The multibody simulation of railway vehicle dynamics needs a reliable and efficient method to evaluate the contact points between wheel and rail, because their positions have a considerable influence on the direction and intensity of the contact forces. In this work, an innovative semi-analytic procedure for the detection of the wheel/rail contact points (named the DIFF method) is presented. This method considers the wheel and the rail as two surfaces whose analytic expressions are known and is based on the idea that in the contact points the difference between the surfaces has local minima and is equivalent to solving an algebraic two-dimensional system. The original problem can be reduced analytically to a simple scalar equation that can be easily solved numerically (since the problem dimension is one, even elementary non-iterative algorithms can be efficient).

The numerical wheel wear prediction in railway applications is of great importance for different aspects, such as the safety against vehicle instability and derailment, the planning of wheelset maintenance interventions and the design of an optimal wheel profile from the wear point of view. For these reasons, this paper presents a complete model aimed at the evaluation of the wheel wear and the wheel profile evolution by means of dynamic simulations, organised in two parts which interact with each other mutually: a vehicle's dynamic model and a model for the wear estimation. The first is a 3D multibody model of a railway vehicle implemented in SIMPACK™, a commercial software for the analysis of mechanical systems, where the wheel–rail interaction is entrusted to a C/C++user routine external to SIMPACK, in which the global contact model is implemented. In this regard, the research on the contact points between the wheel and the rail is based on an innovative algorithm developed by the authors in previous works, while normal and tangential forces in the contact patches are calculated according to Hertz's theory and Kalker's global theory, respectively. Due to the numerical efficiency of the global contact model, the multibody vehicle and the contact model interact directly online during the dynamic simulations.\ud \ud The second is the wear model, written in the MATLAB® environment, mainly based on an experimental relationship between the frictional power developed at the wheel–rail interface and the amount of material removed by wear. Starting from a few outputs of the multibody simulations (position of contact points, contact forces and rigid creepages), it evaluates the local variables, such as the contact pressures and local creepages, using a local contact model (Kalker's FASTSIM algorithm). These data are then passed to another subsystem which evaluates, by means of the considered experimental relationship, both the material to be removed and its distribution along the wheel profile, obtaining the correspondent worn wheel geometry.\ud \ud The wheel wear evolution is reproduced by dividing the overall chosen mileage to be simulated in discrete spatial steps: at each step, the dynamic simulations are performed by means of the 3D multibody model keeping the wheel profile constant, while the wheel geometry is updated through the wear model only at the end of the discrete step. Thus, the two parts of the whole model work alternately until the completion of the whole established mileage. Clearly, the choice of an appropriate step length is one of the most important aspects of the procedure and it directly affects the result accuracy and the required computational time to complete the analysis.\ud \ud The whole model has been validated using experimental data relative to tests performed with the ALn 501 ‘Minuetto’ vehicle in service on the Aosta–Pre Saint Didier track; this work has been carried out thanks to a collaboration with Trenitalia S.p.A and Rete Ferroviaria Italiana, which have provided the necessary techn...

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In railway applications, the estimation of the wear at the wheel-rail contact is an important field of study, mainly correlated to the planning of maintenance interventions, vehicle stability and the possibility to carry out specific strategies for the wheel profile optimization.In this work Authors present a model conceived for the evaluation of the wheel profile evolution due to wear, which is organized in two parts, mutually interactive: a vehicle model for the dynamic analysis and a model for the wear estimation.The wheel wear evolution is reproduced by dividing the overall chosen mileage to be simulated in discrete spatial steps: at each step the dynamic simulations are performed by means of the vehicle model keeping the wheel profile constant, while the wheel geometry is updated through the wear model only at the end of the discrete step. Thus, the two parts of the whole model work alternately until the completion of the whole established mileage. Clearly, the choice of an appropriate step length is one of the most important aspect of the procedure and it affects directly the result accuracy and the required computational time to complete the analysis.The entire model has been validated in collaboration with Trenitalia S.p.A and RFI, which has provided the technical documentation and the experimental results relating to some tests performed on a scenery that exhibits serious problems in terms of wear represented by the vehicle ALn 501 "Minuetto" on the Aosta-Pre Saint Didier line.

