1,721,048 research outputs found
A Non-linear elastodynamic model of a desmodromic valve train
No full textA lumped-parameter model of a motorbike engine's desmodromic valve train is developed for the simulation of the dynamic behaviour of such an uncommon train. The model takes into account several non-linear effects and is highly time-varying. The estimation of the model parameters is discussed and the effectiveness of the model is assessed by a comparison with experimental results. The model is employed to predict the magnitude of forces, impacts and bounces, and to detect unacceptable dynamic phenomena; thus, it may be used as a tool both in design optimization and diagnostics
Use of Acoustic Emission for Cutting Process Monitoring in Turning
No full textThe use of acoustic emission (AE) for monitoring the wear of cutting tools is promising but, since an effective general methodology has not yet been established, further research is required. To scan the relationships between the AE signal on the one hand and both cutting conditions and deterioration phenomena on the other, turning tests were performed in practical conditions. Some AE amplitude parameters and the kurtosis of the squared demodulated signal exhibited a random behaviour plus sudden variations, often related to deterioration phenomena. Two monitoring methodologies are presented and their effectiveness in warning of actual process deterioration is compared. In testing unworn tools the signal amplitude was found to increase with the cutting speed and its continuous part was observed to be almost entirely independent of the feed rate and the depth of cut. Finally, the AE burst frequency was found to match the chip breaking frequency, thus confirming that chip breakage is the ..
Un modello a più gradi di libertà per l'analisi dinamica di trasmissioni con croce di malta
No full textUnusual benign lesion of the thoracic wall. Abrikossoff's myoblastoma. Diagnostic imaging contribution
No full textA novel methodology for dynamic response maximisation in multi-axis accelerated random fatigue testing
No full textThis paper investigates the effects caused by simultaneous multiple random excitations on the fatigue-life of the specimen under test. In multi-axis accelerated fatigue testing, the test specifications are usually provided in terms of PSDs only. However, different combinations of the test specifications can significantly affect the fatigue behaviour of the specimen, resulting in altered failure modes and test durations. In this context, the original contribution of this paper is to provide a novel method for combining the PSD test specifications, which is able to recreate in the laboratory the most severe and damaging vibration environment possible. The aim of the present methodology is to get out the extreme dynamic response of the specimen by fully exploiting the total energy offered by the test specifications. This method avoids the risk of underestimating the fatigue damage to undergo the specimen during laboratory testing. The paper offers the mathematical implementation of the method and its experimental validation achieved throughout an intense test campaign. Fatigue tests have been performed on specially designed specimens, by exploiting a three-axial electro-dynamic shaker
Prognostics of rotating machines through generalized Gaussian hidden Markov models
No full textNowadays, the industrial scenario is driven by the need of costs and time reduction. In this contest, system failure prediction plays a pivotal role in order to program maintenance operations only in the last stages of the real operating life, avoiding unnecessary machine downtime. In the last decade, Hidden Markov Models have been widely exploited for machinery prognostic purposes. The probabilistic dependency between the measured observations and the real damaging stage of the system has usually been described as a mixture of Gaussian distributions. This paper aims to generalize the probabilistic function as a mixture of generalized Gaussian distributions in order to consider possible distribution variations during the different states. In this direction, this work proposes an algorithm for the estimation of the model parameters exploiting the observations measured on the real system. The prognostic effectiveness of the resulting model has been demonstrated through the analysis of several run-to-failure datasets concerning both rolling element bearings and more complex systems
Fourier-Bessel series expansion based blind deconvolution method for bearing fault detection
No full textIn the last few years blind deconvolution techniques proved to be useful in order to extract impulsive patterns related to bearing fault from noisy vibration signals. Recently, a novel blind deconvolution method based on the generalized Rayleigh quotient has been proposed and an iterative algorithm related to the maximization of the cyclostationarity of the source has been defined. This paper presents a new condition indicator that exploit the Fourier-Bessel series expansion for the computation of a new cyclostationarity index that drives the maximization problem for the extraction of the excitation source. The main target of this work is to compare the results obtained through the exploitation of the Fourier-Bessel transform with respect to the classic Fourier transform in term of lower number of cyclic frequencies required for the algorithm. The comparison between the application of the two different methods involves both simulated and real signal taking into account a
bearing fault. The results prove the capability of the new indicator to extract the impulsive source without the need of a set of cyclic frequencies but only with the first one, with a strong reduction of the computational time
Modelling the elastodynamic behaviour of a desmodromic valve train
No full textThis paper deals with a lumped-parameter model of a motorbike engine's desmodromic valve train. The model of such an uncommon cam system is developed and validated with the aid of experimental measurements carried out on a test bench which operates the cam mechanism by means of an electrically powered driveline. The model describes the mechanical system taking into account the mass distribution, the link elastic flexibility, and the presence of several non-linearities. The model parameter estimation is discussed and the effectiveness of the model is assessed by a comparison with experimental results. In addition, the model is employed to estimate the magnitude of contact forces and to predict the system behaviour as a consequence of changes in some design parameters; therefore, it may be used as a tool both in design optimisation and diagnostics
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