1,721,172 research outputs found
Interaction force between magnetic field and ferromagnetic target: analytical, numerical and experimental study
Full text linkIn this study, an analytical model and a finite element (FE) model were developed in order to study the force produced by a permanent magnet on a ferromagnetic target. The study was aimed at estimating the magnetic action in order to design an excitation device for vibration tests. The dynamic analysis of rotating structures as compressors’ bladed wheels requires a solicitation that reflects the operational conditions. If the component is made of ferromagnetic material, it is possible to use magnetic fields for the excitation. The present paper reports the interaction between planar parallel surfaces, first studied analytically and numerically, and the results were compared with experimental results. Then the interaction between sloping surfaces was analyzed, allowing an analytical boundary loss model to be developed. Finally, the FE model was improved to study the interaction between double curvature surfaces. A comparison with experimental results measured on an actual bladed wheel was performed
PROGETTO E OTTIMIZZAZIONE DI UNO SHAKER ELETTROMAGNETICO PER ALTE FREQUENZE
No full textStructures dynamic characterization represents a crucial issue in industry. Experimental tests require
an excitation source able to work in a wide frequency range. Commercial solutions have severe
limitations in terms of working frequency and dimensions. This is the reason why this work presents
the design of a custom electromagnetic shaker developed by University of Pisa. The device is aimed to
the excitation of rotor blades of centrifugal compressors (one shaker on each blade). A really compact
solution was then needed, having a working frequency range of 1–10 kHz. Four different solution
were also analyzed for shaker-structure connection: “beam stinger” (φ = 1 mm), “wire singer” (φ = 0.2
mm), “ball stinger” (φ = 3 mm) with conical slot and spherical slot. Experimental tests performed on a
shaker prototype allowed do verify the performances of the exciters and of the different stingers
STUDIO DELLA FORZA DI INTERAZIONE TRA CAMPO MAGNETICO E BERSAGLIO FERROMAGNETICO MEDIANTE PROVE SPERIMENTALI E MODELLI ANALITICI E NUMERICI
No full textBladed wheels damage detection through Non-Harmonic Fourier Analysis improved algorithm
Full text linkRecent papers introduced the Non-Harmonic Fourier Analysis for bladed wheels damage detection. This technique showed its potential in estimating the frequency of sinusoidal signals even when the acquisition time is short with respect to the vibration period, provided that some hypothesis are fulfilled. Anyway, previously proposed algorithms showed severe limitations in cracks detection at their early stage. The present paper proposes an improved algorithm which allows to detect a blade vibration frequency shift due to a crack whose size is really small compared to the blade width. Such a technique could be implemented for condition-based maintenance, allowing to use non-contact methods for vibration measurements. A stator-fixed laser sensor could monitor all the blades as they pass in front of the spot, giving precious information about the wheel health. This configuration determines an acquisition time for each blade which become shorter as the machine rotational speed increases. In this situation, traditional Discrete Fourier Transform analysis results in poor frequency resolution, being not suitable for small frequency shift detection. Non-Harmonic Fourier Analysis instead showed high reliability in vibration frequency estimation even with data samples collected in a short time range. A description of the improved algorithm is provided in the paper, along with a comparison with the previous one. Finally, a validation of the method is presented, based on finite element simulations results
ANALISI MODALE SPERIMENTALE DI RUOTE PALETTATE TRAMITE STAZIONE ROBOTICA AUTOMATIZZATA
No full textNel presente lavoro vengono descritti il funzionamento e l’impiego di una stazione robotica automatica nell’ambito dell’analisi modale sperimentale di ruote palettate in condizioni free-free, trascurando l’effetto di stress-stiffening. Un robot antropomorfo a sei gradi di libertà posiziona il laser a effetto Doppler che viene impiegato per misurare la velocità di vibrazione di uno o più punti in ognuno dei settori di ripetizione circolare del componente studiato. L’impiego del robot per la movimentazione del sensore richiede un tempo molto ridotto e garantisce una precisione molto maggiore rispetto al posizionamento manuale. Vengono inoltre descritti i test condotti su una girante aperta, confrontando i risultati sperimentali e numerici in termini di matrice MAC e diagramma SAFE. Viene infine affrontata l’analisi del caso particolare di giranti con doppia palettatura, che presentano una maggiore difficoltà di interpretazione dei risultati
TOPICS IN DEPENDENCE MODELLING IN FINANCE
Full text linkTOPICS IN DEPENDENCE MODELLING IN FINANCETOPICS IN DEPENDENCE MODELLING IN FINANC
Excitation device for high frequency vibration analysis: Design and test results
