Rotordynamic Analysis Using XLRotor
Mohsen Nakhaeinejad, Suri Ganeriwala
SpectraQuest Inc., 8201 Hermitage Road, Richmond, VA 23228
Published: November 2008
Abstract
The
rotordynamic analysis of the SpectraQuest Machinery Fault Simulator
(MFS) Magnum is performed in XLRotor to study critical speeds and
imbalance response of the machine. MFS Magnum machine including
motor, shaft, disks, coupling and rolling element bearings is
modeled and the rotordynamic analysis was performed using the
rotordynamic software XLRotor. The stiffness and damping associated
with rolling element bearings of the motor and shaft are calculated
in the software. Different shaft and disk configurations are
introduced to the model, the whole rotating system is solved for
damped critical speeds and mode shapes are obtained. Also, imbalance
response is studied, bearing displacements and dynamic loads on the
bearings are obtained and presented. This study clearly shows the
power of the XLRotor for rotordynamic analysis.
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Introduction
Rotating machinery produces vibration signatures
depending on the structure and mechanism involved. Faults in machine
also can increase and excite the vibrations. Vibration behavior of
the machine due to natural frequency and imbalance is one of the
important topics in rotating machinery which should be studied and
considered in design. All objects exhibit at least one natural
frequency which depends on the structure of the object. The critical
speed of a rotating system occurs when the rotational speed matches
a natural frequency. The lowest speed at which a natural frequency
is encountered is called the first critical. As the speed increases,
additional critical speeds may be observed. Minimizing rotational
unbalance and unnecessary external forces are very important to
reducing the overall forces, which initiate resonance. Due to the
enormous destructive energy and vibration at resonance, the main
concerns when designing a rotating machine are how to avoid
operation at or closed to criticals and how to pass safely through
the criticals in
acceleration and deceleration. Safely refers not only to
catastrophic breakage and human injury but also to excessive wear on
the equipment.
Since the real dynamics of machines in operation is
difficult to model theoretically, calculations are based on the
simplified model which resembles the various structural components.
Obtained equations from models can be solved either analytically or
numerically. Also, Finite Element Methods (FEM) is another approach
for modeling and analysis of the machine for natural frequencies.
Resonance tests to confirm the precise frequencies are often
performed on the prototype machine and then the design revised as
necessary to assure that resonance does not become an issue.
XLRotor as a rotordynamic software provides powerful,
fast and accurate tools to perform rotordynamic modeling and
analysis. Comprehensive capabilities of the software include
analysis of undamped and damped critical speeds, imbalance,
stability, mode shapes, nonlinear transient response, torsion,
synchronous and asynchronous force response, indeterminate static
deflection, rolling element bearings and fluid film bearings. All
model inputs are entered on worksheets and several templates and
modules are available to create the model of each part. Completing
computations and analysis by the software, the results are available
through tables and charts in Excel worksheets.
The objective of this technical note is to study the
rotordynamic behavior of the SpectraQuest Machinery Fault Simulator
(MFS) Magnum including critical speeds and imbalance responses. To
achieve this goal, the MFS Magnum machine including motor, shaft,
disks, coupling and rolling element bearings is modeled and the
rotordynamic analysis was performed using the rotordynamic software
XLRotor. The stiffness and damping associated with rolling element
bearings of the motor and shaft are calculated in the software.
Different shaft and configurations are introduced to the model, the
whole rotating system is solved for damped critical speeds and mode
shapes are obtained. Also, imbalance response analysis for the rotor
in acceleration is studied and dynamic load on the bearings are
obtained and presented.
Fig 1: SpectraQuest’s Machinery Fault Simulator
(MFS) Magnum used for the critical speed test
Fig 2:
Damped 1st Mode Shape
Table 1: Critical speeds of the
SpectraQuest MFS Magnum machine calculated by XLRotor
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