National Repository of Grey Literature 19 records found  1 - 10next  jump to record: Search took 0.01 seconds. 
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.
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.
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.
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.
Vibration stability of a vertical flywheel lifted by permanent magnetic rings
Zapoměl, Jaroslav ; Ferfecki, P. ; Sudhakar, A. ; Kozánek, Jan
Lifting the vertical flywheels by permanent magnets is used to reduce energy losses in the support elements. Unstable equilibrium position of pairs of magnetic rings, material damping of the shaft, and nonlinear properties of the support elements can arrive at the flywheel unstable oscillations. Therefore, evaluation of the vibration stability is the essential part of the design process of flywheel devices. In the developed computational procedure the flywheel is represented by a beam-like body, to which an absolutely rigid disc (the actual flywheel) is attached. The determination of the force action between the magnetic rings is based on mutual interaction between two magnetic dipoles. The steady state response of the flywheel on the unbalance excitation is calculated by means of the trigonometric collocation method and its stability is evaluated by application of the Floquet theorem. The developed computational procedures and learning more on behaviour of flywheels lifted by magnetic rings are the main contributions of the carried out research.\n
The influence of permanent magnetic rings on the loss power, vibration and stability of vertical rotors
Zapoměl, Jaroslav ; Ferfecki, P. ; Sudhakar, A. ; Kozánek, Jan
Lifting the vertical rotors by permanent magnets is an advanced technological solution for\nreducing energy losses in the supports. The disadvantage of this design is the instable interaction between the magnetic rings, which may destabilize the rotor oscillations. The performed research was focused on the development of computational procedures for analysis of energy losses in the supports of magnetically lifted vertical rotors mounted in rolling element bearings and on analysis of stability of their vibration in a wide range of operating speeds. The computational simulations proved efficiency of the magnetic suspension.
Reduction of the energy losses by application of controllable squeeze film dampers
Zapoměl, Jaroslav ; Ferfecki, Petr ; Kozánek, Jan
The unbalance induces lateral oscillations of rotors and forces transmitted to the rotor casing. The squeeze film dampers with integrated rolling element bearings represent a technological solution, which enables to reduce their magnitude, and consequently the resistance against the rotor rotation. To achieve optimum performance of the damping devices, their damping effect must be adapable to the current rotor speed. This paper reports a proposal of a controllable squeeze film damper, the damping effect of which is controlled mechanically by shifting its outer ring in the axial direction. The developed mathematical model of the damper is based on assumptions of the classical theory of lubrication and is completed with implementation of a gas cavitation. The results of the computational simulations show that an appropriate control of the damping force enables to reduce the energy losses in a wide range of operating speeds.
Stability analysis of the lateral oscillations of a flexibly supported viscoelastic Jeffcott rotor
Ferfecki, P. ; Zapoměl, Jaroslav ; Kozánek, Jan ; Dekýš, V.
Material damping has a significant influence on vibrations of flexible rotors and can induce their self excited oscillations. The squeeze film dampers inserted in the rotor supports are frequently used to suppress occurrence of these undesirable operating conditions. Modelling the shaft by means of a Kelvin- Voigt material can arrive at the overestimation of the effect of internal damping on the rotor movement. This was a motivation to develop a procedure based on utilization of the Zener material (standard solid theoretical material) to represent the shaft of a Jeffcott rotor supported by squeeze film dampers. The development and testing of this procedure, the experimental determination of the appropriate material constants, and learning more on the influence of material damping on the stability and vibration attenuation of flexible rotors are the principal contributions of the presented article.
Analysis of the lateral vibration of a flexibly supported unbalanced viscoelastic Jeffcott rotor
Ferfecki, P. ; Zapoměl, Jaroslav ; Kozánek, Jan
The behaviour of flexible rotors is significantly influenced by elasticity and damping properties of the shaft. It implies a special attention has to be paid to appropriate choice of a theoretical material to represent the rotors in mathematical models of rotating machines. The procedure developed in this paper concentrates on the investigation of the steady state component of the Jeffcott rotor vibration induced by the disc unbalance and on magnitude of the forces transmitted to the rotor frame. The shaft material is represented by the Zener standard solid material model. The steady state response was determined by application of the trigonometric collocation method. The results of the simulations confirmed the influence of the input oil pressure on the shape of the rotor orbit and on the oscillation amplitude.
Analysis of the vibrations attenuation of rotors supported by magnetorheological squeeze film dampers as a multiphysical finite element problem
Zapoměl, Jaroslav ; Ferfecki, P.
To achieve optimum performance of rotating machines the damping effect of the damping devices added to the rotor support elements must be controllable. This is enabled by application of squeeze film dampers lubricated by magnetorheological oils. This article deals with approaches, based on utilization of the finite element method, to analysis of mechanical vibrations of rotors effected by several mutually coupled multiphysical phenomena (electric, magnetic, hydraulic) occurring in the damping devices.

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