{"id":233,"date":"2025-10-31T00:25:51","date_gmt":"2025-10-31T00:25:51","guid":{"rendered":"https:\/\/vibromera.eu\/glossary\/shaft-centerline\/"},"modified":"2026-05-23T22:39:14","modified_gmt":"2026-05-23T22:39:14","slug":"shaft-centerline","status":"publish","type":"glossary","link":"https:\/\/vibromera.eu\/tt\/glossary\/shaft-centerline\/","title":{"rendered":"What is the Shaft Centerline in Vibration Monitoring?"},"content":{"rendered":"

Understanding the Shaft Centerline in Vibration Monitoring<\/h1>\n\t\t
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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tOptical Sensor (Laser Tachometer)<\/a>\n\n\t\t\t\t\t\t\t\t<\/h4>\n\n\t\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t
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€<\/span>1,735.00<\/span> + VAT (if applicable)<\/h4><\/div>\n\t\t\t\t\t\t\t\t\t
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In machinery monitoring with proximity probes<\/a>, the shaft centerline position<\/strong> is the average, or steady-state, position of the shaft’s geometric centre within its fluid-film bearing clearance. While a vibration<\/a> measurement \u2014 the AC component of the signal \u2014 describes the shaft’s rapid dynamic motion around<\/em> that average position, the centerline measurement \u2014 the DC component \u2014 describes where<\/em> the average position actually sits inside the bearing. Tracking how that DC position moves over time yields some of the most valuable insight available into bearing load, alignment, and long-term wear, all of which an orbit plot<\/a> alone would miss.<\/p>\n

1. Why the Centerline Position Matters<\/h2>\n

A rotor in a journal bearing<\/a> does not sit at the centre of its bore. At rest it lies at the bottom of the clearance; once running, it climbs the hydrodynamic oil wedge and settles at an offset, eccentric position governed by speed, load, oil viscosity, and the forces fed in through the coupling. That equilibrium position is a direct, physical readout of the steady forces acting on the rotor. Vibration tells you how violently the shaft is shaking; the centerline tells you where it is being pushed. The two answer fundamentally different questions, and a complete diagnosis of a fluid-film machine needs both.<\/p>\n

2. How the Shaft Centerline Position Is Measured<\/h2>\n

The position is derived from the DC voltage output of a pair of X\u2013Y proximity probes \u2014 two probes mounted 90 degrees apart at the same axial plane. The process runs as follows:<\/p>\n

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  1. Probe gap voltage:<\/strong> each proximity probe’s driver outputs a negative DC voltage directly proportional to the gap between the probe tip and the shaft surface. A common calibration is \u2212200 mV\/mil, so the voltage grows more negative as the shaft moves away from the probe. Setting and checking that bias gap correctly is a routine commissioning step, and our Proximity Probe Gap Voltage Calculator<\/a> makes the conversion straightforward.<\/li>\n
  2. Zeroing the position:<\/strong> to establish a reference, the DC gap voltages are typically zeroed or recorded with the shaft at rest at the bottom of its bearing.<\/li>\n
  3. Tracking the average position:<\/strong> as the machine starts up and reaches operating speed and temperature, the shaft lifts on its hydrodynamic oil film. The system continuously monitors the average DC gap voltages from the X and Y probes.<\/li>\n
  4. Plotting the position:<\/strong> by plotting the X and Y DC voltages against each other, the monitoring system displays the shaft’s average position on a 2D graph that represents the full bearing clearance.<\/li>\n<\/ol>\n

    Because the measurement relies on the DC content of an eddy-current<\/a> signal, it demands a permanently installed probe pair and a monitor capable of resolving the slow DC trend \u2014 not a portable, AC-coupled displacement<\/a> reading. This is why centerline monitoring is a feature of installed turbomachinery protection systems rather than of walk-around routes.<\/p>\n

    3. The Diagnostic Value of a Shaft Centerline Plot<\/h2>\n

    A shaft centerline plot<\/strong> shows the path of the average shaft position as the machine’s speed or load changes. On turbomachinery it is a powerful diagnostic tool, and it reveals several conditions that vibration data alone cannot.<\/p>\n

    1. Confirming normal bearing operation<\/h3>\n

    On startup, a healthy rotor in a fluid-film bearing rises and moves sideways as the hydrodynamic oil wedge develops \u2014 the result of the bearing’s geometry and rotation direction. The path it traces on the centerline plot should be smooth and repeatable every time the machine starts. A consistent path confirms that the bearings are generating proper lift and that the rotor is correctly positioned within its clearance<\/a>.<\/p>\n

    2. Diagnosing bearing wear<\/h3>\n

    As a bearing wears, the shaft gradually settles lower and lower within its clearance. By overlaying today’s centerline position on the position recorded a year ago, an analyst can see the trend plainly and predict when the bearing will need replacement \u2014 long before bearing wear<\/a> begins to produce high vibration. The centerline is, in effect, an early-warning channel that leads the vibration trend.<\/p>\n

    3. Detecting changes in alignment or load<\/h3>\n

    The shaft’s position is set by the forces acting on it. If a machine’s alignment changes \u2014 through thermal growth, pipe strain, or a settling foundation \u2014 the bearing forces change, and the centerline position shifts in response. A sudden change in centerline position during otherwise steady-state operation is a strong sign of a significant change in the forces on the rotor and warrants immediate investigation. It is one of the clearest field indicators of developing misalignment<\/a> or a thermal bow<\/a>, and a useful cross-check on foundation<\/a> condition.<\/p>\n

    4. Identifying bearing instabilities<\/h3>\n

    Under certain conditions the shaft never settles into a stable position and instead begins to precess, or whip, within the bearing. This condition \u2014 oil whirl<\/a> or oil whip, a form of rotor instability<\/a> \u2014 shows up as a large, unstable excursion on the centerline plot, distinct from the tidy, repeatable path of a healthy machine. Read alongside the whirl<\/a> signature in the spectrum, it confirms a self-excited problem rather than a forced one.<\/p>\n

    4. Centerline Position Versus Orbit<\/h2>\n

    It is essential to distinguish the two plots that a single pair of proximity probes can produce, because they are read in completely different ways:<\/p>\n