Wyniki wyszukiwania - BazTech

1

Rationale for mu-synthesis control of flexible rotor-magnetic bearing systems

Sawicki J. T.

Acta Mechanica et Automatica

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2008

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Vol. 2, no. 2

67-74

EN

The emergence of sophisticated formal control synthesis tools provokes important questions for any prospective user: why learn to use these new tools, what will they offer me? In synthesis of magnetic bearing controllers, it turns out that the range of stabilizing controllers is often quite narrow so that the difference between a poor controller and an “optimal” one may be small. Hence, the product of formal control synthesis tools often looks and performs much like what a reasonably clever control engineer would produce by hand. This paper demonstrates that the real value of these tools lies in a) generation of a performance benchmark which can be used to firmly establish the best performance relative to a specification and b) change of design parameter space to one which is relatively easy to maintain and represents a durable investment from an engineering process view.

2

A new vibroacoustic method for shaft crack detection

Gosiewski Z., Sawicki J. T.

Mechanics / AGH University of Science and Technology

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2007

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Vol. 26, no. 3

105-111

EN

Various excitation techniques have been developed to extract information on dynamic state of rotating machinery. This paper will address the effectiveness of selected excitations and analysed signals towards health monitoring of rotating machines from the shaft crack point of view. The computer simulation study is based on the uncracked and cracked Jeffcott rotor models and the efficiency of the selected approaches is examined. As the excitation we have considered the rotor unbalance, additional harmonic forces, and input conditions. The combined resonances as diagnostic indicators of the cracked shaft are good seen in the total frequency spectrum in the case when we consider the difference between the spectra of cracked and uncracked shaft. We have also introduced a new model of the rotor which allows us to use different method of signal processing for the crack detection and the evaluation of its deep. So more, in such model we can take into account the number of excitation planes and directions of the applied forces, the choice of the analysed signals (e.g. transient, steady-state).

PL

Pękanie wału prowadzi do asymetrii jego sztywności. Według modelu pękania Mayesa sztywność zmienia się w sposób kosinusoidalny. Tego typu wymuszenie parametryczne prowadzi do kombinowanych rezonansów. Większość autorów badą zachowanie się dynamiczne wirnika w otoczeniu takiego rezonansu. W niniejszym artykule do badań diagnostycznych przyjęto model Jeffcotta dynamiki wirnika. Wyprowadzono równania ruchu uszkodzonego i nieuszkodzonego wału. Analizę drgań tego nieliniowego modelu przeprowadzono z wykorzystaniem metody małego parametru. Zaproponowano podejście transmitancyjne do modelowania uszkodzenia. Przeanalizowano możliwość pobudzania drgań maszyny z wykorzystywaniem różnych typów sygnałów. Z wielu możliwych rozwiązań prowadzących do uzyskania informacji diagnostycznej wybrano do dalszych badań model reprezentowany przez funkcję odpowiedzi częstotliwościowej. Różnica pomiędzy dynamiką uszkodzonego i nieuszkodzonego wirnika została użyta do uwypuklenia pików rezonansów kombinowanych w widmie drganiowym. Na bazie tych wymuszeń określono symptomy, z wykorzystaniem których można budować wskaźniki diagnostyczne. Rozważania analityczne poparto badaniami symulacyjnymi.

3

Cavitation effects on the dynamics of journal bearings

Sawicki J. T.

International Journal of Applied Mechanics and Engineering

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2005

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Vol. 10, no 3

515-526

EN

A numerical procedure incorporating cavitation effects on the dynamics of journal bearings is presented. A two-dimensional linear stability analysis considering the fluid flow in both full film and cavitation regions for a plain cylindrical journal bearing and four multi-lobe bearings are presented. The Lund’s infinitesimal perturbation procedure is applied to Elrod’s universal equation for evaluation of unsteady pressure gradients. Based on JFO theory, the pressure distribution, film rupture, and reformation boundaries can be obtained using Elrod’s universal equation, for a given operating position of the journal. In this work, it is assumed that for infinitesimal perturbation of journal about equilibrium position, the film rupture and film reformation boundaries are same as those obtained for steady state. However, the unsteady pressure gradients in the full film region are evaluated taking into consideration the perturbed flow parameters in the cavitation region, i.e., at both rupture and reformation boundaries. The linearized stiffness and damping coefficients, whirl frequency ratio, and threshold speed for various values of eccentricity and L/D ratios are obtained for a plain cylindrical journal bearing with an axial groove along the load line. Measured data of dynamic coefficients for a 120° partial arc bearing are chosen for comparison with this work. Results show good agreement between the theoretical and experimental results. Results of stiffness and damping coefficients are presented for two-axial groove, elliptical, three-lobe and offset cylindrical bearing for various L/D and eccentricity ratios. A transient analysis of submerged journal bearing incorporating the mechanism of shear between the liquid sublayer and air cavity in the cavitation zone is also presented. Using the mass conservation principles, Elrod’s universal equation is modified to take into consideration the shear of air cavity in the cavitation zone. Results of transient response for the submerged journal bearing using the present approach are compared with the Elrod’s universal equation based on the striated flow in the cavitation region. The limit cycle journal motion using the present approach predicts higher eccentricity ratios.