Influence de la structure sur le comportement des paliers à feuilles

Les principaux modèles structurels d'un palier à feuilles présents dans la littérature considèrent les ondulations du plissé comme des ressorts indépendants. Ces modèles permettent une implémentation relativement simple dans un code de calcul de paliers à gaz. Ils permettent également la prise en compte d'un coefficient d'amortissement de la structure. L'analyse dynamique linéaire présentée dans cette étude met en avant l'intérêt de cette dissipation énergétique sur la capacité d'amortissement et la stabilité du palier. Toutefois cette modélisation possède certaines limites qui seront discutées dans cet article. Pour dépasser ces limites un nouveau modèle analytique structurel considérant le plissé dans son ensemble est développé. Ce modèle est présenté et validé d'une part d'un point de vue théorique par comparaison avec des simulations éléments finis et d'autre part expérimentalement. Enfin l'influence du choix de la modélisation de la structure sur le comportement statique du palier est mise en évidence

Magnétotribologie des contacts glissants ferromagnétiques

Les contacts glissants des couples de matériaux ferromagnétique/ferromagnétique sont largement influencés par la présence d'un champ magnétique dans la zone du tribocontact. L'oxydation des surfaces fraîches cisaillées aux jonctions de contact est activée par l'application du champ magnétique. Cette activation est induite par les forts gradients magnétiques au niveau des jonctions et des phénomènes d'induction engendrés par la variation temporelle et cyclique du champ magnétique à travers les jonctions de contact qui se renouvellent continuellement dans le contact. Les essais expérimentaux montrent que l'application du champ magnétique favorise notamment la formation des oxydes ferromagnétiques pour préserver le passage magnétique à travers le contact et le taux des spins positifs. Dans cette étude nous présenterons les résultats tribologiques et la morphologie des surfaces usées de contact ferromagnétique/ferromagnétique à l'air libre ou bien sous vide poussé 10-5 Pa avec ou sans champ magnétique autour du contact pour comparer la phénoménologie observée. Puis nous discuterons les processus magnétiques et physico chimiques mis en jeu dans le contact glissant magnétisé

Effet thermomécanique sur la lubrification hydrodynamique d'un piston de moteur Diesel

Cet article présente l'analyse du comportement élastohydrodynamique d'un piston à jupe solide de moteur Diesel. Le mouvement secondaire, couplé avec l'effet de la lubrification du piston, est simulé en tenant de l'effet des charges thermomécaniques dues aux charges thermiques et à l'action de pression des gaz de combustion et à l'inertie du piston. Les résultats montrent la grande importance de l'effet des déformations thermiques sur l'EHD du piston

Dynamic analysis of a harmonically excited on-board rotor-bearing system

International audienceThe aim of this paper is to investigate the dynamic behavior of an on-board rotor mounted on elastic bearings in the presence of rigid support movements. The proposed on-board rotor model is based on the Timoshenko beam finite elements. It takes into account the six translations and rotations of its rigid support and the geometric asymmetry of shaft and/or rigid disks. Thus the obtained equations of motion of the rotating rotor in bending contain time-varying parametric terms which can lead to lateral dynamic instability. The influence of support rotational or translational motions is analyzed by means of orbits of the rotor, responses in the time domain and fast Fourier transforms (FFT)

Lubricated revolute joints in rigid multibody systems

The main purpose of this work is to present a general methodology for modeling lubricated revolute joints in constrained rigid multibody systems. In the dynamic analysis of journal-bearings, the hydrodynamic forces, which include both squeeze and wedge effects, generated by the lubricant fluid, oppose the journal motion. The hydrodynamic forces are obtained by integrating the pressure distribution evaluated with the aid of Reynolds’ equation, written for the dynamic regime. The hydrodynamic forces built up by the lubricant fluid are evaluated from the system state variables and included into the equations of motion of the multibody system. Numerical examples are presented in order to demonstrate the use of the methodologies and procedures described in this work.Fundação para a Ciência e a Tecnologia (FCT

Spatial rigid-multi-body systems with lubricated spherical clearance joints : modeling and simulation

The dynamic modeling and simulation of spatial rigid-multi-body systems with lubricated spherical joints is the main purpose of the present work. This issue is of paramount importance in the analysis and design of realistic multibody mechanical systems undergoing spatial motion. When the spherical clearance joint is modeled as dry contact; i.e., when there is no lubricant between the mechanical elements which constitute the joint, a body-to-body (typically metal-to-metal) contact takes place. The joint reaction forces in this case are evaluated through a Hertzian-based contact law. A hysteretic damping factor is included in the dry contact force model to account for the energy dissipation during the contact process. The presence of a fluid lubricant avoids the direct metal-to-metal contact. In this situation, the squeeze film action, due to the relative approaching motion between the mechanical joint elements, is considered utilizing the lubrication theory associated with the spherical bearings. In both cases, the intra-joint reaction forces are evaluated as functions of the geometrical, kinematical and physical characteristics of the spherical joint. These forces are then incorporated into a standard formulation of the system’s governing equations of motion as generalized external forces. A spatial four bar mechanism that includes a spherical clearance joint is considered here as example. The computational simulations are carried out with and without the fluid lubricant, and the results are compared with those obtained when the system is modeled with perfect joints only. From the general results it is observed that the system’s performance with lubricant effect presents fewer peaks in the kinematic and dynamic outputs, when compared with those from the dry contact joint model.Fundação para a Ciência e a Tecnologia (FCT

A comprehensive survey of the analytical, numerical and experimental methodologies for dynamics of multibody mechanical systems with clearance or imperfect joints

"Available online 19 December 2017"A comprehensive survey of the literature of the most relevant analytical, numerical, and experimental approaches for the kinematic and dynamic analyses of multibody mechanical systems with clearance joints is presented in this review. Both dry and lubricated clearance joints are addressed here, and an effort is made to include a large number of research works in this particular field, which have been published since the 1960′s. First, the most frequently utilized methods for modeling planar and spatial multibody mechanical systems with clearance joints are analyzed, and compared. Other important phenomena commonly associated with clearance joint models, such as wear, non-smooth behavior, optimization and control, chaos, and uncertainty and links’ flexibility, are then discussed. The main assumptions procedures and conclusions for the different methodologies are also examined and compared. Finally, future developments and new applications of clearance joint modeling and analysis are highlighted.This research was supported in part by the China 111 Project (B16003) and the National Natural Science Foundation of China under Grants 11290151, 11472042 and 11221202. The work was also supported by the Portuguese Foundation for Science and Technology with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-006941.info:eu-repo/semantics/publishedVersio