Friction Mode Research Articles

Capillary-Stress Controlled Rheometer Reveals the Dual Rheology of Shear-Thickening Suspensions

The rheology of dense colloidal suspensions, which may undergo discontinuous shear thickening or shear jamming, is particularly difficult to analyze with conventional rheometers. Here, we develop a rheometer adapted to colloidal suspensions: the “capillarytron,” which uses the air-suspension capillary interface to impose particle (or osmotic) pressure during shear. The main virtues of this new device are that (i) it gives direct access to the suspension friction coefficient, (ii) it operates for very dense suspensions up to jamming, and, most importantly, (iii) it decouples the stresses developed within the suspension from the applied shear rate. We can, thus, smoothly move through the different frictional states of the system, precisely in the range of volume fractions where discontinuous shear thickening or shear jamming occur under volume-imposed conditions. Our results obtained with the capillarytron provide the first complete characterization of the dual frictional behavior of a model shear-thickening suspension, in agreement with the recently proposed frictional transition scenario. Based on a new concept in rheometry, the capillarytron unlocks the path to pressure-imposed rheology on colloidal and Brownian suspensions. Moreover, its fine control of the particle pressure via the soft capillary interface opens the possibility to explore the flow of “fragile” particles close to jamming, such as Brownian colloids, active particles, and living cells.Received 18 November 2022Revised 15 December 2022Accepted 9 January 2023DOI:https://doi.org/10.1103/PhysRevX.13.011024Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasJammingNon-Newtonian fluidsRheologyPhysical SystemsComplex fluidsGranular fluidsPolymers & Soft Matter

On Friction, Heat Input, and Material Flow Initiation during Friction Stir Welding: Tool and Process Optimization

The Friction Stir Welding (FSW) process depends entirely upon mechanical contact between the tool and the workpiece. As a result of this, all process phenomena and process outcomes such as weld geometry and mechanical properties are governed by FSW’s frictional system. The following work characterizes this system with a focus on process initialization, heat input and material flow. For this purpose, an experimental program for the isolated investigation of the frictional system was carried out. Short-term effects such as contact initiation, run-in behavior and frictional transitions are considered as well as the influences of process parameters and geometry. The system and its behavior are analyzed quantitatively and qualitatively by experiments altering the normal pressure, relative velocity, and tool geometry. The experiments demonstrate a self-similar behavior of the process, including an important wear transition which initiates the material flow, and a subsequent equilibrium of forces, heat balance, and temperatures. The interaction between the tool and the welded material is described, as is the link between the frictional interface and material flow initialization. Based on these findings, recommendations are provided for process optimization and tool design.

Nonlinear Vibration Feature Recognition Method for Reciprocating Compressor Cylinder Based on VMD-Multifractal Spectrum

The failure probability of piston ring abrasion occupies the forefront in reciprocating compressor cylinder assembly. Due to the reciprocating friction between the piston ring and cylinder liner, the local vibration signal contains typical nonstationary characteristics with a seriously overlapped frequency band of impact signals, while the different featured signals are coupled with each other. Therefore, the identification of cylinder friction and abrasion fault modes has always been a hot and difficult problem in fault diagnosis of reciprocating compressors. In order to monitor the friction and abrasion between low linear speed nonmetallic piston ring and cylinder liner of reciprocating compressor in refinery and provide a theoretical basis for preventive maintenance, an analysis method based on VMD-multifractal spectrum was presented in this paper. First, the feasibility of the proposed method was proven based on the cylinder vibration experiment of a reciprocating compressor in the laboratory. Then, an in-service reciprocating compressor in a refinery was taken as the research object, from which the cylinder friction vibration signals were processed by using variational mode decomposition to obtain a band-limited intrinsic mode function (BLIMF). Multifractal detrended fluctuation analysis is employed in the final abrasion pattern identification. The results show that the proposed method based on VMD-multifractal spectrum analysis can effectively obtain the abrasion state of a piston ring, which can avoid blind overhaul or provide the basis for preventive maintenance and a practicable engineering route for the study of the piston ring abrasion degree.

