Vibration Measurement Units
Learn about the key vibration measurement units: mm/s, g, µm, and Hz. Understand what they mean, how they’re used, and why they matter in predictive maintenance.
Understanding Vibration Measurement Units
When measuring vibration in industrial machines or mechanical systems, it’s vital to understand the units used to describe and interpret the data. These units form the foundation of condition monitoring and enable maintenance teams to accurately assess machine health. Whether you use a handheld vibration meter or a sophisticated analyzer, the displayed values always correspond to physical quantities. Let’s break down the main vibration measurement units:
1. Velocity – mm/s (millimeters per second)
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What it measures: The speed at which the machine surface moves during vibration.
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Typical unit: mm/s RMS (Root Mean Square).
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Common use: General vibration severity assessment, following standards such as ISO 10816.
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Why it matters: Best suited for detecting faults related to imbalance, misalignment, and looseness.
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Example: A reading of 4.5 mm/s RMS might indicate moderate vibration on a motor.
2. Acceleration – m/s² or g (gravity)
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What it measures: The rate of change of velocity, i.e., how quickly the vibration speed increases.
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Units:
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m/s² (metric)
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g (gravity), where 1 g = 9.81 m/s² (commonly used for high-frequency measurements).
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Common use: Detecting bearing faults and analyzing high-frequency vibration components.
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Example: A peak acceleration of 12 g could point to a potential bearing defect.
3. Displacement – µm (micrometers) or mils (thousandths of an inch)
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What it measures: The physical movement of the machine surface from its rest position, usually peak-to-peak.
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Units:
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µm (metric)
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mils (imperial), where 1 mil = 0.001 inch.
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Common use: Low-frequency machinery such as turbines, large fans, and compressors.
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Example: A reading of 50 µm peak-to-peak may be acceptable for slow-rotating equipment.
4. Frequency – Hz (Hertz)
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What it measures: The number of vibration cycles per second.
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Units: Hz (cycles per second).
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Common use: Frequency analysis (FFT) for identifying vibration sources and fault frequencies.
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Example: A vibration frequency of 60 Hz corresponds to a motor running at 3600 RPM (since 3600 RPM = 60 revolutions per second × 60).
Additional Notes:
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Velocity is typically used for overall vibration severity as it balances sensitivity across a broad frequency range, especially effective for faults related to machine speed.
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Acceleration emphasizes high-frequency vibration components, making it ideal for fault detection in bearings and gears.
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Displacement emphasizes low-frequency vibration and is useful for slow-speed rotating equipment.
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Vibration signals can be viewed in the time domain (time waveform) or the frequency domain (spectrum via FFT), allowing detailed analysis of vibration behavior and fault diagnosis.
Summary Table
|
Quantity |
Unit |
Description |
Common Use |
|
Velocity |
mm/s RMS |
Speed of vibration |
General fault detection |
|
Acceleration |
m/s² or g |
Rate of velocity change |
High-frequency faults (e.g. bearings) |
|
Displacement |
µm or mils |
Total distance moved (peak-to-peak) |
Low-frequency machinery |
|
Frequency |
Hz |
Cycles per second |
Source identification (FFT) |
Why Understanding Units Matters
Incorrect interpretation of units can lead to wrong diagnoses and unnecessary maintenance costs. By understanding the measurement units, you can:
- Set appropriate alarm limits
- Compare values across equipment types
- Comply with international standards
- Communicate findings effectively with reliability engineers
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