Stability and Damped Critical Speeds of a Flexible Rotor in Fluid-Film Bearings

1974 ◽  
Vol 96 (2) ◽  
pp. 509-517 ◽  
Author(s):  
J. W. Lund
Keyword(s):  
Dynamic Properties ◽  
Fluid Film ◽  
Internal Damping ◽  
Flexible Rotor ◽  
Critical Speeds ◽  
Damping Coefficients ◽  
Fluid Film Bearings ◽  
Multistage Compressor ◽  
Stiffness And Damping ◽  
Rotor Model

A method is described for calculating the threshold speed of instability and the damped critical speeds of a general flexible rotor in fluid-film journal bearings. It is analogous to the Myklestad-Prohl method for calculating critical speeds and is readily programmed for numerical computation. The rotor model can simulate any practical shaft geometry and support configuration. The bearings are represented by their linearized dynamic properties, also known as the stiffness and damping coefficients of the bearing, and the calculation includes hysteretic internal damping in the shaft and destabilizing aerodynamic forces. To demonstrate the application of the method, results are shown for an industrial, multistage compressor.

scholarly journals Identification of Rotordynamic Forces in a Flexible Rotor System Using Magnetic Bearings

2007 ◽  
Author(s):  
Zachary S. Zutavern ◽  
Dara W. Childs
Keyword(s):  
Transfer Functions ◽  
Magnetic Bearings ◽  
Identification Algorithm ◽  
Flexible Rotor ◽  
Domain Identification ◽  
Damping Coefficients ◽  
Reaction Forces ◽  
Rotordynamic Forces ◽  
Stiffness And Damping ◽  
Rotor Model

A method is presented for parameter identification of an annular gas seal on a flexible-rotor test rig. Dynamic loads are applied by magnetic bearings (MBs) that support the rotor. MB forces are measured using fiber-optic strain gauges (FOSGs) that are bonded to the poles of the MBs. In addition to force and position measurements, a finite element rotor model is required for the identification algorithm. The FE rotor model matches free-free characteristics of the test rotor. The addition of smooth air seals to the system introduces stiffness and damping terms for identification that are representative of reaction forces in turbomachines. Tests are performed to experimentally determine seal stiffness and damping coefficients for different running speeds and preswirl conditions. Stiffness and damping coefficients are determined using a frequency domain identification method. This method uses an iterative approach to minimize error between theoretical and experimental transfer functions. Test results produce seal coefficients with low uncertainties.

scholarly journals Identification of Rotordynamic Forces in a Flexible Rotor System Using Magnetic Bearings

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Zachary S. Zutavern ◽  
Dara W. Childs
Keyword(s):  
Transfer Functions ◽  
Magnetic Bearings ◽  
Identification Algorithm ◽  
Flexible Rotor ◽  
Domain Identification ◽  
Damping Coefficients ◽  
Reaction Forces ◽  
Rotordynamic Forces ◽  
Stiffness And Damping ◽  
Rotor Model

A method is presented for parameter identification of an annular gas seal on a flexible-rotor test rig. Dynamic loads are applied by magnetic bearings (MBs) that support the rotor. MB forces are measured using fiber-optic strain gauges that are bonded to the poles of the MBs. In addition to force and position measurements, a finite element rotor model is required for the identification algorithm. The FE rotor model matches free-free characteristics of the test rotor. The addition of smooth air sealed to the system introduces stiffness and damping terms for identification that are representative of reaction forces in turbomachines. Tests are performed to experimentally determine seal stiffness and damping coefficients for different running speeds and preswirl conditions. Stiffness and damping coefficients are determined using a frequency domain identification method. This method uses an iterative approach to minimize error between theoretical and experimental transfer functions. Test results produce seal coefficients with low uncertainties.

Critical Speeds of Turbomachinery: Computer Predictions vs. Experimental Measurements—Part II: Effect of Tilt-Pad Bearings and Foundation Dynamics

1987 ◽  
Vol 109 (1) ◽  
pp. 8-14 ◽  
Author(s):  
J. M. Vance ◽  
B. T. Murphy ◽  
H. A. Tripp
Keyword(s):  
Critical Speed ◽  
Natural Frequencies ◽  
State Of The Art ◽  
Dynamic Properties ◽  
Experimental Measurements ◽  
Critical Speeds ◽  
Damping Coefficients ◽  
Speed Prediction ◽  
Bearing Stiffness ◽  
Stiffness And Damping

This is the second of two papers describing results of a research project directed at verifying computer programs used to calculate critical speeds of turbomachinery. This part describes measurements made to determine the characteristics of tilt-pad bearings and foundation dynamics. Critical speeds of a 166 kg laboratory rotor on tilt-pad bearings are then compared with predictions from a state-of-the-art damped eigenvalue computer program. Measured natural frequencies of a steam turbine are also compared with computer predictions. Accuracy of critical speed prediction is shown to depend on accuracy of 1) the “free-free” rotor models, 2) the bearing stiffness and damping coefficients, and 3) the dynamic properties of the foundation, which can be represented by an impedance that must be determined by experimental measurements.