Investigation of the Wear Resistance of a Journal Bearing with Polymer-Coated Grooved Support Ring

Introduction. In modern heavy-loaded friction units, metallopolymer coated bearings operating in the boundary friction mode are widely used. Their successful application is provided by the viscoelastic deformation of these coatings under load. To pass from boundary friction to liquid friction, it is required to create a bearing hydrodynamic wedge. Currently, the use of journal bearings with polymer-coated grooved support ring is hindered by the lack of a methodology for their calculation. This work analyzes a model of movement of a micropolar lubricant in the operating clearance of a journalbearing with a nonstandard support profile having a PTFE composite coating with a groove on the bearing surface. The study aims at establishing the dependence of the stable hydrodynamic regime on the width of the groove on the surface of the bearing profile. Materials and Methods. Tribological tests of journal bearings with a nonstandard bearing profile having a polymer coating with a groove on the surface were carried out on samples in the form of partial bushes (blocks). Using the equation of movement of a lubricant with micropolar rheological properties, as well as the continuity equation, new mathematical models were obtained that took into account the width of the groove, polymer coating, and nonstandard bearing profile. Results. A significant expansion of the applicability of design models of journal bearings with structural changes has been achieved. Polymer-coated bearings with a groove provided a hydrodynamic lubrication mode. The results obtained allowed us to evaluate the operational characteristics of the bearing: hydrodynamic pressure value, load capacity, and coefficient of friction. Discussion and Conclusions. The design of polymer coated journal bearing and a groove 3 mm wide on the surface of the liner provided a stable ascent of the shaft on the hydrodynamic wedge, which was validated experimentally. The experiments were carried out for journal bearings with a diameter of 40 mm with a groove 1–8 mm wide, at a sliding speed of 0.3–3 m/s and a load of 4.8–24 MPa.

Kinetics of changes of tribotechnical properties of polymeric materials in period of running-in

The article presents the study results of the influence of the type of polymers and friction modes on the duration of the running-in of polymeric materials during friction with various materials. The kinetics of changes in the temperature in the friction zone (Tfr), friction coefficient (f), the height of micro asperities (Rz), and the radius of the tops of asperities (r) during the running-in period have been studied.

Nitriding of Long-Term Holes in the Cyclic-Commuted Discharge

The effect of anhydrous nitriding in a glow discharge on microhardness, phase composition, and wear resistance of long holes in steels C45, 37Cr4, and 41CrAlMo7 with direct current supply and in cyclically switched discharge (CSD) was studied. Nitriding was carried out on a UATR-1 anhydrous nitriding unit with a discharge chamber diameter of 400 mm and a working height of 700 mm. Anhydrous nitriding in a glowing discharge was carried out at a temperature of 560 °С, a voltage of 730 V, a pressure in the chamber of 120 MPa, and the nitriding duration was 6 h. It was established that using holes with a relatively small diameter of glow discharge in a cyclically switched discharge for nitriding creates conditions for obtaining modified layers with higher physical, mechanical, and tribological characteristics. The results of microhardness measurement and their comparison with X-ray phase analysis data confirm the formation of ε, γ, and α phases during nitriding along the entire height of the samples placed in the experimental model. The tests carried out in the dry friction mode showed an increase in the wear resistance of samples made of steel C45, 37Cr4, and 41CrAlMo7 during nitriding in a cyclically switched discharge. To achieve 100 μm wear of 41CrAlMo7 steel during nitriding in CSD, 1400 m of friction path and 1000 m – during nitriding with direct current is required. It was established that using long holes of a glow discharge with different types of power for nitriding creates conditions for obtaining modified layers with variable characteristics. Nitriding of holes with a relatively small diameter of a glow discharge with a different power supply creates conditions for obtaining modified layers with different physicomechanical and tribological characteristics.

