Salvador Cardona

Evaluation of the Influence of a Planned Interference Fit on the Expected Fatigue Life of a Conjugate Cam Mechanism- A Case Study

Due to dynamic effects, clearances, manufacturing and assembly errors in form-closed cam mechanisms, the follower jump can also occur. For conjugate cam mechanisms a technique to avoid the follower jump without the use of a spring involves making the conjugate cam profiles bigger than the kinematical ones by adding an external offset. This strategy produces an interference fit between the conjugate cam profiles and the follower train. This paper presents an ordered procedure to study the influence that the planned interference fit has on the evaluation of the contact forces, the expected fatigue life of the rollers, contact pressures and the lubrication conditions. The study is based on a conjugate cam mechanism with translational roller followers used in a real automatic process for manufacturing muselets. A three-degree-of-freedom dynamic model is proposed and the Hertzian theory for general profiles is used to model the nonlinear contact stiffness between the cams and the crowned rollers. The dynamic model predicts that it is difficult to obtain conjugate cam mechanisms with an infinite expected fatigue life of the rollers just by considering typical achievable manufacturing errors or clearances, and as happens in reality, a set-up process is highly recommended. The procedure is also tested with measured manufacturing errors on a coordinate measure machine—CMM—and with measured radial internal clearances for the rollers measured by an experimental apparatus. Also, to evaluate lubrication conditions, surface finishing measurements have been taken of the cams and the rollers with a surface profiler.

Modelo global de la dinámica de contacto rueda-carril para determinar la vibración de un punto del carril al paso de un tren

The profile irregularities of railway wheels are the main source of vibrations generated by the passage of a train. These vibrations are transmitted to the surroundings, affecting the zones near the railway network. In this work, an analytical wheel-rail contact model based on the hertzian contact theory is presented, with the aim of studying the vibration in a track location. The utility of the linear and nonlinear contact is analyzed. Using this model, the rail response to the presence of wheel flats and other wheel profile irregularities is studied.

A mathematical model to study railway track dynamics for the prediction of vibration levels generated by rail vehicles

This article presents a mathematical model aimed at predicting wheel–rail contact vibration force arising from wheel profile irregularities. Prediction of vibrations caused by a passing train is a basic factor in environmental impact studies related to planning new railway lines. The prediction model is useful during the development of a project as it facilitates selection of the most suitable track for reducing vibration levels. This article presents the model, analyses the influence of the track base on the vibration-generating mechanisms, and compares the dynamic behaviour of the most widely used urban railway tracks.

Integration of Product Conceptual Design Synthesis into a Computer-Aided Design System

Commercial Computer-Aided Design systems have been mainly focused in give support to the process of capturing and representing geometric shapes and incorporating technological information. Conversely, few utilities in these systems are present to facilitate decision making in the early stages of the design process, such as the capture, modeling and conceptual design synthesis of solutions. Typical tasks of the conceptual design process in mechanical design are intended for applications stand-alone or are based on the heuristic knowledge of the designer. Such approaches are non-interoperable with the commercial computer-aided design systems which results in non-continuous information in the design process. This study addresses this subject and proposes a method to improve the integration of product conceptual design synthesis into a Computer Aided Design system. To validate the feasibility of the approach implemented, a prototype application based on a Computer Aided Design system was developed and a study case is presented.

MODELOS PARA LA GENERACIÓN Y TRANSMISIÓN DE VIBRACIONES AL PASO DE UN TREN

El presente articulo resume los modelos empleados para representar las vibraciones generadas al paso de un tren, asi como tambien su transmision al entorno. Estos metodos permiten determinar la respuesta dinamica de la via ferroviaria, considerando las fuerzas desarrolladas en el area de contacto ruedacarril, el perfil de irregularidades de la rueda, la impedancia del terreno y los mecanismos de transmision hacia el hastial del tunel. Adicionalmente, se presenta una revision de las tecnicas de deteccion de vibraciones mas usuales.

Enhancing the Expected Fatigue Life in a Form-Closed Conjugate Cam Mechanism

To solve the indetermination of forces existing in a form-closed cam mechanism formed by conjugate cams, where the contact between the cams and the follower rollers is constantly ensured by only the geometry of the elements, dynamic models that consider the elasticity of the elements must be proposed. Because the stiffness of the main elements is associated with the elasticity of the solids, tight variations in manufacturing and assembly errors modify the effective interference fit, which significantly affects the expected fatigue life of the mechanism, leading to a premature failure of the elements due to surface fatigue. Based on a real industrial application of a conjugate cam mechanism and using lumped-parameter models, the objectives of this paper are: first, to show that it is difficult to achieve a pure form-closed conjugate cam mechanism, with the expected fatigue life of the mechanism, by using only standard tolerance specifications; second, to compare the expected fatigue life and motor torque with other cam mechanism design strategies such as force-closed and the combination of force-closed and form-closed strategies, known as force-closed conjugate cam strategy. This paper based on simulation results demonstrates that this latest strategy can, thanks to a better control of the preload, easily achieve results very similar to the theoretical ones of a form-closed conjugate cam mechanism. A prototype of the mechanism of the force-closed conjugate cam strategy is also built.

Avoiding Early Failures in Conjugate Cam Mechanism by Means of Different Design Strategies

To solve the indetermination of forces existing in a form-closed cam mechanism formed by conjugate cams, where the contact between the cams and the follower rollers is constantly ensured by only the geometry of the elements, dynamic models that consider the elasticity of the elements must be proposed. Because the stiffness of the main elements is associated with the elasticity of the solids, tight variations in manufacturing and assembly errors modify the effective interference fit, which significantly affects the expected fatigue life of the mechanism, leading to a premature failure of the elements due to surface fatigue. Based on a real industrial application of a conjugate cam mechanism and using lumped-parameter models, the objectives of this paper are: first, to show that it is difficult to achieve a pure form-closed conjugate cam mechanism, with the expected fatigue life of the mechanism, by using only standard tolerance specifications; second, to compare the expected fatigue life and motor torque with other cam mechanism design strategies such as force-closed and the combination of force-closed and form-closed strategies, known as force-closed conjugate cam strategy. This paper based on simulation results demonstrates that this latest strategy can, thanks to a better control of the preload, easily achieve results very similar to the theoretical ones of a form-closed conjugate cam mechanism. A prototype of the mechanism of the force-closed conjugate cam strategy is also built.

Influence of the Contact Stiffness Between the Cam and the Roller in the Dynamic Behaviour of the Train Follower Applied in a Conjugate Cam Mechanism

The aim of this study is to analyze the influence that the contact stiffness between the cam and the roller has in the prediction of the dynamic behaviour of the train follower. For this purpose two models based on the Hertzian contact theory are compared. The first model uses the formulation of the general contact assuming a nominal contact point between the cam and the roller. The second model uses the formulation of the cylindrical contact assuming a nominal contact line between them. The objective is to discuss about the convenience of both models when facing the calculations of the deviation from the theoretical motion law, contact forces, contact pressures, life span and surface fatigue in comparison with the surface finishing process for the cam or the roller. The study is set in a conjugate cam mechanism mounted in an automated process for manufacturing wirehoods. Results confirm the goodness of the general contact formulation applied when crowned rollers are used and the results dissuade the cylindrical contact formulation when rollers with cylindrical outer surface are used.