Observations
Concerning Misalignment Vibration Signatures
Lin Liu,
Suri
Ganeriwala
SpectraQuest Inc., 8201 Hermitage Road, Richmond, VA 23228
Published: Nov, 2003
Abstract
Misalignment is a common cause of
machinery malfunction. Considering the importance of alignment, the
vibration spectrum of misalignment is not well documented. This
report is based on the research to determine the unique vibration
signature for misalignment at varying operating and design
conditions such as speed, type and level of misalignment, coupling
types and machinery dynamic stiffness. The SpectraQuest Machinery
Fault Simulator™ was used in the study to create the varying
mechanical conditions. Tri-axial vibration measurements were taken
at each end of the coupling on the motor and rotor bearing housings.
Data was collected at several other locations of the Simulator. The
results indicate that the speed and the coupling type/stiffness have
a strong effect on the vibration spectra. The level and type of
misalignment had a significant effect on the vibration signature. No
unique signature was observed, suggesting that care is needed to
correctly diagnose misalignment.
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Introduction
Misalignment
is a common cause of machinery malfunction. A poorly aligned machine
can cost a factory 20% to 30% in machine down time, replacement
parts, inventory, and energy consumption. A large payback is often
seen by regularly aligning machinery. Operating life is extended and
process conditions are optimized.
Vibration signatures are
widely promoted for studying machine malfunctions. However, the
literature does not present a clear picture of signature
characteristics uniquely attributable to misalignment. Different
authors report different signatures. There are no reports of
systematic, controlled experiments with varying parameters.
This is a report of a
systematic series of experiments designed to elucidate the
consistent features, if any, of vibration signatures for misaligned
machinery. Three machine-operating parameters, coupling types,
amount of misalignment and the motor speed, were systematically
varied while all other parameters were held constant. The machine
was fault-free with the exception of deliberate misalignment, which
was varied systematically. Baseline vibration data was recorded for
each of the test conditions.
Fig: Comparisons of
Vibration Spectra of the Left Bearing Housing and the Motor in
vertical and axial directions of the Misaligned MFS with the Steel
Coupling at Two Different Speeds and 20 mils misalignment. The
graphs illustrate the strong effect of speed on the spectra and its
amplitude. At higher speed note the higher peak at 2X than 1X. Also,
note modulation at the higher harmonics. The frequency scale is
displayed as Order of RPM. Graphs are shifted vertically to clarify
the differences. The vertical scale is 0.2 G’s per block.
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