ISO 21940-12: Mechanical vibration – Rotor balancing – Part 12: Procedures and tolerances for rotors with flexible behaviour

Summary

ISO 21940-12 addresses the complex challenge of balancing flexible rotors. A flexible rotor is one whose shape and unbalance distribution change significantly with rotational speed, particularly as it approaches and passes through its bending critical speeds. Unlike rigid rotors (covered in Part 11), a flexible rotor cannot be balanced at a low speed and be expected to remain in balance at its high service speed. This standard provides the specialized, multi-speed, and multi-plane procedures required to properly balance these complex rotating systems, which are common in high-performance machinery like gas turbines, compressors, and long industrial rolls.

Table of Contents (Conceptual Structure)

The standard provides a framework for understanding and executing the advanced methods required for flexible rotor balancing:

1. 1. Scope and Classification of Flexible Rotors:

   This initial chapter defines the standard’s scope, stating that it applies to rotors that exhibit flexible behavior, meaning their unbalance distribution and/or deflected shape changes with speed. It introduces a crucial classification system to categorize these rotors based on their dynamic characteristics, which is essential for selecting the appropriate balancing strategy. The classes range from:

   • Class 1: Rigid Rotors (covered by ISO 21940-11).
   • Class 2: Quasi-rigid rotors, which can be balanced at a low speed but may require trim balancing at service speed.
   • Class 3: Rotors requiring balancing at multiple speeds, often using the influence coefficient method, typically passing through one or more critical speeds.
   • Class 4 & 5: Highly flexible rotors, such as those in large turbine generators, which require advanced modal balancing techniques to correct multiple bending modes.

   This classification provides a systematic way to determine the complexity of the balancing task and the necessary procedures to achieve a successful balance across the entire operating speed range.

2. 2. Balancing Procedures:

   This chapter forms the technical core of the standard, detailing the advanced, multi-stage procedures necessary for flexible rotors. It explains that a simple low-speed balance is insufficient and must be augmented with high-speed techniques to account for the rotor’s bending. The standard outlines two primary methodologies:

   • The Influence Coefficient Method: This is a versatile and widely used technique. It involves a systematic process of placing a known trial weight in one correction plane at a time and measuring the resulting vibration response (amplitude and phase) at multiple locations and across multiple speeds. This process is repeated for each correction plane. The collected data is used to calculate a matrix of “influence coefficients,” which mathematically defines how an unbalance in any plane affects the vibration at any measurement point and speed. A computer then uses this matrix to solve for the set of correction weights and their angular placements needed across all planes to simultaneously minimize vibration over the entire speed range.
   • Modal Balancing: This is a more physically intuitive method that treats each bending mode of the rotor as a separate unbalance problem. The procedure involves running the rotor at or near a specific critical speed to maximally excite the corresponding mode shape. Vibration measurements are taken to identify the location of the “heavy spot” for that mode, and correction weights are placed at the points of maximum deflection (anti-nodes) for that mode shape to counteract it. This process is then repeated sequentially for each significant bending mode within the rotor’s operating speed range, effectively balancing the rotor one mode at a time.
3. 3. Specification of Balance Tolerances:

   This chapter explains that the simple G-grade tolerances used for rigid rotors are often insufficient for flexible rotors. Instead, it introduces more comprehensive tolerance criteria, which can be based on several factors, including:

   • Limits on the residual modal unbalance for each significant bending mode.
   • Limits on the absolute shaft vibration amplitudes at specific locations and speeds (especially at the service speed).
   • Limits on the transmitted forces to the bearings.
4. 4. Verification of Final Balance State:

   This final section details the acceptance criteria for a successfully balanced flexible rotor. Unlike a rigid rotor, which only needs verification at one speed, a flexible rotor must be confirmed to be in balance throughout its entire operating speed range. After the final correction weights have been applied, the rotor is subjected to a final run-up test. During this run-up, vibration is continuously monitored at key locations (such as bearings and points of maximum deflection). The standard specifies that the rotor is considered acceptably balanced only if the measured vibration remains below the pre-defined tolerance limits at all speeds, particularly while passing through its critical speeds and while dwelling at the maximum continuous operating speed. This comprehensive verification ensures that the complex dynamic behavior of the rotor has been effectively controlled.

Key Concepts

• Flexible vs. Rigid Behavior: The fundamental distinction. A rotor is flexible if its operating speed is a significant fraction (typically >70%) of its first bending natural frequency (critical speed). As the rotor spins faster, centrifugal forces cause it to bend, changing its unbalance.
• Critical Speeds and Mode Shapes: Understanding the rotor’s critical speeds and the associated “mode shapes” (the shape the rotor bends into at that speed) is essential for flexible rotor balancing. Each mode must be treated as a separate balancing problem.
• Multi-Plane, Multi-Speed Balancing: The core methodology. Unlike rigid rotors, which can be balanced in two planes at one low speed, flexible rotors require corrections in multiple planes and measurements at multiple speeds to ensure smooth operation across the entire speed range.
• Modal Balancing: A powerful technique where weights are added to specifically counteract the unbalance associated with each bending mode. For example, to balance the first bending mode, weights are placed at the point of maximum deflection for that mode.

Official ISO Standard

For the complete official standard, visit: ISO 21940-12 on ISO Store

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