Technical insight
Rotor Balance Reports: Unbalance Units and Vibration Evidence
When a supplier sends a balance report, first ask which quantity the result represents. Keep residual-unbalance acceptance and operating-vibration acceptance as separate review items unless the responsible technical authority has explicitly defined how both are covered.
By Geetai Precision
- Published
- Updated
Two acceptance questions
ISO's public abstract for ISO 21940-11:2016 addresses rotors with rigid behaviour, including permissible residual unbalance and its allocation to planes. It excludes rotors with flexible behaviour. [1]
The public abstract for ISO 20816-1:2016 concerns vibration measurement and evaluation on complete machines. Its criteria address operational monitoring and acceptance testing, and it permits different measurement quantities and methods when their definitions and limitations are clear. [2]
Quantities and units
Hofmann's glossary defines unbalance amount by mass and radial distance and lists gmm as an unbalance unit. It defines residual unbalance as what remains after balancing. [3]
ifm lists displacement, velocity and acceleration as vibration quantities. Its units for velocity include millimeters per second, written mm/s. It also distinguishes peak, peak-to-peak and RMS amplitude descriptions. [4]
Treat a residual-unbalance amount in gmm and a vibration-velocity result in mm/s as different evidence. Do not replace one with the other through unit scaling or infer the required acceptance limit from the unit alone.
Report entries and follow-up requests
| Report entry | Buyer request |
|---|---|
| Residual unbalance in gmm | Ask for the final value, unit and referenced plane, plus the governing permissible value and document edition. |
| Vibration velocity in mm/s | Ask for measurement location and direction, rotation rate, operating condition, frequency band, amplitude convention and applicable limit. |
| An overall “pass” without the quantity | Request the underlying result and identify which acceptance requirement the status closes; keep any separately required vibration evidence open. |
Acceptance request checklist
- Retain the reported quantity and its unit exactly; request clarification if a field named “balance” does not explain what was reported.
- Ask whether an unbalance amount is a final residual result or another stage of the assessment, and which plane or planes it represents.
- Keep RMS, peak and peak-to-peak labels with vibration results; ask the report author to explain any unstated convention before comparison.
- Have the responsible technical authority name the requirement and edition, acceptance limit and reviewer for each required report.
- Ask that authority whether the operating conditions represented by the vibration record cover the purchase acceptance question; record unresolved conditions explicitly.
Limitations and drawing review
With the drawing review package, provide the requested report quantities, governing references and acceptance owner. Ask for the proposed documentation scope before releasing the order.
Related review paths
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References
- ISO 21940-11:2016 — public abstract — ISO. Accessed 2026-10-24T11:44:11.000Z.
- ISO 20816-1:2016 — public abstract — ISO. Accessed 2026-10-24T11:44:11.000Z.
- Glossary of balancing technology — Hofmann Mess- und Auswuchttechnik GmbH & Co. KG. Accessed 2026-10-24T11:44:11.000Z.
- Fundamentals of vibration — ifm. Accessed 2026-10-24T11:44:11.000Z.