ISO 21940-14:2012

Overview

ISO 21940-14:2012 - "Mechanical vibration - Rotor balancing - Part 14: Procedures for assessing balance errors" defines procedures to identify, assess and account for errors in rotor unbalance measurements. It specifies balance acceptance criteria (residual unbalance) for both immediate results after balancing and for later user checks. The standard explains error sources and gives methods to reduce them (informative annex), extending earlier guidance (ISO 1940-2) to rotors with flexible behaviour.

Key topics and requirements

• Error classification: systematic errors, randomly variable errors, and scalar errors - each treated differently when assessing measurement uncertainty.
• Identification of error sources: examples include runout, mandrel/shaft eccentricity, bearing runout, tooling fit, residual magnetism, loose parts, thermal or windage effects, and assembly clearances.
• Error assessment methods:
  • Experimental procedures to estimate random errors (multiple runs, vector plots, mean vector and enclosing circle to estimate maximum single-run error).
  • Evaluation of systematic errors by calculation or measurement; where possible these should be eliminated, corrected, or quantified.
  • Treatment of scalar errors where angle is indeterminate but magnitude can be bounded.
• Balance acceptance criteria: residual unbalance limits to determine if balancing operation met required quality - criteria apply both directly after balancing and for subsequent verification by the user.
• Reporting: measured unbalance before and after correction must be recorded if errors are taken into account by calculation.
• Reduction guidance: informative annex lists practical methods to reduce typical errors.

Practical applications and users

Who uses ISO 21940-14:

• Balancing machine manufacturers - to design equipment with minimized systematic errors and to specify performance.
• Rotordynamics and vibration engineers - to assess measurement uncertainty and validate balancing results.
• Maintenance and overhaul teams - to verify that in-shop or in-situ balancing meets acceptance limits.
• Quality assurance and test laboratories - to document balance test procedures, acceptance, and uncertainty. Practical uses include developing balancing procedures, troubleshooting measurement discrepancies, setting reporting protocols, and improving product and process tolerances to reduce unbalance.

Related standards

• ISO 1925 (Vocabulary)
• ISO 1940-1 (Balance quality requirements for rigid rotors)
• ISO 11342 (Balancing flexible rotors)
• ISO 21940-21 (Description and evaluation of balancing machines)

Keywords: ISO 21940-14:2012, rotor balancing, mechanical vibration, balance errors, residual unbalance, balancing machine, systematic errors, randomly variable errors, scalar errors.