Understanding the Shaft Orbit Plot in Vibration Analysis

Definition: What is a Shaft Orbit?

A shaft orbit is a plot that shows the path of a rotating shaft’s geometric center 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 at the end of the shaft. This powerful diagnostic tool is created by using a pair of non-contact proximity probes mounted 90 degrees apart (typically in an X-Y configuration) and plotting their simultaneous displacement signals against each other.

Why is the Orbit Plot So Useful?

While a standard time waveform or FFT spectrum shows vibration in a single direction, the orbit plot combines two directions to provide a complete picture of the shaft’s dynamic motion. This allows vibration analysts to visualize the actual shape and direction of the shaft’s movement, which provides invaluable clues for diagnosing a wide range of machinery faults. The orbit is a cornerstone of rotordynamic analysis, especially for high-speed, critical machinery equipped with fluid-film bearings, such as turbines, compressors, and large generators.

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 diagnose a machine’s condition simply by looking at the orbit’s form.

Circular or Elliptical Orbit

A simple circular or elliptical orbit, centered in the bearing, is typically indicative of 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 is different in the horizontal and vertical directions (anisotropic), which is very common.

Distorted, Figure-8, or Banana-Shaped Orbit

When the orbit is distorted from a simple ellipse, it indicates that multiple frequencies are influencing the shaft’s motion.

• A “banana” or crescent-shaped orbit is often associated with misalignment, where both 1x and 2x frequencies are present.
• A figure-8 shape is a classic sign of a strong 2x frequency component, which is 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. This indicates that the shaft’s motion is being constrained as it makes momentary contact with a stationary component, such as a bearing, seal, or the machine casing.

Highly Irregular Orbits

An orbit that is erratic, unstable, or appears to be filled with “noise” can be a sign of severe mechanical looseness, fluid-induced instability (like oil whirl or whip), or turbulent flow conditions in a pump or compressor.

Direction of Precession: Forward vs. Reverse

The direction in which the orbit is traced relative to the direction of shaft rotation is also a key diagnostic factor.

• Forward Precession: The orbit traces in the same direction as shaft rotation. This is the normal behavior for forces like unbalance.
• Reverse Precession: The orbit traces in the opposite direction of shaft rotation. This is an abnormal condition and can be an indicator of a shaft crack, a severe rub, or certain types of fluid-induced instability.

What an Orbit Plot Shows

In summary, a single orbit plot provides a wealth of information:

• The overall amplitude of shaft vibration.

– The shape of the shaft’s motion, which helps identify the fault type.

• The direction of precession.
• The average position of the shaft within the bearing clearance.

By analyzing the orbit in conjunction with the FFT spectrum and time waveform, analysts can achieve a highly confident and detailed diagnosis of a machine’s dynamic behavior.

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