Measurement Computing Medallion Rotate rev.2.3 - Manual (Page 33)

October 2000 Medallion Rotate Manual
33
T
ORSIONAL
A
NALYSIS
Medallion Rotate can process a tachometer or other machine speed signal
to create an accurate kinematic description of torsional vibrations from a single
measurement. The machine speed signal must be a pulsed signal from a
tachometer, counter, encoder, or other speed sensor that shows torsional
changes.
Note:Torsional analysis creates a time history of the machine speed, similar
to that from a high-resolution DC speed sensor. If you have a high-
resolution DC speed signal, you can simply perform a Waterfall analysis
on the speed signal to view the torsional vibration. See “Waterfall
Analysis.”
Torsional analysis results in a time history of instantaneous shaft speed
with even sample spacing in time (in revolutions per second). You can then use
Waterfall analysis of the torsion file to create a series of spectra displayed on a
three-dimensional (X-Y-Z) plot. The Waterfall plot provides several useful ways
of looking at the data:
• The plot cursors can track the X axis or the Z axis.
• You can choose frequency or orders for the X axis and X axis cursors.
• You can choose spectrum number, RPM, or seconds for the Z axis and
Z axis cursors.
• You can display the data in a traditional spectral Waterfall plot, or as a
Color Contour plot. See “Contour Plot.”
You can use the torsional Waterfall plot to diagnose a variety of problems
directly from the machine speed signal. In some cases, Medallion Rotate can
eliminate the need for slip rings, telemetry, and torsional transducers.
A
PPLICATIONS
Torsional analysis is useful in diagnosing any problems arising from
torsional vibration (variations in rotational speed of a component due to
twisting). This technique can identify problems that do not usually appear in
conventional vibration measurement and analysis. This is particularly useful in
the following applications:
• Finding shaft critical speeds in turbines.
• Identifying torsional resonances in internal combustion engines that
reduce the efficiency.
• Identifying torsional resonances in reciprocating machinery.
• Diagnosing the cause of gear “chatter” from torsional vibration.