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Coupled vibrations in horizontal and vertical rotor-bearing systems
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
2011 (English)Conference paper, Presentation (Refereed)
Abstract [en]

For dynamical systems having several degrees of freedom, motion in one direction can induce motion in the other. This means that there is a certain coupling between these two motions. Coupling can in some cases be a source of instability that causes self-excited vibrations in rotating machinery. In classical modeling of rotor systems, couplings other than those that are the result of gyroscopic effect are normally not considered. This is due to thecomplexity of the reasons for coupling which mainly depends on machinery hardware, for example, the bearings’ design (type) and the asymmetry in machine components.Plain cylindrical hydrodynamic journal-bearings provide high damping to the rotorsystem, but they also cross couple the rotor translational motions. Cross coupling is the main source of oil induced instability; therefore, the rotor speed should not exceed the speed at which oil-induced instability occurs. The inherent nonlinearity of plain cylindrical hydrodynamic journal- bearings becomes strong for eccentricities greater than 60% of the bearing clearance, where most existing linear models are not able to accurately predict the rotor trajectory. Strong nonlinearities together with cross coupling are the source of complex dynamics in fluid-film journal bearings. The journal bearing impedance descriptions method, a method that is valid for all bearing aspect ratios and all eccentricities, was used to evaluate linear analysis of the rotor steady-state imbalance response. The results show that linear bearing models derived from the nonlinear impedance descriptions of the Moes-cavitated (π-film) finite-length bearing can predict the steady-state imbalance response of a rigid symmetric rotor that is supported by two identical journal-bearings at high eccentricities. This is, however, only the case when operating conditions are below the threshold speed of instability and when the system has period one solutions. The error increases in the vicinity of resonance speed. A combi-bearing is a fluid-film lubricated tilting-pad thrust and journal bearings combined together. Thrust bearing is used in vertical rotating machinery and shafts designed to transmit thrust. The total axial load is carried by the single thrust bearing. The analyzed combi-bearing is an existing machine component used in the hydropower unit Porjus U9 situated in northern Sweden. The linearized model shows that the combi-bearing couples the rotor’s lateral and angular motions. However, if the thrust bearing’s pads arrangement is not symmetrical or if all the pads are not angularly equidistant the rotor axial and angular motions are also coupled. This last case of coupling will also occur if the thrust bearing equivalent total stiffness is not evenly distributed over the thrust bearing. A defected pad or unequal hydrodynamic pressure distribution on the pads’ surfaces may be the cause. The Porjus U9’s simulation results show that the combi-bearing influences the dynamic behavior of the machine. The rotor motions’ coupling due to combi-bearing changes the system’s natural frequencies and vibration modes.

Place, publisher, year, edition, pages
Research subject
Solid Mechanics
URN: urn:nbn:se:ltu:diva-37381Local ID: b62d0a37-7901-4d22-9be0-2611b84b78d4OAI: diva2:1010879
Svenska mekanikdagar 2011 : 13/06/2011 - 15/09/2011
Godkänd; 2011; 20111010 (ysko)Available from: 2016-10-03 Created: 2016-10-03Bibliographically approved

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Luneno, Jean-Claude
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