Vibration Amplitude: A Key Indicator of Machine Health

Definition: What is Vibration Amplitude?

Vibration Amplitude is the measure of the intensity or severity of vibration. It quantifies “how much” a machine is vibrating and is one of the most fundamental parameters used in condition monitoring and machinery diagnostics. A change in amplitude over time is often the first indication of a developing mechanical problem. While frequency helps diagnose the *type* of fault, amplitude helps determine its *severity*.

The Importance of Measuring Amplitude

Tracking vibration amplitude is essential for any predictive maintenance program. An increase in amplitude directly correlates with an increase in the dynamic forces acting on a machine’s components. Monitoring these levels helps to:

• Establish a Baseline: Measuring amplitude on a healthy machine provides a baseline for future comparison.
• Trend Machine Health: By plotting amplitude readings over time, engineers can detect gradual deterioration long before a failure occurs.
• Set Alarms: Amplitude levels are used to set alert and danger alarms, notifying personnel when a machine’s condition has worsened significantly.
• Assess Severity: The magnitude of the amplitude is a direct indicator of how severe a problem is, helping to prioritize maintenance actions.

Different Ways to Measure Amplitude

Vibration is a dynamic signal, and its amplitude can be quantified in several different ways. The choice of measurement depends on the type of machine and the information being sought.

1. Peak (Pk) Amplitude

The Peak value is the maximum amplitude reached by the vibration waveform in one direction (either positive or negative) from its zero or equilibrium position. Peak measurements are particularly useful for assessing short-duration, high-impact events, such as those caused by a broken gear tooth or a severe bearing defect. It indicates the maximum stress or force being applied to a component during a vibration cycle.

2. Peak-to-Peak (Pk-Pk) Amplitude

The Peak-to-Peak value is the total distance the vibrating component travels from its maximum positive peak to its maximum negative peak. It represents the overall excursion or total motion of the part. Peak-to-peak is most commonly used for measuring displacement, as it is critical for assessing clearances. For example, it can determine if a rotating shaft is moving enough to risk contact with a stationary bearing housing.

3. RMS (Root Mean Square) Amplitude

The RMS value is the most common and useful measure for assessing overall vibration severity. It is calculated by taking the square root of the average of the squared values of the waveform over time. The key advantage of RMS is that it is directly related to the energy content and destructive power of the vibration. An RMS value considers the entire vibration signal over time, not just the maximum peaks, making it a much more stable and representative measure of a machine’s overall condition. Most international standards, like ISO 10816, use RMS velocity to specify vibration limits.

Relationship Between Pk, Pk-Pk, and RMS

For a perfect sine wave, these values have a simple mathematical relationship:

• Peak-to-Peak = 2 × Peak
• RMS = Peak / √2 ≈ 0.707 × Peak

However, for complex, non-sinusoidal signals found in real-world machinery (which often contain impacts and harmonics), this relationship does not hold. The ratio of Peak to RMS is known as the Crest Factor, which can itself be a useful diagnostic tool.

Which Amplitude Unit to Use?

Amplitude is measured in units of displacement, velocity, or acceleration, and the choice depends on the frequency of interest:

• Displacement (μm, mils): Best for low-frequency vibration (<10 Hz), such as structural movement or unbalance on very slow machines.
• Velocity (mm/s, in/s): The best general-purpose indicator for mid-range frequencies (10 Hz to 1,000 Hz), where most common machine faults like unbalance and misalignment occur.
• Acceleration (g, m/s²): Best for high-frequency vibration (>1,000 Hz), such as gear mesh and bearing faults.

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