Evaluation of the Dynamic Properties of Fluid Film Bearing Elastic Supports

2009 ◽  
Author(s):  
Nicoleta M. Ene ◽  
Florin Dimofte ◽  
Laurentiu E. Moraru ◽  
Abdollah A. Afjeh
Keyword(s):  
Experimental Data ◽  
Experimental Evaluation ◽  
Dynamic Properties ◽  
Fluid Film ◽  
Elastic Supports ◽  
Damping Coefficients ◽  
Turbojet Engine ◽  
Stiffness And Damping ◽  
Experimental Rig

The purpose of this paper is to present the experimental evaluation of the stiffness and damping coefficients of a pair of elastic rings used to support a fluid film bearing into a turbojet engine body. The experimental rig, the method of analysis and the experimental data are presented.

Field Identification of Stiffness and Damping Characteristics of Fluid Film Bearings

2008 ◽  
Author(s):  
Sameh H. Tawfick ◽  
Aly El-Shafei ◽  
M. O. A. Mokhtar
Keyword(s):  
Journal Bearings ◽  
Fluid Film ◽  
Flexible Rotor ◽  
Test Rig ◽  
Vibration Data ◽  
Damping Characteristics ◽  
Fluid Film Bearings ◽  
Field Identification ◽  
Modal Mass ◽  
Stiffness And Damping

A method for field identification of stiffness and damping characteristics of fluid film bearings FFB is derived. The method relies on measuring both the shaft and the housing’s vibration response. Two independent unbalance runs are performed and the synchronous response is recorded. Using the housing vibration data, the amount of unbalance acting on the bearing, as well as the flexible shafts’ “modal mass” can be experimentally determined. Thus, with this method, field engineers can identify the bearings impedance in flexible rotor-bearing systems. A test rig comprising a flexible shaft supported on two cylindrical journal bearings is used to verify the proposed method. The amount of uncertainty in the derived coefficients is calculated.

Modal Response of a Flexible Rotor in Fluid-Film Bearings

1974 ◽  
Vol 96 (2) ◽  
pp. 525-533 ◽  
Author(s):  
J. W. Lund
Keyword(s):  
Fluid Film ◽  
Mode Shapes ◽  
Flexible Rotor ◽  
First Order ◽  
Fluid Film Bearings ◽  
Modal Response ◽  
Governing System ◽  
System Equations ◽  
Multistage Compressor ◽  
Modal Method

An analysis is presented for calculating the response of a general flexible rotor in fluid-film bearings to forced and transient excitation. It is a modal method where the governing system equations are transformed by means of normal coordinates into a set of decoupled, first-order equations which can be solved in closed form. The transformation is based on the orthogonal complex modal functions (“mode shapes”) associated with the eigenvalues of the system. The method has been applied to an industrial multistage compressor. Numerical results are given for the response to selected unbalance distributions and, also, the transient response to a shock pulse.

Identification of Bearing Force Coefficients From Measurements of Imbalance Response of a Flexible Rotor

2004 ◽  
Author(s):  
Luis San Andre´s ◽  
Oscar C. De Santiago
Keyword(s):  
High Performance ◽  
Fluid Film ◽  
Flexible Rotor ◽  
Simple Method ◽  
Force Coefficients ◽  
Fluid Film Bearings ◽  
Support Force ◽  
Bearing Force ◽  
Rotor Structure ◽  
Motion Amplitude

Field identification of fluid film bearing parameters is critical for adequate interpretation of rotating machinery performance and necessary to validate or calibrate predictions from restrictive computational fluid film bearing models. This paper presents a simple method for estimating bearing support force coefficients in flexible rotor-bearing systems. The method requires two independent tests with known mass imbalance distributions and the measurement of the rotor motion (amplitude and phase) at locations close to the supports. The procedure relies on the modeling of the rotor structure and finds the bearing transmitted forces as a function of observable quantities (rotor vibrations at bearing locations). Imbalance response measurements conducted with a two-disk flexible rotor supported on two-lobe fluid film bearings allow validation of the identification method estimations. Predicted (linearized) bearing force coefficients agree reasonably well with the parameters derived from the test data. The method advanced neither adds mathematical complexity nor requires additional instrumentation beyond that already available in most high performance turbomachinery.

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