Обґрунтування механізму функціонування мехатронної системи склоочищення автомобіля

The four-link mechanism of the car's mechatronic windshield cleaning system is substantiated using different ratios of links and analysis of their kinematic and dynamic characteristics. A mathematical model of the glass cleaning mechanism in the Matlab/Simulink environment is presented and its kinematic synthesis is carried out. A mathematical model of the movement of the brush on the glass was developed for the case of close to uniform pressure distribution along the length of the brush, taking into account the unevenness of the law of movement associated with the operation of the transmission mechanism. The influence of the material properties of the rubber element on the quality of glass cleaning and the obtained values of the values characterizing the elastic and dissipative properties of the neck in the limit modes of the system are considered. The possibility of regeneration self-oscillations is shown when the dissipative properties of the neck are reduced as a result of wear or an aggressive environment. The processes of dry friction and hydraulic movement resistance occurring in the system are clarified, the limits of possible visually noticeable self-oscillations depending on the parameters of the friction mode and system operation are shown. It was found that the presence in the system of the possibility of regenerative self-oscillations in critical operating modes requires the introduction of micro-oscillations into the law of movement of the brush on the glass, which can effectively reduce the amplitudes of these self-oscillations. Analytical dependencies are proposed for calculating the parameters of the software, which implements the management of the dynamic characteristics of the system by superimposing the law of motion of micro-oscillations. The necessary hardware and software requirements for the controller are shown, as well as the possibility of saving the windshield wiper control unit as a functional unit of the mechatronic system for ensuring the comfort of movement.

Simulation of transient friction modes of fluoroplastic seals in hydraulic piston pumps

The article considers the issues associated with the friction of radiation-hardened fluoroplastic on steel in a hydraulic fluid environment during the transition period at the beginning of movement. The friction coefficients are studied when starting from a standstill, at the beginning and in steady motion. Changes in the value of the friction coefficient and changes in the amplitude of oscillation of the friction moment are analyzed. Particular attention is paid to the influence of the restart of motion in a friction coupling. Quantitative relationships of tribological properties are established depending on the time of relative sliding of the samples. It is noted that the friction coefficients in different modes are differ a lot, however, the running-in process proceeds quickly and in the practice of operating hydraulic devices, the running-in process of fluoroplastic seals can be ignored

Scaling relationships between viscosity and diffusivity in shear-thickening suspensions.

Dense suspensions often exhibit a dramatic response to large external deformation. The recent body of work has related this behavior to transition from an unconstrained lubricated state to a constrained frictional state. Here, we use numerical simulations to study the flow behavior and shear-induced diffusion of frictional non-Brownian spheres in two dimensions under simple shear flow. We first show that both viscosity η and diffusivity D/ of the particles increase under characteristic shear stress, which is associated with lubrication to frictional transition. Subsequently, we propose a one-to-one relationship between viscosity and diffusivity using the length scale ξ associated with the size of collective motions (rigid clusters) of the particles. We demonstrate that η and D/ are controlled by ξ in two distinct flow regimes, i.e. in the frictionless and frictional states, where the one-to-one relationship is described as a crossover from D/ ∼ η (frictionless) to η1/3 (frictional). We also confirm that the proposed power laws are insensitive to the interparticle friction and system size.

Friction Reduction by Laser Irradiation for a Friction System Using Bearing Steel and Aluminum Alloy in Engine Oil

Aluminum alloy sliding components are widely used in internal combustion engines. However, aluminum easily adheres to the countersurface, resulting in high friction. Hence, laser irradiation was used to oxidize the aluminum surface and promote tribofilm formation using molybdenum dithiocarbamate and zinc dialkyldithiophosphate (ZnDTP). This study investigated laser irradiation to reduce friction between steel balls and aluminum disks in fully formulated engine oil. Laser scanning was used to create 100-μm-pitch concentric circles on the disks. The friction behavior was classified into two modes: no friction reduction (mode-I, μ: 0.09–0.12) and friction reduction after the high-friction period (mode-II, μ: 0.06–0.09). Friction mode transition occurred when the laser energy density exceeded the critical value (Ec). Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy showed that, for mode-I, aluminum adhered to the ball, and no sulfur-containing tribofilm formed. For mode-II, the ZnDTP-derived phosphate film formed on the disk suppressed aluminum adhesion, and a sulfur-containing tribofilm formed on the ball. Micro-Vickers tests and X-ray diffraction showed that an amorphous/nanocrystalline structure formed in the unirradiated area owing to heat diffusion under high-energy-density laser irradiation (>Ec). Results suggest that metallographic structural change in the unirradiated area promotes ZnDTP reactions, causing the mode transition.

