ISO 21940-21: Mechanical vibration – Rotor balancing – Part 21: Description and evaluation of balancing machines

Summary

ISO 21940-21 is a critical standard for both the manufacturers and users of balancing machines. It provides a standardized set of procedures for describing the technical characteristics of a balancing machine and, more importantly, for verifying its performance. The standard outlines specific tests that are performed using calibrated test rotors to prove that a machine is capable of measuring and reducing unbalance to a specified level of accuracy. Compliance with this standard ensures that a balancing machine meets internationally agreed-upon performance criteria, giving users confidence in its precision and reliability.

Table of Contents (Conceptual Structure)

The standard is structured to cover both the description of the machine and the rigorous testing procedures required to certify its performance:

1. 1. Scope and Machine Description:

   This initial chapter clearly defines the standard’s applicability, stating that it applies to all types of balancing machines—both hard-bearing and soft-bearing—that are used for balancing rigid rotors. It establishes a formal framework for how a manufacturer must describe and specify a machine’s technical characteristics. This mandatory information includes the machine’s physical capacity (minimum and maximum rotor mass, diameter, and length), its operational speed range, the type of drive system (e.g., belt-drive, end-drive), and the specifications of its measuring system. This ensures that any user or purchaser has a clear, standardized set of technical data to evaluate the machine’s suitability for their specific rotors.

2. 2. Proving Rotors and Test Masses:

   This crucial chapter details the specifications for the tools used to perform the machine evaluation: the proving rotors and test masses. Proving rotors are not typical production rotors; they are precision-machined, dimensionally stable artifacts with exceptionally low levels of residual unbalance. The standard provides strict requirements for their design, material, and surface finish to ensure they are stable and do not introduce errors into the test. It also mandates that the test masses, which are used to introduce a known amount of unbalance, must be calibrated and traceable to a national standard. By standardizing the test equipment, the standard ensures that the performance tests are repeatable and comparable across different machines and locations.

3. 3. Performance Tests:

   This chapter forms the practical core of the standard, providing a detailed, step-by-step methodology for a series of mandatory tests that objectively quantify the balancing machine’s performance. The two principal tests are:

   • Minimum Achievable Residual Unbalance (MARU): This is the ultimate test of the machine’s sensitivity and precision. Using an already well-balanced proving rotor, the test measures the smallest amount of residual unbalance that the machine can repeatedly and reliably indicate. This value effectively represents the machine’s electronic and mechanical “noise floor,” defining the absolute limit of its measurement capability.
   • Unbalance Reduction Ratio (URR) Test: This test is a direct measure of the machine’s accuracy and efficiency. It begins by adding a known unbalance to the proving rotor. The machine measures this unbalance and calculates the required correction. After this single correction is applied, the residual unbalance is measured again. The URR is the percentage by which the unbalance was reduced. For example, a URR of 95% means the machine successfully removed 95% of the initial unbalance in a single step, which indicates a highly accurate and efficient machine.

   The standard also specifies other vital tests, such as those for plane separation capability (ensuring a two-plane machine can correctly distinguish between static and couple unbalance) and for consistent performance across the machine’s full range of operating speeds.

4. 4. Acceptance Criteria and Documentation:

   This final chapter provides the definitive pass/fail criteria for the performance tests. For example, it will specify a minimum acceptable percentage for the URR test (often 95% or higher) that the machine must achieve to be considered compliant. A machine’s MARU value is not a pass/fail criterion itself but is a declared value that quantifies the machine’s sensitivity. Finally, the standard mandates the creation of a comprehensive test report that documents the conditions of each test and its results. This report serves as the official certificate of performance, providing the end-user with a guarantee that the machine’s capabilities have been verified according to a rigorous, internationally recognized procedure.

Key Concepts

• Hard-Bearing vs. Soft-Bearing Machines: The standard applies to both types. Hard-bearing machines are much more common; they measure centrifugal forces and are permanently calibrated. Soft-bearing machines measure displacement and require calibration for each specific rotor type.
• MARU (Minimum Achievable Residual Unbalance): This is the single most important performance metric for a balancing machine. It represents the “noise floor” of the machine—the smallest unbalance it can reliably detect.
• URR (Unbalance Reduction Ratio): This metric proves the *accuracy* of the machine’s unbalance calculation. A high URR means the machine’s “first shot” correction is very effective, leading to an efficient balancing process.
• Proving Tests: The standard is based on a set of practical, repeatable tests using a standardized test rotor. This provides an objective and comparable method for evaluating any balancing machine, regardless of the manufacturer.

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