Luca Landi

Random Loads Fatigue: The Use of Spectral Methods Within Multibody Simulation

The evaluation of the fatigue damage performed by using the Power Spectral Density function (PSD) of stress and strain state is proving to be extremely accurate for a family of random processes characterized by the property of being stationary. The present work’s original contribution is the definition of a methodology which extracts stress and strain PSD matrices from components modelled using a modal approach (starting from a finite element modelling and analysis) within mechanical systems modelled using multibody dynamic simulation and subject to a generic random load (i.e. multiple-input, with partially correlated inputs). This capability extends the actual stress evaluation scenario (principally characterised by the use of finite element analysis approach) to the multibody dynamic simulation environment, more powerful and useful to simulate complex mechanical systems (i.e. railway, automotive, aircraft and aerospace systems). As regards the fatigue damage evaluation, a synthesis approach to evaluate an equivalent stress state expressed in terms of the PSD function of Preumont’s “equivalent von Mises stress (EVMS)”, starting from the complete stress state representation expressed in terms of PSD stress matrix and easily usable in the consolidated spectral methods, is proposed. This approach allows and has allowed the use of the above methods such as the Dirlik formula as a damage evaluation method. An additional result is the conception and implementation of a frequency domain method for the component’s most probable state of stress, allowing quickly identification of the most stressed and damageble locations. The described methodologies were developed and embedded into commercial simulation codes and verified by using as a test case a simple reference multibody model with a simple flexible component.Copyright

Analytical Model, Multibody Simulation and Validation Tests for Dynamical Instability Reduction of a Grinding Machine With Dampers

The subject of the present paper is the dynamic instability (chatter) of machine tools. The chatter is a self-regenerative oscillatory phenomenon related to the tool-work piece interaction during the process; that leads to an incorrect surface finishing of work piece. In big machine tools passive damping components are often introduced in order to reduce the chatter. Those passive components have to be tuned to the natural frequency of chatter vibrations, measured experimentally or predicted theoretically. In this paper the reduction of chatter for a planar grinding machine is described. At first, a theoretical model for the prediction of the general chatter for planar milling will be described through two lumped parameters and a multi body numerical model. Moreover the correlation between the theoretical model and experimental measures in terms of receptance will be presented. Two different kinds of passive damper systems will be theoretically introduced. In the end, the experimental results for chatter reduction obtained with the passive dampers introduction will be shown.© 2011 ASME

A Methodology for Active Control of Multibody Test-Rig for Virtual Simulation of Vehicles Through Acceleration Inputs

Dynamic simulation through Multibody Systems is more and more used for the design of new industrial products to avoid long and expensive experimental tests on them. The research presented in this paper deals with the realization of an actively controlled virtual test-rig, functional for comfort and durability tests of entire vehicles. The virtual test rig, is capable to impose any experimental acceleration to the centers of the four vehicle wheels through four imposed displacements to the actuators. The active control is implemented with four independent degrees of freedom, one for every actuator. In the paper the active controller logic and its automatic setup will be fully explained. Moreover it will be also explained how to avoid the aliasing and the so called “random signal drift” problems. Finally the results of some virtual pave road experiments to an existing vehicle will be presented.Copyright

Integrated Roller Coaster Design Environment: Dynamic and Structural Vehicle Analysis

In this paper a tool for the multibody simulation and structural analysis of roller coaster (RC) is illustrated. It was developed in order to support the designer and manufacturer of rides during the design stage. The goodness of the modelling and simulation procedure was validated in terms of dynamic behaviour by experimental/numerical comparisons by conducting an experimental tests campaign on the existing Roller Coaster. Another aim was to verify and illustrate the possibility to recover the stress state of RC axles not from a rigid multibody model and by an indirect approach but, directly, by a rigid/flexible model by their modal modelling.Copyright

An Analytical Model for Force Estimation on Arms of Concrete Mixers

Claudio Braccesi Perugia University Dep. of Industrial Engineering Via G. Duranti 1, 06125 Perugia Italy Luca Landi* Perugia University Dep. of Industrial Engineering Via G. Duranti 1, 06125 Perugia Italy ABSTRACT The main task of our research is to achieve a proper analytical model to describe the forces exchanged between the mixing arms of a mechanical mixing machine and concrete during the whole mixing process. The model has been validated through experimental tests on a real type of mixer. The behavior of the sand- cement mix is very different in the three principal phases of a possible mixing operation: Phase 1: first mixing phase, sand and concrete are inserted in the mixer without water, “dry - phase”, Phase 2: a proper quantity of water is added to the dry mix for final mixing, “wet phase”, Phase 3: final mixing for complete homogenization of the mix. The electrical power necessary in the three phases is very different and in close relation to very different theoretical model that we will discuss later in the article. The main correlations between the geometrical shape of a so called omega mixer and the force exchanged is presented in the article. At the end of the article the coupling between the theoretical and the experimental data is presented. 1 INTRODUCTION The mechanical design of a mechanical mixer is based on forces exchanged between the mixing arms - blades and the concrete. These forces change abruptly during the mixing process but the precise entities of these forces is ignored [1]. Also in some latter published article tests on concrete mixing machine have been carried out and a precise correlation between the paste composition and the power necessary for the mixer is investigated [2]. Nowadays when a new machine is to be designed some very rough calculations for power consumption is usually made. So the design process is based on rough calculation of blade forces using an experimental investigation on a particular mixer already designed with a particular concrete mix [3]. These forces cannot be used for the design process of innovative machines nor for force estimation on machine of the “same family” but of different size. For fatigue strength investigation of a new family of mixer is still necessary to build some prototypes of the main components of the machine and some “early design stage problems” for fatigue problems are expected. On the other hand there is a lot of research focused on the behavior of granular material and concrete material as the ones in [4-9]. If high performance concrete is to be obtained, all the studies agree on the necessity of a controlled phase of mixing with an automated process. Moreover the controlled addiction of small quantities of different additives can abruptly change the final behavior of these materials, see for examples the articles [10-12]. In this paper we will show how the well known Brinch-Hansen formula [4] and the Bingham theory of fluids [8] can be used to describe the behaviour of concrete respectively in the so called dry and final mixing phases. The application of these two general different analytical models for concrete in the different phases, to a specific case of mixer will be presented in the paper. The full mixing process will be investigated in the article focusing the force exchanged between the mixer and the concrete. This model covers all the main phases of the granular mixing in the worst case of a wrong loading procedure (higher forces): 1. loading of the dry material in the tank , taking into account the loading law (constant flow– step law), 2. water addition phase , also if the transient behavior of the material is quite impossible to predict, it is

