112 research outputs found
Squeal propensity characterization of brake lining materials through friction noise measurements
No full textDisc brake systems are a technology widely adopted within the automotive and rail industry, especially when high performance is needed. The interaction between the disc and the pads is responsible for friction-induced vibrations, leading often to squeal noise emission. Squeal vibrations are generated by the onset of an unstable mode, which is triggered by an external excitation. Local phenomena occurring at the contact interface, and resulting in friction noise, can be responsible of the dynamic excitation triggering the squeal instability. This work proposes a new approach for characterizing friction lining materials, by measuring the friction noise coming from the contact between different pad materials and a disc rotor, in order to quantify and compare the attitude of materials to trigger squeal. Then, a parametrical analysis has been carried out with the aim of highlighting the influence of the main parameters on the friction noise. When testing the same set of materials on a full brake disc system, the measured friction noise indexes resulted to be strongly correlated with the squeal occurrence, validating the proposed characterization method for the squeal propensity of lining materials
Molecular mechanism of inhibition of the mitochondrial carnitine/acylcarnitine transporter by omeprazole revealed by proteoliposome assay, mutagenesis and bioinformatics.
No full textThe effect of omeprazole on the mitochondrial carnitine/acylcarnitine transporter has been studied in proteoliposomes. Externally added omeprazole inhibited the carnitine/carnitine antiport catalysed by the transporter. The inhibition was partially reversed by DTE indicating that it was caused by the covalent reaction of omeprazole with Cys residue(s). Inhibition of the C-less mutant transporter indicated also the occurrence of an alternative non-covalent mechanism. The IC50 of the inhibition of the WT and the C-less CACT by omeprazole were 5.4 µM and 29 µM, respectively. Inhibition kinetics showed non competitive inhibition of the WT and competitive inhibition of the C-less. The presence of carnitine or acylcarnitines during the incubation of the proteoliposomes with omeprazole increased the inhibition. Using site-directed Cys mutants it was demonstrated that C283 and C136 were essential for covalent inhibition. Molecular docking of omeprazole with CACT indicated the formation of both covalent interactions with C136 and C283 and non-covalent interactions in agreement with the experimental data
Identification of dynamic contact instabilities generated by braking materials
No full textThe occurrence of unstable friction-induced vibrations is a major issue for braking manufacturers, as they lead to annoying noise, structure vibrations and brake surface degradation. Understanding the underlying causes of frictional instabilities, arising during the sliding between two bodies, is necessary for developing solutions and countermeasures. For this purpose, in this work, an experimental and numerical investigation of contact instabilities has been performed. Mode coupling and negative friction-velocity slope instabilities have been numerically investigated by both lumped-parameter and finite element models. As well, an experimental campaign has been carried out for recovering the frictional and vibrational response of braking materials under different boundary conditions. The comparison between numerical and experimental results allows validating a new methodology, based on the study of the phase shift between the tangential and normal vibrational responses, in order to distinguish the different types of contact instabilities
Contact instability identification by phase shift on C/C friction materials
No full textCarbon-carbon (C/C) composite material is currently among the most promising engineering materials for friction applications, where excellent tribological properties, lightweight and good thermal stability are needed. As a result, the industrial demand for C/C composite leads to the need to characterize in detail the frictional and vibrational response of such material, when adopted for high performance braking applications. In this context, the present work shows an experimental and numerical characterization of unstable friction-induced vibrations caused by frictional contact between C/C specimens. The results provide information on the C/C material behavior at high-temperature conditions as well as additional tools to distinguish the occurrence of different vibrational phenomena. The phase shift between vibrational signals has been correlated to different kind of contact instabilities (either mode coupling or negative friction-velocity slope), that can arise and bring to high amplitude oscillations and noise emission. Such correlation has been observed experimentally and reproduced numerically
Numerical and experimental analysis of the bi-stable state for frictional continuous system
