465 research outputs found

    Vibration Pattern Related to Transverse Cracks in Rotors

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    A method for calculating the breathing behavior of transverse cracks of different types in rotating shafts is described. Thermal effects are included. Some results in terms of vibration excitation related to different shapes of cracks are presented

    Deflections and Strains in Cracked Shafts due to Rotating Loads: A Numerical and Experimental Analysis

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    In this article the deflections of a circular cross-section beam presenting a transverse crack of varying depths caused by various loads (bending, torsion, shear, and axial loads) are analyzed with the aid of a rather refined three-dimensional model that takes into account the nonlinear contact forces in the cracked area. The bending and shear loads are applied in several different angular positions in order to simulate a rotating load on a fixed beam or, by changing the reference system, a fixed load on a rotating beam. Torsion and axial loads are fixed with respect to the beam.Results obtained for the rotating beam can then be used for the analysis of cracked horizontal-axis heavy rotors in which the torsion is combined with the bending load. The effect of friction is also considered in the cracked area. The characteristic "breathing" behavior of the cracked area was analyzed and compared to that obtained with a rather simple one-dimensional model. The differences in results with respect to those based on fracture mechanics are emphasized. In order to highlight the effect of the presence of the crack, the deflections of the uncracked beam loaded with the same loads were subtracted from the deflections of the cracked beam.Finally, a cracked specimen was extensively analyzed by means of several strain gauges to study the strain distribution on the outer surface around the crack in various loading conditions. Consistent pre-stresses were found, and they influence the breathing behavior. The experimental results were compared with those obtained using the onedimensional linear model

    The use of orbitals and full spectra to identify misalignment

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    In this paper, a SpectraQuest demonstrator is used to introduce misalignment in a rotating set-up. The vibrations caused by misalignment is measured with both accelerometers on the bearings and eddy current probes on the shaft itself. A comparison is made between the classical spectral analysis, orbitals and full spectra. Orbitals are used to explain the physical interpretation of the vibration caused by misalignment. Full spectra allow to distinguish unbalance from misalignment by looking at the forward and reversed phenomena. This analysis is done for different kinds of misalignment, couplings, excitation forces and combined machinery faults

    Light and short arc rubs in rotating machines: Experimental tests and modelling

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    Rotor-to-stator rub is a non-linear phenomenon which has been analyzed many times in rotordynamics literature, but very often these studies are devoted simply to highlight non-linearities, using very simple rotors, rather than to present reliable models. However, rotor-to-stator rub is actually one of the most common faults during the operation of rotating machinery. The frequency of its occurrence is increasing due to the trend of reducing the radial clearance between the seal and the rotor in modern turbine units, pumps and compressors in order to increase efficiency. Often the rub occurs between rotor and seals and the analysis of the phenomenon cannot set aside the consideration of the different relative stiffness. This paper presents some experimental results obtained by means of a test rig in which rub conditions of real machines are reproduced. In particular shortarcrubs are considered and the shaft is stiffer than the obstacle. Then a model, suitable to be employed for real rotating machinery, is presented and the simulations obtained are compared with the experimental results. The model is able to reproduce the behaviour of the test rig

    Diagnostic Significance of Orbit Shape Analysis and its Application to Improve Machine Faults Detection

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    The full spectrum analysis of rotating machine vibrations is a diagnostic tool that enables the symptoms of some special types of fault to be clearly detected. The Shape and Directivity Index (SDI) of journal filtered orbits is an additional diagnostic parameter whose evaluation can be combined with the full spectrum analysis. The ellipticity of the filtered orbit, as well as the amplitude and the inclination angle of the major axis of the orbit, are parameters whose analysis can provide important diagnostic information. In order to validate the proposed approach, the vibrations of a large turbine-generator unit that was subjected to rotor-to-stator rubs have been analyzed in this paper. The results of this investigation have been used to update the model of the rotor-system that has been used to identify the location and the severity of the fault. In the paper, the improvements in the accuracy of the fault identification provided by the model updating enabled by the SDI analysis are shown

    A sensitivity analysis of vibrations in cracked turbogenerator units versus crack position and depth

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    The dynamic behaviour of heavy, horizontal axis, turbogeneratorunits affected by transverse cracks can be analysed in the frequency domain by means of a quasi linear approach, using a simplified breathing crack model applied to a traditional finite element model of the shaft-line. This allows to perform a series of analyses with affordable computational efforts. Modal analysis combined to a simplified approach for simulating the dynamical behaviour allows to predict the severity of the crack-excited vibrations, resolving the old-age question on how deep a crack must be to be detected by means of vibration measurements of the machine during normal operating conditions. The model of a 320 MW turbogeneratorunit has been used to perform a numerical sensitivity analysis, in which the vibrations of the shaft-line, and more in detail the vibrations of the shaft in correspondence to the bearings, have been calculated for all possible positions of the crack along the shaft-line, and for several different values of the depth of the crack
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