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Developed and distributed by the Rotating
Machinery and Controls Industrial Research Program (ROMAC)
Department of Mechanical,
Aerospace and Nuclear Engineering
University of Virginia
Charlottesville, Virginia
The stability of a rotor-bearing system is of interest to both designers and operators of rotating machinery. If left undetected, an unstable machine can eventually fail due to high subsynchronous vibration levels. Stability can be characterized by the damped eigenvalues of the rotor-bearing system. In particular, if the real parts of all of the complex eigenvalues are less than zero, then the rotor system is considered stable. If any of the eigenvalues has a real part greater than zero, then the system is considered unstable.
Program ROTSTB reads a data file containing the geometry and information of the shaft and information on the bearings and seals in the model. The code constructs the governing equations from which the eigenvalues can be determined. Once the eigenvalues of the rotor system are known, the code determines the characteristic shape of the rotor at each of the modes (damped modeshape).
Due to the speed of computation, the code is ideal for running parameter studies to determine optimum design parameters to produce the best system stability.
Models in the code include:
8-Coefficient
journal bearing models (principal and cross-coupled stiffness and
damping).
Tilting pad journal bearing models where pad rotational
degrees-of-freedom are retained.
Magnetic
journal bearing models (colocated and non-colocated) in which bearing
forces are modeled in terms of open-loop stiffness, actuator gain,
and frequency-dependent transfer functions relating perturbation
currents to journal displacements.
Thrust
bearing models for principal and cross-coupled rotational stiffness
and damping.
Flexible
supports for any of the journal bearing models. Flexible support
models can be entered as single degree-of-freedom systems or as
transfer functions.
Aerodynamic
cross-coupling models
Destabilizing
hysteretic damping
Shear
deformation effects
Linearized
gyroscopic effects
Shaft
rotary inertia effects
Lumped
disk models in terms of added weight and added polar and transverse
mass moments of inertia.
Multiple
speed/bearing cases.
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Cross-Section of Rotor Model |
ROTSTB makes use of the transfer matrix method to solve for the damped eigenvalues and modeshapes of the rotor system. The equations are in a non-dimensional form that allows very accurate values to be calculated.
Degrees-of-freedom away from the main line of the rotor, present in tilting pad and magnetic bearing models for example, are represented as transfer functions relating the forces imparted to the shafting due to excursions of the rotor.
Since the lowest eigenvalues are the modes of interest in rotating machinery, the code is very fast in that it only calculates the number of modes desired.
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Damped Modeshape for First Forward Mode |
ROTSTB produces an ASCII output file containing a listing of the input data, the calculated eigenvalues for the system, the associated modeshape for each of the eigenvalues, and equivalent bearing impedance information. This latter information is useful in determining the effect of flexible supports on the damping in the bearings, for example.
ROTSTB will also produce plot files of the damped modeshapes calculated. These plot files are used with the graphics program TECPLOT to produce 2D and 3D plots of the damped mode shapes. TECPLOT macro files are provided for formatting. The damped modeshapes are presented in terms of both major and minor axis amplitudes (elliptical form) and in terms of displacements in each of two orthogonal planes intersecting the shaft axis.
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ROTSTB can be run on any DOS-based PC compatible with at least 640 KB of RAM. Typical analyses of many mass-station rotors take from seconds to run on the newest PC compatibles to minutes on some of the older models.
ROTSTB can be used with the ROMAC Shell. The Shell is a WindowsTM-based utility used to organize the running of the various ROMAC computer programs. Desirable features of the Shell include menu-driven data file generation, data file validators, and point-and-click operation.