The same proximity probes that draw a shaft orbit also carry a slow, patient signal that most people overlook: a DC component that says where the shaft is sitting on average inside the bearing. A shaft centerline position plot uses that DC gap information from the X and Y probes to track the average location of the journal within the bearing clearance as load and speed change. Watching that resting position move is one of the clearest ways to spot bearing wear, a wipe, or a preload problem. This guide explains what the DC gap means, how to read the centerline, and how it differs from the AC vibration on the same probe.
Shaft Centerline Plot in one line: A shaft centerline position plot shows the average position of the shaft journal within the bearing clearance, derived from the slowly changing DC component of the X and Y proximity-probe signals. As load and speed change, the journal rides up or across on its oil film, and the plot traces where the centerline settles. A shift in that resting position over time flags bearing wear, a wipe, or a preload problem that a vibration reading alone would not reveal.
A proximity probe produces a voltage proportional to the gap between its tip and the shaft surface, and that voltage has two parts. The rapidly changing AC part is the vibration, the wiggle of the shaft as it moves cycle by cycle. The slowly changing DC part, often called the gap voltage, represents the average distance from the probe to the shaft, which is to say the average position of the shaft along that probe's line of sight. That DC average is usually treated as noise in a vibration measurement, but it carries genuinely useful information about where the shaft is sitting.
Combine the DC gap from the horizontal probe with the DC gap from the vertical probe and you get the average position of the shaft centerline in two dimensions within the bearing clearance. Plotting that position, and watching how it moves as the machine's load and speed change, is the shaft centerline position plot. In a fluid-film journal bearing the shaft does not sit at the center of the clearance; it rides on a wedge of oil, resting low and to one side when stopped and climbing up and over as speed and the oil film build. The centerline plot makes that behavior visible.
Because this is a slow-trending measurement rather than a per-cycle one, the centerline position changes over minutes, hours, and the life of the bearing rather than within a single revolution. It answers a different question from the orbit: not how the shaft is moving right now, but where it has come to rest on average. Reading the two together is powerful, since the orbit shows the dynamic motion and the centerline shows the static seat the orbit is drawn around.
A healthy journal bearing establishes a predictable resting position for a given load and speed, with the shaft riding up the oil wedge to a stable point. When the babbitt of the bearing wears, the clearance grows and the shaft's resting position drops or shifts, so a centerline that has migrated from its established position over time is a strong indicator of wear. A bearing wipe, where the shaft has briefly touched and smeared the babbitt, can show up as an abrupt change in resting position because the geometry of the bearing surface has changed.
Preload problems also register on the centerline. If a bearing is assembled or has moved so that it forces the shaft into an unusual part of the clearance, the resting position sits somewhere it should not for the given operating condition, and the plot reveals that offset. The angle between the direction of the applied load and the line from the bearing center to the shaft center is the attitude angle, and it relates directly to the stability of the oil film. A journal riding at an attitude and eccentricity typical of a well-loaded bearing is generally stable, while a shaft that has climbed toward the center of the clearance has a light effective load and is more prone to oil-film instability such as oil whirl.
This is why the centerline plot is valued for early detection: the shifts it shows often precede the point at which vibration alarms would fire. A slowly migrating centerline can warn of developing wear or a changing preload while the orbit and overall levels still look acceptable, because the average position moves before the dynamic motion becomes obviously abnormal. Reading eccentricity and attitude angle from the centerline gives an analyst a window into oil-film condition and stability margin that the vibration signal alone does not provide.
The single most important discipline with this measurement is keeping the DC centerline and the AC vibration distinct, because they come from the same probe but mean opposite things in terms of timescale. The AC vibration is the fast component that feeds overall levels, orbits, and running-speed vectors, and it is what triggers vibration alarms. The DC gap is the slow component that feeds the centerline position, and it is trended over long periods to watch for migration. Mixing them, or letting a system report only one, throws away half of what the probe can tell you.
A cloud SCADA or monitoring historian is well suited to the centerline because the whole value of the measurement is in the long trend. Merobix can log the DC gap values from each proximity probe as their own slow-trending points, separate from the AC vibration channels, so the average shaft position is recorded continuously and its migration over weeks and months is visible at a glance. A resting position that has drifted from its baseline raises attention for a reliability engineer even when the vibration itself is still within limits, which is exactly the early warning the centerline is prized for.
For remote and unmanned sites this separation pays off, because a reviewer sitting far from the machine can see both stories side by side: the vibration trend that governs the immediate protection alarms and the centerline trend that reveals slow bearing changes. When a vibration event does occur, the historized centerline gives context, showing whether the shaft's resting position had already been migrating beforehand. Keeping the DC centerline as its own set of monitored points, alongside but distinct from the AC vibration, is what lets a monitoring system use everything the proximity probe is producing rather than only the fast half.
Both come from the same proximity probe. The AC component is the fast, cycle-by-cycle motion of the shaft and is what feeds vibration levels, orbits, and running-speed vectors. The DC component, or gap voltage, is the slowly changing average distance to the shaft, which represents where the shaft is resting on average in the clearance. The AC part drives vibration alarms; the DC part is trended over long periods to track the shaft centerline position.
A healthy journal bearing gives a predictable resting position for a given load and speed. When the bearing wears, its clearance grows and the shaft's average position drops or shifts. A wipe changes the bearing surface geometry and can move the position abruptly, and a preload problem forces the shaft to an unusual part of the clearance. Because these shifts often appear before vibration alarms would fire, a migrating centerline is an early warning of developing bearing trouble.
The centerline position tells you the shaft's eccentricity and attitude angle within the clearance, which relate to oil-film stability. A well-loaded journal rides at a fairly high eccentricity toward one side and tends to be stable. A shaft that has climbed toward the center of the clearance has a light effective load and a lower stability margin, making it more susceptible to oil whirl. So the centerline gives an indication of how close a bearing is to that kind of instability.
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