Effective Friction Coefficient Research Articles

Simulation of Regular Relief Sliding over an Elastomer

The contact problem concerning sliding of a solid surface with a regular relief along a fixed ring-shaped elastomer under normal load and rotational moment is studied. The concept of an effective (integral) coefficient of friction is introduced, and the aim of this study is to analyze the dependence of this value on the density of contact spots. The problem is solved by direct computer modeling in finite-element formulation. It is shown that dependences of an effective friction coefficient on time have a nonmonotonic cyclic character with transition to the steady-state regime. The amplitude values of an effective friction coefficient are higher for a lower density of contact spots at a fixed value of the normal load on the sample, whereas its averaged value is higher for a higher density of contact spots.

Rheology and microstructure of unsaturated wet granular materials: Experiments and simulations

When dealing with unsaturated wet granular materials, a fundamental question is: What is the effect of capillary cohesion on the bulk flow and yield behavior? We investigate the dense-flow rheology of unsaturated granular materials through experiments and discrete element simulations of homogeneous, simple annular shear flows of frictional, cohesive, spherical particles. Dense shear flows of dry, cohesionless granular materials exhibit three regimes: Quasistatic, inertial, and intermediate [B. Andreotti et al., Contemp. Phys. 55, 151–152 (2013)]. Herewith, we show that the quasistatic and the intermediate regimes persist for unsaturated materials and that the rheology is essentially described by two dimensionless numbers: The reduced pressure P* comparing the cohesive to confining forces and the inertial number I, for a wide range of liquid content. This is consistent with recent numerical simulations [S. Khamseh et al., Phys. Rev. E 92, 022201 (2015)]. Finally, we measure the effective friction coefficient and the solid fraction variation throughout the wet bed. From this, we show that, in the quasistatic regime, the Mohr–Coulomb yield criterion is a good approximation for large enough P*. The experimental results agree quantitatively with the numerical simulation ones, provided the intergranular friction coefficient μ is set to its physical value identified from dry material rheology [M. Badetti et al., Eur. Phys. J. E 41, 68 (2018)]. To directly and quantitatively determine what happens inside the sheared granular bed, x-ray tomography measurements are carried out in a custom-made setup that enables imaging of a wet granular material after different shear histories. For the explored range of liquid content, samples remain homogeneous but exhibit some complex microscopic morphologies far from simple capillary bridges. From the x-ray microtomographic images, we can clearly distinguish liquid capillary bridges and liquid clusters by their morphologies. We see that the total number of capillary bridges decreases when one increases the liquid content and interestingly increases, at the expense of other morphologies, when we increase the shear strain. This explains the concordance between the experimental and numerical measurements since the numerical model is restricted to the pendular state, for which the liquid phase is completely discontinuous and no coalescence occurs between liquid bridges.

Regulation of Pom cluster dynamics in Myxococcus xanthus.

Precise positioning of the cell division site is essential for the correct segregation of the genetic material into the two daughter cells. In the bacterium Myxococcus xanthus, the proteins PomX and PomY form a cluster on the chromosome that performs a biased random walk to midcell and positively regulates cell division there. PomZ, an ATPase, is necessary for tethering of the cluster to the nucleoid and regulates its movement towards midcell. It has remained unclear how the cluster dynamics change when the biochemical parameters, such as the attachment rates of PomZ dimers to the nucleoid and the cluster, the ATP hydrolysis rate of PomZ or the mobility of PomZ interacting with the nucleoid and cluster, are varied. To answer these questions, we investigate a one-dimensional model that includes the nucleoid, the Pom cluster and PomZ proteins. We find that a mechanism based on the diffusive PomZ fluxes on the nucleoid into the cluster can explain the latter’s midnucleoid localization for a broad parameter range. Furthermore, there is an ATP hydrolysis rate that minimizes the time the cluster needs to reach midnucleoid. If the dynamics of PomZ on the nucleoid is slow relative to the cluster’s velocity, we observe oscillatory cluster movements around midnucleoid. To understand midnucleoid localization, we developed a semi-analytical approach that dissects the net movement of the cluster into its components: the difference in PomZ fluxes into the cluster from either side, the force exerted by a single PomZ dimer on the cluster and the effective friction coefficient of the cluster. Importantly, we predict that the Pom cluster oscillates around midnucleoid if the diffusivity of PomZ on the nucleoid is reduced. A similar approach to that applied here may also prove useful for cargo localization in ParABS systems.