The influence of electric and thermal currents on the behavior of hydrogen in friction pairs

Studies have shown that the main influence in brake friction pairs on the behavior of hydrogen is pulsed: specific loads, electric and thermal currents, as well as their gradients. With the growth of impulse specific loads, the electrical conductivity of the contact increases relatively quickly, which increases the thermal effect, but when the load is smoothly reset, it decreases much more slowly and does not coincide with the initial values. The magnitude and direction of the integral e.m.f depends on the physical and mechanical properties of the metals of the friction pair, on the speed and load modes of friction, and on many other factors that cause significant fluctuations and even inversion of this parameter. The effect of thermal current on the wear resistance of parts of friction pairs is related to the peculiarities of the course of relaxation, oxidation, diffusion and other processes that affect each other. Oxide films on the surface of contacting metals have semiconducting properties and high resistivity and can play the role of amplifiers of thermoelectric effects, which can cause the appearance of fairly significant eddy currents in the places of the greatest convergence of contact points and, due to the low thermal conductivity of oxides, contribute to localized discharge. Impulsive high-frequency mechanical oscillations in the friction zone are inseparable from thermoelectric relaxation processes, and these factors always jointly affect the spread of experimental values of wear resistance of parts.

APPLICATION OF TRIBOTECHNICAL DIAGNOSTIC METHODS IN CASE OF DETERMINATION OF COEFFICIENT OF OPERATIONAL RELIABILITY OF MACHINE TOOLS

The submitted paper analyses the check of operational capacity and of economic efficiency of the selected machines in case of the use of different types of oil, determination of mode of friction and of machine wear and specification of interval of oil change. The object of the paper is a machine park in the industrial company comprising of five machine tools. Particularly, it is the case of two grinding machines, two lathes and one milling cutter. Having been removed from the greasing systems, the oils were used to gain information on technical condition of the respective machines.

Tribological properties of doped zirconium dioxide crystals under water lubrication

The issues of a ceramic plain bearing performance under friction in the water are discussed in this article. Bearing bushings made from nanostructured partially stabilized zirconium dioxide crystals doped with rare earth elements under friction on steel and ceramic shafts are investigated. The statement about the possibility of using zirconium ceramic materials in bearings of multistage submersible centrifugal pumps for oil production is substantiated. The efficiency of alloying crystals to improve crack resistance is analyzed. Particular attention is paid to the tribological properties of the bearing when using steel and ceramic shafts. The results of comparative tribological tests of samples of the studied materials, bench tests of full-scale bearing bushings carried out under water and dry friction are presented. Based on the study, it is proposed to make bushings from a zirconium dioxide crystal for working on a steel shaft. It has been established that this plain bearing can withstand the emergency mode of friction under water or dry friction for half an hour.

Effects of Graphene Redox on Its Triboelectrification at the Nanoscale

Controlling triboelectrification behaviors of graphene-based materials plays a key role in the output performance and reliability of the graphene-based triboelectric nanogenerators. The electrical and mechanical properties of graphene-based materials are sensitive to their redox, and the study of the redox effects on their triboelectrification at nanoscale is the basis for regulating the triboelectrification performance. Here, the triboelectrification behaviors of graphene oxide (GO) and thermally and chemically reduced graphene oxide (RGO) were explored by scanning probe microscopy. It is found that the triboelectric charges on GO are localized in the rubbed region due to the sp3 hybridized structure. However, on RGO, triboelectric charges are distributed over the whole sheet due to the restored sp2 structure. In the rubbing process with insufficient contact, wrinkles in RGO, contact forces, and friction modes affect the triboelectric charging process dramatically. These influences are related to the resistance threshold of nanoscale tip–sample contact that is related to the present tip–sample potential difference, the contact force, the roughness, and the friction mode. When the tip–sample contact is sufficient enough, the charge transfer in the triboelectric charging process can reach equilibrium, and the roughness, contact force, and rubbing mode have no effect on the triboelectrification. Moreover, the triboelectric potential is linearly dependent on the initial potential, lifting (positive) with a negative initial potential and dropping (negative) with a positive initial potential. Tunneling triboelectrification on RGO is also affected by the initial potential, presenting a localized distribution of triboelectric charges in the rubbed area. Our findings would provide guidance for regulating the triboelectric charges on graphene-based materials and designing high-performance graphene-based triboelectric nanogenerators.

