What is permissible residual unbalance?

Permissible residual unbalance is the maximum amount of unbalance allowed to remain in a rotor after balancing, defined by ISO 21940-11 (formerly ISO 1940). It depends on the rotor mass, operating speed, and the required balance quality grade G.

How do I choose the right balance grade G?

Balance grade G is selected based on rotor type. G6.3 is typical for fans, pumps, and electric motors. G2.5 is for gas turbines and turbochargers. G1.0 is for grinding machine drives. G16 is acceptable for agricultural machinery.

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

ISO 21940-11 replaced the older ISO 1940-1 standard. The technical content is essentially the same — the formula and balance grades remain unchanged. The new number reflects the reorganization of the ISO 21940 series.

How do I split unbalance between two correction planes?

For rotors balanced in two planes, the total permissible unbalance is distributed between the planes based on their distance from the center of gravity. A common simplification is to allocate 50% to each plane when the CG is approximately centered.

What units is unbalance measured in?

Unbalance is typically measured in g·mm (gram-millimeters) or oz·in (ounce-inches). The SI unit is kg·m, but g·mm is more practical for most industrial rotors. 1 oz·in ≈ 720 g·mm.

Free Engineering Tool

Permissible Residual Unbalance Calculator

Calculate the maximum allowable residual unbalance for any rotor per ISO 21940-11. Enter mass, speed, and balance quality grade.

ISO 21940-11 ISO 1940-1 G0.4 – G4000

Rotor Mass (kg)

Service Speed (RPM)

Balance Grade G

Correction Planes

Quick presets

Results

Total Permissible Unbalance

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Specific Unbalance (eccentricity)

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Per-Plane Unbalance

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Angular Velocity ω

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Balance Grade

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Permissible Specific Unbalance

Per ISO 21940-11, the permissible specific unbalance (eccentricity) is:

G — balance quality grade (mm/s)
n — service speed (RPM)
ω — angular velocity (rad/s)

Total Permissible Residual Unbalance

Where m is rotor mass in kg. Result U_per is in g·mm when e_per is in μm and m in kg.

Balance Quality Grades

Grade	e·ω (mm/s)	Typical Application
G0.4	0.4	Gyroscopes, spindles
G1.0	1.0	Grinding machine drives
G2.5	2.5	Gas/steam turbines, turbochargers
G6.3	6.3	Fans, pumps, motors, general machinery
G16	16	Agricultural machinery, automobile wheels
G40	40	Car wheels, rims
G100	100	Complete reciprocating engines
G250	250	Four-stroke engines on rigid mounts
G630	630	Crankshaft drives (fast diesels)
G1600	1600	Slow diesels on rigid mounts
G4000	4000	Crankshaft drives (large diesels)

Practical Example

Example — Centrifugal Fan Rotor

Given: Rotor mass = 120 kg, Speed = 1500 RPM, Grade = G6.3

ω = 2π × 1500 / 60 = 157.08 rad/s

e_per = 6.3 × 1000 / 157.08 = 40.11 μm

U_per = 40.11 × 120 = 4,813 g·mm

Per-plane (2 planes, 50/50): 4,813 / 2 = 2,407 g·mm

⚠️ Note: For disc-like rotors, 50/50 split between planes is acceptable when CG is centered. For long rotors, use the lever rule based on actual CG position relative to correction planes.

Permissible Residual Unbalance Calculator | ISO 21940-11 | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Permissible Specific Unbalance

Total Permissible Residual Unbalance

Balance Quality Grades

Practical Example