ISO 21940-12:2016

Overview

ISO 21940-12:2016 - Mechanical vibration - Rotor balancing - Part 12: Procedures and tolerances for rotors with flexible behaviour - provides practical guidance for balancing rotors that exhibit flexible (non‑rigid) dynamic behaviour. The standard presents typical rotor configurations, balancing procedures for low and high speeds, methods to assess the final state of balance, and guidelines for balance quality criteria. ISO 21940-12:2016 is intended as guidance rather than an acceptance specification and excludes detailed treatment of structural resonances.

Key Topics and Requirements

• Fundamentals of flexible rotor dynamics
  • Mode shapes, unbalance distribution and rotor response to unbalance.
• Rotor classification and configurations
  • Typical flexible rotor layouts and provision for correction planes.
• Balancing procedures
  • Low‑speed procedures (Procedures A–F) including single‑plane, two‑plane, component and staged balancing, and optimum‑plane methods.
  • High‑speed procedures (Procedures G–I) covering multiple‑speed balancing, service‑speed balancing and fixed‑speed approaches.
• Evaluation criteria and tolerances
  • Methods to derive balancing machine vibration limits and residual unbalance tolerances from machine or rotor requirements.
  • Criteria for choosing evaluation methods based on vibration limits or residual unbalance.
• Evaluation procedures
  • Vibration‑based assessment in balancing machines, test facilities or on site.
  • Residual unbalance assessment including modal unbalance concepts and evaluations at service speed in specified test planes.
• Informative annexes
  • Practical aids such as conversion factors, example calculations of equivalent modal unbalances, procedures to distinguish rigid vs flexible behaviour, and methods to compute correction masses.

Practical Applications

ISO 21940-12:2016 is applicable where rotors can flex under operating speeds and bending influences performance. Typical uses:

• Balancing of turbine shafts, long pump or compressor rotors, generator rotors and other industrial rotating machinery with flexible modes.
• Establishing shop balancing procedures and acceptance checks prior to assembly.
• Deriving balancing machine limits from installed machine vibration requirements.
• Guiding investigations when more accurate determination of required balance quality is necessary.

Who should use this standard

• Mechanical design and rotating‑equipment engineers
• Test labs and balancing service providers
• Maintenance and reliability teams responsible for vibration troubleshooting
• Quality assurance and procurement professionals specifying balancing requirements

Related standards

• ISO 21940 series (Part 11: rigid rotors; Part 13: in‑situ balancing; Part 21: balancing machines)
• ISO 10816 and ISO 7919 (vibration evaluation guidance referenced for deriving limits)

Note: ISO 21940-12:2016 complements other rotor balancing parts and provides procedures specifically tailored to the challenges of flexible rotor dynamics.