Rockwell Automation 1441-DYN25-Z Dynamix 2500 Data Collector - Manual (Page 171)

Rockwell Automation Publication 1441-UM001B-EN-P - September 2012 171
Multi-channel Measurements Chapter 6
Figure 2 - 1X Orbit Plot
If the machine has unbalance only, the displayed orbit will be dominated by a
once-per-revolution signal and have a circular or elliptical shape.
To get a better impression of the actual shaft motion and to identify other
common machine anomalies, such as misalignment, shaft whirls, rubs, and
looseness, all signal frequencies are required up to several orders of shaft speed.
The orbit shape then becomes more complex, and it may be highly elliptical,
non-elliptical, or it may have several inner and outer loops.
Inner loops presented in a Lissajous orbit plot can indicate a hit and bounce type
condition. This phenomenon occurs when the shaft comes in contact with the
bearing surface and bounces off.
In early stages of contact, a flat spot in the orbit plot will be presented. As the
condition becomes more severe, the number of loops present will increase and
become tighter. Detailed analysis of the orbit plot can indicate exactly where the
shaft is coming into contact with the bearing in question. This figure shows a
non-filtered orbit plot of the same shaft as discussed above.
Figure 3 - Non-filtered Orbit Plot
In the Filtered plot, only 1X frequency information appears in the orbit. In this
case, the rub condition presented would have been overlooked if only a 1X orbit
was taken. In general, a filter of OVERALL in the Emonitor software specified
for the orbit plots will give the most useful information.
To adequately define an orbit of a machine shaft, it is first necessary to know the
approximate shaft speed and how many orders of the shaft speed are to be
included in the measurement. A spectrum analysis (FFT) of the signals can be
used to determine the number of orders.
Unless the measurement is synchronized with a trigger (or a once-per-rev phase
reference pulse), it is not possible to relate the orbit shape to the rotational
Inner loops