Study of the friction and vibration characteristics of the braking disc/pad interface under dry and wet conditions

Abstract The joint analysis of friction coefficient and time-frequency characteristics of vibration is employed in this study to explore braking behaviours under dry and wet conditions. The results indicate that there is a weak braking period in the braking process under wet conditions, which would last until the effective friction coefficient is achieved. The duration of the weak braking period is dominant at low initial braking velocities and would be prolonged by strong water flow but shortened by a higher braking pressure. A partial transition from wet to dry stage on the front part of braking interface is deduced in the weak braking period, causing an obvious increasing trend of friction coefficient in this period and asymmetric abrasion of braking surface.

Ultra-Low Friction on Tetrahedral Amorphous Diamond-Like Carbon (ta-C) Lubricated with Ethylene Glycol

Lubricated tetrahedral amorphous carbon coatings can show a very complex tribological behavior. In particular, friction regimes with extremely low friction have been observed. In tribological experiments with a ta-C/steel friction pair that was lubricated with ethylene glycol, we observed a sudden and very strong decrease in the effective friction coefficient from 0.45 to 0.01 after running-in. By varying different components of the tribological system after this abrupt decrease we investigated the role of the counter-body, the lubricant and the coating. To investigate the surface chemistry, static time-of-flight secondary ion mass spectrometry (ToF-SIMS), dynamic secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS) measurements were performed. Using deuterated lubricants, ToF-SIMS measurements allowed us to distinguish adsorption of hydrogen and hydroxyl-groups from the lubricant from the adsorption from the environment. Deuterated hydroxyl-groups from the lubricant adsorbed to the surface during the experiment. In particular, more adsorbed deuterated hydroxyl-groups were detected prior to the sudden decrease in the friction coefficient. Thus, the sudden decrease in the coefficient of friction was most likely caused by an interplay between the lubricant, the ta-C coating and the counter-body which lead to the formation of transfer and adsorption layers.

The Model of Friction and Wear of the Elastic Coating against a Wavy Punch under Vibrations

A mechanical model of friction and wear of an elastic coating against a wavy punch under forced oscillations (vibrations) is proposed. The model allows predicting reduction in the effective coefficients of friction and wear depending on the parameters of forced vibrations and waviness of the punch. Qualitative agreement between the obtained theoretical dependences and known experimental results is demonstrated. The proposed model can be used for correct interpretation of the results of tribological experiments.

Shear localization and wall friction in confined dense granular flows

In this work, we discuss experiments and discrete element simulations of wall-bounded shear flows of slightly polydisperse spheres under gravity. Experiments were performed in an annular shear cell in which the bottom bumpy wall rotates at fixed velocity, while a pressure is applied at the top bumpy wall. The coaxial cylinders delimiting the flow are flat, frictional and transparent, allowing visualization of the flow. Velocity profiles were obtained by particle image velocimetry, and are characterized by an exponential profile, the decay length of which depends on the applied load, but not on the wall velocity. A force sensor was installed at different vertical positions on the outer sidewall in order to measure wall forces. The effective streamwise and transverse wall friction coefficients were thus estimated, showing wall friction weakening in creep zones. In order to better understand these results, contact dynamics simulations were carried out in a simplified configuration (Artoni & Richard, Phys. Rev. Lett., vol. 115 (15), 2015, 158001). In this case, profiting from the possibility of varying the particle–wall friction coefficient, different flow regimes were observed. In particular, shear can either be localized (1) at the bottom or (2) at the top of the shear cell, or (3) it can be quite evenly distributed in the vertical direction. Through an averaging technique that explicitly takes into account gradient effects (Artoni & Richard, Phys. Rev. E, vol. 91 (3), 2015, 032202), relevant, coarse-grained, continuum fields (solid fraction, velocity, stresses, velocity fluctuations) were obtained. They allow a discussion of the relevance of velocity fluctuations (i.e. granular temperature) for describing non-locality in granular flow. The case of solid-like fluctuations is also addressed. Finally, a simplified stress analysis is devoted to explain the emergence of complex shear localization patterns by the heterogeneity of effective bulk friction, which is due to the joint effect of gravity and wall friction.

