ISO 1940-2: Mechanical vibration – Balance quality requirements – Vocabulary

Summary

ISO 1940-2 serves as a foundational terminology standard for the entire field of rotor balancing. Its primary purpose is to define and standardize the vocabulary used when discussing balancing concepts, procedures, and equipment. By providing clear and unambiguous definitions for key terms, this standard ensures that engineers, technicians, manufacturers, and customers can communicate with precision and without misunderstanding. It is the essential “dictionary” that supports other balancing standards like ISO 1940-1.

Note: This standard has been formally replaced by ISO 21940-2, but its defined terms remain the basis of modern balancing vocabulary.

Table of Contents (Conceptual Structure)

The standard is structured as a comprehensive glossary, with terms grouped into logical categories:

1. 1. Scope:

   This initial section defines the standard’s singular purpose: to establish a clear, unambiguous, and internationally agreed-upon vocabulary for the field of rotor balancing. It clarifies that the terms defined within are intended for use in engineering, manufacturing, quality control, and technical communication to prevent misunderstandings. By creating a common language, the standard facilitates global trade and collaboration, ensuring that a term like “dynamic unbalance” has the exact same meaning whether used by an engineer in Germany, Japan, or the United States.

2. 2. Terms Related to the Rotor:

   This chapter defines the physical object being balanced. It provides the formal definition of a Rotor as a body capable of rotating about a fixed axis. More importantly, it establishes the critical distinction between a Rigid Rotor and a Flexible Rotor. A rigid rotor is defined as a rotor whose unbalance can be corrected in any two arbitrary planes and, after correction, the residual unbalance does not change significantly at any speed up to the maximum service speed. In contrast, a flexible rotor is defined as one that deforms elastically at its service speed, and whose unbalance state must be corrected at or near its service speed in more than two planes. This distinction is the most important in all of balancing, as it dictates the entire balancing procedure, the equipment required, and the complexity of the task.

3. 3. Terms Related to Unbalance:

   This core section provides the physics-based definitions for the condition that balancing aims to correct. It defines Unbalance as the condition that exists when the principal axis of inertia of a rotor is not coincident with its rotational axis. This misalignment causes centrifugal force, leading to vibration. The standard then defines the three distinct types of unbalance:

   • Static Unbalance: The condition where the principal axis of inertia is displaced parallel to the rotational axis. It is caused by a single “heavy spot” and can be detected by placing the rotor on knife-edges, where it will roll to the bottom. It causes in-phase vibration at the bearings.
   • Couple Unbalance: The condition where the principal axis of inertia intersects the rotational axis at the rotor’s center of gravity. It is caused by two equal and opposite heavy spots in two different planes, creating a “wobble” or rocking motion. It can only be detected when the rotor is spinning and causes out-of-phase vibration at the bearings.
   • Dynamic Unbalance: The most common condition, where the principal axis of inertia is neither parallel to nor intersects the rotational axis. It is a combination of both static and couple unbalance.

   This section also defines Residual Unbalance as the small amount of unbalance that remains after the balancing process is complete.

4. 4. Terms Related to the Balancing Process:

   This chapter defines the actions and components involved in performing the balancing procedure. It formally defines Balancing as the process by which the mass distribution of a rotor is checked and, if necessary, adjusted to ensure that the residual unbalance is within a specified tolerance. It then defines the key physical and procedural elements:

   • Correction Plane: A plane perpendicular to the rotor axis in which mass is added or removed to correct for unbalance.
   • Correction Mass: The actual mass (e.g., a steel weight) that is added to, or removed from, the rotor at a specific radius and angle within the correction plane.
   • Single-Plane (Static) Balancing: A procedure that corrects for only the static component of unbalance, typically performed in one correction plane.
   • Two-Plane (Dynamic) Balancing: A procedure that corrects for both static and couple unbalance by making adjustments in at least two separate correction planes.
5. 5. Terms Related to Balancing Machines:

   This final section defines the equipment used to perform the balancing task. It provides a definition for a Balancing Machine as a device that measures unbalance in a rotor so that the mass distribution can be corrected. It then defines the two primary types based on their suspension characteristics:

   • Soft-Bearing Balancing Machine: A machine with a suspension system that is very flexible, at least in the horizontal direction. The rotor is run at a speed well above the suspension’s natural frequency, and the machine measures the physical displacement of the rotor. These machines must be calibrated for each specific rotor geometry.
   • Hard-Bearing Balancing Machine: A machine with a very stiff suspension system. The rotor is run at a speed well below the suspension’s natural frequency, and the machine’s sensors measure the centrifugal forces produced by the unbalance. These machines are permanently calibrated and can measure a wide range of rotors without rotor-specific calibration, making them much more common in modern industry.

Key Concepts

• Clarity and Consistency: The main goal is to eliminate ambiguity. When a standard or a customer specifies “dynamic unbalance,” this document ensures everyone has the same, precise understanding of what that means.
• Foundation for Other Standards: This vocabulary is the language used in all other major balancing standards (like those covering tolerances, machines, and procedures), making it an indispensable companion document.
• Technical Precision: The definitions are technically precise, often rooted in the physics of rotating bodies, ensuring that they are robust and applicable to complex engineering analyses.

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