Simcenter Testing Solutions Orbit Plots

Direct YouTube link: https://youtu.be/qK9K4rQf88Y


What is an Orbit Plot?

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.
 
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:
 
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.
 
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 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.
 

• 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.

• 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:

 

• 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.

 

• View the data! Select the "View" checkbox next to one of the channels to display it as shown in Figure 16.

 

• To process the data, click on the "Roror Dynamics" workbook shown in Figure 17.

• 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.

• 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:

• Browse to the newly created orbit data in the Navigator. Drag and drop the orbit into the display (Figure 20).