Jean-François Brunel

PROGRESSIVE WAVINESS DISTORTION: A NEW APPROACH OF HOT SPOTTING IN DISC BRAKES

ABSTRACT This paper deals with the mechanisms of the appearance of hot spots in railway brake disks. From infrared measurements, a classification of hot spots is presented. The most damaging thermal gradients are identified as macroscopic hot spots uniformly distributed on the friction surface. An explanation of the origin of these macroscopic hot spots is presented based on a scenario of progressive distortion of the disk. Three major effects have to be considered: the structural response of the disk and the pads; realistic boundary conditions, both geometric and loading; and the thermoelastoplastic behavior of the material. Finite element simulations based on the proposed model agree with experimental observations.

Copper Substitution and Noise Reduction in Brake Pads: Graphite Type Selection

Graphite is commonly used in brake pads. The use of graphite powder has the main goal of solid state lubrication and friction coefficient stabilization. In this article results on resin bonded brake pads with focus on noise performance and heat dissipation are presented. Experimental tests are based on model friction materials with a known formulation and a reduced number of components for a better identification of the role of the graphite type. Results clearly indicate that both noise performance and thermal conductivity are strongly affected by the type of graphite. Guidelines for the selection of graphite types for optimized friction materials are given.

Monitoring of temperature and emissivity during successive disc revolutions in braking

The main difficulties of brake disc infrared temperature measurements are the unknown of disc surface emissivity, non-uniform and time-varying, and the high rotation speed and deceleration of the disc, which makes it difficult to follow the special distribution of the temperature. To provide information on the emissivity variation during braking, brake disc temperature and emissivity were investigated by an original optic-fiber two-color pyrometer combined with an infrared camera, which allows monitoring the true friction areas. In addition, an optical top-tour was used to identify disc revolutions. The evolutions of the surface temperature and emissivity on successive disc revolutions were successfully determined. It was showed that circumferential thermal gradients form on the surface and maintain their angular position, they can, however, win or lose in intensity with braking progress.

Analysis of the attenuation of railway squeal noise by preloaded rings inserted in wheels

Squeal from railway wheels occurring in short radius curves produces a very intense and highly annoying noise in the range 400-8000 Hz. When the excitation, due to lateral forces acting on the wheel, cannot be avoided, additional systems can be added on the wheel to limit acoustic emission. A very economical approach is the use of metal rings inserted into grooves machined in the wheels. Unfortunately the effectiveness of these so called damping rings varies from one wheel to another and for different rings. Because the mechanisms of attenuation are not well understood, these variations have not to date been explained. The aim of this paper is to clarify the attenuation mechanisms for damping rings especially for the first three axial wheel modes, which are the predominant sound radiated ones in curve passage and for which the effectiveness of the treatment is lower. It has been generally assumed that friction between the ring and the groove has been the mechanism for squeal noise attenuation. Here it is shown that the vibration attenuation is due to modal coupling between the wheel and the ring. The validity of this proposed mechanism is investigated using experimental measurements and theoretical and numerical models. The results presented here will provide an avenue for optimization of the damping ring noise control treatment to obtain significant levels of squeal noise attenuation notably for the first three axial modes.

A Multiscale Model of a Disc Brake Including Material and Surface Heterogeneities

During friction it is well known that the real contact area is much lower to the theoretical one and that it evolves constantly during braking. It influences drastically the system’s performance. Conversely the system behavior modifies the loading conditions and consequently the contact surface area. This interaction between scales is well-known for the problematic of vibrations induced by friction but also for the thermomechanical behavior. Indeed, it is necessary to develop models combining a fine description of the contact interface and a model of the whole brake system. This is the aim of the present work.A multiscale strategy is propose to integrate the microscopic behavior of the interface in a macroscopic numerical model. Semi-analytical resolution is done on patches at the contact scale while FEM solution with contact parameters embedded the solution at the microscale is used. Asperities and plateaus are considered at the contact interface. FFT techniques are used to accelerate the resolution at the micro-scale. As an example the multiscale model is applied into a complex value analysis used to identify modal coupling in NVH simulations. With this model the interaction between non uniform surface and system dynamic behavior is clearly shown. The contact surface variations clearly affect the modal coupling and therefore noise propensity.

Three-dimensional finite element model of an automotive clutch for analysis of axial vibrations:

Recent progress made in making clutches lightweight and in reducing torsional vibrations has led to the emergence of new vibroacoustic phenomena due to the out-of-plane vibrations of the clutch. These vibrations are generated along the axial direction of the clutch system. Contrary to torsional vibrations that can be simulated by unidirectional lumped-mass models, axial vibrations require a three-dimensional approach to describe the dynamics of the components. Finite element models are widely used to design or optimize the components of the clutch. However, no finite element model of a complete clutch has been developed for analysis of axial vibrations. The main difficulties are the complexity of the system, the great number of non-linearities, the difficulties of instrumentation for validation and the variation in the behaviour along the engagement stroke. This paper provides a validated three-dimensional finite element model of the dynamic behaviour of the clutch for studying axial vibrations. The model simulates the axial static and dynamic characteristics of the clutch. Model accuracy and validation are achieved by comparing the numerical results with the experimental data. The results show the crucial parameters to consider in order to reproduce correctly the non-linear axial behaviour and the dynamic characteristics of the clutch system, namely the orthotropic behaviour of the friction material and the geometrical parameters of some components that should be detailed. Validation is carried out on each component and for the system assembly by dedicated experiments and instrumentation.