Mohsen Nakhaeinejad & Suri Ganeriwala
Rotordynamic Analysis Using XLRotor
SpectraQuest Inc., 8227 Hermitage Road, Richmond, VA 23228
Published: November, 01 2008
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.
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: SpectraQuestÃƒÂ¢Ã¢â€šÂ¬Ã¢â€žÂ¢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