5,210 research outputs found
Analisi del cedimento strutturale del braccio di una gru portuale
Full text linkNel presente lavoro vengono discusse le cause del cedimento strutturale del braccio di una gru portuale a portata variabile. L’ispezione delle sezioni di rottura ha mostrato in particolare la presenza di una grossa fessura preesistente al momento del crollo, che si estendeva per circa metà sezione di uno degli elementi tubolari principali che costituiscono il traliccio del braccio. Al fine di stabilire le modalità del crollo, nel lavoro vengono presentati un modello per l’analisi della propagazione della fessura, che fa uso del metodo delle weight functions ed un modello per l’analisi di collasso della sezione. Le analisi condotte hanno permesso di concludere che il crollo si è verificato per collasso plastico della sezione ed hanno messo in evidenza come la fessura in esame, per una parte considerevole della vita, sia propagata in condizioni stabili con velocità di avanzamento circa costante
A new formulation of the understeer coefficient to relate yaw torque and vehicle handling
Full text linkThe handling behaviour of vehicles is an important property for its relation to performance and safety. In 1970s, Pacejka did the groundwork for an objective analysis introducing the handling diagram and the understeer coefficient. In more recent years, the understeer concept is still mentioned but the handling is actively managed by direct yaw control (DYC). In this paper an accurate analysis of the vehicle handling is carried out, considering also the effect of drive forces. This analysis brings to a new formulation of the understeer coefficient, which is almost equivalent to the classical one, but it can be obtained by quasi-steady-state manoeuvres. In addition, it relates the vehicle yaw torque to the understeer coefficient, filling up the gap between the classical handling approach and DYC. A multibody model of a Formula SAE car is then used to perform quasi-steady-state simulations in order to verify the effectiveness of the new formulation. Some vehicle set-ups and wheel drive arrangements are simulated and the results are discussed. In particular, the handling behaviours of the rear wheel drive (RWD) and the front wheel drive (FWD) architectures are compared, finding an apparently surprising result: for the analysed vehicle the FWD is less understeering than for RWD. The relation between the yaw torque and the understeer coefficient allows to understand this behaviour and opens-up the possibility for different yaw control strategies
Analysis of the catastrophic failure of a dockside crane jib
No full textThe catastrophic failure of a crane jib is discussed in the paper. The final failure was determined by an existing crack, which had propagated up to a considerable part of the cross section of one of the main tubular elements, which constituted the jib frame. The crack originated from a seam weld, which was used to join a stiffener on top of the tubular element where some cracks were found few years before the failure and that, actually, resulted to be a misguided attempt to strengthen the jib structure at that point. A stress analysis of the crane jib is firstly carried out, in order to analyse the stress levels nearby the failure region and to discuss the fatigue design with reference to current standards. In the second part of the paper, a crack-growth model is developed in order to achieve an estimate of the propagation period and a simple collapse model, based on beam theory, is used together with the material properties determined by experimental tests, to determine whether the final failure was determined by fracture or by plastic collapse. The analyses conclude that the fatigue design requirements were not satisfied and that the final failure was determined by plastic collapse of the critical cross section, after a relatively long stable propagation period. Some considerations about the necessity of scheduled and qualified inspections are also drawn
L’impatto della progettazione sulla sicurezza degli apparecchi di sollevamento
No full textNella presente memoria vengono presentate le valutazioni tecniche relative a due incidenti industriali
che hanno interessato due apparecchi di sollevamento di notevoli dimensioni. Viene presentata la
ricostruzione dei due incidenti, sino alla determinazione delle cause del cedimento e degli aspetti
progettuali ad esse correlati, al fine di mettere in risalto l’impatto della progettazione meccanica sulla
sicurezza dei luoghi di lavoro e l’importanza della programmazione di controlli periodici qualificati.
Gli incidenti esaminati sono stati il crollo di una gru mobile portuale, avvenuto in seguito alla
propagazione di un difetto di fatica originatosi in prossimità di un cordone di saldatura ed il
deragliamento di una gru a cavalletto, che si è verificato in presenza di forte vento. Entrambi gli
incidenti sono riconducibili ad errori di progetto, nel senso del mancato rispetto dei criteri di
dimensionamento indicati negli standard di riferimento
Simulation of motor-scooter drop testing by a multi-body finite element approach
No full textThe drop test is employed in the motor-scooter industry for evaluating the frame
performance under particularly severe conditions. In this work, a numerical procedure developed
for simulating the test is presented. The procedure includes a multi-body model for the evaluation of the maximum loads acting on the scooter frame during impact at ground. Such loads were then employed in a non-linear static finite element analysis in order to obtain the permanent frame deformation.
