ISO 2372: Mechanical vibration of machines with operating speeds from 10 to 200 rev/s

Summary of a Withdrawn Standard

ISO 2372 is a historical, withdrawn standard that was one of the first widely adopted international guides for evaluating machine vibration. Published in 1974, it provided a simple method for assessing the vibration severity of common industrial machines by comparing a single, broadband vibration measurement to a chart. For decades, it was the go-to reference for “how much vibration is too much?”

While revolutionary for its time, it has since been replaced by the much more detailed and sophisticated ISO 10816 and ISO 20816 series of standards. Understanding ISO 2372 is important for its historical context and for interpreting older maintenance documents that may still reference its classifications.

The Core Concepts of ISO 2372

The methodology of ISO 2372 was based on a few simple but effective principles:

1. 1. Measurement Parameter:

   The standard’s core principle was to quantify vibration severity using a single, repeatable metric. It specified that the measurement should be the broadband RMS (Root Mean Square) Velocity, captured in a frequency range of 10 Hz to 1,000 Hz (600 to 60,000 CPM). RMS velocity was chosen because it is directly related to the destructive energy of the vibration, making it a robust indicator of a machine’s condition regardless of its rotational speed. The measurement was to be taken on the non-rotating parts of the machine, typically on the bearing housings, as this was the most practical and accessible location to assess the forces being transmitted to the machine’s structure.

2. 2. Machine Classification:

   Recognizing that a small pump and a large turbine cannot be held to the same vibration standard, ISO 2372 grouped machinery into broad categories. This allowed for different vibration limits to be applied based on the machine’s size, power, and the flexibility of its support structure. The classifications were:

   • Class I: Individual parts of engines and machines, integrally connected with the complete machine in its normal operating condition (production electric motors up to 15 kW are typical examples).
   • Class II: Medium-sized machines (typically electric motors with 15 to 75 kW output) without special foundations, or rigidly mounted engines or machines up to 300 kW on special foundations.
   • Class III: Large prime movers and other large machines with rotating masses mounted on rigid and heavy foundations which are relatively stiff in the direction of vibration measurement.
   • Class IV: Large prime movers and other large machines with rotating masses mounted on foundations which are relatively soft in the direction of vibration measurement (e.g., a turbo-generator set on a lightweight, flexible steel frame).
3. 3. Vibration Severity Chart:

   The heart of the standard was its evaluation chart. This chart provided specific RMS velocity values that corresponded to different condition levels for each of the four machine classes. The quality bands were typically designated with qualitative judgments that were easy to understand and apply. For each machine class, a specific range of velocity values was assigned to categories such as:

   • A (Good): Newly commissioned or well-maintained machines.
   • B (Satisfactory): Acceptable for long-term, unrestricted operation.
   • C (Unsatisfactory): Not acceptable for long-term operation. Machine should be monitored and scheduled for maintenance.
   • D (Unacceptable): Vibration levels are damaging and require immediate action to avoid failure.

   This chart-based approach made it simple for a technician with a basic vibration meter to take a measurement, look up the machine class on the chart, and make a clear determination of the machine’s health.

Why It Was Replaced

ISO 2372 was a major step forward, but it had limitations that the modern standards have addressed:

• Oversimplification: Grouping all machines into just four classes was too broad. The modern ISO 10816/20816 series provides much more specific guidance for different machine types (pumps, fans, compressors, etc.).
• Foundation Influence: The distinction between “rigid” and “soft” foundations was often ambiguous and difficult to apply consistently.
• Lack of Diagnostic Information: The standard only provided a single overall number. It gave no information about the frequencies present in the vibration signal, and therefore could not help diagnose the *cause* of the problem (e.g., unbalance vs. misalignment).
• Evolving Technology: The standard was developed before the widespread availability of digital, FFT-based vibration analyzers.

Legacy and Importance

Despite being withdrawn, the legacy of ISO 2372 is significant. It established RMS velocity as the primary metric for overall vibration severity, a practice that continues in today’s standards. Many simple vibration meters and screening tools still use the color-coded Green/Yellow/Red alarm levels that are based on the principles of the original ISO 2372 chart.

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