Understanding Peak Amplitude (Pk & Pk-Pk)
Peak (Pk) amplitude is a measure of the maximum level of a vibration signal, taken from the zero or equilibrium position to the highest point the waveform reaches. For a clean sinusoid it represents the largest instantaneous level of vibration that occurred during the measurement. It is one of the three ways of expressing amplitude — alongside Peak-to-Peak and RMS — and the choice between them is one of the most fundamental decisions in vibration analysis.
Peak-to-Peak (Pk-Pk) amplitude is the total excursion of the waveform, measured from the most negative trough to the most positive crest. For a pure sine wave the Peak-to-Peak value is exactly twice the Peak value. Both quantities are read from the time waveform rather than from a spectrum.
Example — for a sine wave that oscillates between +5 mm/s and −5 mm/s:
- ದಿ Peak (Pk) amplitude is 5 mm/s.
- ದಿ Peak-to-Peak (Pk-Pk) amplitude is 10 mm/s.
1. Peak, Peak-to-Peak and the Relationship to RMS
For an ideal sinusoid the three measures are linked by fixed ratios: the RMS value equals the Peak divided by √2 (about 0.707 × Peak), and the Peak equals 1.414 × RMS. Crucially, these tidy conversions only hold for a pure sine wave. The moment a signal contains impacts, noise or multiple frequencies, the true peak and the RMS-derived peak diverge — which is exactly why a genuine, sample-by-sample peak detector behaves differently from one that simply scales an RMS reading.
2. When Are Peak Measurements Used?
While RMS is the most common metric for assessing the overall energy and destructive potential of vibration, Peak and Peak-to-Peak values come into their own in two specific situations.
a) Assessing Clearance and Mechanical Space
Peak-to-Peak displacement is a critical measurement, particularly for machines with journal bearings monitored by proximity probes. The Pk-Pk value tells the analyst the total distance the shaft travels within its bearing clearance. If that figure begins to approach the bearing’s physical clearance, it is a clear warning of a severe problem that could end in catastrophic contact between the rotor and the stationary parts. Because it speaks the language of mechanical gaps — micrometres or mils of actual movement — Peak-to-Peak displacement is the natural unit for clearance questions.
b) Detecting Impacts and Transients
Peak amplitude is highly sensitive to short-duration, high-energy events such as impacts. A cracked gear tooth, or a rolling element rolling over a spall in a bearing race, produces a sharp spike in the time waveform. The Peak acceleration value rises steeply during these events even while the overall RMS value remains low, which makes Peak measurements a valuable tool for early bearing and gear fault detection. The ratio of Peak to RMS is itself a recognised diagnostic indicator — the Crest Factor — and a rising Crest Factor is often the very first sign that an impacting fault is developing.
3. Limitations of Peak Measurements
The main weakness of relying on Peak amplitude alone for general condition assessment is that it captures only a single instant. It says nothing about the energy content of the rest of the signal, the way RMS does. A waveform with one sharp, isolated spike may show a high Peak but a low RMS, suggesting it is not especially destructive. Conversely, a complex waveform made up of many moderate peaks can carry a high, genuinely damaging RMS while no single Peak looks alarming in isolation. A further practical issue is repeatability: because a true peak is a one-off maximum, a single stray transient or a burst of electrical noise can inflate it, so peak readings are often captured with a peak-hold function over several seconds and interpreted alongside RMS rather than on their own.
4. Summary: Pk vs. Pk-Pk vs. RMS
The three measures are complementary, not competing — the skill lies in choosing the right one for the question being asked:
- Use RMS for trending overall vibration severity and judging the general health of a machine; it relates best to the destructive energy of the vibration and underpins standards such as ISO 20816.
- Use Peak-to-Peak (displacement) when the concern is physical clearance and the absolute motion of a component — above all, shafts running in journal bearings.
- Use Peak (acceleration) to detect and quantify sharp, impacting events, which are frequently the earliest sign of bearing and gear faults.
In practice an analyst keeps all three in view at once. A portable two-channel instrument such as the ಬ್ಯಾಲೆನ್ಸೆಟ್-1ಎ reports overall levels and the underlying time waveform together, so the engineer can read RMS for severity, Peak-to-Peak for clearance, and Peak for impact content from the same measurement without compromise.
