Negative Friction-velocity Slope Research Articles

Analysis of friction-induced vibration and wear characteristics during high-speed train friction braking process

This study explored the evolution of friction-induced vibration (FIV) and wear in high-speed train friction braking through experiments, theoretical analysis, and numerical simulation. Experimental tests revealed that vibration behavior transitions from high-frequency modes at high speeds to low-frequency stick-slip at lower speeds. Theoretical analysis indicated that brake disc speed, coefficient of friction (COF), and contact stiffness affect system stability. Specifically, the system experiences high-frequency vibrations at high speeds due to mode coupling, and the friction block surface primarily undergoes abrasive wear mechanisms. At lower speeds, the system demonstrates low-frequency stick-slip vibrations induced by the combined effects of mode coupling and negative friction velocity slope (NFVS) characteristics, and the friction block surface mainly experiences adhesive wear mechanisms.

A Root Cause of Curve Squeal: Self-Excited Frictional Vibration of a Wheelset–Track System

Abstract When a metro vehicle navigates a tight curve, it invariably causes a squealing noise. Most researchers proposed the negative friction–velocity slope as the generation mechanism for curve squeal. However, some phenomena of railway curve squeal are still difficult to explain. The purpose of this paper is to establish a friction-coupling finite element model of the wheelset–track system that can accurately predict curve squeal and to examine the influence of different wheelset–track structures on the trend of curve squeal occurrence. It has been proposed that curve squealing is caused by self-excited frictional vibrations. The complex eigenvalue analysis (CEA) and transient dynamics analysis were applied to predict the unstable vibration of curve squeal. The impacts of the coefficient of friction between the rail and the wheel, wheel web plate shape, fastener damping, gauge, and negative friction–velocity slope on the curve squealing noise were studied. The results demonstrated that the predicted frequency corresponded to the main frequency of the squealing noise measured on-site. Simultaneously, a friction coefficient of 0.25 or higher resulted in a curve squeal with a frequency of 2153.9 Hz. An S-shaped web plate wheel, appropriate fastener damping, and gauge can reduce unstable vibrations and curve squealing when a metro vehicle navigates a tight curve. The negative friction–velocity slope has less influence on the tendency of curve squealing.

Modelling and stability analysis of a high-speed train braking system

The stability of a braking system is crucial for the steady and safe operation of trains. To investigate the dynamic behaviors and stability of a train braking system as influenced by key parameters, a numerical model integrating mode-coupling, negative friction–velocity slope (NFVS), and stick–slip theories was established. Additionally, the wheelset connected with the braking system was contained in the model considering that mode-coupling and stick–slip characteristics were affected by system structures. Based on this model, the influence and mechanism of stiffness, damping, and NFVS characteristics on mode-coupling of the braking system were revealed. The results showed that when the tangential and normal contact stiffness values of the pad were close, the system experienced mode-coupling instability and vibrations in these two directions were coupled. When the tangential and normal damping values of the pad were unequal, the energy dissipation in the two directions became different and the system instability was enhanced. Additionally, the system complex eigenvalues always had positive real parts and the dynamic behaviors were more complicated by considering NFVS at relatively low speeds. This is because NFVS characteristics can be equivalent to negative damping values, which cause the difference in tangential and normal damping values and continuously input energy into the braking system, then weakening the system stability.

A hybrid friction-induced vibration form: Experimental measurement and mechanism discussion

Friction-induced vibration (FIV) has strong nonlinear properties and complicated mechanism. In this research, the friction characteristics of a block-on-disc system with the variation of the speed are experimentally investigated. At a relatively medium speed, a hybrid vibration form composed of low-frequency stick–slip and high-frequency intermittent unstable vibration is discovered and denoted as stick–unstable sliding. Further, a mathematical model integrating the negative friction–velocity slope (NFVS) and mode-coupling theories is established to analyze the FIV and noise mechanisms at different speed levels. The results show that the FIV at relatively low and high speeds is respectively affected by the NFVS and mode-coupling. At a relatively medium speed, the NFVS and mode-coupling co-exist and then generate the stick–unstable sliding.

Contact instability identification by phase shift on C/C friction materials

Carbon-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.

Revealing transitions in friction-excited vibrations by nonlinear time-series analysis

We 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.

Effect of groove surface on friction noise and its mechanism

In this study, friction noise according to whether grooves exist was measured using a reciprocating sliding device, and the relationship between the groove and friction noise mechanism was investigated. Experimental results revealed that both non-groove textured and groove textured surfaces produced friction noise at approximately 5 kHz. However, the friction noise onset time was relatively faster and the sound pressure level was higher for the non groove textured surface. The tribo-surfaces of the plate and ball which change in time for the non-groove textured and groove textured surfaces were observed using a microscope. For the non-groove textured surface, adhesive wear was commonly observed on the tribo-surface, and for the groove textured surface, wear particles accumulated within the groove due to the movement of the ball, and adhesive wear appeared more slowly. As a result, the friction coefficient was relatively larger for the non-groove textured surface, facilitating the mode coupling instability. Using the pin-on-disk system, the propensity for the negative friction-velocity slope was also measured with regard to the non-groove textured surface.

