1,721,210 research outputs found
Structural modifications for squeal noise reduction
No full textBrake squeal is an example of noise caused by vibrations induced by friction forces that can lead to a dynamic instability. The onset of squeal is due to the lock-in phenomenon, i.e. the instability occurring when two modes of the system coalesce and one becomes unstable. The sensitivity of the onset of instabilities with respect to the system parameters is due to the high modal density and to the large uncertainties on the dynamics of the single brake. This makes impossible the design of a 'squeal-free' brake apparatus. This paper suggests an innovative approach to avoid the growth of squeal vibrations, by exploiting the introduction of lumped structural modifications in the disc. The modification rotates with the disc while the disc modes are fixed in space. Depending on the relative position between the concentrated modification and the nodes of the disc modes, the natural frequencies of the disc shift back and forward. This expedient allows to change continuously the lock-in conditions between the system modes that are involved in squeal. The vibrations due to the lock-in between the pair of modes have not enough time to increase because of the repetitive look-out and they can be reduced until they completely disappear
Fingertip scanning on a surface: friction coefficients and induced vibrations
No full textTactile perception happens when scanning the fingertip on object surfaces (haptic sensing), by activating the receptors that are located in the skin. The skin deformation due to the contact stresses and the vibrations induced by the sliding contact activate the mechanoreceptors which generate electric impulses and allow the brain to identify objects and perceive information about their surfaces.
In this paper, the friction coefficient between a real human finger and both rigid surfaces and fabrics is recovered, as a function of the contact parameters (load and scanning speed). Then, the analysis of the vibration spectra is carried out to investigate the characteristics of the induced vibrations, measured on the fingernail, as a function of surface features and contact parameters.
Different behaviour of the friction coefficient is recovered scanning rigid surfaces or textiles, with respect to the contact parameters.
Results show that frequency spectrum distribution when touching a rigid surface is mainly determinate by the relative geometry of the two contact surfaces, while, when scanning a fabric, the structure and the deformation of the textile itself affect largely the spectrum of the induced vibration. Some main characteristics of the measured vibrations are found to be representative of tactile perception, by comparison with psychophysical and neurophysiologic works in literature
Characterization of the High Frequency Squeal on the Laboratory Brake Set-up
No full textBrake squeal is one of the major issues in the design process of an automotive brake and the development of a robust procedure of a squeal-free design is still under investigation. The high frequency squeal is the most frequent noise generated by automotive brakes and is characterized by a wavelength of the disc mode comparable to the length of the brake pad. In 2002 one of the authors presented a new experimental set-up called "laboratory brake" that is a good compromise between simple test rigs, such as the beam-on-disc, and the experimental set-ups that use real brakes. The beam-on-disc set-up is a useful tool to understand the mechanism leading to instability, but it does not simulate appropriately a real brake. On the other hand, real brakes are too complex for fundamental investigation and for efficient modeling. The laboratory brake has now been modified and used to understand the behavior of the high frequency squeal and the relations between the squeal characteristics and the modal interaction of disc and caliper
Dynamic analysis of a tactile device for mimicking mechanical stimuli responsible for texture perception
No full textFriction-induced vibrations are one of the main mechanical stimuli at the origin of tactile perception, allowing perception and discrimination of surface textures. While acoustic waves and electromagnetic waves are successfully reproduced for mimicking the auditive (loudspeakers) or visual (monitors) stimuli, the stimuli at the origin of tactile perception are still not fully understood and are still not reproduced. This work presents the development and the dynamic analysis of a device, allowing the reproduction of vibrations induced by the sliding of the finger on a surface. The overall bio-electro-mechanical transfer function, including the fingertip, mechanical device and its control electronics, has been first characterized. The system is highly nonlinear, due to the contact nonlinearities and the nonlinearities proper of the tissues of the fingertip, and a parametrical analysis has been developed for investigating the effect of the contact parameters (contact force, subject, etc.) on the transfer function of the overall biomechanical system. Then, the vibrations measured on the nail of the subject, during the exploration task of different surfaces, have been reproduced by the tactile device. A first validation is obtained by the comparison of the original and the simulated vibration spectra. Then, a discrimination campaign has been developed to verify the ability in discriminating different textures, both during the exploration of real textures and when mimicking the respective vibrational stimuli. The obtained spectra can be correlated with both correct discrimination results and discrimination errors, allowing the identification of the spectral features responsible for texture perception and discrimination
