by Ray Kelm, P.E.
Orbital analysis of machine vibration data is the topic of this paper. It begins by defining what an orbit plot is and how it is properly constructed. The keyphasor, blank-bright spot on the plot and how to identify forward versus reverse precession are discussed. Both 1x rpm compensation and unfiltered compensation of orbits are described. The “loop rules” are defined with examples to determine the frequency content of an orbital plot. How loading or a restriction to shaft motion effects the orbit shape is explained. Within the paper, orbital plots of both shaft vibration data (proximity probes) and bearing pedestal vibration data (accelerometers) are presented. The usefulness of orbit plots when transient events occur in machinery is shown. For someone wanting an introduction to orbital analysis, this well organized, brief paper is the ticket.
Orbit analysis is a useful tool for any vibration analyst that deals with rotating machinery, especially those machines that are equipped with permanently installed proximity probes for vibration measurement. The process is essentially an extension of time waveform analysis as it uses time domain data from two orthogonal probes plotted on an X-Y graph with consideration for the physical location of the probes as installed on the subject machine. This paper will discuss the construction of orbit plots, uses of the orbit for analysis, examples of field data, and some of the pitfalls that may be encountered when applying this analysis technique.
A massive amount of information is contained in vibration data measured from a rotating machine. Orbit plots are a way to present some of this data to highlight some of this information that may provide details about the nature of the vibration. If an analyst has the ability to examine the path a rotor takes as it vibrates in its bearings, the analyst can have a better understanding of what is causing the vibration. This is exactly what the orbit plot provides; a map of the path the rotor travels on while vibrating.
Orbits plots, or Lissajous patterns if you are a fan of electrical engineering terminology, are generated by plotting the scaled vibration signals from two vibration probes mounted at the same axial position on a rotating machine. The two measurements fully define the vibratory motion of the rotor at the axial position of the probes. Figure 1 illustrates how the vibration signals observed by each of the two proximity probes on the machine are combined to produce an orbit plot. The proximity probes output a voltage proportional to the physical gap between the probe tip and the target surface. This voltage has both a DC component and an AC component. The AC component represents the vibration of the rotor. For an orbit plot, the DC component is ignored and only the AC component of the signal is considered. This makes the orbit plot always centered around the origin in a normal orbit plot.
To get the most out of orbit plots, it is necessary to have a phase reference measurement from the shaft as it rotates. This can come from a Keyphasor® probe looking at a keyway or key on the shaft or some sort of optical or laser tachometer triggered by a target on the shaft. This phase reference, or trigger, allows the user to extract frequency content from the orbit plot.”
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