Understanding Velocity in Vibration Analysis

Definition: What is Vibration Velocity?

Velocity is the rate of change of displacement with respect to time. In vibration analysis, it is a measure of “how fast” a component is moving. Of the three primary vibration parameters (displacement, velocity, and acceleration), velocity is the most widely used for assessing the overall health and vibration severity of general rotating machinery in the most common frequency range.

Why is Velocity the Standard for Severity Measurement?

Velocity has become the industry standard for general-purpose vibration monitoring for several key reasons:

• Best Indicator of Destructive Energy: The destructive energy of vibration is most directly related to its velocity. A given level of velocity corresponds to a consistent level of vibration severity across a wide range of machine speeds and types.
• “Flat” Frequency Response: In the most critical frequency range for machine diagnostics (typically 10 Hz to 1,000 Hz or 600 to 60,000 CPM), velocity provides the most balanced or “flat” view. This means it is almost equally sensitive to low-frequency problems like unbalance and higher-frequency problems like misalignment, making it an excellent all-around indicator.
• Basis for International Standards: Global standards for machinery vibration, such as ISO 20816 (which replaced the older ISO 10816), use RMS (Root Mean Square) Velocity as the primary metric for defining acceptance limits and alarm levels for different classes of machines.

Units and Measurement

Common Units

Vibration velocity is typically expressed in one of two units:

• mm/s (millimeters per second): The standard SI unit, commonly used in most parts of the world.
• in/s (inches per second): The standard imperial unit, common in the United States.

Velocity is almost always measured and trended as an RMS value, as this best represents its energy content.

How is it Measured?

Velocity can be measured in two primary ways:

1. Directly with a Velocity Transducer: An electrodynamic velocity sensor generates a voltage directly proportional to the velocity of the vibration. These sensors were common but have largely been superseded by accelerometers.
2. By Integrating an Accelerometer Signal: This is the most common method today. A robust accelerometer measures acceleration, and a data collector or monitoring system electronically integrates that signal to calculate velocity. This approach combines the wide frequency range and reliability of an accelerometer with the diagnostic advantages of the velocity parameter.

Velocity’s Role in Diagnostics

A high overall velocity level indicates that a machine has a problem, but it doesn’t specify the cause. The next step is to look at the velocity spectrum (FFT) to see which frequencies are contributing to the high overall value.

• High velocity at 1x RPM points to unbalance.
• High velocity at 2x RPM points to misalignment.
• A series of velocity peaks at running speed harmonics indicates looseness.

While acceleration is better for very high-frequency faults (bearings, gears) and displacement is better for very low-frequency issues (structural motion), velocity remains the single most valuable parameter for a “big picture” view of a machine’s dynamic health in its primary operating range.

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