86,642 research outputs found
Enhanced brush model for the mechanics of power transmission in flat belt drives under steady–state conditions: Effect of belt elasticity
No full textThe present paper is an extension of a previously published paper by the authors, where the “brush” model was adopted for the contact stresses between the belt and pulleys. In this paper, the axial stiffness of the belt is introduced, while in previous work, particularly suited for belt with stiff reinforcement fibers, the belt was assumed to be inextensible. The complete set of equations is derived in steady state conditions and the relationship between belt tension and belt speed is introduced based on the continuity condition. The belt tension can be obtained by solving a second order differential equation, for which a closed form solution is given. A numerical procedure is, however, necessary for determining the solution of a given transmission with assigned rotational speed at the driving pulley and resistant moment at the driven pulley. It is shown how the contact angle at which stick–slip phenomenon may occur is influenced by the belt stiffness and the way the transmission effiency is reduced. Allowing to analyze the mechanics of flat belt transmission, the model can be considered a useful tool for the designer
“Brush model” for the analysis of flat belt transmissions in steady-state conditions
Get PDFIn the present work a novel mathematical model for the analysis of the contact actions between belt and pulleys, particularly suited for flat reinforced rubber belt, is presented. The model considers the tension member, composed of the reinforcement fibers, inextensible, and the rubber matrix, which is subjected to tangential stress, as a continuum bed of elastically deformable bristles, fixed to the tension member on one side and in contact with the pulley on the other side. The deformation of the matrix is inversely proportional to the bending stiffness of the bristles, while friction conditions determine the local adhesion/sliding behavior between belt and pulleys. The proposed model can give a detailed description of the contact conditions along the whole contact arc and is able to describe the stick–slip phenomenon which has been experimentally observed by some authors. The model assesses also the power losses due to the contact stresses and to the elastic deformation of the matrix. The results of the model are discussed in comparison with results from classical models, Grashof and Firbank models, available in the technical literature
Analysis of belt transmissions capabilities using the brush model
Get PDFThe mechanics of power transmission is usually modeled by two different theories: the creep theory and the shear theory. Recently, the authors introduced an alternative theory based on the brush model, which allows to compute the tangential stress distribution along the winding arc of pulleys. The brush model is able to predict the speed loss along the driving and driven pulley as a function of the transmission parameters (e.g. pre-load, friction, pulley radii etc.) and the operating parameters (i.e. angular speed and resistant torque). In addition, the energy efficiency of the system is obtained by knowing the speed loss and the energy dissipation; this contribution can be subdivided into energy loss due to friction and energy loss due to the non-recoverable elastic deformation of the bristle.
In the present paper, using the previously developed model, a sensitivity analysis aimed at mapping the transmission capabilities as a function of geometry and operating parameters is proposed. These results, given as look-up table (or contour plot), are very important in mechanical systems simulation (e.g. real-time systems, hardware in the loop systems) since they allow to introduce the phenomenological behavior of the pulley-belt transmission without introducing complex models in the simulation
Modello a spazzola per le azioni di contatto tra cinghia e puleggia, per trasmissioni con cinghie piatte in regime stazionario
No full textValidation of the brush model for the analysis of flat belt transmissions in steady-state conditions by finite element simulation
No full textIn this paper a finite element (FE) model for the analysis of the contact stresses in flat belt transmissions was developed, with the intent of comparing the numerical with the theoretical results of the brush model and those of the classical Euler–Grashof (creep) model. The FE model consists of two pulleys and a belt composed of a thin layer of inextensible reinforcement fibers and a rubber matrix in contact with the pulley. The analysis is performed incrementally, under quasi-static conditions; as a consequence, any inertia effect is not accounted for. In the paper, the capabilities of the analyzed models are discussed. The brush model is generally better correlated with the FE results, both in terms of tangential stress along the winding arc and belt tension and it is capable of estimating the power losses due to friction with low computational and time effort. In addition, the effect of the belt thickness on the tangential stress at the entrance and the exit from the pulley, which are generally neglected by simplified model, are highlighted by the FE analysis
