Rockwell Automation 1441-PEN25-Z Enpac 2500 Data Collector - Manual (Page 160)

160 Rockwell Automation Publication GMSI10-UM002D-EN-E - August 2012
Chapter 6 Multi-channel Measurements
One key parameter is the measure of relative phase between two points on a
machine, which is easily calculated as a cross channel measurement. Phase
measurements across a coupling can easily show misalignment. Phase
measurements can also be used around a bearing to determine if it is cocked.
It is even possible for a significant loss of mass, such as a thrown blade, to result in
little or no change in vibration amplitude, but a very clear shift in phase. With the
two-channel cross phase feature, there is no need for a reference mark on the
shaft. Therefore, there is no need to stop the machine, nor is there a need for any
triggering equipment - unless the requirement is to measure phase relative to a
known reference such as a shaft keyway.
Orbit Measurements You can set up and collect orbit measurements with the Enpac 2500 using the
two input channels and displacement transducers. Because sensor orientation is
not loaded and unloaded with routes, the Enpac 2500 data collector assumes the
trigger input comes from top dead center. From there, it calculates the relative
sensor angles to be 90 degrees apart. The difference between Channel 1 and
Channel 2 is always 90 degrees.
Understanding an Orbit Plot
Another set of key measurements with a two-channel collector results in an orbit
plot. An orbit plot is derived from the output of two non-contact pickups
(proximity probes) mounted radially, 90 degrees apart at the bearing.
Figure 26 - Relative Sensor Angles
If viewed in real time, the signals produce a trace on the screen of a Lissajous
pattern, corresponding to the total shaft motion, which is really the orbit of the
shaft in the bearing.
A filtered orbit can help exclude any frequencies not related to running speed or
its harmonics. The slight elliptical shape of the 1X orbit plot shown in
1X Orbit
Plot on page 161 shows a small unbalance condition.
In this case, a 1X filter was applied to the measurement for isolating potential
running speed related anomalies. It is important to note, however, that other non-
1X related frequencies may be present that are missed if a filtered orbit is used.