1,720,955 research outputs found
Vibrazioni Non Lineari e Caos negli Ingranaggi Spiroconici
No full textIl presente studio analizza le dinamiche complesse di un sistema reale di ingranaggi spiro conici, utilizzato nei sistemi di trasmissione per elicotteri, impiegando diversi strumenti di analisi dinamica non lineare. Una coppia di ingranaggi può essere soggetta a tre differenti tipi di contatto: contatto frontale, separazione tra i denti ed impatto posteriore. Lo studio prende in esame tutte le possibili tipologie di contatto, calcolando la rigidezza di ingranamento dentale sia nel moto in avanti sia in quello inverso. Per condurre un'analisi quasi-statica precisa, si utilizza un programma basato sul metodo degli elementi finiti non lineari, Transmission3D, al fine di determinare la rigidezza di ingranamento del sistema. Le equazioni del moto sono poi derivate applicando il teorema di Lagrange. Le equazioni differenziali non lineari, con parametri variabili nel tempo, rappresentano un sistema dinamico non regolare e non autonomo. Il gioco tra i denti introduce non linearità nel sistema, mentre la rigidezza variabile di ingranamento conduce a un’eccitazione parametrica. Il programma GearDyns-SBG è stato sviluppato per risolvere le equazioni del moto in differenti condizioni operative. La metodologia dinamica impiegata include simulazioni dirette e inverse per tracciare i rami stabili.
Sono presi in considerazione tre modelli dinamici principali per valutare la dinamica della coppia di ingranaggi spiroconici, considerando una rigidezza media di ingranamento, una rigidezza adattiva e una rigidezza costante dell'albero. L’approccio ibrido analitico-computazionale tiene conto della non linearità dovuta allo stress da contatto hertziano. La forza dinamica di ingranamento è stata calcolata utilizzando una combinazione di tecniche di interpolazione polinomiale e analisi del contatto dentale a carico. L’accuratezza dell’approccio della rigidezza adattiva di ingranamento è stata validata confrontando i risultati ottenuti con quelli di una simulazione non lineare basata sul metodo degli elementi finiti. Inoltre, la dinamica del sistema è stata studiata considerando l’effetto della rigidezza torsionale dell’albero. Gli alberi possono variare per lunghezza e diametro, influenzando significativamente la risposta del sistema. Al fine di comprendere meglio l’effetto della rigidezza torsionale dell’albero, è stato sviluppato un modello con tre gradi di libertà. L’effetto della rigidezza dell’albero è stato inoltre indagato dal punto di vista progettuale.
Questo studio fornisce un contributo significativo alla comprensione dei sistemi dinamici non lineari, offrendo metodi affidabili per prevedere e analizzare il comportamento del sistema in diverse condizioni operative. L'analisi ha rivelato importanti approfondimenti sulla dinamica del sistema, in particolare attraverso l'uso dello spettro FFT e delle analisi delle serie temporali non lineari. Vengono utilizzati diagrammi ampiezza-frequenza, mappe di Poincaré, diagrammi di fase e grafici di ricorrenza per visualizzare le risposte dinamiche, rivelando comportamenti complessi come la separazione tra i denti, il contatto posteriore e forti irregolarità nelle serie temporali. Fenomeni come boundary crises e le cascate di raddoppio del periodo sono esplorati, sottolineando il ruolo del massimo esponente di Lyapunov e della dimensione frattale nella caratterizzazione del caos. Ulteriori misure, come le analisi delle biforcazioni tridimensionali, offrono una comprensione più approfondita della dinamica del sistema, catturando fenomeni come chaos-chaos transitions, route-to-chaos, il ciclo boom-and-bust e il fenomeno della trappola. Lo studio confronta la dinamica del sistema in diverse condizioni operative ed esamina la periodicità delle risposte del sistema.This study delves into the intricate dynamics of a real spiral bevel gear system, used inside a helicopter transmission system, through various nonlinear dynamic analysis tools. In a real scenario, a gear pair may experience three different contacts: forward contact, tooth separation, and backside impact. In this study, all possible types of contacts are considered, which leads to calculating the tooth mesh stiffness in both the forward and the reverse motions due to the geometry of the spiral bevel gears. A nonlinear finite element method-based program, Transmission3D-Calyx, is used to perform a precise semi-static analysis to determine the system's mesh stiffness. Because of the mismatch between the tooth surface of the gear and pinion, a loaded tooth contact analysis and an unloaded tooth contact analysis are conducted. The motion equations of the system are obtained by applying Lagrange's theorem. The nonlinear governing differential equations with time-varying parameters represent a non-smooth and non-autonomous dynamical system. Indeed, the backlash introduces nonlinearity into the system, and time-dependent mesh stiffness leads to parametric excitation. The GearDyns-SBG program is developed to solve the governing equations under different working conditions. The considered dynamical methodology includes backward and forward simulations to track stable branches, enhancing the understanding of the system's behavior.