INFLUENCE OF LUBRICANT ON THE TECHNICAL LEVEL OF HIGH-SPEED GEARBOXES WITH ASYMMETRICAL PROFILE

The issues of efficiency improvement in hydrodynamic (liquid) or limit (semi-liquid) modes of friction of gears with an asymmetric profile and the effect of the liquid on tooth damage, power loss and temperature in contact, as well as the role of an elastic medium that dampens shocks and changes contact lines, are considered. In many gears, the tooth load per tooth is much higher and applied for longer periods of time. The asymmetrical shape of the tooth reflects this functional difference. In the case of asymmetric gears, the standard symmetrical toothed tool rail is modified by changing the pressure angle of one of the side surfaces. However, this simplistic approach to the design of an asymmetric transmission significantly limits the ability to maximize the performance of a wide range of possible applications of these transmissions. As the frequency of impact interactions increases, the lubricating layer of high-speed gears with an asymmetric profile ceases to respond to pressure changes, and the load value, which determines the smallest layer thickness over time, approaches the minimum stationary component of the spectrum of dynamic loads. At the same time, noise and vibration levels are slightly reduced. Therefore, in the calculations of the strength, contact endurance and seizure of high-speed gears with an asymmetric profile, the complex influence of the criterion parameters of the lubricant must be taken into account not only when determining the allowable stresses, but also when determining the amount of dynamic load amplification due to the impact interaction of the teeth.

OPTIMIZATION OF CUTTING MODES ON HEAVY MACHINES

The work is devoted to improvement of the design theory and development on this basis of adaptive hydrostatic supports of new generation, as well as development of methods for optimal design of spindle assemblies and guides with such supports. It is established that the use of fluid friction modes in heavily loaded nodes is very relevant for heavy numerically controlled precision machine tools. Hydrostatic spindle bearings are used in heavy lathes, in which the required accuracy of rotation is not provided by rolling bearings. An adaptive system for controlling the pressure in the pockets of hydrostatic supports and the tensioning force of the positioning drive has been developed. The system makes it possible to adjust the transfer function and exclude the possibility of oscillations by rather simple means. By means of the system the accuracy and productivity of machining on metal-cutting machines of different technological purposes is significantly increased. The reliability of the supports is increased, since the throttle elements are in motion during operation, which prevents the throttle gap from becoming overgrown. Stiffness and load-bearing capacity of the lubricant layer are increased. Debugging work on setting the working pressure in the support pockets is eliminated. The mechatronic system of adaptive control of pressure in the pockets of hydrostatic spindle assembly with increased accuracy is developed. Adaptive regulators of support power supply systems with spindle position feedback have been developed. A new design of hydrostatic bearing sleeve with built-in capacitive clearance sensors is developed

Crustal Structure of the Nankai Subduction Zone Revealed by Two Decades of Onshore‐Offshore and Ocean‐Bottom Seismic Data: Implications for the Dimensions and Slip Behavior of the Seismogenic Zone

