Why do couplings need to be balanced?

Couplings rotate at shaft speed and any unbalance creates a centrifugal force proportional to mass × eccentricity × ω². Even small coupling unbalance can cause significant vibration at high speeds, especially in precision machinery.

What balance grade should I use for couplings?

Most flexible couplings use G6.3 or G16. Precision couplings for turbomachinery may require G2.5 or G1.0. API 671 specifies that coupling unbalance should not exceed the lesser of the calculated ISO grade value or specific API limits.

How is coupling unbalance different from rotor unbalance?

The formula is identical — U = e × m, where e = G×1000/ω. However, coupling unbalance must be considered separately because it acts at the coupling location, which may amplify its effect depending on the distance to bearings.

What is the API 671 coupling balance requirement?

API 671 requires coupling components to be balanced individually to a level that the residual unbalance does not exceed 10W/n (oz·in) or the ISO 21940-11 Grade G1.6 calculation, whichever is less. W = journal weight (lb), n = max speed (RPM).

Should I balance a half-coupling or complete coupling?

Each half-coupling should be balanced individually, as the complete coupling may not always be assembled in the same orientation. Use the mass of each half-coupling for the calculation, then verify the assembled coupling balance.

Free Engineering Tool — #016

Coupling Unbalance Tolerance Calculator

Calculate the permissible unbalance for couplings using ISO 21940-11 balance quality grades. Enter coupling mass, maximum speed, and grade.

ISO 21940-11API 671Couplings

Coupling Mass (kg)

Maximum Speed (RPM)

Balance Grade G

Calculate For

Quick presets

Results

Permissible Coupling Unbalance

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Specific Unbalance (Eccentricity)

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Angular Velocity ω

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Balance Grade

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Effective Mass Used

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Permissible Coupling Unbalance

The same ISO 21940-11 formula applies to couplings:

G — balance quality grade (mm/s)
ω — angular velocity = 2π × n / 60 (rad/s)
m — coupling mass (kg)

API 671 Requirement

For special-purpose couplings per API 671, the permissible unbalance of each coupling component shall not exceed:

Where W is the journal static weight in pounds and n is max continuous speed in RPM.

Practical Example

Example — Disc Coupling

Given: Coupling mass = 15 kg, Speed = 3600 RPM, Grade = G6.3

ω = 2π × 3600 / 60 = 376.99 rad/s

e_per = 6.3 × 1000 / 376.99 = 16.71 μm

U_per = 16.71 × 15 = 250.7 g·mm

⚠️ Note: For half-coupling balancing, use half the total coupling mass. Always verify assembled coupling balance after installation.

Coupling Unbalance Tolerance Calculator | ISO 21940 | Free Online Tool

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

Results

Permissible Coupling Unbalance

API 671 Requirement

Practical Example