Critical speed

About: Critical speed is a research topic. Over the lifetime, 2764 publications have been published within this topic receiving 31365 citations.

Active Control of Rotor Vibrations by Two Feedforward Control Algorithms

Abstract: Resonance vibrations (critical speeds) play a significant role in rotor vibration control. Active vibration control methods for rotors are studied to develop solutions to enhance machines' dynamic b ehavior, durability, and operating range. This paper reports rotor vibration attenuation with a supplementary electromagnetic actuator located outside the rotor bearing span. Feedback and feedforward control system design are shown, and comparative experiments on two active vibration control methods for mass unbalance compensation are reported. The methods compared are adaptive FIR filter with the least mean squares (LMS) algorithm and convergent control (CC) method with a frequency-domain adaptation algorithm. The methods were experimentally validated on the rotor test rig (rotor weight 2.7 kg, length 560 mm, and first critical speed about 50 Hz). The feedback system provided wideband damping in the sub- and supercritical regions. The feedforward systems attenuated vibratory responses at the speed of rotation and its harmonic. The attenuation achieved was about 20 dB depending on the rotor speed. Also, discrete-time CC algerithm is shown to have a feedback equivalent circuit. The significance of feedback control lies in making the system phase-characteristics sufficiently smooth for feedforward control methods. Then, feedforward algorithms provided a good vibration damping performance over the operating range. CC was found to be a more effective and simpler algorithm for the purpose than the adaptive FIR filter with the LMS algorithm. The equivalent feedback circuit derived for CC, and systems similar to CC, facilitates their stabil ty and robustness analysis.

Comparison of Tilting-Pad Journal Bearing Dynamic Full Coefficient and Reduced Order Models Using Modal Analysis

Abstract: There is significant disagreement concerning the frequency response of tilting pad journal bearings (TPJBs) due to non-synchronous excitations. Two linear models for the frequency dependence of TPJBs have been proposed. The first model, the full-coefficient or KC model, considers Np tilting pads and rotor motions for Np + 2 degrees of freedom. Dynamic reduction of the KC model results in eight frequency-dependent stiffness and damping coefficients. The second model, based on results from bearing system identification experiments, yields twelve frequency-independent stiffness, damping, and mass (KCM) coefficients. Experimental data has been presented to support both models. There are major differences in the two approaches. The analysis in this paper takes a new approach of considering the pad dynamics explicitly in a state-space modal analysis. TPJB shaft and bearing pad stiffness and damping coefficients are calculated using a well known laminar, isothermal analysis and a pad assembly method. The TPJB rotor and pad full system eigenvalues and eigenvectors are then evaluated using state-space methods, with rotor and bearing pad inertias included explicitly in the model. The full bearing coefficient results are also non-synchronously reduced to the 8 stiffness and damping coefficients are and expressed as shaft complex impedances. The system identification method is then applied to these complex impedances, and the state space modal analysis is applied to the resulting KCM model. The damping ratios, natural frequencies, and mode shapes from the two bearing representations are compared. Two example TPJBs are examined in detail. The analysis indicated that four underdamped modes, two forward and two backward, dominate the rotor response over excitation frequencies from 0 to running speed. The full coefficient, non-synchronously reduced model predicts additional critically damped or overdamped modes due to the additional degrees of freedom as compared to the identified KCM model. The KCM model results in natural frequencies that are 63–65 percent higher than the full coefficient model. The difference in modal damping ratio estimates depend on the TPJB considered, with KCM being 7–17 percent higher than the full coefficient model. The full coefficient model also indicates that the bearing pads contribute significantly to the underdamped modes. The results indicate that the system identification method results in a reduced order model of TPBJ dynamic behavior. Additionally, the differences in the modal calculated system natural frequency and modal damping have potential implications for rotordynamic analyses of flexible rotors, such as critical speed and stability analyses.Copyright © 2009 by ASME

Hunting stability of high-speed railway vehicles on a curved track considering the effects of steady aerodynamic loads

Abstract: Aerodynamic loads may have effects on the hunting stability, and the factor of curved track makes it more complicated. Therefore, considering the steady aerodynamic loads generated by crosswind and airflow in the opposite advancing direction of train, the hunting stability of high-speed railway vehicle on a curved track is studied in this paper. The changes of gravitational restoring force and creep coefficients which are caused by aerodynamic loads are considered, and the change of equilibrium position due to aerodynamic loads, centrifugal force and the factor of curved track is also in consideration. A mathematical model of a high-speed railway vehicle during curve negotiation with aerodynamic loads is set up. A program based on the model is written and verified. Using this program, the linear critical speed considering the effects of aerodynamic loads is determined by the eigenvalue analysis. This paper investigates the critical speeds in three aerodynamic conditions. Considering the aerodynamic loads,...

Frictionally excited thermoelastic instability in the presence of contact resistance

Abstract: In sliding systems, frictional heating generates a well-known instability above a certain critical speed Vcr, which is a function of geometrical and material properties only. Similar instabilities are known to occur in the static problem, driven by temperature differences, in the presence of thermal contact resistance. Thermal contact resistance at the interface has seldom been considered and gives rise to full coupling of the problem. Generally, the resistance decreases non-linearly when pressure is increased. Here, the critical condition (in terms of heat flux and sliding speed) for the stability of the uniform pressure solution for a half-plane in frictional contact with a rigid wall at fixed temperature is studied for a general resistance function R(p). It is found that the heat flux direction increases the instability as in the case of zero speed, i.e. when directed into the half-plane (which is the only distortive material), whereas frictional heating can have also a stabilizing effect, for a given ...