An ordinary spectrum tells you what frequencies are present and how strong each one is, but it cannot tell you which way the shaft is whirling at each frequency, and for some faults that direction is the whole diagnosis. A full spectrum plot solves this by using both XY proximity probes together to split each frequency into a forward-whirling and a reverse-whirling part. Reading the balance between those positive and negative frequency lines exposes oil whirl, rubs, and asymmetric stiffness that an amplitude-only spectrum leaves ambiguous. This guide explains how the full spectrum is built from two probes, why precession direction is diagnostic, and how a monitoring system captures the data it needs.
Full Spectrum Plot in one line: A full spectrum plot is a directional frequency spectrum computed from a pair of XY proximity probes rather than from a single probe. Instead of showing only the amount of vibration at each frequency, it separates each frequency into forward precession and reverse precession components, drawn as positive and negative frequency lines. This reveals the direction the shaft is whirling at each frequency, which distinguishes faults such as oil whirl and rubs that an ordinary single-probe half spectrum cannot separate.
An ordinary spectrum, sometimes called a half spectrum, is computed from a single transducer. It shows the amplitude of vibration at each frequency along a positive frequency axis, and it is excellent at answering which frequencies are present and how large they are. What it cannot express is direction, because one probe only measures motion along its own line of sight and has no way to know whether the shaft is circling clockwise or counterclockwise at any given frequency. All directional information is lost the moment you reduce the motion to a single axis.
The full spectrum recovers that direction by using both probes of an XY pair together. Because the two probes are mounted ninety degrees apart, their combined signal describes the shaft's motion in the full plane of the bearing, the same information that draws an orbit. Mathematically, the two probe signals are combined so that each frequency is resolved into two counter-rotating parts: one component whirling in the forward direction, the same sense as shaft rotation, and one whirling in the reverse direction. The full spectrum plots these on an axis that runs both positive and negative.
The result is a spectrum where each frequency can appear on the positive, forward side, the negative, reverse side, or both. A purely circular forward whirl at some frequency shows up as a single line on the forward side, a purely reverse whirl shows on the reverse side, and an elliptical or line motion shows as a pair of forward and reverse lines whose relative sizes describe the shape and tilt of the ellipse. Reading the two sides together is what makes the full spectrum a directional tool rather than just a longer list of frequencies.
Several important faults are defined by the direction of whirl, and this is where the full spectrum earns its keep. Oil whirl, an instability of the oil film in a fluid-film bearing, produces a subsynchronous vibration that whirls in the forward direction, the same sense as rotation, typically at a bit under half running speed. On a full spectrum that shows as a strong forward line on the low-frequency, forward side. Seeing that the subsynchronous component is predominantly forward is a key part of confirming an oil-film instability rather than some other subsynchronous source.
Rubs and asymmetric conditions announce themselves through reverse components. When a shaft rubs against a stationary part, or when the support stiffness differs between two directions, the motion is no longer a clean forward circle and reverse-precession components appear. A significant reverse line at a frequency that would normally be pure forward is a strong hint of a rub or of asymmetric stiffness, because those conditions force the shaft into an elliptical or reversing path. An amplitude-only spectrum would show the frequency but give no clue that its whirl direction has become abnormal.
The general power of the full spectrum is that it separates faults that share a frequency but differ in direction, which a half spectrum simply cannot do. Two machines could show the same line at the same frequency in an ordinary spectrum, yet one is a benign forward response and the other a reverse-precessing rub, and only the directional view tells them apart. This is why the full spectrum is treated as a specialist companion to the ordinary spectrum: the ordinary spectrum finds the frequencies of interest, and the full spectrum resolves the direction that pins down the mechanism.
The full spectrum has one firm prerequisite: it can only be computed where a genuine XY pair of proximity probes exists in the same bearing plane, because the direction information comes from combining the two orthogonal signals. A single probe, however good, can never yield a full spectrum. So a machine that is to benefit from directional analysis has to be instrumented with orthogonal probe pairs, and the monitoring system has to preserve both signals in a way that keeps their timing relationship intact, since the combination depends on the two being sampled together.
A cloud monitoring platform supports this by capturing the raw X and Y waveforms with their phase relationship preserved, so the forward and reverse components can be resolved when a directional view is needed. Merobix keeps the everyday overall levels and running-speed vectors trending continuously for alarming, while retaining the underlying XY data so that a full spectrum can be produced on demand for a machine that is showing a subsynchronous or otherwise puzzling signature. That way the directional tool is available for the deeper investigation without cluttering the routine monitoring an operator watches day to day.
This division of labor suits remote and unmanned installations well. The continuous trends and simple spectra handle the bulk of monitoring and trigger attention when something moves, and when the situation calls for it a reliability engineer can pull the full spectrum from the retained XY data to check whether a subsynchronous component is forward, pointing at oil whirl, or shows a reverse content, pointing at a rub or asymmetry. Having the directional analysis reachable from the same historized data means the specialist view is a click away rather than a site visit away, which is what makes advanced diagnosis practical across a distributed fleet.
A half spectrum is the ordinary spectrum computed from a single probe, showing the amplitude of vibration at each frequency with no direction information. A full spectrum is computed from a pair of XY probes and resolves each frequency into forward-whirling and reverse-whirling components, plotted on an axis that runs positive for forward and negative for reverse. The full spectrum therefore shows the direction of precession at each frequency, which the half spectrum cannot.
Oil whirl is a fluid-film instability that produces a subsynchronous vibration whirling in the forward direction, the same sense as shaft rotation, usually a little under half running speed. On a full spectrum it appears as a strong forward line on the low-frequency side, and confirming that the subsynchronous component is predominantly forward helps distinguish oil whirl from other subsynchronous causes. An amplitude-only spectrum would show the frequency but not reveal that its whirl is forward.
Yes. A full spectrum requires a genuine XY pair of proximity probes mounted ninety degrees apart in the same bearing plane, because the direction information comes from combining the two orthogonal signals with their timing preserved. A single probe cannot produce a full spectrum no matter how it is processed. The monitoring system also has to retain both probe signals together so the forward and reverse components can be resolved when a directional view is needed.
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