ISO 21940-11:2016

Overview

ISO 21940-11:2016 - "Mechanical vibration - Rotor balancing - Part 11: Procedures and tolerances for rotors with rigid behaviour" defines procedures and unbalance tolerances for balancing rotors that behave as rigid bodies up to their maximum service speed. The standard specifies how to determine the permissible residual unbalance, the required number of correction planes, how to allocate residual unbalance to tolerance planes, and how to account for errors in the balancing and verification process. It applies only to rotors with rigid behaviour (not flexible rotors).

Key technical topics and requirements

• Permissible residual unbalance: Defines the magnitude of residual unbalance that may remain after balancing and how to specify it for a given rotor or application.
• Correction planes: Establishes when one or two correction planes are sufficient and guidance for rotors needing more than two correction planes.
• Allocation to tolerance planes: Procedures to distribute the allowable residual unbalance among tolerance planes and, subsequently, to correction planes.
• Representation of unbalance: Vectorial representations of resultant unbalance and couple/moment unbalance for rigid rotors.
• Similarity considerations: Rules for scaling tolerances according to rotor mass and service speed (permissible specific unbalance).
• Verification and error accounting: Methods to include measurement and process errors when verifying that a rotor meets the permissible residual unbalance; identifies regions of uncertainty and how to interpret out-of-tolerance readings.
• Experimental evaluation and practical methods: Guidance on deriving tolerances from experiments, bearing-force limits, or established field experience.
• Assembled rotors: Guidance on balancing entire rotor assemblies versus component-level balancing.
• Supporting informative annexes: examples of specifications, bearing-force–based tolerances, experience-based tolerances, and allocation rules.

Practical applications and who uses this standard

ISO 21940-11:2016 is intended for engineers and professionals involved in:

• Rotating equipment design (OEMs, mechanical designers)
• Maintenance and reliability engineering (predictive maintenance teams)
• Vibration analysts and balancing machine operators
• Test laboratories and quality control for rotating machinery Practical uses include specifying balance quality for new rotors, setting acceptance criteria for repaired or refurbished rotors, selecting the number of correction planes, and defining verification procedures that account for measurement uncertainty.

Related standards

• ISO 21940 series (other parts covering measurement, terminology and flexible behaviour)
• ISO 21940-12: Procedures and tolerances for rotors with flexible behaviour
• ISO 21940-14: Detailed assessment of balancing errors
• ISO 19499: Fundamentals and introduction to rotor balancing

Keywords: ISO 21940-11:2016, rotor balancing, mechanical vibration, permissible residual unbalance, correction planes, unbalance tolerances, balancing procedures, rigid rotors.