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Computational Analysis Of Dynamic Behaviour Of Journal Bearings
Rak, Vladimír ; Kamenický, Ján (referee) ; Pochylý, František (referee) ; Zapoměl, Jaroslav (referee) ; Malenovský, Eduard (advisor)
This work deals with computational modelling of static and dynamic analyses of journal bearings, with analyses of stability of oil-film motion and analyses of response of the rotor assemblies. At our workplace a new theoretical approach to the modelling of the static and dynamic behaviour of the rigid rotating body in liquid is used. The approach is based on the application of the Navier-Stokes motion eq., equation of continuity and boundary conditions eqs. It is possible to separate the motion of the rigid body and liquid from each other using suitable transformation relations and then it is also possible to separate the stationary and nonstationary motions from each other. A method of control volumes is used for these analyses. The real Bézier body is used for the description of the geometrical configuration and also for the approximation of velocity and pressure functions. Combined the ALE (Arbitrary Lagrange-Euler) method is used, because it´s necessary to generate a new net (to perform new meshing) for a change of the shaft position. The additional effects of the liquid (additional mass, stiffness and damping), which we solved in dynamic analysis, are the function of the single parameter only – the shaft-centre position. There is a large advantage in comparison with the standard approach, which is based on application of the Reynolds liquid eq. Author solving the models of the long and short journal bearing with different geometry, especially the elliptical and cylindrical bearings, with incompressible and compressible journal bearing liquid. If the journal bearing problem is solved, it is possible to include the additional effect of the liquid to the right side of the motion equation of a model rotor assembly. Author analyze a model rotor assemblies with two degrees of freedom, which is supported inside of the two journal bearings on the ends of the rotor (Jeffcott rotor assembly). Author modelling and solveing a response of the model rotor assembly on the forced steady-state vibrations, which was actuating by the unbalanced matter.
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A short study on self-balancing of vertical rotors mounted in passive contactless bearings
Zapoměl, Jaroslav ; Kozánek, Jan
Energy losses and wear of the support elements of high-speed rotors can be reduced by mounting the rotors in stable passive contactless bearings, the operation of which is based on magnetic levitation. The goal of the conducted research was to investigate applicability of selfbalancing device added to vertical rotors supported by bearings showing low stiffness and damping, which is a specific property of passive magnetic bearings. This paper deals with applicability and efficiency of self-balancing devices added to vertical rotors supported by bearings having very low stiffness and damping, which corresponds to the properties of magnetic frictionless bearings.
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Study of the oscillation of a pendulum in a magnetic field
Zapoměl, Jaroslav ; Kozánek, Jan ; Košina, Jan ; Cibulka, Jan
Realization of some technological processes requires application of rotating machines with a vertical rotor mounted in rolling element bearings. One of the requirements put on their operation is minimization of energy losses in the support elements. The proposed design variant consists in supporting the vertical rotor by one rolling element bearing placed at its upper part and by one axial magnetic bearing mounted in its lower end. The magnetic bearing is composed of an electric coil coupled with the stationary part and of a permanent magnet attached to the rotating part. The magnetic force attracts the permanent magnet, which reduces radial displacement of the lower end of the rotor. The magnetic field between the magnets represents a force coupling between the rotor and the stationary part, which shows some stiffness and affects the system natural frequencies. The controlled change of the stiffness makes it possible to reduce the rotor lateral oscillations in the resonance area. The investigated pendulum is coupled with the frame by a revolute joint at its upper end. The permanent magnet is attached to its lower end. The electric coil is coupled with the stationary part and placed under the pendulum. The system is excited by a moment of harmonic time history, which sets the pendulum into a seesaw motion.\n\n
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Design of the magnetically sensitive hydrodynamic bearing for the experimental rotor rig
Ferfecki, P. ; Zapoměl, Jaroslav ; Molčan, M.
