ISO 2372: Mechanical Vibration of Machines with Operating Speeds from 10 to 200 rev/s

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ISO 2372 is a historical, withdrawn standard that was one of the first widely adopted international guides for evaluating machine vibration.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 gave maintenance engineers a refreshingly simple way to answer the perennial question “how much vibration is too much?”: take a single broadband velocity reading on a bearing housing, look it up against a chart, and read off the machine’s condition. For two decades it was the default reference, and it shaped the vocabulary of vibration severity that the industry still uses today. It has since been superseded — first by ISO 10816 and now by the current ISO 20816 series — but understanding it remains valuable for interpreting older maintenance records and for appreciating why the modern standards are built the way they are.

1. A Standard Born of Necessity

Before ISO 2372, judging machine health from vibration was largely a matter of individual experience and rules of thumb that varied from plant to plant. The standard’s contribution was to put that judgement on a common, repeatable footing. It applied to machines with operating speeds from 10 to 200 rev/s — that is, roughly 600 to 12,000 rpm — covering the great majority of industrial pumps, fans, motors and generators of its era. By tying a clear pass/fail verdict to one easily measured number, it made condition monitoring accessible to any technician with a basic vibration meter, not just to specialists.

2. The Core Concepts of ISO 2372

The methodology rested on a few simple but effective principles:

1. Measurement parameter.

   The standard quantified severity using a single, repeatable metric: broadband RMS (root mean square) velocity, captured over 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, which makes it a robust indicator of condition largely independent of rotational speed. The reading was to be taken on the non-rotating parts of the machine — typically the bearing housings — as the most practical and accessible place to assess the forces being fed into the structure. This “velocity on the bearing housing” convention is the same one carried forward into ISO 20816-3 today, and you can convert a spectrum to an overall figure with an overall vibration level calculator.

2. Machine classification.

   Recognising that a small pump and a large turbine cannot be held to the same yardstick, ISO 2372 sorted machinery into four broad groups, so that different limits could be applied according to size, power and the flexibility of the support structure:

   • 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 of 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, heavy foundations that are relatively stiff in the direction of vibration measurement.
   • Class IV: large prime movers and other large machines with rotating masses mounted on foundations that are relatively soft in the direction of vibration measurement (for example, a turbo-generator set on a lightweight, flexible steel frame).

   The split between Classes III and IV hinged on foundation stiffness — a stiff base transmits and constrains vibration differently from a soft one, so the same machine could warrant different limits depending on how it was mounted.

3. Vibration severity chart.

   The heart of the standard was its evaluation chart. For each of the four classes it assigned specific RMS velocity bands to qualitative condition grades that were easy to understand and apply:

   • A (Good): newly commissioned or well-maintained machines.
   • B (Satisfactory): acceptable for long-term, unrestricted operation.
   • C (Unsatisfactory): not acceptable for prolonged operation; the machine should be monitored and scheduled for maintenance.
   • D (Unacceptable): vibration is damaging and demands immediate action to avoid failure.

   This chart-based approach let a technician take one reading, find the machine class, and reach a clear verdict on its health in seconds. Notice that a fixed velocity — say 4.5 mm/s — could be “Satisfactory” for a large Class IV machine yet “Unsatisfactory” for a small Class I one; the class determined the boundary.

3. Why It Was Replaced

ISO 2372 was a major step forward, but it carried limitations that the modern standards set out to fix:

• Oversimplification: sorting all machines into just four classes was too coarse. The modern ISO 10816 / 20816 series gives far more specific guidance tailored to individual machine types — pumps, fans, compressors, hydro and gas turbines and more.
• Foundation ambiguity: the distinction between “rigid” and “soft” foundations was often hard to judge and applied inconsistently from one assessor to the next.
• No diagnostic information: the standard delivered a single overall number. It revealed nothing about the frequencies present in the signal and therefore could not point to the cause — it could flag a sick machine but not distinguish unbalance from misalignment.
• Evolving technology: it was written before the widespread availability of digital, FFT-based vibration analysers that today resolve a reading into its component frequencies almost instantly.

4. Legacy and Modern Practice

Despite its withdrawal, the legacy of ISO 2372 is profound. It established broadband RMS velocity on the bearing housing as the primary measure of overall severity — a convention that survives intact in today’s standards. Many simple meters and screening tools still display the colour-coded green / yellow / red alarm bands that descend directly from the original four-grade chart, mapping neatly onto the modern alarm and trip levels used in continuous monitoring. If you are working from a current zone-based limit instead, the vibration severity chart bridges the old and new schemes.

In day-to-day field work the principle endures even as the document is retired. A portable analyser such as the ব্যালানসেট-১এ measures broadband velocity on the bearing housings just as ISO 2372 prescribed, then goes further — resolving the same signal into a spectrum so the engineer can not only grade severity against ISO 20816 zones but also identify the offending frequency and, where the fault is unbalance, correct it on the spot by ক্ষেত্রের ভারসাম্য. In that sense the modern workflow does exactly what ISO 2372 did, and then answers the question the old chart never could: why.

Official ISO Standard

For the complete official standard, visit: ISO 2372 on ISO Store

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