ISO 21940-11: Mechanical vibration – Rotor balancing – Part 11: Procedures and tolerances for rotors with rigid behaviour

Summary

ISO 21940-11 is the modern, authoritative standard for the balancing of rigid rotors. It officially replaces the very well-known and widely used ISO 1940-1 standard. This updated document provides a comprehensive framework for specifying, achieving, and verifying the balance quality of rotors that do not deform significantly at their service speed. It retains the core concepts of its predecessor, like the G-grades, but refines them, expands the list of machine types, and provides more detailed procedural guidance for a more robust balancing process.

Table of Contents (Conceptual Structure)

The standard is structured to guide the user logically through the entire balancing process, from specification to verification:

1. 1. Scope and Balancing Requirements:

   This initial chapter defines the standard’s focus, specifying that it applies exclusively to rotors that exhibit rigid behavior. A rigid rotor is defined as one that can be corrected in any two arbitrary planes and, after correction, its residual unbalance does not significantly exceed the specified tolerance at any speed up to the maximum service speed. The chapter establishes the fundamental goal of balancing: to reduce the mass eccentricity to a level where the centrifugal forces and vibrations caused by the remaining unbalance are acceptably low for the machine’s intended operation. It sets the stage by clarifying the underlying assumptions and objectives of the rigid rotor balancing process.

2. 2. Balance Tolerance Specification:

   This is the central chapter for defining “how good” a balance job needs to be. It carries forward the internationally recognized concept of Balance Quality Grades (G) from the previous ISO 1940-1 standard. A G-Grade is a constant value representing the product of the rotor’s eccentricity (e) and its maximum service speed (Ω), where G = e·Ω. This chapter provides an extensive and updated table listing hundreds of different rotor types—from small electric armatures to massive steam turbines—and assigns a recommended G-Grade to each. Using this table, an engineer can specify a G-Grade (e.g., G6.3 for pumps, G2.5 for turbines). The standard then provides the crucial formula to convert this grade into a practical, measurable tolerance: the permissible residual specific unbalance (e_per), which is then multiplied by the rotor mass to get the final unbalance tolerance in units like gram-millimeters.

3. 3. Allocation of Tolerance to Correction Planes:

   This chapter provides the essential mathematical framework for two-plane balancing. Once the total permissible residual unbalance for the entire rotor is calculated (from the G-Grade), this value must be distributed between the two chosen correction planes. This section offers explicit formulas and vector diagrams to guide the balancing technician on how to correctly apportion the total tolerance into individual tolerances for each plane. It explains that the distribution depends on the geometry of the rotor, specifically the distance of the correction planes from the rotor’s center of gravity and the bearing locations. Adhering to these allocation procedures is critical for correcting both static and couple unbalance and ensuring that the dynamic forces at the bearings are minimized across the rotor’s length.

4. 4. Procedures for Verifying Residual Unbalance:

   This chapter outlines the methodology for the final acceptance test on the balancing machine. After the final correction weights have been applied, a verification run is performed. The standard specifies that the machine should measure the remaining unbalance in each correction plane. The measured values are then compared to the individual plane tolerances that were calculated in the previous step. The rotor is considered to have passed the balancing procedure only if the measured residual unbalance in *both* planes is less than or equal to the specified tolerance for each plane. This section emphasizes the importance of using a properly calibrated balancing machine and accounting for any tooling errors to ensure the verification measurement is accurate and reliable.

5. 5. Reporting:

   To ensure full traceability and clear communication of the balancing results, this final chapter specifies the minimum information that must be documented in a formal balancing report. This includes administrative details (like the date and operator’s name), a complete identification of the rotor (part number, serial number), and all key balancing parameters. Crucially, the report must state the specified balance quality grade (e.g., G6.3), the rotor’s maximum service speed, and its mass. The report must then clearly document the initial unbalance measurements and, most importantly, the final measured residual unbalance values for each correction plane, confirming that they are below the calculated tolerances. This creates a permanent, verifiable record that the rotor has been balanced in accordance with the standard.

Key Concepts and Updates

• Modernization of ISO 1940-1: This standard is the official replacement for ISO 1940-1. It maintains the same fundamental principles but reorganizes the content, updates the G-grade tables with more rotor types, and provides clearer, more explicit procedural guidance. The core formula remains the same.
• Emphasis on the Process: Compared to its predecessor, ISO 21940-11 places more emphasis on the entire balancing *process*, from specifying the tolerance to allocating it correctly between planes and properly verifying the final result.
• Rigid Rotor Assumption: It is critical to remember that this standard applies only to *rigid* rotors. These are rotors where the unbalance distribution does not change significantly when the rotor is brought up to its service speed. For rotors that bend or deform at speed, the more complex procedures in ISO 21940-12 (for flexible rotors) must be used.
• G-Grades Remain Central: The concept of Balance Quality Grades (G) remains the cornerstone of the standard, providing a simple yet powerful way to specify the required precision for a vast range of machinery.

Official ISO Standard

For the complete official standard, visit: ISO 21940-11 on ISO Store

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