Peak vs. Peak-to-Peak Amplitude in Vibration Analysis

Definition: Distinguishing Peak and Peak-to-Peak

Peak (Pk) and Peak-to-Peak (Pk-Pk) are two of the primary ways to quantify the amplitude, or magnitude, of a vibration signal. While closely related, they measure different aspects of the waveform and are used for different diagnostic purposes.

Peak (Pk) Amplitude

The Peak value represents the maximum excursion of the waveform from its zero or equilibrium position in one direction (either positive or negative). It captures the single most intense moment of the vibration cycle. If the waveform is symmetrical, the positive and negative peak values will be the same.

Peak-to-Peak (Pk-Pk) Amplitude

The Peak-to-Peak value is the total distance or displacement measured from the maximum positive peak to the maximum negative peak of the waveform. It represents the full range of motion or the overall excursion of the vibrating component during one cycle.

For a simple, symmetrical sine wave, the relationship is straightforward:

Peak-to-Peak = 2 × Peak

However, for the complex waveforms found in real machinery, this simple relationship may not hold true if the signal is not symmetrical around the zero line.

When to Use Peak (Pk) Measurement

Peak amplitude is most useful for indicating the level of short-duration, high-energy events or impacts. It is a measure of the maximum stress or force being applied to a component. Therefore, Peak measurements are particularly valuable for:

• Detecting Impacts: A cracked gear tooth, a severe bearing defect, or loose parts will generate sharp impacts that cause high peak values in the time waveform.
• Assessing Stress: Since stress is often related to the maximum deflection, the peak value can be a better indicator of potential fatigue failure than an averaged value like RMS.
• Setting Protective Alarms: For some machinery, alarms may be set on peak values to protect against sudden, damaging events.

Peak values are typically used when analyzing acceleration signals, as this highlights the impulsive forces within the machine.

When to Use Peak-to-Peak (Pk-Pk) Measurement

Peak-to-Peak amplitude is the measurement of choice when the primary concern is the total displacement or travel of a component. Its most important application is:

• Clearance Analysis: Pk-Pk displacement is used to determine if a rotating shaft is moving enough to make contact with stationary parts like bearing housings or seals. It provides a direct measure of the physical space being occupied by the vibrating part.
• Shaft Vibration Monitoring: For critical turbomachinery monitored with proximity probes, vibration limits and alarms are almost always specified in Peak-to-Peak displacement (e.g., in mils or micrometers).
• Low-Speed Machine Analysis: On very slow-moving machines, the total movement of the components is often the most critical health indicator.

Comparison with RMS

It is important to contrast these measurements with the RMS (Root Mean Square) value, which measures the overall energy content of the vibration.

• RMS is best for trending overall machine health and conforms to international standards for vibration severity.
• Peak is best for detecting impulsive events and assessing maximum stress.
• Peak-to-Peak is best for evaluating total movement and clearances.

A comprehensive analysis often involves looking at all three parameters. The ratio of the Peak value to the RMS value, known as the Crest Factor, is itself a powerful diagnostic indicator. A high Crest Factor indicates the presence of sharp impacts within the signal, even if the overall energy (RMS) is low, often pointing to early-stage bearing or gear faults.

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