Understanding the Shaft Centerline in Vibration Monitoring

Definition: What is the Shaft Centerline Position?

In the context of machinery monitoring with proximity probes, the shaft centerline position is the average, or steady-state, position of the shaft’s geometric center within its fluid-film bearing clearance. While vibration measurements (the AC component of the signal) describe the shaft’s rapid dynamic motion *around* this average position, the shaft centerline measurement (the DC component of the signal) describes *where* this average position is located. Tracking changes in this DC position over time provides critical insights into bearing load, alignment, and long-term wear.

How is the Shaft Centerline Position Measured?

The shaft centerline position is measured using the DC voltage output from a pair of X-Y proximity probes (two probes mounted 90 degrees apart). Here’s how it works:

1. Probe Gap Voltage: Each proximity probe’s driver outputs a negative DC voltage that is directly proportional to the gap between the probe tip and the shaft surface. For example, a common calibration is -200 mV/mil, meaning the voltage becomes more negative as the shaft moves away from the probe.
2. Zeroing the Position: To establish a reference, the DC gap voltages are typically “zeroed” or recorded when the shaft is at rest at the bottom of its bearing.
3. Tracking the Average Position: As the machine starts up and comes to operating speed and temperature, the shaft lifts on its hydrodynamic oil film. The proximity probe system continuously monitors the average DC gap voltages from the X and Y probes.
4. Plotting the Position: By plotting the X and Y DC voltages against each other, the monitoring system can display the shaft’s average position on a 2D graph that represents the bearing clearance.

The Diagnostic Value of a Shaft Centerline Plot

A shaft centerline plot, which shows the path of the average shaft position as the machine’s speed or load changes, is a powerful diagnostic tool for turbomachinery.

1. Confirming Normal Bearing Operation

On startup, a healthy rotor in a fluid-film bearing will rise and move horizontally as the hydrodynamic oil wedge develops. The path it takes on the centerline plot should be smooth and repeatable every time the machine starts. This plot confirms that the bearings are generating the proper lift and that the rotor is positioned correctly.

2. Diagnosing Bearing Wear

Over time, as a bearing wears, the shaft will gradually settle lower and lower in its clearance. By overlaying today’s shaft centerline position with the position from a year ago, an analyst can clearly see this trend and predict when the bearing will need to be replaced, long before it starts causing high vibration.

3. Detecting Changes in Alignment or Load

The shaft’s position in its bearing is determined by the forces acting on it. If a machine’s alignment changes (for example, due to thermal growth or pipe strain), the forces on the bearings will change, which will, in turn, cause the shaft centerline position to shift. A sudden change in the centerline position during steady-state operation is a strong indicator of a significant change in the forces acting on the rotor and warrants immediate investigation.

4. Identifying Bearing Instabilities

Under certain conditions, the shaft may not settle into a stable position and can begin to precess or “whip” within the bearing. This condition, known as oil whirl or whip, is visible as a large, unstable movement on the centerline plot.

Centerline Position vs. Orbit

It is important to distinguish between the two plots derived from proximity probes:

• The Shaft Centerline Plot uses the DC voltage to show the *average* position of the shaft. It is used to analyze slow changes over time (trends) or during startup/shutdown.
• The Shaft Orbit Plot uses the AC voltage to show the *dynamic motion* of the shaft around its average centerline position. It is used to diagnose specific faults like unbalance and misalignment.

Together, these two plots provide a complete and detailed picture of the health and behavior of the rotor within its bearings.

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