ISO 21940-31:2013

Overview

ISO 21940-31:2013 - Mechanical vibration - Rotor balancing - Part 31: Susceptibility and sensitivity of machines to unbalance - specifies methods to determine a machine’s vibration sensitivity to unbalance and gives evaluation guidelines based on how close relevant resonance rotational speeds are to the operating speed. This part of ISO 21940 deals only with once-per-revolution (1×) vibration caused by unbalance, provides a classification scheme for machine susceptibility, and offers numerical sensitivity ranges intended mainly for simple rotor systems (preferably with a single resonance in the operating range).

Key topics

• Scope and focus: Assessment methods for 1× vibration due to unbalance; guidance on applying sensitivity values rather than acceptance limits.
• Machine susceptibility classification: Machines are grouped by likelihood of unbalance change into Type I (low), Type II (moderate) and Type III (high) susceptibility. Correction factors adjust sensitivity ranges for different types.
• Modal sensitivity: Expressed via the modal amplification factor (M_n) for each resonance speed; sensitivity ranges and characteristics are provided to indicate how much unbalance amplifies vibration near resonance.
• Operational considerations: Preference for systems with a single resonance speed or widely separated resonances (e.g., >~20% separation). Guidance on operating and transient speeds and the impact of damping.
• Experimental methods: Procedures for measuring sensitivity under operating conditions, including Nyquist and Bode diagram techniques, local sensitivity measurement, and damped unbalance sensitivity analysis.
• Limitations: Sensitivity values are guidance to avoid gross deficiencies and to identify when more detailed investigation is needed; they are not intended as formal acceptance specifications.

Applications

• Practical use cases for ISO 21940-31:2013 include:
  • Evaluating whether a machine is likely to require in-situ trimming or frequent rebalancing.
  • Assessing risks when a machine’s resonance speed is near normal operating speed.
  • Guiding design or remediation decisions (modify resonance speed, increase damping, improve supports) to reduce unbalance sensitivity.
  • Supporting commissioning and troubleshooting of rotating machinery-pumps, turbines, compressors, generators, fans, centrifuges, roll machinery.
• Typical users: rotating machinery designers, vibration analysts, maintenance engineers, balancing specialists, commissioning teams, and reliability engineers.

Related standards

• ISO 21940 series (rotor balancing): Parts 1, 2, 11, 12, 13, 14, 21, 23, 31, 32 - ISO 21940-31 links directly with other balancing guidance in the series.
• ISO 1925 (Balancing - Vocabulary) and relevant parts of ISO 7919 (machine vibration assessment) for complementary criteria and terminology.

ISO 21940-31:2013 is a practical tool for understanding and managing unbalance sensitivity in rotating machinery; when used with measurement and balancing procedures from the wider ISO 21940 family it helps improve machine reliability and reduce vibration-related downtime.