Understanding Vibration Severity

Definition: What is Vibration Severity?

Vibration severity is a generic term for a single, overall value used to characterize the health of a machine based on its level of vibration. Instead of looking at a complex spectrum, a severity measurement condenses the machine’s condition into one simple number. This value can then be compared to standardized charts to quickly determine if the machine is running smoothly, requires monitoring, or is in danger of failure. The goal of a severity measurement is to provide a simple, reliable, and universal indicator of a machine’s dynamic stress and condition.

The Standard for Measurement: RMS Velocity

Over decades of research and practice, the industry has established that the single best parameter for gauging the severity of vibration on most common rotating machinery is RMS (Root Mean Square) Velocity. This is because:

• The destructive energy of vibration is most closely related to velocity.
• A given velocity level corresponds to a consistent level of severity across a wide range of machine types and speeds.

For this reason, international standards like the ISO 20816 series use RMS velocity (measured in mm/s or in/s) as the basis for their evaluation criteria.

The ISO 20816 Vibration Severity Charts

The International Organization for Standardization (ISO) has developed a series of standards to classify machinery health. The ISO 20816 standard (which superseded the older ISO 10816) is the most widely used framework. It provides vibration severity charts that are used globally as a reference for acceptance testing and routine condition monitoring.

The core concept of the standard is to:

1. Classify the Machine: Machines are grouped into different classes based on their size, type, and foundation (e.g., “Group 1” for large turbines, “Group 2” for medium-sized pumps and motors).
2. Measure RMS Velocity: Broadband RMS velocity is measured on the machine’s bearing housings in the horizontal, vertical, and axial directions.
3. Compare to the Chart: The highest measured value is compared to the chart for that machine class.

The Four Severity Zones

The ISO charts divide machinery health into four distinct zones:

• Zone A (Green): The vibration of newly commissioned machinery would normally fall into this zone. Represents a very smooth, healthy condition.
• Zone B (Yellow): Indicates a condition that is acceptable for unrestricted long-term operation. This is the normal operating range for most machines.
• Zone C (Orange): The vibration levels are considered unsatisfactory for long-term continuous operation. Machines in this zone should be monitored closely, and maintenance should be scheduled to find and correct the root cause of the increased vibration.
• Zone D (Red): The vibration values are considered to be of sufficient severity to cause damage to the machine. Machines operating in this zone are in a critical state and may require immediate shutdown.

Using Vibration Severity in a Predictive Maintenance Program

Vibration severity charts are a cornerstone of predictive maintenance. By taking regular, monthly readings of the overall RMS velocity on a machine and plotting the trend, maintenance teams can:

• Quickly Screen Assets: Easily identify which machines in a plant are healthy and which require further attention.
• Provide an Early Warning: A trend of rising vibration that moves from Zone B into Zone C gives an early warning of a developing problem.
• Justify Maintenance Actions: The clear, standardized zones provide an objective basis for recommending maintenance work. It’s much easier to justify a repair when you can show that a machine has entered the “Unsatisfactory” (Zone C) or “Damage” (Zone D) range.

While a detailed spectral analysis is needed to find the *root cause* of a problem, the simple vibration severity measurement is the essential first step that tells you *that* a problem exists.

← Back to Main Index