ISO 21940-21:2022

Overview - ISO 21940-21:2022 (Rotor balancing machines)

ISO 21940-21:2022 specifies requirements for the description and evaluation of balancing machines used in rotor balancing. It applies to both hard-bearing and soft-bearing balancing machines that support and rotate:

• rotors with rigid behaviour at balancing speed (as defined in ISO 21940-11), and
• rotors with shaft elastic behaviour when balanced using low-speed procedures (as defined in ISO 21940-12).

The standard defines how to report machine capacity, performance and testing methods so users can verify machine capability, reproducibility and suitability for production or laboratory balancing tasks.

Key technical topics and requirements

ISO 21940-21 organizes machine evaluation around practical, measurable data and test procedures. Important topics covered include:

• Capacity and performance data: rotor mass and unbalance limitations, rotor dimensions, drive data, torque, brakes, motors and speed regulation.
• Machine features and measuring systems: principle and arrangement of operation, unbalance indicators, plane separation systems (single- and multi-plane), calibration and setting of measuring systems for both hard- and soft-bearing machines.
• Production efficiency: metrics such as time per balance cycle and the unbalance reduction ratio (R).
• Proving rotors and test masses: standardized and user-defined proving rotors, test mass types (U and R), permissible errors and materials.
• Verification tests: procedures to establish minimum achievable residual unbalance (U), R tests, couple moment interference tests, compensator tests and simplified test options.
• Installation and supply requirements: foundations, electrical/air supplies and environmental considerations relevant to accurate balancing machine performance.

Practical applications and users

This standard is intended for professionals who need to specify, verify or operate balancing machines:

• Balancing machine manufacturers - to document machine performance and provide user data sheets.
• Test laboratories and calibration facilities - to perform verification tests and report machine capability.
• Maintenance and reliability engineers - to select appropriate balancing equipment for rotors (fans, motors, turbines, shafts).
• OEMs and production managers - to assess production efficiency and ensure consistent unbalance reduction in manufacturing environments.
• Quality and compliance teams - to demonstrate that balancing equipment meets recognized international criteria.

Adopting ISO 21940-21 supports consistent rotor balancing results, traceable verification procedures and improved reliability of rotating machinery.

Related standards

• ISO 21940-11 - Rotor behaviour: rigid rotors at balancing speed (referenced in scope).
• ISO 21940-12 - Low-speed balancing procedures for rotors with shaft elastic behaviour.

Keywords: ISO 21940-21, rotor balancing, balancing machines, hard-bearing, soft-bearing, unbalance reduction, verification tests, production efficiency.