Rockwell Automation 1441-PEN25-Z Enpac 2500 Data Collector - Manual (Page 162)

162 Rockwell Automation Publication GMSI10-UM002D-EN-E - August 2012
Chapter 6 Multi-channel Measurements
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
position of the shaft. There is a mark on the orbit to show the location of the
shaft at the time of the trigger event.
Assuming pickups are located 90 degrees apart and there is a phase reference
pickup providing a once-per-revolution pulse to trigger the data collection, you
can use orbits to distinguish common faults in machines equipped with plain
bearings.
A typical machine with normal to moderate unbalance, and no other major
faults, shows a simple orbit, corresponding mostly to the once-per-rev
component, which really dominates.
See 1X Orbit Plot on page 161 .
Misalignment tends to add significant levels of vibration to the shaft at once-per-
revolution, but also at higher orders of running speed. The once-per-revolution
component from misalignment is not in phase with the unbalance component.
The effect is to make the orbit plot less circular and more elliptical or even non-
elliptical, for example, a banana-shape or a figure-eight pattern.
Resonance and excessive bearing wear also tend to produce elliptical orbits. If the
contribution is due to resonance, this condition can be pinpointed if the orbit
changes noticeably with changes in running speed.
Figure 29 - Resonance and Excessive Bearing Wear
If a shaft comes close to a bearing surface or actually contacts it, the orbit takes on
a different set of characteristics. Shaft rub tends to flatten the orbit, associated
with the increased resistance to the radial movement of the shaft as it comes close
or contacts the bearing surface, once per revolution. The orbit of a shaft with rub
has a shape similar to whirl, except that the internal loop remains fixed and does
not rotate, as it is occurring at an exact submultiple of the shaft rotation speed.