A normal vibration spectrum comes from one probe looking at motion in a single direction, and it tells you how much vibration exists at each frequency but nothing about which way the shaft is orbiting. A full spectrum fuses the signals from two probes mounted at right angles into a directional spectrum that splits each frequency into a forward and a reverse component. That extra dimension, the direction the shaft precesses, carries diagnostic information that an ordinary half-spectrum simply throws away. It is what lets an analyst separate faults that would otherwise look identical.
Full spectrum in one line: A full spectrum is a directional vibration spectrum built by mathematically combining the signals from two orthogonal proximity probes into forward-precession and reverse-precession components at each frequency. Unlike a normal half-spectrum, which shows only amplitude versus frequency in one direction, a full spectrum reveals the direction the shaft is orbiting at each frequency, which sharpens the diagnosis of misalignment, rubs, and fluid-induced instabilities.
Turbomachinery shafts are commonly watched by a pair of proximity probes mounted ninety degrees apart, an X probe and a Y probe, that together measure how the shaft center moves in its bearing. Each probe on its own produces an ordinary spectrum describing motion along its own axis. A conventional analysis treats these separately or looks at just one, which is why it is sometimes called a half-spectrum: it captures amplitude at each frequency but discards the relationship between the two probe signals that encodes direction.
A full spectrum keeps that relationship. By combining the X and Y signals with their correct phase relationship, the math splits the vibration at each frequency into two parts: energy that is orbiting in the same direction as shaft rotation, called forward precession, and energy orbiting against rotation, called reverse precession. The result is a spectrum with a center line for zero and frequency lines extending both to the right for forward components and to the left for reverse components, so every frequency now carries a direction as well as an amplitude.
That direction is real physical information about the shape and sense of the shaft's orbit at each frequency, not a display trick. A perfectly circular forward orbit shows up as pure forward energy, a flattened or reversing orbit shows up as a mix of forward and reverse energy, and the ratio between them describes how elliptical or how backward the motion is. Because different faults distort the orbit in characteristic ways, reading forward and reverse content separately gives the analyst a lever that a single-direction spectrum does not offer.
Misalignment is a classic case. It tends to constrain the shaft into a flattened, elliptical orbit rather than a clean circle, and an ellipse contains reverse-precession energy alongside the forward energy. On a full spectrum, a two-times running-speed component with significant reverse content is a stronger pointer toward misalignment or a preload than the same amplitude would be on a half-spectrum, because the ordinary spectrum shows only that there is energy at two times, not that the orbit is squashed. The direction content adds evidence the plain amplitude cannot.
Rubs and fluid instabilities each leave directional fingerprints too. A subsynchronous fluid-film instability such as oil whirl is strongly forward, so a forward-dominated component sitting below running speed reinforces the diagnosis of a fluid instability rather than something else living at a similar frequency. A rub, by contrast, can throw energy into reverse-precession components and fractional frequencies as the shaft contacts the stationary part and gets kicked backward, so reverse content at frequencies where you would not otherwise expect it is a clue that something is touching that should not be.
The general value is separation. Two faults can produce peaks at the same frequency and the same amplitude on a half-spectrum and be impossible to tell apart, yet differ sharply in how they orbit the shaft. The full spectrum resolves that ambiguity by showing the forward-to-reverse balance, turning a single ambiguous peak into two distinguishable signatures. This is why full spectrum is a favored tool for the machines where getting the diagnosis right matters most, the large turbines, compressors, and pumps carried on fluid-film bearings and watched by XY probes.
The prerequisite for a full spectrum is a genuine pair of orthogonal probe signals captured together with their timing preserved, because the direction information lives entirely in the phase relationship between X and Y. If the two channels are sampled at slightly different times or their phase is not kept aligned, the forward and reverse split is corrupted and the directional content becomes meaningless. This makes synchronized multi-channel acquisition, not just single-channel logging, the real requirement behind the technique.
That requirement is a good fit for permanent instrumentation on critical machines, where XY proximity probes are already installed at each bearing and wired back to a monitoring rack. A platform that acquires the X and Y channels for a bearing simultaneously and preserves their relative timing can compute the full spectrum on demand and store both the raw orbit and the directional spectrum. Merobix brings those paired probe signals into a browser-accessible history with their timing intact, so the forward and reverse content can be reconstructed later rather than only viewed live at the machine.
Keeping the directional data in a continuous cloud history changes how the diagnosis is used. Instead of an analyst having to be at the console during an event, a developing rub or a growing instability can be examined after the fact by pulling up the full spectrum from stored XY data, and today's forward-reverse balance can be compared against a healthy baseline captured months earlier. For remote turbomachinery where dispatching a specialist is slow and expensive, having the paired-probe record already sitting in the history is what makes directional diagnosis a routine check rather than an emergency response.
A half-spectrum is an ordinary single-direction spectrum showing amplitude against frequency from one probe. A full spectrum combines two orthogonal probes to split each frequency into forward-precession and reverse-precession components, adding the direction the shaft is orbiting. The full spectrum therefore carries directional information that a half-spectrum discards, which helps distinguish faults that look identical on amplitude alone.
Reverse precession is vibration energy where the shaft orbit moves against the direction of shaft rotation at that frequency. Its presence usually signals that the orbit is elliptical or distorted rather than a clean forward circle, which points toward conditions like misalignment, preload, or a rub. A large reverse component at a given frequency is a clue the orbit is being constrained or disturbed in a particular way.
You need two vibration sensors mounted orthogonally and sampled together, and on turbomachinery those are typically XY proximity probes watching the shaft. In principle any pair of orthogonal transducers with preserved relative timing can be combined, but proximity probes are the usual source because they measure shaft motion directly at fluid-film bearings. A single sensor cannot produce a full spectrum, since the direction information only exists in the relationship between two orthogonal channels.
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