1,721,091 research outputs found
Contact stiffness estimation for PMMA/STEEL contact pair
Full text linkModelling of frictional contact systems with high accuracy needs the knowledge of several contact parameters that are mainly related to the properties of the contact interfaces. While the interface parameters cannot be directly obtained by performing local measurements, the values estimated by means of analytical/numerical models are not reliable to describe the contact behavior, which affects in a prominent way the complex contact phenomena. This work presents a newer approach for identifying reliable values of the normal contact stiffness between rough surfaces in both sliding and sticking conditions as a function of contact pressure, surface roughness and materials. The combination of dynamic experimental tests, on a dedicated set-up, with finite element modelling allowed for an indirect determination of the normal stiffness at the contact
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
Design of a tribometer for investigating tactile perception
No full textThe understanding of the tactile perception mechanism implies the reproduction and measurement of friction forces and vibrations induced by the contact between the skin of human fingers and object surfaces. When a finger moves to scan the surface of an object, it activates the receptors located under the skin allowing the brain to identify surfaces and information about their properties. The information concerning the object surface is affected by the forces and vibrations induced by the friction between the skin and the rubbed object. The vibrations propagate in the finger skin and are converted into electric impulses sent to the brain by the mechanoreceptors. Because of the low amplitude of the induced vibrations, it results quite hard to reproduce the tactile surface scanning and measuring it without affecting measurements by external noise coming from the experimental test-bench. In fact the reproduction of the sliding contact between two surfaces implies the relative motion between them, which is obtained by appropriate mechanisms having a more or less complicated kinematics and including several sliding surfaces (bearings, sliders, etc.). It results quite difficult to distinguish between the vibrations coming from the reproduced sliding and the parasitic noise coming from the other sliding contact pairs. This paper presents the design and validation of a tribometer, named TRIBOTOUCH, allowing for reproducing and measuring friction forces and friction induced vibrations that are basilar for a clear understanding of the mechanisms of the tactile sense
Estimation of the normal contact stiffness for frictional interface in sticking and sliding conditions
Full text linkModeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a newer approach for identifying reliable values of the normal contact stiffness between surfaces in contact, in both sliding and sticking conditions. The combination of experimental tests, on a dedicated set-up, with finite element modeling, allowed for an indirect determination of the normal contact stiffness. The stiffness was found to increase with increasing contact pressure and decreasing roughness, while the evolution of surface topography and third-body rheology affected the contact stiffness when sliding
Dynamic and energy analysis of frictional contact instabilities on a lumped system
Full text linkWhen dealing with complex mechanical systems, the frictional contact is at the origin of significant changes in their dynamic behavior. The presence of frictional contact can give rise to mode-coupling instabilities that produce harmonic friction induced vibrations. Unstable vibrations can reach large amplitude that could compromise the structural and surface integrity of the system and are often associated with annoying noise emission. The study of this kind of dynamic instability has been the subject of many studies ranging from both theoretical and numerical analysis of simple lumped models to numerical and experimental investigation on real mechanical systems, such as automotive brakes, typically affected by such issue. In this paper the numerical analysis of a lumped system constituted by several degrees of freedom in frictional contact with a slider is presented, where the introduction of friction can give rise to an unstable dynamic behavior. Two different approaches are used to investigate the effects of friction forces. The first approach, the Complex Eigenvalues Analysis, allows for calculating the complex eigenvalues of the linear system that can be characterized by a positive real part (i.e. negative modal damping). The complex eigenvalues and eigenvectors of the system are investigated with respect to friction. In the second approach a non linear model has been developed accounting for the stick-slip-detachment behavior at the interface to solve the time history solution and analyze the unstable vibration. The effects of boundary conditions and of system parameters are investigated. Results comparison between the two different approaches highlights how nonlinearities affect the time history solution. The lumped model allows for a detailed analysis of the energy flows between the boundary and the system during self-excited vibrations, which are at the origin of the selection between the predicted unstable mode
Analysis of squeaking frictional noise of hip endoprostheses
No full textThe ceramic-on-ceramic bearings are characterized by low wear rates and an excellent
biocompatibility, motivating for the recent evolution of ceramic bearing surfaces for total hip arthroplasty and
allowing the development of implants characterized by longer longevity. Nevertheless, recent worrisome
rates of squeaking noise occurrence are reported for this kind of prosthesis.
Although recent clinical literature focuses on this topic, the origin and factors leading to squeaking are not
completely identified. The aim of this work is the development and validation of a numerical model able to
predict the friction induced vibrations at the origin of the squeaking noise emission. A complex eigenvalue
analysis of the hip prosthesis allowed for predicting the frictional instabilities recovered experimentally on a
dedicated set-up. The model allowed as well for recovering the same in-vivo squeaking frequencies and for
comparing squeaking propensity of different prosthesis designs
Interaction between contact behaviour and vibrational response for dry contact system
No full textThis work wants to provide insights on the coupling between contact behaviour (local scale) and vibrational response (global scale) which brings to different contact scenarios arising in dry frictional systems. A newer setup, named TriboWave, has been developed in order to reproduce and investigate the system response to frictional contact, under well-controlled boundary conditions. The experimental results highlighted how a simple frictional system can switch from stable friction-induced vibrations to unstable vibrations, i.e. either macroscopic stick–slip instabilities or mode coupling instabilities. The effect of the contact surface roughness on the reproduced frictional scenario has been investigated too
Experimental Analysis of friction induced vibrations at the finger contact surface
No full textWhen a finger moves to scan the surface of an object (haptic sensing), the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) located in the skin, allowing the brain to identify objects and perceive information about their properties. The information about the surface of the object is transmitted through vibrations induced by friction between the skin and the object scanned by the fingertip. The mechanoreceptors transduce the stress state into electrical impulses that are conveyed to the brain. A clear understanding of the mechanisms of the tactile sensing is fundamental to numerous applications, like the development of artificial tactile sensors for intelligent prostheses or robotic assistants, and in ergonomics. While the correlation between surface roughness and tactile sensation has already been reported in literature, the vibration spectra induced by the finger-surface scanning and the consequent activation of the mechanoreceptors on the skin have received less attention. In this paper, frequency analysis of signals characterizing surface scanning is carried out to investigate the vibration spectrum measured on the finger and to highlight the changes shown in the vibration spectra as a function of characteristic contact parameters such as scanning speed, roughness and surface texture. An experimental set-up is developed to recover the vibration dynamics by detecting the contact force and the induced vibrations; the bench test has been designed to guarantee reproducibility of measurements at the low amplitude of the vibrations of interest, and to perform measurements without introducing external noise. Two different perception mechanisms, as a function of the roughness wavelength, have been pointed out. The spectrum of vibration obtained by scanning textiles has been investigated
- …
