Understanding Vibration Severity

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Dynamic balancer “Balanset-1A” OEM

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Vibration severity is a generic term for a single, overall value used to characterise the health of a machine from its level of vibration. Rather than interpreting a whole spectrum, a severity measurement condenses the machine’s condition into one number that can be compared against standardised charts to judge, at a glance, whether the machine is running smoothly, needs watching, or is heading for failure. The aim is a simple, reliable, and universal indicator of a machine’s dynamic stress and condition — the screening figure that tells you that a problem exists, before deeper analysis tells you what it is.

1. The Standard Parameter: RMS Velocity

Over decades of research and practice, the industry has settled on a single best parameter for gauging severity on most common rotating machinery: RMS (root-mean-square) velocity. Two reasons stand behind that consensus:

• The destructive energy carried by vibration correlates most closely with velocity, because velocity reflects both how far and how fast a part moves.
• A given velocity level corresponds to a consistent degree of severity across a wide range of machine types and speeds, which makes one chart broadly applicable.

For this reason the international standards use RMS velocity — in mm/s or in/s — as the basis for their evaluation criteria. The choice is also why velocity is preferred over displacement (which dominates only at low frequency) or acceleration (which dominates at high frequency) for general machine-health screening. The RMS value is normally read as a broadband, or overall, level; our Overall Vibration Level calculator shows how that single figure is built up from the individual peaks in a spectrum.

2. The ISO 20816 Vibration-Severity Charts

The International Organization for Standardization has developed a family of standards to classify machinery health. The ISO 20816 series — which superseded the older ISO 10816 — is the most widely used framework, providing severity charts referenced worldwide for both acceptance testing and routine condition monitoring. The method has three core steps:

1. Classify the machine: machines are grouped by size, type, and foundation — for example large turbines in one group, medium-sized pumps and motors in another. The detailed limits for the common industrial range of roughly 15 kW to 50 MW live in ISO 20816-3.
2. Measure RMS velocity: broadband RMS velocity is read on the machine’s bearing housings in the horizontal, vertical, and axial directions.
3. Compare to the chart: the highest measured value is compared with the chart for that machine class.

You can look up the boundaries directly with our ISO 10816 / 20816 Vibration Severity Chart, or evaluate a reading against the zone limits with the ISO 20816-1 Zone calculator and the machine-specific ISO 20816-3 limits tool.

The Four Severity Zones

The ISO charts divide machinery health into four zones:

• Zone A (green): the vibration of newly commissioned machinery would normally fall here — a very smooth, healthy condition.
• Zone B (yellow): acceptable for unrestricted long-term operation; the normal operating range for most machines.
• Zone C (orange): unsatisfactory for long-term continuous operation. Machines here should be watched closely, with maintenance scheduled to find and correct the root cause of the elevated vibration.
• Zone D (red): severe enough to cause damage. Machines operating here are in a critical state and may require immediate shutdown.

3. Using Vibration Severity in a Predictive-Maintenance Programme

Severity charts are a cornerstone of predictive maintenance. By taking regular — typically monthly — readings of the overall RMS velocity on a machine and trending them over time, maintenance teams can:

• Screen assets quickly: identify at a glance which machines in a plant are healthy and which need attention.
• Get an early warning: a rising trend that crosses from Zone B into Zone C provides early warning of a developing problem, often months before failure.
• Justify maintenance actions: the standardised zones give an objective basis for recommending work — it is far easier to authorise a repair when you can show a machine has entered the “unsatisfactory” (Zone C) or “damage” (Zone D) range.

The power of the approach lies in the trend as much as the absolute number: a steady reading well inside Zone B is reassuring, while a value still technically in Zone B but climbing steeply each month is a clearer call to act.

4. From Severity to Root Cause — and Back to Health

A severity reading is a screening tool, not a diagnosis. It tells you a machine is rough, but a detailed spectral analysis is what reveals why — distinguishing unbalance at 1× from misalignment, looseness, or bearing wear. In practice the overall level and the spectrum are used together: the severity number flags the asset, and the spectrum directs the repair.

This is exactly where a portable analyser proves its worth in the field. An instrument such as the Balanset-1A reads the overall RMS velocity for an immediate ISO 20816 severity check, then provides the spectrum and the 1× amplitude-and-phase needed to confirm whether unbalance is the cause — and, if it is, to balance the rotor in its own bearings and watch the severity fall back into Zone A or B on the spot. Measuring before and after in the same units closes the loop: the same number that raised the alarm verifies the fix.

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