On a machine with fluid-film bearings, the shaft does not spin perfectly on center. It traces a small path inside the bearing clearance as it turns, and that path is a rich fingerprint of the machine's condition. An orbit plot draws that path by combining the readings of two proximity probes mounted at right angles, so you can literally see the shaft's centerline moving. The shape of the orbit, and timing marks placed on it, let an analyst tell unbalance from misalignment from a rub from an oil-film instability. This guide explains how two probes build an orbit, and how to read its shape and its keyphasor dots.
Orbit plot in one line: A shaft orbit plot is a picture of the path that a shaft's centerline traces inside its journal bearing as it rotates, built by plotting the signals from two proximity probes mounted ninety degrees apart against each other. One probe gives the horizontal motion and the other the vertical motion, and together they draw the two-dimensional orbit the shaft follows. The shape of that orbit, and the once-per-turn keyphasor dots marked on it, let an analyst distinguish faults such as unbalance, misalignment, a rotor rub, and oil whirl in fluid-film-bearing machines.
An orbit needs two proximity probes on the same bearing, mounted at ninety degrees to each other around the shaft, commonly called the X and Y probes. Each probe measures the changing gap to the shaft along its own direction, so as the shaft moves within the bearing clearance, one probe reports the horizontal component of that motion and the other reports the vertical component. Neither probe alone shows the shaft's path; each only sees motion along its single axis. It is the pair, read together, that captures the full two-dimensional movement.
The orbit plot is formed by plotting the two probe signals against each other rather than against time. The horizontal probe's instantaneous reading sets the horizontal position on the plot and the vertical probe's reading sets the vertical position, so at every instant the two together mark one point, and as the shaft turns those points trace out a closed loop. That loop is the orbit: a direct, to-scale picture of where the shaft centerline goes during a revolution, as if you were looking down the shaft and watching its center move around inside the bearing.
Because it shows actual shaft motion inside the bearing, the orbit is a tool for fluid-film bearing machines specifically, where the shaft has room to move within an oil film and proximity probes are already fitted. On a rolling-element machine there is essentially no clearance for the shaft to trace an orbit and no proximity probes to build one from. So the orbit belongs to the world of large turbines and compressors with journal bearings, where seeing the shaft's path directly reveals things that case vibration cannot.
The shape of the orbit is the primary clue. A machine with pure unbalance tends to trace a roughly circular or slightly elliptical orbit, because the once-per-turn force from the heavy spot pushes the shaft around in a smooth loop. When the orbit becomes flattened into a strong ellipse or a figure that is much longer in one direction than the other, it points toward a preload or a stiffness that differs between directions, which is characteristic of misalignment or an external force pushing the shaft to one side. The overall roundness or flatness of the loop is therefore a first-order diagnostic.
More complex shapes carry more specific meaning. A misalignment often produces a figure-eight or banana-shaped orbit, a distinctive form that comes from strong harmonics of running speed adding to the fundamental motion. A rotor rub, where the shaft is contacting something it should not, tends to produce a distorted, flattened, or clipped orbit with sudden truncations where the contact stops the shaft's motion in one region. The orbit's departure from a clean loop, and the particular way it is distorted, narrows down which of these mechanisms is at work.
Oil whirl and related fluid-film instabilities show up in a way that is unmistakable once you know it. Because these instabilities occur at a frequency below running speed, the shaft does not close its loop in one revolution, so the orbit does not repeat as a single stationary loop but appears as a looping, precessing pattern that seems to rotate or drift, often with two loops per keyphasor timing mark rather than one. This subharmonic behavior is a signature that case vibration and simple amplitude readings would miss, and it is one of the strongest reasons to look at the orbit on a journal-bearing machine.
The orbit becomes far more informative once a once-per-turn timing reference, the keyphasor, is added to it. A keyphasor is a pulse generated once every shaft revolution from a mark on the shaft, and on the orbit it places a bright dot at the point the shaft occupies at that instant each turn. Because the dot lands at the same rotational moment every revolution, its position on the orbit is a direct phase reference, and comparing it to the heavy spot or to a change over time tells the analyst where in the rotation the shaft is when it reaches a given part of its path. That phase information is essential for balancing and for interpreting the orbit.
The keyphasor dots also reveal precession and the frequency of the motion. For simple once-per-turn motion there is a single dot on the orbit, because the shaft returns to the same spot each revolution. When a subharmonic instability like oil whirl is present, the shaft takes more than one revolution to close its pattern, so multiple keyphasor dots appear spread around the orbit, and their number tells you the ratio of the whirl frequency to running speed. The direction the dots march around the loop shows whether the precession is in the same direction as rotation or against it, which further separates one fault type from another.
For remote monitoring, orbit data is captured by a system that samples the X and Y probes together along with the keyphasor and reconstructs the orbit, which a cloud platform such as Merobix can present and store alongside the machine's other measurements. Trending how the orbit shape and size change over startups and over time turns a one-off diagnostic snapshot into a history, so an analyst can see an ellipse growing, a figure-eight developing, or whirl dots appearing without standing at the machine. Because the orbit shows the shaft directly, it complements the overall levels and spectra that a monitoring platform trends, giving turbomachinery operators the detailed shaft view they need next to the broad screening numbers.
It is made from two proximity probes mounted ninety degrees apart on the same bearing, one measuring horizontal shaft motion and the other vertical. Their two signals are plotted against each other rather than against time, so at each instant the horizontal reading sets the horizontal position and the vertical reading the vertical position, and as the shaft turns the points trace a closed loop. That loop is the orbit, a direct picture of the path the shaft centerline follows inside the bearing during a revolution.
A roughly circular orbit suggests unbalance, while a strongly flattened ellipse points to misalignment or a directional preload pushing the shaft to one side. A figure-eight or banana shape is characteristic of misalignment, and a distorted or clipped orbit suggests a rotor rub where the shaft is contacting something. A looping, precessing orbit that does not close in one revolution, showing multiple keyphasor dots, indicates a subharmonic instability such as oil whirl. The departure from a clean single loop, and how it is distorted, points to the fault.
The keyphasor dots are bright marks placed on the orbit at the instant a once-per-turn timing pulse occurs each revolution, generated from a reference mark on the shaft. Because a dot lands at the same rotational moment every turn, its position gives a phase reference used for balancing and interpreting the orbit. A single dot means simple once-per-turn motion, while multiple dots spread around the orbit reveal a subharmonic instability, and the direction the dots march shows whether the shaft precesses with or against rotation.
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