Investigations on Additive Manufacturing of Ti–6Al–4V by Microplasma Transferred Arc Powder Deposition Process

This paper presents investigation findings on additive manufacturing (AM) aspects of Ti6Al4V by microplasma transferred arc powder deposition (μ-PTAPD) process in continuous and dwell-time mode. Pilot experiments were conducted to identify feasible values of six important parameters of μ-PTAPD process for single-layer deposition followed by 27 main experiments varying three parameters. Energy consumption aspects were used to identify optimum values of parameters varied during main experiments for multilayer deposition. It revealed that higher values of flow rate of powder and travel speed of deposition head result in smaller values of power consumption per unit flow rate of powder and energy consumption per unit traverse length. Continuous and dwell-time modes were used to study deposition characteristics, microstructure, lamellae widths, wear characteristics, tensile properties, fractography of tensile specimen, wear mechanism, and microhardness of multilayer depositions. Dwell-time deposition yielded higher effective wall width (EWW), deposition efficiency (DE), yield strength, ultimate strength, microhardness, surface straightness, lower strain, wear volume and friction coefficient, and smaller lamellar width. It had good deposition quality with fine partial martensite and basket-weave microstructure. Fractography analysis exhibited fine dimple rupture for dwell-time multilayer deposition and occurrence of elongated regions for continuous multilayer deposition. Wear of dwell-time multilayer deposition occurred by microploughing and microcutting resulting in smaller wear debris. Comparison of Ti6Al4V depositions by different processes revealed that dwell-time μ-PTAPD process is cost-effective than laser-based processes and energy efficient than pulsed plasma arc process.

Influence of train axle load on wheel-rail interface friction

The implementation of high speed railways in India are in their early phases and vast studies are being conducted for the effective implementation of the system. Maintaining the traction required for efficient operation of trains at high speeds requires the net minimum frictional coefficient to be maintained at the wheel-rail interface, across the desired range of axle loads and target speeds of train operation. With the increase in speed, wheels of the train tend to lift-off, effectively reducing the interface friction at the wheel-rail contact. In this study, the effect of increasing the axle load on the effective frictional coefficient available at wheel-rail interfaces during high speed railway operations were quantified by numerical simulations, and the results reveal that the net frictional coefficient available at the wheel-rail contacts have a trend to increase with an increase in the applied axle loads. Conversely, this instead points out that decreasing the axle loads for achieving higher speeds of train operation will result in reduction of traction, and there will be chances of wheel-slips on the rails during rapid acceleration and emergency braking situations. Modifying the rail surface friction using top-of-rail friction modifiers are a much easier alternative to making amendments in the wheel-rail profiles, for maintaining sufficient traction on rails. In the wheel-rail interfaces with high friction, lubrications in the boundary regime, mixed lubrication or hydrodynamic lubrication are applied on the basis of Stribeck curve, adopting the optimum frictional coefficients for avoiding rolling contact fatigue and high rates of wear due to plastic deformations. When there is less frictional grip, a coating of top-of-rail positive friction enhancers becomes necessary, for increasing the adhesion characteristics and achieving positive creep. This study emphasizes the importance of optimizing the contact at wheel-rail interfaces, to maintain the frictional coefficients to neither cause loss of traction due to lower grip, nor be on the higher side to cause rolling contact fatigue and high rates of material wear.

