Introduction For decades, ISO 1940 was the definitive international standard for balancing rotating rigid rotors. However, this standard has been superseded and replaced by the ISO 21940 series. While many engineers still refer to "ISO 1940" out of habit, understanding the structural and technical differences between the two is critical for compliance with current best practices.

| Aspect | Verdict | | :--- | :--- | | | No difference – fully compatible. | | Calculation Method | No difference – identical physics. | | Standard Structure | Major difference – single document vs. modular series. | | Handling of Flexible Rotors | Major difference – new standard provides dedicated guidance. | | Current Validity | ISO 1940 is withdrawn. Use ISO 21940-11. |

| Feature | ISO 1940-1 (Old) | ISO 21940 (Current) | | :--- | :--- | :--- | | | Single document covering all aspects. | Modular series (e.g., Part 11: Rigid Rotors; Part 21: Description of Shaft & Fitment). | | Rotor Classification | 11 quality grades (G 0.4 to G 4000). | Identical quality grades (G 0.4 to G 4000) – no change . | | Calculation Method | Specific formula for permissible residual unbalance per plane. | Identical formula – no change . | | Correction Methods | Addressed within the single document. | Moved to separate parts (e.g., Part 21 for correction planes). | | Flexible Rotors | Briefly mentioned but not detailed. | Handled in dedicated parts (e.g., Part 12: Flexible Rotors). | | Vibration vs. Unbalance | Focused on unbalance as a mechanical property. | Clarifies use of vibration measurement as a proxy for unbalance effect. |

Update your internal specifications to reference ISO 21940-11:2012 (or latest revision) instead of ISO 1940. Continue using the same G-grade values as before.