In a hydrodynamic bearing, the relative motion between the rotating shaft and stationary housing is separated by a thin film of a lubricant. The magnetically sensitive lubricants with the promise of a semiactive control are able to improve the vibration response of the rotor system supported by the hydrodynamic bearings. A design of the magnetically sensitive hydrodynamic bearing is briefly introduced in this paper. The presented experimental rotor rig is proposed to analyse the large range of load conditions and rotational speeds of the bearing. The designed hydrodynamic bearing is intended to test the ferrofluids, magnetorheological oils, and nano-micro composite magnetic fluids. The apparent viscosity of the magnetically sensitive lubricant is altered by the magnetic field generated by the electric coil and thus the position of the bearing journal is shifted. The experimental measurement results show that the rheological behaviour of the lubrication layer with a magnetorheological fluid is significantly influenced by a magnetic field. The raising magnitude of the current in the electric coil leads to an increase in the bearing's load performance.
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Analysis of vibration of rotors with a thick rigid disc on the overhanging end supported by hydrodynamic bearings and loaded by uncertain unbalance effects
Zapoměl, Jaroslav ; Ferfecki, P. ; Molčan, M. ; Kozánek, Jan
Rotors of some rotating machines consist of a shaft and a rigid disc attached to its overhanging end. Because of manufacturing and assembling inaccuracies, the disc is unbalanced. In general case, the disc principal axis of inertia deviates from the shaft axis (both axes are skew lines) and the disc center of gravity is not situated on the shaft center line. Then the disc inertia effects are the source inducing the rotor lateral vibration. As magnitude of parameters specifying the disc unbalance is uncertain, application of an adequate method is required to perform the analysis. In addition, if the rotor is supported by hydrodynamic bearings, their nonlinear properties lead to rising complexity of the system vibration and to reducing its predictability. In the presented paper, the motion equation of a rigid disc has been derived. The shaft was represented by a beam body that was discretized into finite elements. The stiffness and damping parameters of the hydrodynamic bearings were linearized in vicinity of the rotor equilibrium position. Then vibration of the rotor is governed by a set of linearized motion equations. The fuzzy numbers were applied to consider uncertainty of the disc unbalance parameters. The presented procedure provides the approach to computational analysis of rotating machines with the rotor having a disc attached to its overhanging end and the geometrical, mechanical, or technological parameters of which have uncertain values.
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Effect of the controllable bearings lubricated by magnetically sensitive oil on vibration of rigid rotors
Zapoměl, Jaroslav ; Ferfecki, P.
Dynamical properties of hydrodynamic bearings depend of the Sommerfeld number. To avoid rupture of the oil film of heavily loaded bearings or inducing self-excited oscillation of rotors operating at high angular velocities the Sommerfeld number should remain in a certain interval that depends of the bearing size and design. The only parameter that can be changed during the rotor running to adapt the Sommerfeld number and thus the bearing performance to the current operating conditions is the dynamical viscosity. This is offered by application of magnetic fluids as lubricants. This paper reports on a new design concept of a controllable hydrodynamic bearings lubricated by composite liquid. The research is focused on investigation of the possibilities of the control process on the load capacity of the bearings and on the rotor vibration amplitude attenuation.
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Efficient numerical computation of the steady-state response and stability analysis of the rotor systems with squeeze film dampers
Molčan, M. ; Ferfecki, P. ; Zapoměl, Jaroslav
The aim of this paper is to demonstrate capabilities of the created numerical procedure, which is based on harmonic balance method. Furthermore, the procedure incorporates the alternating frequency-time domain technique and the arc-length parameterization to solve the steady-state response of nonlinear systems in efficient manner, including unstable branches. The stability of the motion was assessed by two methods: the 2n-pass method and Hill’s method. The procedure was verified on an example from literature to prove its sufficient accuracy and subsequently, the procedure was applied on the finite element model of the rotor systém mounted on the squeeze film dampers. The carried out computational simulations confirmed that the created procedure is efficient for the strongly nonlinear response and it gives similar results as the time integration.
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