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Power losses in high-speed gears come from the friction between the teeth (sliding and rolling), the lubrication process (dip or jet lubrication), the pumping of a gas -lubricant mixture during the meshing, and the losses associated with windage effects. The authors have developed different approaches to analyse the contribution of each power loss source, namely:(a) the windage losses based on simplified air flow models neglecting the influence of the lubricant, which compare well with the experimental evidence from a specific test bench; (b) an original model of gas trapping in the intertooth time-varying spaces, which has been validated using the experimental findings on a spur gear test rig in which pressure transducers have been placed at the bottom of the space between two teeth; (b) tooth friction, by introducing into a three-dimensional dynamic model of gears a new traction law based on measurements from a two-disc machine which accounts for lubricant properties and surface finish.The combination of the three models allows for the prediction of gear efficiency over a wide range of speeds and loads for jet-lubricated geared units. This approach also makes it possible to improve tooth geometry and gearbox design to minimize power losses and heat generation. The procedure is illustrated in several industrial applications.

Power losses in high-speed gears come from the friction between the teeth (sliding and rolling), the lubrication process (dip or jet lubrication), the pumping of a gas-lubricant mixture during the meshing and the losses associated with windage effects. The objective of this paper is to present a number of preliminary experimental and theoretical findings on the prediction of windage losses. Experiments were conducted on a test bench whose principle consists in driving a gear to a given speed and then measuring its deceleration once it has been disconnected from the motor. The transmission between the motor and the rotor is ensured by a friction wheel which also plays the role of speed multiplier. A pneumatic jack either imposes a sufficient contact pressure between the driving wheel and the rotor (transmission of rotation), or is used for separating the parts when the maximum speed is reached. A disk and 4 different gears were tested in the absence of a lubricant at speeds ranging from 0 to 12 000 rpm. Two different theoretical approaches have been developed: i) a dimensional analysis based upon the dimensionless groups of terms which account for the flow characteristics (Reynolds number), the gear geometry (tooth number, pitch diameter, face width) and the speed, ii) a quasi-analytical model considering in detail the fluid flow on the gear faces and inside the teeth. It is found that both approaches give good results in comparison with the experimental evidence and two analytical formulae aimed at predicting windage losses in high-speed gears are proposed.

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BOTTA-ZIMMERMANN M. (Mme) UNITE DE RECHERCHE EN GENIE CIVIL-Développement Urbain BOULAYE G.

Accurate modelling of friction coefficient is of primary importance in efficiency, vibration and failure analyses of enclosed gear drives. After showing the influence of surface/lubricant interactions on friction, the authors used a semi-empirical model which can take all these aspects into account. Lubricant is modelled as an Eyring–Reynolds fluid and rough surfaces are described with two parameters via a stochastic approach. A specific two-disc machine is used to perform series of friction measurements on smooth and rough discs. Smooth discs allow to operate under full film lubrication and to measure a reference shear stress of lubricant, whereas rough discs reproduce gear tooth roughness and generate a representative value of friction on asperities. The purpose of this present paper is to describe calculations of this physical-based friction coefficient model and to present the experimental process. On the basis of new results, the impact of a surface finishing process is assessed as well as the consequence of calculating friction coefficient based on oil injection instead of local bulk temperature.

RésuméEn France, la réglementation concernant l'évaluation du niveau de sécurité des linéaires de digues a été renforcée. Elle nécessite l'évaluation probabiliste du risque de défaillance des ouvrages. Or ceux-ci et leurs mécanismes de défaillances demeurent dans une large mesure mal connus, conduisant à une prédominance de l'exploitation du jugement expert dans les études de diagnostic. Ainsi une erreur de jugement peut avoir des conséquences catastrophiques et la production d'une information fiable induit que l'ingénieur est capable de décrire fidèlement ses évaluations ainsi que l'incertitude qui s'y rattache. Répondant à ce besoin, notre recherche, au sein du projet INCERTU, vise à produire des approches et des outils scientifiques pertinents pour la collecte et le traitement des déclarations d'experts fiables ou assorties d'un degré de vraisemblance. Nous présentons ici nos recherches concernant l'identification et le traitement des biais susceptibles d'entacher les évaluations expertes. SummaryIn France, the regulation on the evaluation of linear levees security level has been strengthened. It requires the probabilistic assessment of failure risk of structures. However thereof and their failure mechanisms remain largely not well known, leading to a predominance of the exploitation of the expert judgment in diagnostic studies. Thus an error in judgment can have catastrophic consequences and the production of reliable information induces the engineer is able to accurately describe these evaluations and the uncertainty associated whit it. To meet this need, our research, within INCERTU project, aims to produce relevant scientific approaches and tools for the collection and treatment of reliable expert statements or accompanied by a degree of verisimilitude. We present our research on the identification and treatment of biases may taint the expert evaluations.

Comparaison entre l'approche de Marston et l'approche allemande de conception structurelle des conduites rigides enterrées Résumé Dans différents pays, la théorie de Marston est utilisée pour estimer le chargement appliqué sur les conduites rigides enterrées. En Allemagne, on utilise une autre approche applicable aux conduites rigides et souples. Cette note présente une comparaison entre les deux approches. Elle est réalisée sur des conduites en grès. Le rapport entre les chargements calculés par les deux méthodes démontre que l'approche de Marston est très conservatrice pour les conduites de petit diamètre installées dans un sol granulaire bien compacté car elle néglige le soulagement par les côtés et sous-estime le frottement entre le sol de remblaiement et les parois de la tranchée.