Three main different dynamical systems are considered to evaluate the dynamics of the spiral bevel gear pair with consideration of an average-mesh stiffness (average slope approach), an adaptive-mesh stiffness (hybrid analytical-computational approach), and a constant shaft stiffness. The hybrid analytical-computational approach accounts for the nonlinearity due to Hertzian contact stress. The dynamical mesh force was computed using a combination of polynomial interpolation techniques and loaded tooth contact analyses. The accuracy of the adaptive-mesh stiffness approach was validated by comparing its results with those obtained from a verified nonlinear finite element method simulation, demonstrating robust agreement and the efficacy of our approach. Moreover, the nonlinear dynamic scenario of spiral bevel gears is investigated by considering the effect of torsional shaft stiffness. Shafts can have different lengths and diameters, which significantly affects the dynamic response of the system. For a deeper understanding of the torsional shaft stiffness effect, a system with three degrees of freedom is developed. Besides, the effect of shaft stiffness was investigated from a design point of view.
This comprehensive study contributes to a deeper understanding of nonlinear dynamical systems and offers reliable methods for predicting and analyzing their behavior under various conditions. Our analysis reveals significant insights into system dynamics, particularly through the use of FFT spectrum, nonlinear time series analyses. Amplitude-frequency analyses, Poincaré maps, phase diagrams, and recurrence plots are used to visualize the dynamic responses, revealing complex behaviors including tooth separation, backside contact, and strong irregularities in time histories. We explore phenomena such as boundary crises and period-doubling cascades, emphasizing the role of the largest Lyapunov exponent and fractal dimension in characterizing chaos. Additional measures such as 3D bifurcation analyses offer deeper insights into the system's dynamics, capturing phenomena like chaos-chaos transitions, route-to-chaos, boom-and-bust cycles, and trapping phenomena. The investigation compares the dynamics of the system under different working conditions and examines the periodicity of the system’s responses
Effects of laser surface texturing on the dynamic performance of spur gears
No full textLaser Surface Texturing (LST) has been successfully applied to improve the tribological performance of reciprocating surfaces. In this context, although the relative motion between the meshing teeth is a combination of sliding and rolling, gears provided with LST could further reduce losses and vibrations in power transmissions. In this study, experimental results on the efficiency and the vibrations of gears are reported. The tests were carried out through a test bench that allows for evaluating the power losses of the transmission and the dynamic transmission error thanks to a set of torquemeters and accelerometers. Different load and speed conditions were investigated, and a comparison of the results between textured gears and standard carburized gears is provided
Loaded and unloaded tooth contact analysis of spiral bevel gears in consideration of misalignments
No full textBevel gear pairs are employed extensively in transmission systems, such as vehicle transmissions (rear axle drive), aircraft engines/turbines and helicopter gears, to transfer power between non-parallel shafts at high speed or high torque. The most complex form of bevel gear is the spiral bevel gear (SBG). SBG pairs are commonly used in applications that require high load capacity at higher operating speeds than are typically possible with other types of bevel gear. When manufactured in a metal-cutting process, spiral bevel gears can either be produced using single indexing (a face-milling method, which is considered in this study) or continuous indexing (a face-hobbing method). Due to manufacturing imperfections and the flexibility of components, the system might experience misalignments that intensify or exert a destructive effect on the gear vibration, which causes disruption in the stress distribution, thereby decreasing the lifetime of the gearbox. The main purpose of this study is to carry out loaded tooth contact analysis (LTCA) and unloaded tooth contact analysis (UTCA) for an SBG pair in the presence of two types of misalignment, axial and radial misalignment, and represent their effects on the mesh stiffness (MS). To calculate the MS, it is essential to determine the geometrical mismatch between two mating tooth profiles by means of UTCA. To conduct LTCA, three main approaches can be utilised: the finite element method (FEM) and experimental and analytical approaches. Due to the development of software packages during the last decade, Transmission3D-Calyx, an FEM-based software, is used in this study to carry out LTCA and UTCA. Finally, the MS for different misalignment cases is compared to represent the effect of misalignment on the SBG pair
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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