Most vibration data plots, including: Timebase, Orbit, Polar, and Bode, present dynamic vibration data. For seismic transducers, this dynamic data is inertially referenced to ground. For the case of the shaft relative proximity probe, the dynamic vibration occurs about a –dc gap voltage, with the –dc gap voltage being proportional to the average distance from the probe tip to the target (shaft). These data plots display dynamic vibration data, but do not show changes in the average shaft radial position, an important response characteristic of the rotor system.
When a rotor system with hydrodynamic bearings changes speed or load, the stiffness and damping characteristics of the bearings are also modified. As a result, changes in the average radial position of the shaft will also be observed. Primary and secondary machinery malfunctions such as misalignment, fluid-induced instability, and rubs, to name a few, can produce significant changes in the rotor’s radial position within the bearings and / or seals. These changes in radial shaft position can be directly observed via the average shaft centerline data plot. Correlation of shaft average centerline data with other vibration and process data provides a much more complete understanding of the total response of the rotor system. Hence, it is important to recognize that changes in shaft radial position are just as important an indicator of machinery health as changes in the dynamic motion (vibration) of the rotor and must not be overlooked during the analysis.
The Average Shaft Centerline data plot is designed to show changes in the average radial position of the shaft; thus, the plot is effectively low-pass filtered and does not display dynamic (vibration) data. However, when the information contained within the average shaft centerline plot is combined with other information such as known bearing and seal clearances, shaft orbit data, process information, etc. a detailed picture of shaft motion relative to bearing and seal clearances and / or radial loading is obtained.
The Average Shaft Centerline plot is most often used to display changes in shaft position versus speed, but it is also used to display changes in shaft position versus time, so the changes can be correlated to changing operating conditions, i.e. load.
There are two pieces of information always available from the output of a proximity probe transducer. First is the “ac” or more correctly stated, the instantaneously varying –dc voltage which is proportional to the dynamic motion of the shaft centerline, i.e. vibration. Vibration data plots, i.e. orbit, bode, polar, spectrum, timebase, etc. are constructed from this information. (Refer to the Note below.)
The second piece of information from the proximity probe is the average –dc value referred to as the probe gap voltage. The –dc gap voltage is directly proportional to the average distance from the probe tip to the target (shaft). The average position information is contained in the -dc gap voltage of the transducers over the linear range for which they operate. The Average Shaft Centerline plot depicts changes in the average radial position of the shaft centerline in two dimensions and is constructed from the -dc gap voltage part of the proximity probe transducer signals.
Industry standard transducers (5mm and 8mm diameter transducers with a scale factor of 200 mv/mil), are linear over a range from approximately -4 to -20 volts dc with a corresponding physical gap of 10 mils to 90 mils from the probe tip to the target (shaft). If the gap voltages from two orthogonal transducers are low-pass filtered (to only pass the dc component), the results represent the average radial position of the shaft in the plane of the transducers.
This average position information from two, orthogonal, coplanar transducers is combined to produce a point on a XY (Cartesian) plot. The point represents the shaft at a particular location inside the bearing clearance. When a set of this data is collected versus time or speed, the plot becomes an Average Shaft Centerline plot. The plot has equal scaling in both horizontal and vertical directions and is square in shape. Shaft rotation and plot orientation references are indicated at the top of the plot. For a horizontally oriented machine, “Up” is normally used as the orientation reference to free space. The direction of shaft rotation is referenced to the established view of the machine train, typically from “driver” to “driven”. The data may be plotted vs. rotative speed, time, eccentricity ratio, or sample number. If the dimensions of a nearby bearing or seal are known, then a clearance circle can be added to the Average Shaft Centerline Data Plot. This circle (or ellipse) then represents the available diametral clearance in either the bearing or seal.
The position of the shaft within the clearance circle is often expressed as the eccentricity ratio. The eccentricity ratio is a dimensionless quantity representing the average radial position of the shaft within the bearing (or seal). The eccentricity ratio is obtained by measuring the distance between the centerline position of the shaft and the center of the bearing (or seal), divided by the radial clearance. Consequently, the eccentricity ratio can vary from a value of 0 (shaft centered in the bearing) to 1 (shaft touching the bearing or seal wall). Eccentricity ratio has a direct effect on stiffness, damping, and stability.
If the measurement plane is close to the bearing or seal, and the clearance circle is accurately depicted, then the Average Shaft Centerline Data plot is representative of the actual shaft average radial position and provides a good indicator of approaching or exceeding the limit of the available clearance. If the measurement plane is some distance away from the bearing or seal, shaft deflection and mode shape can affect the apparent shaft radial position and must be taken into consideration.
The instantaneously varying –dc voltage (vibration) occurs about the –dc gap value. The vibration data is depicted by the shaft Orbit, while the change in shaft radial position is depicted by the Average Shaft Centerline Data. The combination of the Orbit and the Average Shaft Centerline Data Plot in one data plot format is called the DC Orbit Data Plot.
The vibration data is obtained from the “ac” portion of the proximity probe signal, while the Average Shaft Centerline Data is obtained from the –dc gap voltage portion of the proximity probe signal.
Changes in radial shaft position can occur with or without changes in the observed vibration response. Consequently, changes in shaft radial position are important to document in order to properly understand the total response of the rotor system.
Note: The dynamic (vibration) signal from a proximity transducer is commonly referred to as the “ac” component of the signal from the proximity probe. In reality, it is not an ac signal since the polarity of the voltage does not fluctuate between + and – values. The dynamic signal from a proximity transducer is a varying, –dc signal.
Fundamentals of Rotating Machinery Diagnostics; Bently, Donald E.; Hatch Charles T.; Chapters 6 and 7; 2001; ISBN 0-9714081-0-6
Machinery Malfunction Diagnostics and Correction; Eisenmann Sr., Robert C.; Eisenmann Jr., Robert C.; pp 410-415, 418, 578, 752; Chapters 9, 11, 14; 1998; ISBN 0-13-240946-1
 Thomas, G. Richard; NO VIBRATION – NO PROBLEM: OR IS THERE?; The 24th Canadian Machinery Vibration Association Machinery Vibration Seminar and Annual General Meeting; 25 – 27 October 2006; Montreal, Quebec, Canada