The Peak, Peak to Peak and RMS values in vibration analysis

In vibration analysis, correctly interpreting an asset’s signals, such as Peak, Peak-to-Peak and RMS, and being able to act on them is crucial to the success of predictive maintenance.

In general, interpretation involves analyzing vibration signals in the time domain, that is, plotted on a t-axis. In this way, it is possible to analyze changes or patterns of vibration by means of different parameters.

In this article, we cover some of the metrics that support this interpretation: peak, peak-to-peak, RMS and crest factor, which usually complements the first three.

What is Peak Value in vibration analysis?

This is the value of the largest amplitude present in the signal, also called “True peak value” in many platforms. Its increase may indicate the appearance of impacts in the signal.

The peak value represents a point value and does not take into account the total signal energy, that is, any fluctuation or transient vibration of higher value can influence this metric.

In the example below, the bearing housing of a discharge pulley of a conveyor belt presents vibration amplitudes close to 1g, with the axial axis presenting higher values.

When we look closely, we can see that the peak of this signal of duration 4s corresponds to 1.1194g at time 2.52s. Note that in this case no other parts of the signal matter, since the peak value is only the highest amplitude value.

What is Peak-to-Peak value?

Another complementary metric is the Peak to Peak value. This value is the measurement from the minimum peak to the maximum peak of the signal.  Like the peak value, its increase can indicate the emergence of impacts.

Similarly, the Peak to Peak amplitude does not provide any information about the amount of energy within the vibration signal, it only provides information about the highest and lowest values.

Following in the previous example, it is possible to observe that the largest value is 1.1194g, while the smallest value corresponds to -0.6306g, occurring at time 2.87s of the signal. By calculating the distance from one point to the other we obtain a Peak to Peak value of 1.75g for the axial axis. 

Once again, see that the rest of the signal does not matter. A clearer example of this is the following signal where we can see even more significant impacts. Notice how these impacts break away from the rest of the signal and provide a final Peak to Peak value of:

1,4099 g + 0,9965 = 2,4064 g

Joel Nunes, expert in vibration analysis at Dynamox, comments that the peak to peak metric is essential for monitoring low rotation equipment.

Assets of this type generally have low vibration amplitudes / low energy values, which hinders the use of metrics such as RMS (we’ll discuss more about it below) and favors the use of metrics such as peak or peak to peak.

Peak and peak-to-peak are point metrics that represent impacts well. To enrich the analysis, however, you also need a metric that better represents the total signal energy. That’s where RMS comes in.

However, it is necessary to enrich any analysis by also looking at a metric that better represents the total energy, that is, the total signal content. An excellent metric for this is the RMS value.

What is RMS in Vibration Analysis?

RMS (Root Mean Square): Effective value of the signal. It is calculated based on the entire sample using the following formula:

More practically, it is a measure of the vibratory energy of the equipment. 

Unlike the Peak and Peak to Peak values, the RMS value is not a point value, but rather a representation of the total signal energy. 

As a failure progresses in a monitored asset, the RMS value tends to evolve. This happens because the number of peaks increases, thus impacting the total signal energy. 

In general, the RMS value shows little change in the early stages of mechanical failures, because there is little change in the total signal energy. However, as the failures worsen, the RMS value tends to increase more significantly.

Most vibration analysis software already displays the calculated RMS value for user interpretation.

RMS in Vibration Analysis: A Practical Example

Let’s look at an example of a ball mill gearbox. This gearbox has an error in the transmission of power and movement of the external gear, crown and pinion.

Notice in the upper left corner that both the Peak to Peak (PP) and the RMS value are shown by the software. The difference between them is significant precisely because the RMS takes into account the entire signal.

As the failure gets worse, this value will tend to increase, indicating a higher level of vibration at the monitored point, with more impacts and higher amplitudes.

Look at the case of this same gearbox a few months later (the same scale was used for both graphs plotted in the velocity domain):

A signal trend graph with well-adjusted limits can be useful in tracking the evolution and worsening of this and other types of failure. 

How RMS is used in Vibration Analysis

Below is a graph with RMS values in velocity for the horizontal axis of the equipment.

The point highlighted in green corresponds to the first waveform shown above, with a still controlled RMS value. The red point corresponds to the last waveform, with higher RMS values, according to the evolution of the reported defect.

See that for this second case, alarm levels A2 (most critical level) had already been reached a few months ago, pointing to this evolution.

According to Joel Nunes, one of the benefits of RMS in vibration analysis is how stable and predictable this metric tends to be.

RMS is less affected by outliers than peak and peak-to-peak, so it usually follows a more consistent trend that is easier to track.

In addition, standards such as ISO 10816/20816 usually specify severity limits in RMS, which reinforces its use as a tracking reference.

Two other vibration metrics are kurtosis and skewness. These statistical parameters are important because they qualitatively describe the shape of a vibration signal.

Since a digital signal is made up of discrete points in time, any vibration signal can be treated as a statistical distribution of acceleration, velocity or displacement points.

Peak, Peak-to-Peak and RMS: comparing the three metrics

What is crest factor and when does it complement Peak and RMS?

Crest factor is the ratio between the peak value and the RMS value of the same signal.

It fills the gap between these two metrics: while RMS measures total energy and peak measures the single highest value, crest factor shows how “impulsive” the signal is, meaning how much the peaks stand out from the average energy of the rest of the signal.

In bearings, this behavior usually follows a characteristic pattern. In the early stages of a failure, impacts are still rare and isolated, so the peak grows faster than RMS and the crest factor rises.

As the failure progresses and impacts become more frequent, the total signal energy (RMS) starts to keep pace with the peak. The crest factor then tends to level off or even drop, even though the signal is more degraded.

That’s why crest factor works as a complementary early-stage failure indicator, especially in bearings, when used alongside peak, peak-to-peak and RMS.

What is the spectrum in Vibration Analysis?

The spectrum is the full range of frequencies, from the lowest-frequency signal to the highest.

Spectral analysis examines a signal within this range, continuously monitoring the spectrum to capture transient effects, low-power signals and other sources of interference, helping identify potential failures.

Peak, peak-to-peak and RMS are the starting point of the analysis. The next step is recognizing in the spectrum what is driving those amplitudes.

The Illustrated Guide – Vibration Analysis shows exactly that, with visual cases of real mechanical and electrical failures. Download it for free:

Frequently Asked Questions About Peak, Peak-to-Peak and RMS