Understanding Peak Amplitude (Pk & Pk-Pk)

1. Definition: What is Peak Amplitude?

Peak (Pk) Amplitude is a measure of the maximum amplitude of a vibration signal, measured from the zero or equilibrium position to the highest positive point of the waveform. In a simple, clean sinusoidal signal, this represents the maximum instantaneous level of vibration that occurred during the measurement.

Peak-to-Peak (Pk-Pk) Amplitude is the total excursion of the waveform, measured from the most negative peak to the most positive peak. For a pure sine wave, the Peak-to-Peak value is exactly twice the Peak value.

These measurements are typically taken from the time waveform signal.

Example: For a sine wave that oscillates between +5 mm/s and -5 mm/s:

• The Peak (Pk) amplitude is 5 mm/s.
• The Peak-to-Peak (Pk-Pk) amplitude is 10 mm/s.

2. When Are Peak Measurements Used?

While RMS (Root Mean Square) is the most common metric for assessing the overall energy and destructive potential of vibration, Peak and Peak-to-Peak values are important for specific situations:

a) Assessing Clearance and Mechanical Space

Peak-to-Peak Displacement is a critical measurement, particularly for machines with journal bearings that are monitored by proximity probes. The Pk-Pk value tells the analyst the total distance the shaft is moving within its bearing clearance. If this value approaches the bearing’s physical clearance, it is a clear indication of a severe problem that could lead to catastrophic contact between the rotor and stationary parts.

b) Detecting Impacts and Transients

Peak amplitude is highly sensitive to short-duration, high-energy events like impacts. A cracked gear tooth or a rolling element in a bearing passing over a spall will create a sharp spike in the time waveform.

The Peak Acceleration value will be very high during these events, even if the overall RMS value is still low. This makes Peak measurements a valuable tool for early fault detection. The ratio of Peak to RMS amplitude is known as the Crest Factor, which is itself a key diagnostic indicator.

3. Limitations of Peak Measurements

The main limitation of relying solely on Peak amplitude for general condition assessment is that it only represents a single point in time. It doesn’t account for the energy content of the entire signal in the way that RMS does.

A signal with a single, sharp spike might have a high Peak value but a low RMS value, indicating it may not be very destructive. Conversely, a highly complex signal with many moderate peaks could have a high, destructive RMS value but a Peak value that doesn’t look as alarming in isolation.

4. Summary: Pk vs. Pk-Pk vs. RMS

• Use RMS for trending overall vibration severity and assessing the general health of a machine. It relates best to the destructive energy of the vibration.
• Use Peak-to-Peak (Displacement) when concerned with physical clearances and the absolute motion of a component, especially shafts in journal bearings.
• Use Peak (Acceleration) to detect and quantify sharp, impacting events, which are often the first sign of bearing and gear faults.

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