No full textUnstable friction-induced vibrations are considered an annoying problem in several fields of engineering. Although several theoretical analyses have suggested that friction-excited dynamical systems may experience sub-critical bifurcations, and show multiple coexisting stable solutions, these phenomena need to be proved experimentally and on continuous systems. The present work aims to partially fill this gap. The dynamical response of a continuous system subjected to frictional excitation is investigated. The frictional system is constituted of a 3D printed oscillator, obtained by additive manufacturing that slides against a disc rotating at a prescribed velocity. Both a finite element model and an experimental setup has been developed. It is shown both numerically and experimentally that in a certain range of the imposed sliding velocity the oscillator has two stable states, i.e. steady sliding and stick–slip oscillations. Furthermore, it is possible to jump from one state to the other by introducing an external perturbation. A parametric analysis is also presented, with respect to the main parameters influencing the nonlinear dynamic response, to determine the interval of sliding velocity where the oscillator presents the two stable solutions, i.e. steady sliding and stick–slip limit cycle
Localization of mitochondrial carnitine/acylcarnitine translocase in sensory neurons from rat dorsal root ganglia
No full textThe carnitine/acylcarnitine transporter is a transport system whose function is essential for the mitochondrial β-oxidation of fatty acids. Here, the presence of carnitine/acylcarnitine carrier (CACT) in nervous tissue and its sub-cellular localization in dorsal root ganglia (DRG) neurons have been investigated. Western blot analysis using a polyclonal anti-CACT antibody produced in our laboratory revealed the presence of CACT in all the nervous tissue extracts analyzed. Confocal microscopy experiments performed on fixed and permeabilized DRG neurons co-stained with the anti-CACT antibody and the mitochondrial marker MitoTracker Red clearly showed a mitochondrial localization for the carnitine/acylcarnitine transporter. The transport activity of CACT from DRG extracts reconstituted into liposomes was about 50 % in respect to liver extracts. The experimental data here reported represent the first direct evidence of the expression of the carnitine/acylcarnitine transporter in sensory neurons, thus supporting the existence of the β-oxidation pathway in these cells
Kinetic mechanism of antiports catalyzed by reconstituted ornithine/citrulline carrier from rat liver mitochondria
No full textThe transport mechanism of the reconstituted ornithine/citrulline carrier purified from rat liver mitochondria was investigated kinetically. A complete set of half-saturation constants (K(m)) was established for ornithine, citrulline and H(+) on both the external and internal side of the liposomal membrane. The internal affinity for ornithine was much lower than that determined on the external surface. The exclusive presence of a single transport affinity for ornithine on each side of the membrane indicated a unidirectional insertion of the ornithine/citrulline carrier into liposomes, probably right-side-out with respect to mitochondria. Two-reactant initial velocity studies of the homologous (ornithine/ornithine) and heterologous (ornithine/citrulline) exchange reactions resulted in a kinetic pattern which is characteristic of a simultaneous antiport mechanism. This type of mechanism implies that the carrier forms a ternary complex with the substrates before the transport reaction occurs. A quantitative analysis of substrate interaction revealed that rapid-equilibrium random conditions were fulfilled, characterized by a fast and independent binding of internal and external substrates
Numerical and experimental analysis of nonlinear vibrational response due to pressure-dependent interface stiffness
No full textModelling interface interaction with wave propagation in a medium is a fundamental requirement for several types of application, such as structural diagnostic and quality control. In order to study the influence of a pressure-dependent interface stiffness on the nonlinear response of contact interfaces, two nonlinear contact laws are investigated. The study consists of a complementary numerical and experimental analysis of nonlinear vibrational responses due to the contact interface. The laws investigated here are based on an interface stiffness model, where the stiffness property is described as a nonlinear function of the nominal contact pressure. The results obtained by the proposed laws are compared with experimental results. The nonlinearity introduced by the interface is highlighted by analysing the second harmonic contribution and the vibrational time response. The analysis emphasizes the dependence of the system response, i.e., fundamental and second harmonic amplitudes and frequencies, on the contact parameters and in particular on contact stiffness. The study shows that the stiffness-pressure trend at lower pressures has a major effect on the nonlinear response of systems with contact interfaces
Tribological and vibro-acoustic behaviour of a lubricated contact subjected to the stick-slip phenomenon: the case of the spring-brake system