Investigation of friction induced vibration in lead screw system using FE model and its experimental validation

Finite Element Method (FEM) is proposed to model the friction-induced vibration in the gear system with a lead screw and nut. In order to validate it, the experiment was conducted by using the lead screw test set-up. In the frequency spectrogram, there were two distinctively different patterns of the unstable modes: a mode where the frequency significantly changed and another where the frequency remained almost unchanged. Under a constant friction coefficient, the complex eigenvalue analysis in the FE model demonstrated that the system frequencies altered with respect to the transfer distance of the nut. The unstable modes were found to occur due to mode-coupling between the paired bending modes of the screw. Meanwhile, an unstable mode in which the frequency showed little changes in relation to the translation distance of the nut, was found to be an unstable torsion mode, which was caused by the negative friction-velocity slope.

Wet Clutch Friction Interfaces under Water-Contaminated Lubricant Conditions

ABSTRACTThe performance of wet clutches used for automatic transmissions or other applications usually includes the desired positive friction characteristics and a shudder-free torque generation. Changes in the operating variables such as the lubricant conditions influence the formation of a tribofilm, and friction characteristics and can alter the degradation of the friction interfaces. In this work, the friction characteristics and degradation of the paper–steel friction interfaces were monitored when a commercial fully formulated automatic transmission fluid (ATF) was contaminated with water. It was found that water in ATF influenced the clutch stability by increasing the mean coefficient of friction (µ) and the negative friction–velocity slope. Surface studies of the posttest friction interfaces clearly indicated reduced surface porosity and permeability, increased wettability, and changed elemental composition on the contacting surfaces after tested with water-contaminated ATF. Moreover, water-contaminated paper liners' thermal decomposition shifted to a lower temperature compared to an uncontaminated liner during thermal analyses. These results displayed faster degradation and reduced service life of the clutch friction interfaces due to water contamination. The resultant surface condition can be associated with the observed unstable friction and negative friction–velocity slopes.

Friction-induced noise of gear system with lead screw and nut: Mode-coupling instability

The mode-coupling instability in the gear system with a lead screw and nut is investigated. The actual gear geometry and the contact kinematics are developed in analytical the model. The complete set of vibration modes including axial, torsion and transverse displacements is applied to seek the solution of the linearized gear system. From the linear stability analysis, the bending mode pair as well as the torsion and axial mode pair have the strong tendency towards the mode-coupling instability. It points out that squeak noise in the lead screw system can occur even for a constant friction coefficient without the negative-friction velocity slope. The closed-form solution and numerical calculation also show that the rotating direction can drastically change the onset of mode-coupling instability.

The onset condition of friction noise in ball joint under concentric loading

This study provides the general methodology describing the propensity of friction noise in ball joint system. The dynamic ball joint model is theoretically proposed under the concentric loading condition with nonlinear friction curve. In ball joint system, contact condition becomes non-conformal due to clearance between the ball head and cup. The results from stability analysis reveal that the ball joint system becomes dynamically unstable under the sufficiently small contact area and a negative friction-velocity slope. It is also shown that the lower bending modes of the beam have the higher propensity for modal instability.

Dynamic instability of a spherical joint under various contact areas

The contact area and the negative friction velocity slope on dynamic instability of spherical joint are theoretically investigated. The contact area may be influenced by the various loading and design conditions between a ball and socket. The non-conformal contact is adopted for the static and dynamic derivations. This contact geometry is selected as the major design parameter in this study. The numerical results reveal that the smaller contact area produces the higher propensity of modal instability.

마찰 곡선을 고려한 Pin-on-disk 마찰소음 해석 및 검증

This study provides the numerical finite-element method(FEM) estimating the friction noise induced by the negative slope in the friction-velocity curve. The friction noise due to the friction-velocity curve is experimentally investigated through the pin-on-disk setup. The measured squeal frequency is estimated by FEM. The friction curve is measured by the friction test, then it is applied to the com- plex eigenvalue analysis. The results shows that the experimental squeal frequency can be determined by the FEM analysis. Also, it is emphasized that the negative friction-velocity slope is essential in generating friction noise in the pin-on-disc system.

마찰 곡선에 따른 마찰 소음 특성

The characteristics of friction noise due to the friction-velocity curve is experimentally investigated through the pin-on-disk setup. The rotation speed of the disk is controlled in order to produce the sliding speed variation. Then, the friction coefficient and the corresponding friction noise are simultaneously measured with respect to the sliding speed between the steel disk and aluminum pin. The experimental results show that the negative friction-velocity slope is essential in generating friction noise.