Full text linkIn the present paper, the design and optimization of a high frequency excitation source is presented. The device was developed for a harmonic response analysis test bench, aimed at dynamic characterization and resonance prediction of mechanical structures. A wide frequency range must be covered, depending on the analyzed structure: the range 1–10 kHz was considered in the present work. The device was designed for a test bench aimed at investigating the vibrational response of centrifugal compressor bladed wheels. A really compact solution was needed since the final test bench provides one exciter for each blade (up to 20 devices on the circumference hoop). Both contact and contactless solutions were considered, but only the contact solution was found to fulfill all the specifications. Finally, different stinger solutions were proposed and compared in the paper. The investigated solutions were: a beam stinger (diameter 1 mm); a wire stinger (diameter 0.2 mm); and a ball stinger (diameter 3 mm) with two different support solutions. Experimental tests performed on a device prototype allowed to verify the specifications fulfillment and to choose the best stinger solution for the application
DISPOSITIVO PARAPIOGGIA PER ZAINO
No full textViene previsto un dispositivo parapioggia (1) per zaino comprendente un telo di copertura (10) definente un asse preferito di sviluppo (10a) ed estremità (10b) opposte tra loro rispetto all’asse preferito di sviluppo (10a); una struttura di ancoraggio (20) atta a vincolare il dispositivo parapioggia (1) allo zaino (1a); un montante di sostegno (50) atto a sostenere il telo di copertura (10); un organo di apertura (30) comprendente prime aste (31) vincolate alle estremità (10b), seconde aste (32) interposte tra le prime aste (31) e cerniere (33, 33a, 33b) atte a incernierare tra loro le aste (31, 32) definendo per l’organo di apertura (30) un sistema articolato; un apparato di comando (40) comprendente una guida (41) e un cursore (42) solidale ad una cerniera (33, 33a, 33b) e atto a scorrere lungo la guida (41) comandando una rotazione reciproca delle aste (31, 32) così da variare l’estensione dell’organo di apertura (30) lungo l’asse preferito di sviluppo (10a) definendo una configurazione di apertura in cui le estremità (10b) sono allontanate tra loro e il telo di copertura (10) è disteso e presenta estensione massima così da riparare dalla pioggia e una configurazione di chiusura in cui le estremità (10b) sono avvicinate tra loro e il telo di copertura (10) presenta estensione minima
Non-Harmonic Fourier Analysis for bladed wheels damage detection
No full textAbstract The interaction between bladed wheels and the fluid distributed by the stator vanes results in cyclic loading of the rotating components. Compressors and turbines wheels are subject to vibration and fatigue issues, especially when resonance conditions are excited. Even if resonance conditions can be often predicted and avoided, high cycle fatigue failures can occur, causing safety issues and economic loss. Rigorous maintenance programs are then needed, forcing the system to expensive shut-down. Blade crack detection methods are beneficial for condition-based maintenance. While contact measurement systems are not always usable in exercise conditions (e.g. high temperature), non-contact methods can be more suitable. One (or more) stator-fixed sensor can measure all the blades as they pass by, in order to detect the damaged ones. The main drawback in this situation is the short acquisition time available for each blade, which is shortened by the high rotational speed of the components. A traditional Discrete Fourier Transform (DFT) analysis would result in a poor frequency resolution. A Non-Harmonic Fourier Analysis (NHFA) can be executed with an arbitrary frequency resolution instead, allowing to obtain frequency information even with short-time data samples. This paper shows an analytical investigation of the NHFA method. A data processing algorithm is then proposed to obtain frequency shift information from short time samples. The performances of this algorithm are then studied by experimental and numerical tests
Structural Behaviour of GFRP Reinforced Glass Beams
No full textThe study of the structural behavior of some particular glass beams is presented. Two series of beams were constructed, the former alternating glass sheets with sheets of GFRP (Glass Fibre Reinforced Polymers) and the second by coupling glass and glass fibres pultruded profile, with the aim to realize tensile resistant beams and solve the problem of the brightly of the glass and the great dispersion of the tensile strength values. Structural behaviour was studied by subjecting the beams to four-point bending tests. The behaviour of the beams can be clearly seen: all beam specimens show a pre-critical phase characterized by a linear behavior and a post-critical phase represented by a sequence of drops and increases in load. The tests made it possible to evaluate the benefit obtained by applying the reinforcement both in the elastic phase and in the post-critical phase
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