What is minimum detectable unbalance (Umin)?

Umin is the smallest unbalance that a balancing machine can reliably detect and display. It depends on the machine type, sensor sensitivity, display resolution, rotor mass, and balancing speed. A lower Umin means better sensitivity.

What is the difference between soft and hard bearing balancing machines?

Soft bearing machines operate below the first critical speed of the rotor-bearing system, measuring displacement. Hard bearing machines operate well below critical speed with stiff supports, measuring force. Hard bearing machines are generally faster to set up but may have slightly lower sensitivity for light rotors.

How does rotor mass affect achievable balance grade?

For a given Umin, a heavier rotor will have a better (lower) achievable residual specific unbalance (e = Umin/m). This means heavier rotors can potentially achieve better balance grades on the same machine.

What balance grades can typical machines achieve?

High-quality industrial balancing machines typically achieve G1.0 to G2.5. Laboratory-grade machines can achieve G0.4 or better. The achievable grade also depends on rotor speed — higher speeds make finer grades easier to achieve.

How is machine sensitivity verified per ISO 21940-21?

Machine sensitivity is verified by placing a known test unbalance on a proving (test) rotor and confirming the machine can detect and display it correctly. The test is repeated at different angular positions and the results must be within specified tolerances, following the Umar and URR test procedures of ISO 21940-21 (formerly ISO 2953).

Free Engineering Tool

Balancing Machine Sensitivity Calculator

Calculate minimum detectable unbalance (Umin) and achievable balance grade for soft or hard bearing balancing machines per ISO 21940-21 (formerly ISO 2953).

ISO 21940-21UminSoft / Hard Bearing

Results

Minimum Detectable Unbalance (Umin)

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Residual Specific Unbalance (eper)

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Achievable Balance Grade

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Unbalance Reduction Ratio (URR)

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Service Speed ω

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Minimum Detectable Unbalance

The minimum detectable unbalance (Umin) of a balancing machine depends on sensor sensitivity, electronic noise floor, and display resolution:

Achievable Balance Grade

Where ω is the angular velocity at service speed (not balancing speed).

Practical rule: Umin should be well below the permissible residual unbalance (Uper) — typically no more than about 10% — so that measurement uncertainty does not dominate the result. Balancing machines themselves are described and evaluated per ISO 21940-21 (formerly ISO 2953), which defines the minimum achievable residual unbalance (Umar) and unbalance reduction ratio (URR) tests.

The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.