An orbit shows how a shaft vibrates, but it says nothing about where the shaft is actually sitting inside its bearing. A shaft centerline plot answers that different question by using the steady, averaged part of the same proximity-probe signals to map the shaft's resting position within the bearing clearance, and how that position moves as load and speed change. Because the shaft's rest position is set by the oil film supporting it, the centerline plot reveals things the vibrating orbit cannot: preload, bearing wear, and the condition of the oil film. This guide explains how averaged DC gap builds the plot and what its position and movement mean.
Shaft centerline plot in one line: A shaft centerline plot maps the average resting position of a shaft within its bearing clearance, built from the slowly varying DC gap voltages of two proximity probes rather than their vibrating AC signals. As the shaft rides up on its oil film with speed and shifts under load, the plot shows where its centerline sits inside the clearance circle. Because that position is set by the supporting oil film, the centerline plot reveals preload, bearing wear, and oil-film condition, which the AC orbit data alone cannot show since the orbit only depicts vibration around the rest position.
A proximity probe's signal has two parts. The rapidly changing AC part is the vibration, the small oscillation of the shaft that an orbit plots. The slowly changing DC part is the average gap between the probe and the shaft, and that average gap tells you the mean position of the shaft relative to the probe. The orbit throws the DC part away and keeps only the vibration; the centerline plot does the opposite, keeping the averaged DC gap from each probe to find where the shaft is sitting rather than how it is shaking.
With the two orthogonal probes on a bearing, the averaged DC gap from each gives one coordinate of the shaft's rest position, and together they locate the shaft centerline as a single point within the bearing clearance. Plotting that point against the outline of the available clearance shows how much of the clearance the shaft has taken up and in which direction it has moved. This is a fundamentally different measurement from the orbit: the orbit is the little loop the shaft traces, while the centerline point is the location of the middle of that loop within the bearing.
The plot becomes powerful when the shaft position is tracked across changing conditions. At rest the shaft sits at the bottom of the bearing. As it comes up to speed, the oil film builds and lifts and pushes the shaft up and to one side, so the centerline moves to a new position, and it moves again as load changes. By recording the centerline point at different speeds and loads, the plot traces the path the shaft's rest position takes, and that path, compared to what a healthy bearing should do, is the diagnostic.
The shaft's rest position within the clearance reflects the oil film that is holding it up, so the centerline plot is really a window onto the oil film. As speed rises, a healthy journal bearing lifts the shaft along a predictable arc to a position offset from bottom center at an attitude angle, sitting comfortably within its clearance on a well-formed film. When the observed position and the path taken to reach it match that expectation, the film is doing its job. Departures from the expected position and path are what flag trouble.
Preload shows up as a shaft that sits pushed toward one side of the clearance more than the operating conditions should cause. An external force, a misalignment, or a distorted bearing bore pressing the shaft off its natural position moves the centerline to an unexpected location, and the centerline plot makes that displacement visible as a resting point that is not where a free, well-aligned rotor would sit. Because preload changes the rest position rather than the vibration amplitude, it is exactly the kind of condition the centerline plot catches that an orbit might not.
Wear and oil-film problems also read directly from the centerline position over time. As a bearing wears, its clearance grows and the shaft can settle lower or move differently than it did when new, and a centerline plot compared against an earlier baseline shows that drift. A shaft that fails to lift as expected with speed suggests a poor or breaking-down film, while a shaft riding unusually high or in an odd position can indicate too much clearance or a film problem. Tracking where the shaft rests, and how that has changed, is how the centerline plot diagnoses the slow degradation of the bearing and its film.
The orbit and the centerline are complementary because they use the two halves of the same signal and answer different questions. The orbit uses the AC part and shows how the shaft vibrates around wherever it happens to be sitting; it is blind to the rest position because it discards the DC gap. The centerline uses the DC part and shows where the shaft is sitting; it is blind to the vibration. A machine can have a perfectly acceptable orbit while its shaft is slowly sinking through a wearing bearing, and only the centerline plot would show it. That is why serious journal-bearing monitoring watches both.
The conditions the centerline reveals are also the ones that develop slowly and quietly. Bearing wear, growing clearance, creeping preload, and a gradually degrading oil film do not necessarily produce dramatic vibration until late, but they do move the shaft's rest position steadily. A vibration-only view can look stable while these long-term problems advance, so the centerline plot's ability to show a slow drift in position is its unique contribution. It catches the deterioration that amplitude and orbit data cannot, before it turns into a vibration problem or a bearing failure.
For remote monitoring, the centerline is well suited to a cloud platform because it is fundamentally a trend of position over time. A system that captures the averaged DC gap from the proximity probes can log the shaft centerline continuously, and a platform such as Merobix can trend how that position shifts across startups, load changes, and months of running. An operator can then see the shaft's rest position drifting toward a bearing edge, or failing to lift as it once did, and recognize wear or a film problem developing long before it forces an outage. Because it is a slow-moving positional trend rather than a fast waveform, the centerline sits naturally alongside the other trended measurements a monitoring platform keeps for the machine.
They use the two parts of the same proximity-probe signals to answer different questions. The orbit uses the fast AC part to show how the shaft vibrates around its position, while the centerline plot uses the slow averaged DC gap to show where the shaft is resting within the bearing clearance. The orbit is blind to the rest position because it discards the DC gap, and the centerline is blind to the vibration. A machine can have an acceptable orbit while its shaft is slowly sinking through a worn bearing, which only the centerline plot reveals.
It reveals the shaft's resting position within the bearing clearance and how that position moves with speed and load, which reflects the oil film supporting the shaft. From that it shows preload, when the shaft sits pushed to one side more than conditions warrant, bearing wear, when growing clearance lets the shaft settle differently than when new, and oil-film condition, when the shaft fails to lift as expected or rides in an unusual position. These are slow-developing conditions that vibration data alone can miss.
Each proximity probe's signal has a slowly changing DC part that represents the average gap to the shaft, meaning the shaft's mean position along that probe's axis. Taking the averaged DC gap from two orthogonal probes gives two coordinates that together locate the shaft centerline as a point within the bearing clearance. Plotting that point across different speeds and loads traces the path the shaft's rest position takes, and comparing that path against a healthy baseline is the diagnostic.
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