Variation of effective frictional coefficient at wheel-rail contact interfaces during high speed railway operations

Numerical simulations were performed for the analysis of wheel-rail interactions in high speed railway networks, and the variation of effective frictional coefficient at the wheel-rail interface during high speed train operations was quantified, with respect to the increase in speeds of trains. The analyses were performed for railway operations ranging from speeds of 160 to 200 km/h, which are the maximum speeds recommended by Indian Railways for safe operation of trains in the broad gauge railway tracks in India. Transient structural analyses were performed using ANSYS, and it was observed that the interface frictional coefficients at wheel-rail interface had a trend to decrease with an increase in the speed of train. This reduction in the coefficient of friction at wheel-rail interface would contribute to a loss of traction due to reduced grip, resulting in locomotive wheel-slips, wastage of motive power and reduced braking efficiency of train wheels on rails during high speed railway operations. Electronic wheel-spin detectors shall be used to control and alter the power supplied by high speed locomotives to train wheels, for maintaining sufficient traction on the rails. Modification of rail and wheel surfaces using friction modifiers will help in maintaining the frictional grip necessary for maintaining safety in railway operations at the targeted maximum speeds of train transit.

The effective temperature for the thermal fluctuations in hot Brownian motion.

We revisit the effective parameter description of hot Brownian motion-a scenario where a colloidal particle is kept at an elevated temperature than the ambient fluid. Due to the time scale separation between heat diffusion and particle motion, a stationary halo of hot fluid is carried along with the particle resulting in a spatially varying comoving temperature and viscosity profile. The resultant Brownian motion in the overdamped limit can be well described by a Langevin equation with effective parameters such as effective temperature THBM and friction coefficient ζHBM that quantifies the thermal fluctuations and the diffusivity of the particle. These parameters can exactly be calculated using the framework of fluctuating hydrodynamics and require the knowledge of the complete flow field and the temperature field around the particle. Additionally, it was also observed that configurational and kinetic degrees of freedom admit to different effective temperatures, THBMx and THBMv, respectively, with the former predicted accurately from fluctuating hydrodynamics. A more rigorous calculation by Falasco et al. [Phys. Rev. E 90, 032131-10 (2014)] extends the overdamped description to a generalized Langevin equation where the effective temperature becomes frequency dependent and consequently, for any temperature measurement from a Brownian trajectory requires the knowledge of this frequency dependence. We use this framework to expand on the earlier work and look at the first order correction to the limiting values in the hydrodynamic limit and the kinetic limit. We use the linearized Stokes equation and a constant viscosity approximation to calculate the dissipation function in the fluid. The effective temperature is calculated from the weighted average of the temperature field with the dissipation function. Further, we provide a closed form analytical result for effective temperature in the small as well as high frequency limit. Since hot Brownian motion can be used to probe the local environment in complex systems, we have also calculated the effective diffusivity of the particle in the small frequency limit. To look into the kinetic temperature, the velocity autocorrelation function is computed from the generalized Langevin equation and the Wiener-Khinchine theorem and numerically integrated to evaluate THBMv as a function of the ratio of particle density and fluid density ρP/ρ0. The two limiting cases of ρP/ρ0 → 0 and ρP/ρ0 → ∞ is also discussed.

Origin of a foreland-dipping seismogenic zone and its basal decollement in southwestern Taiwan

An unusual foreland-dipping seismicity band has been identified at a depth between 7 and 25 km beneath the Coastal Plain, a foreland basin, in southwestern Taiwan. This seismogenic zone is closely associated with abrupt changes in Vp/Vs based on our new high-resolution seismic tomography. Rocks with high Vp/Vs ratios of up to 1.84 were found directly above the inclined seismicity band. Below it, the Vp/Vs ratio dramatically drops to about 1.72–1.78. We found that this change may result from illite-bearing slates transforming to muscovite-bearing phyllite/mica-schist. Previous studies suggested that the frictional instability regime for illite–quartz gouge and muscovite–quartz gouge falls in the range of 250–500 °C. We calculated the isotherms based on regional heat flow data and found that the foreland-dipping seismicity band can well be bounded between the 250 °C and 500 °C isotherms. In addition, in the same temperature range wet illite/hydromuscovite would transform to muscovite and release water. This dehydration may elevate pore pressure and facilitate the formation of an incipient decollement at the base of the seismicity band in front of the mountain belt. The hypothesis that such a decollement exists is supported by the existence of a corresponding zone of low resistivity and also by the geometry of the earthquake of October 22, 1999. The latter clearly shows a fault branched off the basal decollement with an angle about 45°. This finding implies that the effective friction coefficient of the basal decollement is at least 23% less than that of the overlying rocks.

