Understanding Vibration Displacement

Definition: What is Displacement in Vibration?

Displacement is a measure of the total distance that a vibrating object moves from its position of rest (equilibrium). It quantifies “how far” a component is moving back and forth. As the most straightforward representation of vibratory motion, displacement is a fundamental parameter in vibration analysis, especially for low-frequency applications and for assessing mechanical clearances.

Why and When to Measure Displacement

While velocity is the most common parameter for overall machine health, displacement is the preferred measurement in several specific, critical scenarios:

• Low-Frequency Analysis: Vibration energy is proportional to displacement at very low frequencies. For slow-speed machinery (typically below 600 RPM or 10 Hz), such as large fans, cooling towers, and paper machines, displacement is the most sensitive and representative indicator of vibration severity.
• Assessing Clearances: Displacement provides a direct measurement of the physical movement of a part. This is crucial for determining if a rotating shaft has enough clearance to avoid rubbing against stationary components like bearings or seals.
• Structural Deflection: When analyzing the movement of structures like bases, frames, or piping, displacement is used to understand the mode shapes and ensure deflections are within design limits.
• Balancing of Low-Speed Rotors: During the balancing of large, slow-moving rotors, displacement measurements are often used to determine the amount of unbalance.

Units and Measurement

Common Units

Vibration displacement is typically expressed in one of two units:

• Mils: The industry standard in the United States, where 1 mil equals one-thousandth of an inch (0.001″).
• Micrometers (µm): The standard SI unit, where 1 micrometer equals one-millionth of a meter. (1 mil ≈ 25.4 µm).

Displacement is almost always measured in Peak-to-Peak (Pk-Pk) units, as this value represents the total travel of the component, which is the most important factor for clearance analysis.

How is it Measured?

Displacement can be measured in several ways:

1. Proximity Probes: This is the most common method for measuring shaft vibration. A non-contact eddy current probe is mounted on a stationary part and measures the changing gap between its tip and the rotating shaft. This directly measures the relative displacement of the shaft.
2. Integration from Accelerometers: A standard accelerometer measures acceleration. Its signal can be electronically integrated once to get velocity, and a second time to get displacement. This is a common feature in modern data collectors, but it can be prone to noise and errors at very low frequencies.
3. Laser Displacement Sensors: These non-contact sensors use a laser beam to provide highly accurate displacement measurements.

Displacement’s Role in Diagnostics

While high displacement at the shaft’s rotational frequency (1X RPM) on a low-speed machine often points to unbalance, its primary diagnostic power comes from its relationship with velocity and acceleration. For a given amount of vibration energy:

• At low frequencies, displacement will have the highest amplitude.
• At mid-range frequencies, velocity will have the highest amplitude.
• At high frequencies, acceleration will have the highest amplitude.

Because of this, analysts use displacement to focus on low-frequency phenomena that might be completely invisible in an acceleration spectrum. A machine could be undergoing severe, damaging low-frequency movement that generates very little acceleration, making displacement a critical part of a complete diagnostic toolkit.

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