The experimental activity carried out for vehicle characterization and the experimental apparatus
set up for tests are also presented in detail. The multi-code approach appeared versatile and
straightforward, and furnished results in satisfactory agreement with experiments
Gantry crane derailment and collapse induced by wind load
No full textThe catastrophic derailment of a gantry crane is analysed in the paper. Under the action of a persistent and very strong wind, with gusts at about 110 km/h, the crane started moving and, after having travelled for about 60 m, derailed and collapsed. Fortunately, no injured operator resulted from the event.The crane was endowed with passive rail clamps, which had to be dimensioned with reference to a very strong, i.e. out-of-service, wind intensity, given in standards. The analysis of technical data of the crane and of the installed clamps brought to the conclusion that a trivial error was made in the determination of the out-of-service wind load, since no aerodynamic coefficient was taken into account in the evaluation of the wind thrust. Due to this, the clamps didn't respond to the current regulations and, actually, were not capable of keeping the crane standing for the strongest wind load, which may occur in the region of the plant installation.The analysis, based on crane geometry and wind recordings, shows that, actually, the clamps capacity was exceeded by the wind load at the moment of the accident. A plane analysis of the crane loading condition, based on a beam model, allowed to explain the way the crane collapsed, with the rotation about a vertical axis of one of the leg, and to estimate the loads exchanged between the crane bogies and the rail, that caused the rupture of several rail anchoring bolts.The accident was caused by a design error and this points out the necessity of accurate evaluations, with multiple and independent checks, in particular in case of large plants with significant risk for operators, as in this case, or for the environment
Simulazione multibody di veicoli a 2 ruote. Alcune esperienze presso l'Università di Pisa
No full textStability analysis of a novel four-wheeled motorcycle in straight running
Full text linkThis paper deals with the stability analysis of a novel four wheeled tilting motorcycle. This new kind of vehicle is equipped with two front steering wheels and two rear wheels; the front and rear suspension mechanisms allow the scooter to roll freely as a standard motorcycle. An analytical model of the vehicle with locked suspensions, rigid and lenticular tires was developed in order to study the stability of the system in straight running and to carry out a modal analysis focusing on the main normal modes. The frame and all the vehicle components were considered as rigid bodies, the driver was considered fixed to the frame and the tires behavior was described by a linear function of the camber and side-slip angles. The action of the driver on the handlebar was not taken into account, considering the vehicle running in straight without any control system. The stability analysis shows that the eigenmodes of the four-wheeler have several common features with the capsize, weave, wobble and rear wobble modes which characterize the dynamics of two and three wheeled motorcycles. A sensitivity analysis showing the influence of the front trail on stability and the vehicle eigenmodes is presented. A graphical and intuitive representation of the eigenvectors is also given
Elastokinematic analysis of compound crank axle suspensions
No full textIn the present paper a theoretical model for the analysis of compound crank axle is presented. It is based on the set of equilibrium, constitutive and congruence equations of the axle; to this aim, the cross-member is described by its stiffness matrix, the longitudinal arms are assumed as rigid and the bearing which connects the axle to the body as ideal. For this configuration, a closed form solution for the elastokinematic analysis is obtained in the small displacements field, for the pure roll condition. This can be summarized by the knowledge of the position, in a 3D space, of the instantaneous axis of rotation of each wheel; it is shown how the instantaneous axis of rotation is determined by both the axle geometry and the stiffness properties of the cross member. An iterative procedure has been also developed for the large displacement analysis. Results, in terms of camber and toe angle alteration, are discussed in comparison with those obtained by finite element analyses
Automatic transmission for small displacement motor-scooters
No full textThis work deals with an automatic power train for two-wheelers and it is carried out
in the framework of the Italian MUSS project. Environmental issues are currently within the main
topics on which vehicle design is focused. In this context, two wheelers have to face many challenges and one of them is to reduce the loss of efficiency in the power train. Motor-scooters are currently quite always equipped with CVT transmission with rubber belt. This kind of transmission can be very
cheap to manufacture, it has good comfort performance but has a mechanical efficiency of just 80% – 85% in the best working conditions. In this work the study of an automatic transmission and its
application to a small displacement motor-scooter are presented. In order to keep low manufacturing
costs and a high comfort typical of the CVT, improving its efficiency at the same time, the studied
system is based on a discrete ratio gear box with mechanical control. The transmission has gears and
centrifugal clutches and free-wheels which mechanically control the gear shift. A dynamic model of the transmission and results of a typical acceleration manoeuvre are presented and discussed in the paper
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