A viscoplastic shear-zone model for deep (15–50 km) slow-slip events at plate convergent margins

A key issue in understanding the physics of deep (15–50 km) slow-slip events (D-SSE) at plate convergent margins is how their initially unstable motion becomes stabilized. Here we address this issue by quantifying a rate-strengthening mechanism using a viscoplastic shear-zone model inspired by recent advances in field observations and laboratory experiments. The well-established segmentation of slip modes in the downdip direction of a subduction shear zone allows discretization of an interseismic forearc system into the (1) frontal segment bounded by an interseismically locked megathrust, (2) middle segment bounded by episodically locked and unlocked viscoplastic shear zone, and (3) interior segment that slips freely. The three segments are assumed to be linked laterally by two springs that tighten with time, and the increasing elastic stress due to spring tightening eventually leads to plastic failure and initial viscous shear. This simplification leads to seven key model parameters that dictate a wide range of mechanical behaviors of an idealized convergent margin. Specifically, the viscoplastic rheology requires the initially unstable sliding to be terminated nearly instantaneously at a characteristic velocity, which is followed by stable sliding (i.e., slow-slip). The characteristic velocity, which is on the order of <10−7 m/s for the convergent margins examined in this study, depends on the (1) effective coefficient of friction, (2) thickness, (3) depth, and (4) viscosity of the viscoplastic shear zone. As viscosity decreases exponentially with temperature, our model predicts faster slow-slip rates, shorter slow-slip durations, more frequent slow-slip occurrences, and larger slow-slip magnitudes at warmer convergent margins.

Wedge geometry, frictional properties and interseismic coupling of the Java megathrust

The mechanical interaction between rocks at fault zones is a key element for understanding how earthquakes nucleate and propagate. Therefore, estimating frictional properties along fault planes allows us to infer the degree of elastic strain accumulation throughout the seismic cycle. The Java subduction zone is an active plate boundary where high seismic activity has long been documented. However, very little is known about the seismogenic processes of the megathrust, especially its shallowest portion where onshore geodetic networks are insensitive to recover the pattern of elastic strain. Here, we use the geometry of the offshore accretionary prism to infer frictional properties along the Java subduction zone, using Coulomb critical taper theory. We show that large portions of the inner wedge in the eastern part of the Java subduction megathrust are in a critical state, where the wedge is on the verge of failure everywhere. We identify four clusters with an internal coefficient of friction μint of ∼ 0.8 and hydrostatic pore pressure within the wedge. The average effective coefficient of friction ranges between 0.3 and 0.4, reflecting a strong décollement. Our results also show that the aftershock sequence of the 1994 Mw 7.9 earthquake halted adjacent to a critical segment of the wedge, suggesting that critical taper wedge areas in the eastern Java subduction interface may behave as a permanent barrier to large earthquake rupture. In contrast, in western Java topographic slope and slab dip profiles suggest that the wedge is mechanically stable, i.e deformation is restricted to sliding along the décollement, and likely to coincide with a seismogenic portion of the megathrust. We discuss the seismic hazard implications and highlight the importance of considering the segmentation of the Java subduction zone when assessing the seismic hazard of this region.

MEASUREMENT OF THE STATIC FRICTION COEFFICIENT BETWEEN A poly-V belt 5pk AND A PULLEY UNDER DRY CONDITIONS

The paper presents the results of experimental measurements of static friction forces between a poly-V belt 5pk and a pulley on a specialised research stand. An average effective static coefficient is assumed depending on the wrap angle and preload force. Different shapes and positions of curves of the measured values for lower wrap angles are found, with similar curves in the set of measurements with higher angles. The Nelder-Mead optimisation method is proposed to approximate the measured results by a nonlinear function and to achieve good accordance. The dependence of the effective friction coefficient on the rest time between measurements is also presented.