No full textThe main objective of the PhD thesis is the analysis of stick-slip phenomena in lubricated contacts, by merging the physical, experimental and numerical points of view and proposing a novel methodological approach, applied here to an industrial case. Nowadays, in the field of applied mechanics and tribology, one of the most important challenges is the ability to predict and reduce surface damages, failure of machine components and undesirable frictional and dynamic characteristics. In particular, the appearance of friction-induced vibrations at the contact is hardly controllable and can result in high local contact pressure, elevated stresses, system oscillations, discontinuous motion and premature failure]. Understanding the conditions for which the system is more predisposed to the stick-slip phenomenon may allow preventing the appearance of such instabilities, and the related vibrations and noise emission. Friction-Induced Vibrations are a phenomenon that engages multiple scientific challenges, due to the complexity of their physics. Focusing the attention on stick-slip phenomena, these are generally characterized by a saw-tooth displacement-time evolution. Each change in the contact parameters influences directly the dynamic and frictional response of the system, due to the mutual influence of the local scale (contact) and the system scale (macroscopic frictional and vibrational response). Moreover, the presence of a lubricant, and in particular of grease, at the contact interface, increases the complexity of the phenomenon, from both a tribological and dynamic point of view. Lubricated systems are supposed to reduce the frictional losses and wear, but they can also collaborate in the appearance of dynamic contact instabilities, due to the friction-velocity characteristics when passing from boundary to mixed contact regimes. The complex rheology of a grease, function of both the matrix, additive and oil responses, becomes then a key point for the occurrence and evolution of stick-slip. Despite the great importance of this phenomenon, from both scientific and industrial points of view, a lack emerges into the literature about stick-slip of lubricated interfaces. The few works are manly focused on molecular dynamics simulations and numerical modelling of the dynamical response of the system. The different role of the grease components, during the sliding, is nowadays still not clear. Nevertheless, the complex rheology of a grease, function of both the thickener, the base oil and the additives, is a key point for understanding and controlling the occurrence and evolution of stick-slip. Moreover, a general approach is needed to account for the coupling between the local phenomena (e.g. lubricated contact response) and the system dynamic response. Aiming to improve the understanding of stick-slip in lubricated contacts, the present Ph.D. work proposes a novel methodological approach to the stick-slip problem of a lubricated contact, referring to a real industrial case, in order to deploy the obtained results in a more realistic and detailed manner. The subject of the investigation is a mechanical brake used in tubular electric actuators, which can present frictional instabilities originated at the lubricated contact between the two main brake components. The methodology used is twofold: i) on one hand, experimental tests are carried out to understand the local frictional response of the lubricated contact; ii) on the other hand, a lumped model is created in order to simulate and analyse the system dynamic response. Introducing the information about the local lubricated contact behaviour (friction law), achieved experimentally, into the numerical model, it is possible to investigate the parameters for which the system is more predisposed to the stick-slip phenomenon and recreate a representative scenario of its appearance. Particular attention has been placed on the analysis of the lubricant rheology, dealing with different types of lubricants and regimes of lubrication, with both oils and greases. The frictional response has been thus related to the different contributions of the grease components (i.e. thickener, base oil and additives) on the rheology at the interface. The obtained local information has been then integrated in the lumped model to evaluate the unstable dynamic response of the entire system (i.e. the stick-slip phenomena) and identify the lubrication parameters that most influence its appearance. The numerical analysis had the dual objective of understanding the role of the local contact response in the system instability and investigating the stick-slip occurrence as a function of the key system parameters. The obtained results allowed to identify the lubrication components, and the respective friction-velocity curves, more favourable for the stick-slip occurrence. Combining the grease rheology evolutions with the stick-slip dynamic response represents a further challenge in both the domains of research. The Ph.D. thesis has been developed in collaboration between the Sapienza University of Rome, Department of Mechanical and Aerospace Engineering (Rome, Italy), the Institut National des Sciences Appliquées (INSA) of Lyon, laboratory LaMCoS (Lyon, France), and the company SOMFY S.A. (Cluses, France)
- …
