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, maximum service speed, and the required balance quality grade G.

How do I choose the right balance grade G?

Use the grade specified by the rotor drawing, manufacturer, applicable product standard, or engineering agreement. Example application lists are informative and must not be used as a universal automatic grade selector.

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 permissible residual unbalance for a rigid rotor using ISO 21940-11. Enter rotor mass, maximum service speed, and the specified balance quality grade.

ISO 21940-11:2016 + Amd 1:2022 ISO 1940-1 G0.4 – G4000

Rotor Mass (kg)

Maximum 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 — maximum 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)
G0.4	0.4
G1.0	1.0
G2.5	2.5
G6.3	6.3
G16	16
G40	40
G100	100
G250	250
G630	630
G1600	1600
G4000	4000

Practical Example

Example calculation — specified grade G6.3

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.

Residual Unbalance Calculator | ISO 21940-11 | Vibromera

Results

Permissible Specific Unbalance

Total Permissible Residual Unbalance

Balance Quality Grades

Practical Example

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