Analysis and Optimization of a Spring Based Clamping System

Clamping systems based on a spring mechanism are widely used in tooling machine industry due to its simplicity, low cost and high reliability.This paper explores the theoretical formulation of a general clamping mechanism based on direct spring actuation, in terms of the two different requirements: a proper clamping force and a limited maximum stress for a better fatigue duration.First the influence of the main “key design parameters” on the mechanism characteristic will be recovered, the results show clearly that, the simple re-engineering of typical solution does not meet the requirements for a new multi-purpose tooling machine.Moreover the improved performance obtainable for the clamping system using a “preloaded mechanism” will be evidenced thorough an analytical formulation taking into account the different behaviors of the mechanism during the preloading phase and the clamping phase.The goodness of the new analytical model will be verified through analytical and FEM non-linear model comparison for an industrial clamping system based on a flexural spring both in terms of clamping force and of fatigue durability.Copyright

Non Linear Multibody Modelling for the Vibrational Prevision of the Shift Lever of Automotive Gearboxes

The main task of this paper is to identify and formalize “design methodologies” useful to highlight/solve any possible problem due to the vibrations of a new gear box command system in each design stage of this component. In particular, we focused on the so-called phenomenon of “shift lever vibration” (SLV - vibration of the shift leverage in the car cabin). This phenomenon can occurs if a natural frequency of the control system is excited by a movement of the so called shift collars in the gear box. Therefore an unwanted low level of comfort and high level of noise and vibration can be experienced by the car driver. In this paper we will explain in detail the SLV phenomenon by measuring a real shift command during acceleration/deceleration manoeuvres. Then we will present two different types of multibody models developed to predict this phenomenon: the first simple-linear one is useful to “predict” the first natural frequency of the shift lever vibration. The first one can be used in the early design stage of the main components of the gearbox control. It is useful to the designer to perform a sensitivity analysis of the most important parameters to be taken into account to prevent the SLV in the cabin of the car. The second complete nonlinear multibody model is useful, not only to take into account all the non-linear aspects of the phenomenon (different behaviours in acceleration and deceleration manoeuvres), but also to predict the level of acceleration in the shift lever. The results obtainable with the multibody approach will be then compared with the results from experimental data of a real shift command.Copyright

Parametric Multibody Modeling of Anthropomorphic Robot to Predict Joint Compliance Influence on End Effector Positioning

Nowadays, in the field of robotic, one of the most important objectives is to reduce robot error positioning and improve its dynamic behaviour. One of the main source of error in end effector positioning is due to the joint compliance: robot joint components under operating conditions can be deformed as a function of their stiffness/damping properties. Generally, for industrial robots, harmonic drive gearings are used, their principal characteristics are high transmission ratio and law weight, on the other hand, to realize high transmission ratio, harmonic drive gearings work on inner gear elastic deformation, conferring to the robot joints an excessive compliance that, in some robot applications, cannot be neglected. In this research activity multibody modelling and simulation approach has been used to analyse joint compliance influence on robot position accuracy. The principal aim of this work was the formulation of a modelling procedure that starting from classical robots modelling approach (i.e. Denavit Hartenberg) defines an universal database and a parametric modelling procedure that allows the designer to use any multibody commercial codes to analyse anthropomorphic robots considering or not the compliance effect. All the procedure was developed and managed into a numerical code environment (Matlab/Simulink).An example of commercial anthropomorphic robot was considered by assuming its principal kinematic and dynamic characteristics. Parametric models of the robot have been developed in two different multibody modelling environments (Simmechanics, Adams/View). Moreover the models structure has been built in order to control the robot movements both in motion (open loop) or in force (closed loop). In this case they are interfaced with Simulink code in a so called co-simulation approach that allows to developed a generic control system and test it by using one or more models, less or more refined.Copyright

New dual meshless flexible body methodology for multi-body dynamics: Simulation of generalized moving loads

Abstract One of the main limits on the design phase of new mechanical components is the limited integration between the computer aided design (CAD) and the computer aided engineering (CAE) phases [1,2]. At present the virtual validation phase, carried out using various methods such as the finite element method (FEM) and multibody analysis (MBS), wastes a lot of time on model meshing, simplifications and database transfers. In recent years some new so-called meshless methodologies for CAx phase integration have been improved. In this paper the feasibility of a new meshless methodology based on spline geometry description is presented. Using this method it is possible to employ directly the B-spline CAD description to describe the physical behaviour of a general flexible body for all the CAE validation phases. Moreover, this method, taking advantage of its continuous formulation, is able to solve some important unsolved problems such as the generalized interactions between flexible bodies in multi-body analyses. In this paper the full method formulation is presented through curved beam implementation and examples.