# Vibration Analysis of Large Rotating Machinery

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### WHEN USED TOGETHER, SHAFT ORBIT AND CENTERLINE ANALYSIS CAN PINPOINT THE ROOT CAUSE OF MALFUNCTIONS

For vibration analysis of large rotating machinery, shaft orbit and centerline analysis are useful diagnostic techniques for the specialist’s toolbox. Especially when used together, they can yield diagnostic information for evaluating machinery condition and pinpointing the root cause of malfunctions.

Shaft orbit and centerline analysis apply to equipment such as turbines, generators, motors, pumps, compressors and fans with fluid-film radial bearings. By design, radial bearings allow for a nominal amount of shaft movement within the bearing clearance.

Shaft vibration measurements relative to the bearing are made possible by pairs of radial- mounted, non-contact proximity transducers installed in an X-Y configuration 90 degrees apart at each bearing location (Figure 1). Because the frequency range of proximity transducers starts at DC (0 Hz), the information from each X-Y probe pair enables two-dimensional views of shaft dynamic motion and average position within the bearing clearance.

The shaft orbit presentation is constructed by plotting the AC vibration signals from each probe pair together in X-Y format. The resulting graph in displacement units represents the dynamic deflection of the vibrating shaft within the bearing. One way to conceptualize how a shaft orbit develops is to first consider a shaft rotating at low speeds with little dynamic motion.

At low speeds, the resulting orbit presentation will not be much more than a dot on the graph, assuming that the shaft is straight and there is little noise in the signal from scratches or other surface irregularities.

Then, consider the same rotating shaft with the introduction of increased unbalance force at higher speeds. The shaft will now begin to exhibit dynamic response as a result of the unbalance force. Because unbalance is an outward force that is also rotating at the rpm of the machine, shaft deflection will tend to follow a circular path at a frequency equal to running speed (Figure 2).

Most shaft orbits are slightly elliptical due to the net effect of steady state side loads, or preloads, acting on the shaft. Examples include reaction from discharge flow on a pump, partial arc admission of steam into a turbine, and gravity on a horizontal machine. A significant aspect of orbit analysis, therefore, is differentiating between effects of normal preloads and significant deviations from the expected shape and form that are the result of machinery malfunctions.

While orbits are derived from the AC portion of the signals captured from an X-Y probe configuration, shaft centerline plots are derived from the DC portion. The DC signal from each probe is proportional to the average gap between the probe tip and the shaft surface.

#### Shaft centerline display

When changes in DC voltages from the two probes are converted to displacement units and plotted together in X-Y format, the resulting two-dimensional graph is called the shaft centerline. The most common application of the shaft centerline graph is to display changes in shaft centerline position within the bearing, such as might be experienced during a startup or shutdown, or which may occur at constant speed over time given changes in process load.

The shaft centerline display is most useful when centerline position data points are referenced to the bearing clearance boundary. This reference can easily be established on a horizontal machine by capturing DC gap reference voltages from the X and Y probes with the machine at rest, preferably following a shutdown to minimize hysteresis in the data from thermal effects.

The DC gap voltages captured at rest can then be used to reference the plot starting point at the bottom of the bearing clearance boundary. The resulting graph displays shaft centerline position changes relative to the starting point.

Some change in shaft centerline position with increased machine speed is normal and expected. This is because a radial fluid-film bearing provides hydrodynamic support of the rotor on the oil wedge which develops underneath the shaft as it rotates. The supporting oil wedge generally delivers more lift to the shaft at higher rotational speeds, causing the gap to change between the shaft and the observing proximity probes.