Peak vs. Peak-to-Peak Amplitude in Vibration Analysis
Peak (Pk) and Peak-to-Peak (Pk-Pk) are two of the primary ways to quantify the amplitude, or magnitude, of a vibration signal. Although closely related, they measure different features of the waveform and serve different diagnostic purposes. Choosing between them — and knowing how each relates to RMS — is one of the first skills a vibration analyst develops, because the same physical vibration can look mild or alarming depending on which descriptor you quote.
1. Definition: Distinguishing Peak and Peak-to-Peak
Both values are read from the time waveform — the trace of instantaneous amplitude against time — but they describe its geometry in two distinct ways.
Peak (Pk) Amplitude
The peak value is the maximum excursion of the waveform from its zero or equilibrium position in one direction, positive or negative. It captures the single most intense instant of the vibration cycle. For a symmetrical waveform the positive and negative peaks are equal; for an asymmetrical one they differ, and instruments may report the larger of the two as the true peak.
Peak-to-Peak (Pk-Pk) Amplitude
The peak-to-peak value is the total distance from the maximum positive peak to the maximum negative peak — the full range of motion, or overall excursion, of the vibrating component during one cycle. For a clean, symmetrical sine wave the relationship is simple:
Peak-to-Peak = 2 × Peak
For the complex, asymmetrical waveforms that real machinery produces, this tidy factor of two may not hold — the wave is rarely centred perfectly on the zero line, so doubling the peak can over- or under-state the true total travel. When displacement matters, measure Pk-Pk directly rather than inferring it.
2. When to Use Peak (Pk) Measurement
Peak amplitude is most useful for flagging short-duration, high-energy events or impacts. It reflects the maximum stress or force applied to a component, which makes it valuable for:
- Detecting impacts: a cracked gear tooth, a spalled bearing, or a loose part fires sharp impulses that drive high peak values in the time waveform long before the averaged level rises.
- Assessing stress: because fatigue damage tracks maximum deflection, the peak value is often a better warning of impending failure than an energy average like RMS.
- Setting protective alarms: on some machines, alarms are placed on peak values to guard against sudden, damaging transient events.
Peak values are usually taken from acceleration signals, where the impulsive forces inside the machine — the hallmark of incipient gear and bearing damage — stand out most clearly. A related instrument feature, peak hold, captures and retains the highest value seen during a measurement so a fleeting impact is not missed.
3. When to Use Peak-to-Peak (Pk-Pk) Measurement
Peak-to-peak amplitude is the measurement of choice when the concern is the total physical travel of a component, almost always expressed as displacement:
- Clearance analysis: Pk-Pk displacement reveals whether a rotating shaft is moving enough to contact stationary parts such as bearing housings or seals, giving a direct measure of the physical space the vibrating part sweeps out.
- Shaft vibration monitoring: on critical turbomachinery watched by proximity probes, limits and alarms are nearly always specified in peak-to-peak displacement — in mils or micrometres — under standards such as ISO 7919.
- Low-speed machines: on very slow rotors the total movement of the parts, rather than their energy, is usually the most meaningful health indicator.
A vibration displacement calculator converts a measured velocity at a known frequency into the equivalent peak-to-peak micrometres, which is handy when comparing a velocity reading against a displacement-based shaft limit.
4. Comparison with RMS
It is essential to contrast Pk and Pk-Pk 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 is the basis of international vibration severity standards such as ISO 20816 (the modern replacement for ISO 10816), which set their mm/s zone limits in RMS velocity.
- Peak is best for catching impulsive events and gauging maximum stress.
- Peak-to-Peak is best for evaluating total movement and running clearances.
For a pure sine wave the three are linked by fixed factors (Pk ≈ 1.414 × RMS, Pk-Pk = 2 × Pk), and a vibration unit converter applies them instantly; but real signals break those ratios, which is exactly where the diagnostic value lies.
5. The Crest Factor: Where the Three Meet
A thorough analysis examines all three parameters together, and their interplay is captured by the Crest Factor — the ratio of peak to RMS. A high crest factor signals the presence of sharp impacts even when the overall energy (RMS) is still low, which is a classic early indicator of bearing or gear faults. Watching the crest factor climb while RMS is still calm often gives the earliest possible warning of a developing defect, well before any single descriptor would trip an alarm on its own. In the field, a portable two-channel analyser such as the Balanset-1A records the time waveform directly and reports peak, peak-to-peak and RMS side by side, so an engineer can read all three descriptors — and the crest factor between them — right at the machine without any post-processing.