Theoretical model of ball joint squeak

The theoretical model for the ball joint squeak problem is proposed. The ball joint is modeled as a sphere attached to a rotating flexible beam and it is in contact with a semi-spherical rigid socket. The relative rotation and vibration between the ball and socket produces friction stresses causing the dynamic instability of the system. The squeak propensity of the ball joint system is numerically investigated through the eigenvalue sensitivity analysis at the sliding equilibrium. Stability analysis shows that the bending modes of the beam can produce the dynamic instability under the negative friction–velocity slope. It is also highlighted that the squeak propensity under the poor friction characteristics can be controlled by the system parameters such as the sliding speed, the contact stiffness, the radius of the ball, the axial load and the tilting angle.

Effects of Lining Thickness on Squeal in Drum Brake Assembly: Experimental Investigations

This paper presents the effects of brake lining thickness due to wear on drum brake squeal. Brake lining will be worn out and subsequently its thickness will be reduced after a few number of braking applications. Hence dynamic properties of the lining, such as its natural frequency, might be changed. In this work, two different sets of brake lining, i.e., new and worn lining are used to investigate its effect on squeal generation. First, modal testing is performed to determine natural frequencies of those brake linings at free-free boundary condition. Later, squeal tests are carried out using brake dynamometer and squeal frequency is measured up to 10 kHz. Several squeal results are plotted over brake operating conditions to observe the influence of different lining thickness. In addition to these, squeal mechanisms, i.e., modal coupling due to closeness of the natural frequency between drum brake components and negative damping due to negative friction-velocity slope that contribute to the squeal generation are also investigated and discussed.

The onset of friction-induced vibration and spragging

The minimal dynamic models are used to investigate the influences of geometric coupling on frictionally-excited system. At a certain set of system parameters, spragging can occur where the steady-sliding is indeterminate. It is found that the sprag condition becomes the boundary of the regime for the non-existence of the equilibrium in the parametric space. Equilibrium instability induced by the negative friction-velocity slope and mode-coupling is determined for the regime in which the steady-sliding is defined. Its ensuing nonlinear response is shown to reach the stick-slip limit cycle.

A NEW DYNAMICAL FRICTION MODEL

This paper proposed a new dynamical friction model structure which allows accurate modeling both in sliding and presliding regimes. Transition between these two regimes is accomplished without a switching function. In the presliding regime, the model has the characters of a Bouc–Wen model, so it can adapt to various hysteretic behaviors. While in the gross-sliding regime, the model is equivalent to a general velocity-dependent friction model. Therefore, the dynamical characters of friction force in this regime, such as negative friction-velocity slope, can be described also. Furthermore, the transition between these two regimes is not abrupt, but smooth, and needs no extra function. The application of this model is demonstrated by describing experimentally obtained friction force for a guide–rail system. Numerical simulation results for a vibration system with friction show the practicability of the model to predict dynamic characteristics such as hysteretic behavior in presliding, velocity-dependent behavior in sliding, and stick-slip behavior.

Experimental investigation into squeal under reciprocating sliding

Experiments on squeal under reciprocating sliding were performed by means of a ball against a block. Vibration accelerations, sound pressure level of squeal and tangential force were measured simultaneously. Under certain test conditions, the reciprocating sliding can create a whole process from squeal generation to disappearance. Based on power spectral density (PSD) and short-time Fourier transform (STFT) analyses on the vibration accelerations in that process, it was found that the dominant frequencies of the friction-induced vibrations associated with squeal is not varied. Examination of the friction–velocity slope shows that there is no invariable correlation between the negative friction–velocity slope and occurrence of squeal. Squeal can occur in regions with both negative and positive friction–velocity slopes.

Correlation of a negative friction–velocity slope with squeal generation under reciprocating sliding conditions

Under reciprocating sliding, it is found that the friction strongly depends on both displacement and frequency. This characteristic may be used to verify the negative friction–velocity slope mechanism of squeal generation. Test parameters such as the frequency and amplitude of reciprocating sliding are found to have a significant influence on the friction. In the present test, extensive test parameters are included. The frequency is ranged from 0.1 to 5 Hz, amplitude from 0.5 to 5 mm. In order to assure the accuracy of friction measurement, the friction force is measured with two different types of load cells: stress-strain and piezoelectric. Two different types of test apparatus are used to explore the dependence of the friction on both displacement and frequency. One of the test apparatus has a rotational degree of freedom between the frame fastening a specimen and load cell, which can almost keep a constant nominal normal force between two matched specimens regardless of the misalignment and wear of the specimens. The other does not have the degree of freedom. The reciprocating sliding is imposed in both sine and triangle waves. The test result shows that a negative friction–speed slope is not always responsible for vibration and associated squeal. Many squeals are also found to occur in the areas with positive friction–speed slopes.