AbstractTwo‐decades of onshore‐offshore, ocean‐bottom seismometer, and passive‐source data are integrated to obtain high‐resolution 3‐D constraints on the architecture of the Nankai subduction zone. Our model reveals large along‐strike variability in the width of the outer‐forearc, with the crustal backstop (VP ≥ 5 km/s) extending within 50 km of Nankai Trough offshore Tokai and Kii Peninsula, but is a factor of 2 further landward (90–115 km) offshore Cape Muroto and Kyushu. The crustal backstop broadly coincides with the down‐dip extent of shallow slow‐earthquakes and the up‐dip extent of co‐seismic slip and geodetic locking, suggesting a link with the shallow frictional transition. In the inner‐forearc, high seismic‐velocities (VP > 6.6 km/s) in Kii Peninsula reflect Miocene plutons, impacting the nucleation and segmentation of large megathrust earthquakes. The complex geometry of the subducting slab drives spatial variability in the location of the down‐dip frictional transition. Variability in the width of the outer‐forearc, the dip of the subducting slab, and the depth distribution of deep slow‐slip events combine to produce large changes in the width of the seismogenic zone. This is most pronounced across Bungo channel, where the seismogenic zone narrows from 90 to 120 km in Shikoku to <60 km offshore Kyushu. Pinching of the seismogenic zone may explain the smaller area, and thus magnitude, of earthquakes offshore Kyushu and the reduction in slip‐deficit rate. Our results suggest crustal‐scale architecture is a key driver of profound along‐strike transitions in the location of frictional transition zones, and the width and slip behavior of the intervening seismogenic zone at Nankai Trough.

Linear stability analysis of the condition for vibration during frictional slip

Slip along a frictional contact between elastic bodies can be stable or unstable, leading to stick-slip motion. Frictional slip can also be associated with vibrations. The condition for these vibrations and their characteristics remains poorly understood. To address this issue, which is relevant to engineering and earth science, we carry out a linear stability analysis of a spring-and-slider system obeying rate and state friction. We first identify the solution space for the linearized equation and define the conditions for different slip modes from the real and imaginary parts of the solution. We then derive asymptotic equations for all boundaries between overdamped stable sliding, inertial/non-inertial underdamped oscillation, stick-slip, and harmonic vibration. Finally, we verified the conditions with numerical simulations. Our work provides rigorous criteria regarding the conditions for the various frictional slip modes and the emergence of vibrations. It can help design appropriate approaches for suppressing undesired vibrations in mechanical systems and investigate the mechanisms generating vibrations (tremor) associated with fault slip in nature.

Tribological characteristics of polymer composites based on polytetrafluoroethylene and UVIS-AK-P carbon fibers under loading

Experimental and computational results of tribotechnical tests of polymer composites based on polytetrafluoroethylene and UVIS-AK-P discrete carbon fibers are presented. The PV factor is determined in the dry friction mode at a sliding speed of 0,2 m/s. Using the "steel disk — polymer disk" mathematical friction model, the maximum load was determined taking into account the allowable temperature of 120 °С for the friction surfaces of the materials under study. Keywords: polytetrafluoroethylene, wear resistance, friction coefficient, wear rate, carbon fiber. ppavlina@yandex.ru, markovamusya@mail.ru, roman_tikhon@mail.ru

Improvement of tribological characteristics of coupling parts "shaft-sleeve" with polymer and polymer-composite materials

The article provides an analytical justification of the flow of tribological processes of coupling of "shaft-sleeve" parts, which simulates the functioning of sliding bearings and cylindrical joints of machines. The main attention is paid to such characteristics as contact pressure, static and dynamic forces, the criterion of the product of the total pressure on the sliding speed, the work of friction forces and its transition into thermal energy in the friction zone for polymer (based on polyamide P-68) and polymer-composite coatings (based on P-68 with kaolin filler) on the working surfaces of the parts. A comparative analysis of the functioning and tribological characteristics of the couplings of parts without coatings is presented.
 Experimentally, on the basis of tests of samples on the MИ-1M friction machine, a significant reduction in wear and an increase in the relative wear resistance of samples with polymer-composite coatings in the modes of friction without lubrication (by 1.3...1.4 times) and marginal friction (in 1.2...1.3 times), as well as a decrease in the temperature in the friction zone (365 K and 347 K) compared to the polymer coating.