were updated.