The numerical simulation of system dynamics is today a standard in the design of railway vehicles; their typical applications are the suspension kinematics, handling performance and ride comfort as well as the generation of load data for lifetime prediction. One of the key points in this type of simulation is the model of the wheel/rail interaction, in other words the definition of the forces exchanged between the wheels and the rail in the contact points. The direction and the magnitude of the contact forces depend on the number and the location of the contact points. The procedure that allows one to define the geometry of the contact then has a significant effect on the reliability of the simulation. The component of the normal contact force to the contact surfaces can be defined as a function of the relative indentation between the surfaces. The component of the contact force tangent to the contact surfaces depends on the relative speeds between the surfaces in the contact area (wheel sliding). The authors have been working on the definition of efficient and reliable models of the interactions between the wheels and the rails and in particular for the definition of the contact points. Different algorithms have been analyzed and compared, based on semi analytical approaches and on neural networks. The paper will summarize the proposed methods and the results obtained from the simulation of two different sceneries. Two different models have been used in this test: the first one was realized with a commercial software, while the second one was developed and implemented by the authors. There is a global agreement between the models, although some differences can be seen during the transients due to the different methods for the determination of the contact points for the integration.

Friction dampers are one of the most common structures used to alleviate excessive vibration amplitudes in turbomachinery applications. There are very well-known types of contact elements exploited efficiently, such as underplatform dampers. However, different design approach is sometimes needed to maximize the effectiveness further. In this paper, computational forced response prediction of bladed disks with a configuration of the secondary structure commonly used by Baker Hughes design, the so-called mid-span dampers, is presented. Mid-span dampers are metal devices positioned at the middle section of the airfoil span and come into contact with the blade by the centrifugal force acting during rotation. Proposed damping mechanism is applied to a realistic steam turbine bladed disk under cyclic symmetric boundary conditions. Friction contact is modeled through a large number of contact nodes between the blade and the damper by using a 2D friction contact element with variable normal load. Harmonic Balance Method and Alternating Frequency/Time approach are utilized to obtain nonlinear algebraic equations in frequency domain and nonlinear forced response is computed by using Newton-Raphson method. The results obtained by numerical simulations show that mid-span dampers are an efficient configuration type of a damping mechanism to be used in the design of the bladed disks for nonlinear vibration analysis.

A better integration and interoperability between rail and road transportation is a key factor in order to reduce pollution and increase railway freight traffic.Development of special freight wagons like "SAADKMS" for the transportation of trucks by railway is a successful solution that is meeting an increasing consensus and popularity among many European countries. In order to accelerate truck loading on wagons and reduce the limitation of normal clearance (structure/vehicle/loading gauges) it is necessary to reduce the wheel diameter as much as possible. This is not a drawback-free solution since an excessive reduction of wheel diameter involves many troubles concerning the stability of the vehicle, maximum axle load, wear of bearings and rolling surfaces of rails and axles. Also designing the braking system is very complicated because the reduced number of encumbrances available makes the placement of internal disks on the axles difficult.In order to solve these problems a very original solution concerning wheelset, wheel profiles and more general bogie design have been applied in the development of "SAADKMS" freight wagons so the resulting vehicle is very different from the conventional one. As a matter of fact, many past experiences and know-how for conventional freight wagons are not applicable for this kind of application, so numerical simulations are very important to deeply understand the behaviour of the system and propose criteria for further optimization.The authors of this paper have developed models on commercial multibody software in order to simulate the behaviour of the "SAADKMS" freight wagon in different conditions (stability, steering performances). Also important results such as position of the contact point or wear-number are shown to be useful parameters for further optimization of the rolling surface profiles.

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