In rotating systems, like engines and turbines, excessive vibration can lead to poor machine performance or damage. An orbit plot is a useful tool in diagnosing the root cause(s) of rotating machinery issues. An orbit plot is the visual representation of the centerline of a rotating shaft. The shape of the orbit often holds important clues as to the source of excessive vibration.
This article will discuss the theory, required instrumentation, and the necessary steps to construct an orbit plot.
Contents: 1. Orbit Plot Background 1.1 Measurement Locations and Response Planes 1.2 Proximity Probe 1.3 AC versus DC 1.4 Two Probes 1.5 Key Phasor 1.6 Filtered versus Unfiltered 2. Diagnosing Faults with an Orbit Plot 2.1 Unbalance 2.2 Misalignment 2.3 Fluid Instability 3. Creating an Orbit Plot in Simcenter Testlab
1. Orbit Plot Background
Ideally, there will be no motion of the shaft centerline as it rotates. In the real-world, vibrations caused by imbalance, misalignment, bearing-fluid interactions, or other sources will cause the shaft centerline to vibrate or “orbit” as shown in Figure 1.
Figure 1: Left - Ideally, a shaft (red cross section) should rotate (moving solid blue line) about its center. Right - If a shaft does not rotate about is center, an orbit pattern (dotted blue line) emerges
It is possible to use the shape and magnitude of the shaft orbit to identify the root cause of the vibration.
Some of the measurement equipment needed to create an orbit plot are discussed in the next sections.
1.1. Measurement Locations and Response Planes
A conceptual layout of the instrumentation for creating an orbit plot is shown in Figure 2 below:
Figure 2: Instrumentation locations (X and Y probes in blue and green) required for creating two orbit plots from different planes (transparent white) along a rotating shaft (blue). In this case, the planes correspond to the location of two bearings.
In Figure 2, the rotating system consists of a single shaft supported by two bearings. Two displacement probes are located at each bearing (X in green, Y in blue). The probes measure the displacement in the X and Y direction of the plane of the bearing. The plane of the bearing is also called the Transducer Response Plane (shown in transparent white) in this case.
The displacement measurements can be combined to determine the full motion of the shaft in the transducer response plane. Multiple orbit plots can be constructed to get a clear picture of the motion of the entire shaft.
1.2 Proximity Probe
Orbit plots are typically constructed from displacement measurements. The displacement of a shaft can be measured with a sensor called a proximity probe. An example is shown in Figure 3.
Figure 3: Proximity probe
A proximity probe is a noncontact eddy current displacement sensor. The probe outputs a voltage proportional to the distance between the sensor and the test object. In this case it measures the distance between the sensor and the rotating shaft as shown in Figure 4:
Figure 4: Top - A shaft (cross section in red) rotates within the bearing clearance (black circle) and is measured by a proximity probe (green). Bottom - The displacement measured by the shaft is plotted over time.
Figure X shows the motion of the shaft moving within the bearing. As the shaft orbits it will move closer to and then further away from the proximity probe. In this simple case the displacement of the shaft is sinusoidal in nature.
1.3 AC versus DC displacement
The proximity probe can measure both the AC (fluctuating) and the DC (offset) components of the displacement signal. The DC component is dependent on the offset between the proximity probe and the center line of the shaft. It can be used to determine the absolute position of the shaft centerline in the bearing. The AC component of the signal describes the orbit of the shaft.
Figure 5 shows how the distance from the shaft to the proximity probe causes a DC offset.
Figure 5: Left and Right - The distance between the shaft and probe produces a proportional DC offset in the proximity probe signal.
If the shaft starts rotating (and orbiting), the signal from the proximity probe will include both a DC and an AC component as shown in Figure 6.
Figure 6: An orbiting shaft creates an alternating signal (green) and a signal offset (dashed red)
Notice that the shape of the orbit is the same for both shafts, but the center of the orbit is in a different location. The DC component of the signal describes the center of the orbit within the bearing.
An orbiting shafts signal has both a fluctuating component (AC) and an offset (DC).
1.4 Two Probes
With one proximity probe it is possible to understand the motion of the shaft in one direction (in this case the X direction). If another transducer is added at an orientation 90 degrees from the first, data is also available in the Y direction.
The displacement signals from proximity probe X and proximity probe Y can be used to completely describe the motion of the shaft in the transducer response plane. These measurements can be plotted versus each other to show the orbit of the shaft. This is illustrated in the animation shown in Figure 7.
Figure 7: Displacement (center graph) from two proximity probes (Y-blue and green-X on left) are plotted against each other form an orbit plot (purple on right)
Issues that can be identified by an orbit plot are described in sections further ahead in the article.
A keyphasor is an additional sensor that is instrumented on the shaft. The key phasor gives very important information about the rotational speed of the shaft and the phase relationship between signals measured by the proximity probes.
The keyphasor consists of a sensor and a mark on the shaft. When the key phasor “see’s” the mark, a large voltage spike is produced. This information is used to annotate the data. It indicates that a new cycle, or that a new rotation of the shaft is beginning as shown in Figure 8.
Figure 8: In addition to data from the proximity probes (green and blue data), a keyphasor (black data) marks each rotation.
