Time History: Orbit and Shaft Centerline

Orbit and Shaft Centerline processing requires a time history analysis file that includes XY signal pairs with a good reference channel that can be used to define the speed signal. Time History files do not inherently "know" how to calculate the speed channels.

To begin an Orbit or Shaft Centerline analysis, first go to the respective tab in Aurora Client.

When first entering the tabs, a popup dialog window will open for the initial configuration of the Orbit / shaft centerline analysis.

Choose the XY channel pair and select a channel to use for defining a Tacho (speed) channel. This will open a new dialog box. Again, select the speed channel, specify the 100% frequency value in Hz (in the example, the channel is a 1/rev, and 36 Hz corresponds to 2200 RPM). If the Speed counting doesn't creating the correct RPM values, then re-do the Define Tacho option using Generic Cycle Counting and specify the high and low trigger thesholds.

When the ‘Plot’ button is selected an X-Y plot of speed against time will be produced.

Click through the "OK" buttons and the initial analysis plot will be displayed.

The top two plots show the currently selected channel in the time history file. The upper plot is over the full time duration of the TH file and the lower plot shows the data from the zoom window highlighted in blue. Left clicking on the plot will drop a cursor for the point where the analysis is being performed. The polar plot shows the orbit. It will be necessary to zoom into the plot to get a good view of the orbit. Left click and drag a window to the right and surround the data points to zoom in. The X and Y channels are shown to the right of the polar plot and span the zoomed envelope plot from above.

Make a window around the data points to zoom in on the polar plot
Make a zoom window around the data points on the polar plot to zoom

The polar plot will re-center based on where the zoom window. To zoom out, make a box to the left.

Just like all other Aurora Windows, it is possible to zoom from any plot.

To show more or less periods within the Polar Plot, right click on the plot and select Orbit Settings, then go to Period Display. Two options are provided - one where the software automatically locks one full cycle periods based on the tacho channel settings and one to manually specify how many full periods to plot based on the the cursor location or the zoom window.

Automatic locking only works correctly with a 1/rev signal. Additionally, if the tacho definition is wrong, e.g. it doesn't properly count the cycles, then the software can incorrectly identify the full period of data and might show an incorrect number of points for an orbit.


Generic Cycle Count

The Generic Cycle Counting tool carries out a similar function used in the Tacho Speed Counting option and is used for analysing the frequency content of signals, such as flow meter data or a tachometer. When ‘Generic Cycle Counting’ option is selected the ‘Generic Cycle Counting Options’ window is displayed.


In the ‘Channel /Parameter Options’ the user first selects which time history channel is to be analysed. In order to plot the amplitude against time the ratio between the signal frequency and the EU is required. The user must then specify low and high cycle threshold values to ensure a suitable crossing point is used to identify each pulse.

In the ‘Period Characteristics’ section the user enters a tacho frequency value which is the shaft speed that would give the characteristic of the tacho. If EU is selected the unit entered in the ‘EU Name’ field is used. The ‘elapsed time’ value is the time taken for the number of cycles defined in the field below (number of period cycles over elapsed time).

In the ‘Analysis Options’ section the user selects whether to display % or EU. If the ‘Use smoothing’ option is selected a rolling average calculation is applied to the tacho so that the plot is smoother. The number of smoothing cycles needs to be specified which determines the number of values used to calculate the rolling average. The higher the number of smoothing cycles the smoother the plot.

When the ‘OK’ button is selected an X-Y plot of processed steady state EU against time will be produced.

Shaft Centerline

The shaft centreline plot is used to assess where the shaft is sitting within the bearing clearance. 

  1. The user must first select the 2 proximity probes for the X and Y axes to be used from the lists available. 
  2. Next the filter type must then be specified.
    1. none 
    2. rolling average 
    3. low pass filter 
  3. The tacho must be selected. If ‘none’ is displayed in the drop down list the user can select the ‘Define New Tacho’ button. This brings up the generic cycle counting GUI where a tacho can be defined. This newly defined tacho is then available for selection in the select tacho list.
  4. The user must then enter values for the delta time and delta speed triggers which will be used to select the data points in the analysis for plotting. When the speed of the selected tacho has changed by the value entered in the ‘Delta Speed Trigger’ that data point is plotted. However, if the time since the last data point is greater than the ‘Maximum Delta Time’ then that point is plotted.
  5. If ‘OK’ is then selected the ‘shaft centreline’ tab is then displayed with 4 plots.
An envelope plot is displayed at the top of the screen. The channel displayed can be selected from the envelope plot’s right mouse menu. The user can select a zoom area from the Envelope Plot. The area selected on the envelope plot is displayed on the other 3 plots. The two time history zoom plots display the channels x and y used in the analysis.
All of the plots on the ‘shaft centreline’ screen can be saved to csv using the option in their right mouse menu.

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