Understanding the Shaft Orbit Plot in Vibration Analysis
A shaft orbit is a plot that shows the path traced by a rotating shaft’s geometric centre during one or more revolutions. It is a two-dimensional view of the shaft’s motion within its bearing clearance, as if you were looking straight down the end of the shaft. This powerful diagnostic is created by mounting a pair of non-contact proximity probes 90 degrees apart — typically in an X-Y configuration — and plotting their simultaneous displacement signals against each other rather than against time.
1. Why the Orbit Plot Is So Useful
A standard time waveform or Wigo wa FFT shows vibration in a single direction. The orbit plot combines two perpendicular directions to give a complete picture of the shaft’s dynamic motion, letting analysts visualise the actual shape and direction of the movement rather than inferring it. That extra dimension provides invaluable clues for diagnosing a wide range of faults, which is why the orbit is a cornerstone of rotor-dynamic analysis — especially for high-speed, critical machinery fitted with fluid-film journal bearings, such as turbines, compressors, and large generators.
Because the probes measure displacement directly against the bearing, the orbit also reveals where the shaft sits within its clearance, not just how far it moves — information a casing-mounted accelerometer simply cannot provide. Adding a tachometer or Keyphasor mark stamps each revolution onto the orbit, fixing its phase and making precession direction unambiguous.
2. How to Interpret Orbit Shapes
The shape, size, and orientation of the orbit are direct indicators of the forces acting on the rotor. A skilled analyst can often diagnose a machine’s condition simply by looking at the orbit’s form.
Circular or Elliptical Orbit
A simple circular or elliptical orbit, centred in the bearing, typically indicates a well-behaved rotor dominated by a single frequency — most often unbalance. The orbit will be circular if the bearing support stiffness is the same in both directions (isotropic) and elliptical if the stiffness differs horizontally and vertically (anisotropic), which is very common in practice because most machines are stiffer vertically than horizontally.
Distorted, Figure-8, or Banana-Shaped Orbit
When the orbit is distorted from a simple ellipse, multiple frequencies are influencing the shaft’s motion:
- A “banana” or crescent-shaped orbit is often associated with misalignment, where both 1× and 2× frequencies are present.
- A figure-8 shape is a classic sign of a strong 2× component, a textbook indicator of shaft misalignment. A figure-8 with an internal loop often suggests a more severe condition or the presence of a rub.
Orbits with Kinks or Sharp Corners
Sharp changes in direction, flat spots, or “kinks” in the orbit are strong evidence of a rotor-to-stator rub. They show that the shaft’s motion is being momentarily constrained as it makes contact with a stationary component such as a bearing, a seal, or the machine casing.
Highly Irregular Orbits
An orbit that is erratic, unstable, or appears filled with “noise” can signal severe mechanical looseness, fluid-induced instability such as oil whirl or whip, or turbulent flow conditions in a pump or compressor.
3. Direction of Precession: Forward vs Reverse
The direction in which the orbit is traced, relative to the direction of shaft rotation, is itself a key diagnostic factor:
- Forward precession: the orbit traces in the same direction as shaft rotation. This is the normal behaviour for forces such as unbalance.
- Reverse precession: the orbit traces opposite to shaft rotation. This is an abnormal condition and can indicate a shaft crack, a severe rub, or certain types of fluid-induced instability.
Distinguishing the two requires the once-per-revolution phase mark; without it the orbit shows shape but not the sense in which it is travelled, and the diagnostic distinction is lost.
4. The Orbit in Field Diagnostics
Permanently installed proximity probes feeding a monitoring system are the textbook way to capture an orbit on large critical machines, but the underlying principle — two perpendicular displacement signals plotted against each other with a phase reference — is equally available to the field engineer. A portable two-channel instrument such as the Balancet-1A measures synchronous amplitude and phase on two planes at once, so once an orbit or spectrum has confirmed that unbalance is the dominant force, the same setup carries straight through to field balancing and verification of the corrected state. When the orbit instead reveals misalignment, a rub, or instability, that finding redirects the work toward alignment or a mechanical fix rather than balancing.
5. What an Orbit Plot Shows
In summary, a single orbit plot delivers a wealth of information at a glance:
- The overall amplitude of shaft vibration.
- The shape of the shaft’s motion, which helps identify the fault type.
- The direction of precession, forward or reverse.
- The average position of the shaft within the bearing clearance — its shaft centreline.
Analysed in conjunction with the FFT spectrum and the time waveform, the orbit lets an analyst reach a highly confident and detailed diagnosis of a machine’s dynamic behaviour — combining the “how much,” the “what shape,” and the “which way” into one decisive picture.