On the need for new approaches for brake squeal prediction and suppression
No full textBrake squeal is a dynamic instability of the mechanical system caused by friction forces at the contact interface. Notwithstanding several decades of investigations and an increased understanding of brake noise, commercial brake design is still not capable of avoiding squeal instability. The actual predictive tools employed by the designers, mostly based on complex eigenvalue analysis, appear inadequate for reliable squeal prediction. It is now time for investigating alternative approaches using the knowledge developed on squeal generation and its dependence on fundamental parameters (e.g. damping). In the example presented here, attention is no longer focused on the design of brakes with a view to avoid coincidence of natural frequencies, which is impossible in practice, but rather on the suppression of the squeal-causing vibrations during their generation. Structural alterations made to the rotor allow continuous modification of the system dynamics during disc rotation
Revealing transitions in friction-excited vibrations by nonlinear time-series analysis
No full textWe study the transitions in friction-induced vibrations (FIV) experimentally. The measurement data stem from a highly sophisticated setup specifically designed to study FIV problems and where the relative motion between the samples is achieved using air bearings and a voice-coil motor. This peculiarity ensures avoiding parasitic vibrations and makes the setup particularly suitable to perform measurements of very low vibration levels. The relative sliding velocity decays along the measurement to zero, which provokes several types of FIV. We employ advanced time-series analysis techniques, such as spectral analysis, attractor reconstruction and recurrence plot analysis to study the dynamical transition from steady sliding to high-frequency FIV and stick-slip vibrations in detail. For different specimens, self-excited vibrations are observed stemming from an instability that is driven by a negative friction-velocity slope characteristic as well as for constant friction values. Prior to instability, it is observed that highly irregular oscillations decay and most of the vibration energy focuses in a low-frequency mode of the experimental setup. The analysis of the FIV range illustrates a plethora of qualitatively different dynamics that can be detected, characterized and visualized using advanced signal processing. Particularly, we report on period-1 and period-2 limit cycles, quasi-periodic motion, weakly chaotic attractors and different types of stick-slip vibrations. The analysis of transitions between those dynamic regimes reveals beating phenomena, sudden energy exchange between different modes and intermittent dynamics. The results of this study aim to provide a step forward on the application of nonlinear dynamics post-processing tools for identifying and characterizing the different frictional stable and unstable scenarios
Competition between 3rd body flows and local contact dynamics
No full textWhen contact occurs between two bodies in relative
motion, instability state – i.e. stick-slip or
stick-slip-separation up so several kHz – often occur in
the contact . Such phenomenon has been
numerically highlighted and studied by Baillet and
Massi using dynamic finite element modelling
(FEM) with constant Coulomb’s friction coefficient.
Nevertheless, such model doesn’t take account for the
3rd body flow inside the contact and its interactions
with contact dynamics. Recently, Renouf has
coupled finite element method with discrete element
method (DEM) to compensate for this lack.
The purpose of the present work is so to validate
experimentally the numerical results obtained from
both contact dynamics and 3rd body flows . An
experimental set-up, called “PhotoTrib”, has so been
developed to reproduce the birth conditions of both
instability state and 3rd body particles
Steady state of modal coupling instabilities as a dynamic energy equilibrium
No full textContact interaction between different parts of mechanical system is at the origin of phenomena known in literature as friction induced vibrations. In this paper friction induced unstable vibrations are analyzed by an energy point of view. The analysis is here developed on a polycarbonate disc, which is constrained on the external circumference and in frictional contact with a steel cylinder rotating at its inner circumference. In a classical approach the stability of the system can be evaluated by a complex eigenvalue analysis, performed with the commercial finite element code ANSYS. Then, the steady state of the system can be evaluated by a transient non linear analysis, performed with the explicit finite element code PLAST2D. Power flows are analyzed during the transient response to observe the effect of the local friction coefficient and the boundary conditions on the different terms of the energy balance
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