Multi-body simulation of a rope-driven automated people mover
Get PDF In this paper, a multibody model of the automated people mover, PisaMover, is presented. PisaMover is a rope-driven small train, composed of a few cabins, which makes use of train-inspired bogies. The aim of the model was to support the design of the architecture of the suspensions and to select the proper characteristics of the elastic and damping elements in order to fulfill comfort needs of passengers and to resolve the constraints related to the layout of the vehicle and the guide-ways. For this purpose, attention was especially focussed on the definition of the railway path and the modeling of the forces of the supporting and guidance wheels. A simplified model of the rope was implemented, which neglects the rope elasticity and computes the rope force direction taking account of the different positions of the sheaves along the path. The multibody simulation allowed to select the most appropriate suspension system and to properly define the elastic and damping characteristics of the shock absorbers, with respect to the technical constraints. A modal analysis was performed, and several dynamic on-track simulations were carried out to infer the effect of dampers’ design on the comfort of passengers. Within the validity of the simplifying assumptions, the multibody simulation also allowed to obtain a fairly good estimate of the loads necessary for the design of the main structural components. </jats:p
Fatigue assessment of structural components through the Effective Critical Plane factor
Get PDFThe integrity assessment of structural components under complex loading conditions relies on the evaluation of the fatigue damage typically arising from stress concentrations, such as geometric irregularities, notches, weld beads, grooves etc.. Various methodologies, including the Notch Stress Approach (NSA), the Theory of Critical Distances (TCD), the Strain Energy Density (SED), and the Critical Plane (CP) concept, have been pivotal in assessing fatigue strength for notched and welded components. Recent works combine some of the above mentioned methodologies, while other works propose to vary the embedded parameters accounting for the loading type or the fatigue lives, trying to improve the accuracy of the fatigue assessment process. This paper introduces a novel approach, the Effective Critical Plane (ECP), which is founded on the critical plane concept. The CP factor is, however, calculated starting from an averaged, over a small volume, stress- strain field. The size of the averaging volume is assumed to be a material parameter and is determined by a best fitting procedure over different experimental data sets. The novel approach is illustrated by means of the Fatemi-Socie and the Smith-Watson-Topper CP damage factors. Its potential application to other CP formulations is straightforward, as well. Literature experimental data for low carbon steel specimens possessing different notches and loading conditions are used to validate the method's capability in accurately determining the fatigue life and to set the radius of the averaging volume for the given material and CP parameter. A spherical volume or circular area are used in case of fully 3D or 2D numerical models, respectively. Results are compared to those of some of already existing methods, namely SED, TCD and the Modified W & ouml;hler Curve Method
Progettazione di prove sperimentali per la calibrazione del modello di rottura di Johnson-Cook
Get PDFFrequency analysis of random fatigue: Setup for an experimental study
Get PDFThe frequency-domain approach to fatigue life estimation in random loading has been largely investigated due to its computational advantages, and several methods for the frequency translation of the most common time-domain methods have been proposed. Between the most known frequency methods there are the Bendat's Method, valid for narrow-band signals, and the Dirlik's formula, which is considered the best result for wide-band signals. However, the great part of the frequency methods takes the rainflow count as a reference time-domain method and uses the rainflow damage computation as the exact value to emulate. Therefore, very few experimental data for fatigue life of mechanical components subject to random loads are available in the literature. This work presents the setup for a series of experimental tests for specimens subjected to random loads, aiming at achieving experimental data to compare with the results provided by frequency methods. After a brief description of the materials used for the setup, the two-step test concept is described: firstly, the specimen will be subjected to random loads obtained by a certain PSD for an amount of time which should nominally cause a 30% of damage; then, the fatigue test will be ended on a resonance testing machine to compute the actual residual fatigue life of the specimen; this two-step testing also allows to reduce the time requested for the tests. The test bench developed for the experimental investigation is described in the paper, together with the results of some preliminary tests, aimed at verifying the feasibility of the conceived procedure
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