Braced Frame Symmetrical and Asymmetrical Friction Connection Performance

This paper describes quasi-static testing of Asymmetrical Friction Connection (AFC) and Symmetrical Friction Connections (SFC) in steel braces. It is shown that stable energy dissipation mechanisms have been achieved in braces using Bisalloy 500 shims on the sliding surface. When incorporated into a moment frame, the braces and the moment resisting frames underwent large displacements without significant frame yielding. The effective coefficient of friction is shown to be dependent on prying.

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

The grain size distribution and composition of the Touzhai rock avalanche deposit in Yunnan, China

The September 23, 1991 rock avalanche that occurred in Touzhai, Zhaotong, Yunnan province of China claimed 216 lives and caused heavy property losses. The Touzhai avalanche possesses typical and representative feature of high-mobility. Field surveys are applied in the exploration of the grain size distribution and deposit composition, and the lab analyses provide important information about the occurrence mechanism of the rock avalanche flow. The Touzhai rock avalanche has a volume of 9millionm3 that traveled about 3.4km along Touzhai valley within 3min, with an effective coefficient of friction of 0.22. The deposit of the Touzhai rock avalanche contains a wide spectrum of grain sizes (<5μm–7.82m) and the proportion of the boulders (>50cm) outcropped in the deposit surface is 0.2%. Compared with the upper reaches, a greater number of boulders are found to exist in the middle- and lower reaches of the rock avalanche flow. The non-uniformity coefficient, curvature coefficient and fractal dimension of the deposit (<150mm) are 21.1, 2.93 and 2.53, respectively, indicating that the particles are poorly sorted. The changes in the grain size distribution from the upper part of the rock avalanche flow to the terminus are not obvious. There is no sorting or stratification in terms of structure and composition of the deposit profiles. The source rock mass is highly fragmented, having been subjected to initial discontinuities, tectonic movement and weathering, but undisaggregated, which is responsible for the broad grain size distribution of the deposit. The montmorillonite and kaolinite formed by the weathering of Emeishan basalt, together with the fine particles created by the fragmentation of Emeishan basalt saprolite, might have pervaded the spaces among the coarse debris and trapped the compressed air, which may have supported the debris in the rapid rock avalanche flow.

A study of the rheology and micro-structure of dumbbells in shear geometries

We study the flow of frictional, inelastic dumbbells made of two fused spheres of different aspect ratios down a rough inclined plane and in a simple shear cell, using discrete element simulations. At a fixed inclination angle, the mean velocity decreases, and the volume fraction increases significantly with increasing aspect ratio in the chute flow. At a fixed solid fraction, the shear stress and pressure decrease significantly with increasing aspect ratio in the shear cell flow. The micro-structure of the flow is characterized. The translational diffusion coefficient in the normal direction to the flow is found to scale as Dyy=bγ̇d2, independent of aspect ratio, where b is a constant, γ̇ is the shear rate, and d is the diameter of the constituent spheres of the dumbbells. The effective friction coefficient (μ, the ratio of shear stress to pressure) increases by 30%-35% on increasing the aspect ratio λ, from 1.0 to 1.7, for a fixed inertial number I. The volume fraction (ϕ) also increases significantly with increasing aspect ratio, especially at high inertial numbers. The effective friction coefficient and volume fraction are found to follow simple scalings of the form μ = μ(I, λ) and ϕ = ϕ(I, λ) for all the data from both systems, and the results are in reasonable agreement with kinetic theory predictions at low I. The computational results are in reasonable agreement with the experimental data for flow in a rotating cylinder.

Friction Characteristics of Synchronization Process Based on Tribo‐Thermodynamics

In order to improve the shift control accuracy and shift quality, the temperature and friction coefficient changing regularities of a friction cone during the synchronization process were investigated. The thermal‐structural coupling model was established through tribo‐thermodynamic analysis. The relevant experiment was carried out as well. The results show that the error between the experimental and simulated results is within 3%. Besides, the maximum temperature of the synchronous ring friction surface increases 1.8°C for every additional 50 N of shift force, while increases 1.1°C for every additional 200 r/min shift speed difference. Moreover, the friction coefficient declines rapidly first and then tends to be stable slowly during the synchronization process. The result of friction coefficient changing regularity lays a good theoretical basis for establishing an effective friction coefficient compensation control strategy.