Two cycles are shown (a red and a blue curve) with a key phasor mark in Figure 9:
Figure 9: Plot of two revolutions (red and blue) designated by key phasor marks (circled in black)
The bottom axis (degrees) can be used to visualize the position of the shaft at a specific angular position of the keyphasor. The dot in the orbit plot indicates that one revolution of the shaft has ended and a new one has begun.
1.6 Filtered versus Unfiltered
An orbit plot can be viewed "filtered" or "unfiltered".
The unfiltered (sometimes called direct) orbit contains unmodified data from the rotating system (all order content, all frequencies, etc). It is the result of simply plotting the raw information from the X and Y proximity probes as shown on the left side of Figure 10.
Figure 10: Unfiltered (left) versus filtered (right) Orbit plot
Alternatively, it is possible to filter and only display specific order information in the orbit. For example, the plot on the right in Figure 10 shows a filtered orbit. It is only showing the 1X (or first order) vibration of the shaft.
This is useful to examine a specific phenomena (like unbalance) in more detail.
2. Diagnosing Faults with an Orbit Plot
Several issues in a rotating assembly can be understood by using an orbit plot.
2.1 Circular Orbit
A circular orbit (Figure 11) can be due to unbalance.
Figure 11: Circular orbit is due to imbalance
Unbalance is caused when the center of gravity of the shaft is not aligned with the center of rotation. The larger the diameter of the orbit, the greater the unbalance present.
An oval orbit indicates an external force is restricting the motion of the shaft as shown in Figure 12.
Figure 12: Oval orbit indicates a restrictive force is present
Some possible reasons include:
Bearing Stiffness: bearing is much less stiff in the lateral direction than the vertical.
Shaft Misalignment: A preload due to a shaft misalignment when it was installed could constricts the movement in the Y direction.
2.3 Other Faults
Rotating shafts are commonly supported by fluid bearings. Fluid bearings contain a thin layer of oil or other fluid which supports the shaft and allows it to operate with minimal friction (Figure 13):
Figure 13:A double orbit could indicate a fluid instability
When a shaft supported by a fluid bearing starts to rotate the fluid inside the bearing is also set in motion. The average fluid velocity inside the bearing has a velocity that is slightly lower than half (½ X) the rotating speed of the shaft. When the shaft compresses the fluid, the fluid creates a supporting force that contains a tangential and a radial component. At high RPM the tangential force can grow strong enough to cause an instability in the bearing. This creates a large amplitude vibration with a frequency that is equal to the average fluid velocity.
With Revision 2021.2 or higher of Simcenter Testlab, use the following steps to utilize orbit plots:
Under "Tools -> Add-ins". turn on "Rotor Dynamics" (Figure 14). Also required are Signature Throughput Processing, Angle Domain Analysis, and Customized Metrics calculator. Two new workbooks will be added to the currently running instance of Simcenter Testlab.
Figure 14: Turn on "Rotor Dynamics"
The "Rotor Dynamics" workbook will be added to the workflow. If using tokens, this will require 88 tokens total: Angle Domain Processing (42 tokens), Signature Throughput Processing (26 tokens), Roror Dynamics (20).
In the Navigator worksheet of Simcenter Testlab, select the time data to process and add it to the input basket as shown in Figure 14:
Figure 14: Right click on the time data acquired from proximity probes and choose "Replace in Input Basket"
Go to the Time Data Selection worksheet. Make sure the data source is set to “Input Basket”. Select "Replace" button in the Dataset area as shown in Figure 15.
Figure 15: In Time Data Selection, choose "Replace" to use the time data in the Input Basket.
View the data! Select the "View" checkbox next to one of the channels to display it as shown in Figure 16.
Figure 16: Click on the "View" checkbox (left side) next to any time history to visualize it.
To process the data, click on the "Roror Dynamics" workbook shown in Figure 17.
Figure 17: Rotor Dynamics workbook.
The Rotordynamics interface is shown below. It Includes four main areas as shown in Figure 18: Rotor Parameters, Angle Domain (AD) Resampling Parameters, Acquisition Parameters, and Processing.
Figure 18:The Rotor Dynamics menu has four key areas: Rotor Parameters (top, red), Angle Domain Parameter (middle left, green), Acquisition Parameters (middle right, blue), and Processing (botton, purple)
The sections in the menu are as follows:
a) Rotor Parameter: Select the appropriate Keyphasor signal. If the correct directions are specified for the proximity probes Auto select X-Y transducer pairs can be selected. The Sense of rotation and the probe angular positions can be specified for annotation on the orbit plot.
b) AD Resampling Parameters: The Rotor Dynamics tool resamples the time histories into the angle domain. The resampling tachometer and the angle domain resampling parameters are specified here.
c) Acquisition Parameters: The number of orbits to calculate is set under acquisition parameters. In this example the software will calculate twenty orbits.
d) Processing: The final processing parameters are specified here. Additional calculations are available. Orbit plots, Centerline plots, Polar Plots, Full spectrum waterfalls can all be calculated. In addition, Slow roll compensation is available. When ready select the “Calculate” button.
The resulting orbit plots are stored in the active section of the Simcenter Testlab project.
To view the data, go back to the Navigator worksheet. In the plot area, select the “Create a Picture” button. Select the Orbit plot as shown in Figure 19:
Figure 19: Under the "Create a Picture..." button, select "Orbit".
Browse to the newly created orbit data in the Navigator. Drag and drop the orbit into the display (Figure 20).