Understanding Frequency in Vibration Analysis

Definition: What is Vibration Frequency?

Frequency is a measure of how often a repetitive event occurs in a given unit of time. In vibration analysis, it quantifies “how fast” an object is oscillating or vibrating. It is the single most important parameter for diagnosing the root cause of a machinery problem. While amplitude tells us the *severity* of a vibration, frequency tells us the *source*.

The Diagnostic Power of Frequency

The core principle of vibration diagnostics is that different mechanical and electrical components generate vibration at specific, predictable frequencies when they start to fail. By identifying the frequencies present in a machine’s vibration signature, an analyst can pinpoint the exact component that is causing the problem. This is analogous to how a doctor uses a stethoscope to listen for specific sounds that indicate different medical conditions.

Each potential fault has a characteristic frequency signature:

• An issue with the entire rotating assembly, like unbalance, will show up at the frequency of the shaft’s rotation (1x running speed).
• An issue related to the alignment between two shafts, like misalignment, will typically appear at twice the running speed (2x).
• A defect on a rolling-element bearing will generate frequencies based on its precise geometry and the shaft speed.
• Problems with gears will create signals at the Gear Mesh Frequency (GMF), which is the number of teeth multiplied by the gear’s speed.

Units of Frequency

Frequency is expressed in several different units, and it’s important to be familiar with all of them:

1. Hertz (Hz)

This is the standard international (SI) unit for frequency. One Hertz is equal to one cycle per second. This is the most common unit used in technical and scientific contexts.

2. Cycles Per Minute (CPM)

CPM is widely used in industrial maintenance because it relates directly to the rotational speed of machinery, which is typically measured in Revolutions Per Minute (RPM). Since one minute has 60 seconds, the conversion is simple:

CPM = Hz × 60

For example, a vibration at 30 Hz is equal to 1,800 CPM.

3. Orders

Orders are multiples of the machine’s primary running speed. The running speed itself is the 1st order. A vibration at twice the running speed is the 2nd order, three times is the 3rd order, and so on. This is particularly useful for analyzing machines that change speed, as the orders will remain constant even as the absolute frequency in Hz or CPM changes. For example, unbalance will always be at the 1st order, regardless of the machine’s speed.

How is Frequency Determined?

The frequencies present in a vibration signal are identified using a Fast Fourier Transform (FFT). An accelerometer collects a raw time waveform signal, and the FFT algorithm processes this signal to create a frequency spectrum. This spectrum is a graph that clearly displays all the individual frequencies that make up the complex vibration, allowing the analyst to identify the peaks and match them to potential fault sources.

The Relationship Between Frequency, Velocity, and Acceleration

For a given level of vibrational energy, the amplitudes of displacement, velocity, and acceleration are highly dependent on frequency. This is why different measurement units are preferred for different frequency ranges:

• Low Frequencies: Displacement is highest.
• Mid Frequencies: Velocity is highest.
• High Frequencies: Acceleration is highest.

Understanding frequency is the key that unlocks the diagnostic potential of vibration analysis, transforming raw data into actionable maintenance information.

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