Change In Friction Coefficient Research Articles

Relationship of evolution of slip-surface structure in Indian sandstone to changes in friction coefficient for a wide range of slip rates

Although many experimental studies have investigated the frictional properties of sandstone, few of them have considered the evolution of slip-surface structures with changes of friction coefficient over a wide range of slip rates. Here we report the results of rock-on-rock rotary shear experiments on specimens of Indian sandstone at slip rates from 2 × 10−4 to 1 m s−1. The resultant slip behaviors and microscale surface structures indicate two types of dependency on slip rate. At high slip rates (≥5 × 10−1 m s−1), the friction between the sandstone blocks fluctuated markedly and the surfaces of the blocks were intensively worn. At low slip rates (≤1 × 10−1 m s−1), the friction decreased gradually with increasing slip distance and the slip surfaces became reflective. The high slip-rate experiments produced abundant rock fragments (∼10–100 μm diameter), whereas the slip surfaces after low slip-rate experiments were highly polished fault mirrors accompanied by abundant rounded ultrafine grains (∼100 nm diameter). Such slip-rate-dependent evolution of slip-surface structures in Indian sandstone may arise from the relatively low cohesiveness of that rock. This insight may further our understanding of faulting and sliding mechanisms in sandstone near the ground surface.

Investigation of friction and vibration performance of lithium complex grease containing candle soot on aviation electrical machine

The prolonged operation of the motor in the all-electric aircraft can cause the fragmentation and shortening of grease thickener fibers, ultimately resulting in the lubricating grease becoming ineffective. In order to avoid grease failure causing motor winding burnout, this paper uses a four-ball friction testing machine to test the tribological properties of six composite greases containing candle soot with different concentrations, and analyzes the changes in friction coefficient and wear diameter. In addition, by building a test platform for motor vibration and noise, this paper studies the influence of lithium complex grease on vibration and noise characteristics of aviation electrical machine at different propeller speeds. The results show that the addition of 0.04wt% candle soot lithium grease has the best tribological properties and vibration inhibition properties, because the candle soot in the oil film changes the sliding friction into rolling friction in the contact area of some friction pair. The temperature rise test of aviation electrical machine shows that the lithium complex grease plays an active role in the heat dissipation performance of the high-speed rotating motor.

EXPERIMENTAL, TRIBOLOGICAL STUDIES ON SINTERED Al–xV2O5–2B4C COMPOSITES USING BIO-OILS

This reports the tribological behavior of sintered aluminum composite with bio-oil as lubricant. The composite is fabricated by powder metallurgy route, and the aluminum is reinforced with the particles V2O5 and B4C. The bio-lubricant oil is Pongamia, Jatropha, and blended Pongamia with Jatropha of equal volume. The wear test was carried out at the sliding speed of 3[Formula: see text]m/s and load ranging up to 180 N, and the wear behavior of Al–xV2O5–2B4C composite. Then the composite’s friction coefficient and weight loss were tested on a pin-on-disc tribometer. Further, pinworn surfaces were analyzed by Scanning Electron Microscope. The mix of Pongamia with Jatropha of equal volume resulted in a good change in specific wear rate and coefficient of friction of composites.

Effect of time-varying friction coefficient on self-excited vibration of floating spline-rotor system

Different lubrication states seriously affect the stability and safety of the aviation power transmission system. This study investigates how time-varying friction coefficients impact the stability of an aviation spline-rotor system using a spline-rotor model that considers uneven stress distribution on the tooth surface. Experimental tests with a high-frequency friction and wear test machine yield time-varying friction coefficients for splines with different lubrication, surface roughness, and materials. A sharp change of friction coefficient for the sample occurs under wear debris lubrication. The average friction coefficient for the sample with roughness 1.6 is lower than those with roughness 0.8 and 3.2. Under the same lubrication conditions, the friction coefficient of Resin matrix composites is smaller than that of metallic materials. Dynamic simulation of the spline-rotor model considering time-varying friction coefficients shows rich self-excited vibration behaviors influenced by lubrication, surface roughness, and materials.

Charge pattern detection through electrostatic array sensing

As reported in previous work, electrostatic charge induced by contact potential difference (CPD) arising from oxidational wear can be detected by electrostatic button sensors. However, to detect signs of localised wear and to investigate the mechanisms involved, the charge distribution on worn regions needs to be measured. Therefore, this paper investigates the evolution of surface charge during wear processes, its interplay with friction, and the correlation between charge distribution and surface chemistry. An electrostatic bar sensor and an array sensor were initially calibrated using CPD patterns generated by dissimilar metal inserts in steel plates. After appropriate signal processing, the sensor outputs exhibited excellent agreement with the electric field strength modelled using COMSOL Multiphysics. Subsequently, the bar sensor was integrated into a reciprocating tribometer for in-situ detection of oxidational wear of steel-on-steel rubbing contacts, followed by ex-situ array sensing of worn regions. Positive picocoulomb-level surface charge during the evolution of oxidational wear were successfully detected by the bar sensor and were found to correlate to changes in friction coefficient. The array sensor effectively mapped the distribution of surface charge. EDS mapping suggested patchy formation of Fe3O4 layers over the worn areas, and these patches correlated to the surface charge map. Increased electrostatic charge levels were associated with higher concentrations of oxidational wear. Therefore, this paper evaluates the potential of electrostatic array sensors to spatially resolve surface charge patterns induced by surface chemistry transformations, which enables the monitoring of localised and smaller-scaled machinery component deterioration and provides additional information for machinery diagnosis.

TRIBOLOGICAL PROPERTIES OF A SILICONE-BASED COMPOSITEWITH INORGANIC ADDITIVES

The paper presents the results of experimental studies, including tribological tests of silicone-based compositeswith additions of hexagonal boron nitride (hBN) and titanium (Ti). The tests were conducted on a BrukerUMT2 tribotester and using a pin-on-disk setup developed by the authors, without a lubricating medium,and they employed a steel ball made of 100Cr6 steel and a sample made of the composite. During the tests,the products were not removed from the contact area. The paper analyzes the influence of additives on thetribological properties of the composite, i.e., the coefficient of friction (COF) as a function of distance and thewear of the tested samples. In the case of samples containing hBN, the COF decreases with an increase in itscontent. After reaching a volumetric percentage concentration of 20%, it begins to stabilize with the increasein mass loss. The profiles of COF changes as a function of distance for samples with different additivecontents are comparable. The self-lubricating properties of hBN have been confirmed. The addition of Tireduces the COF value, which decreases with the increase in the Ti content. Samples with a mass percentageconcentration exceeding 100% of the Ti content have a COF value equal to the initial value for silicone. Thecomposite containing hBN has a lower COF value than samples with the Ti addition, and the wear tracks ontheir surface are narrower and shallower.

A novel fiber‐reinforced polymer rope: Simulation and theoretical investigation

AbstractThe study proposed using twisted rope made of flexible fiber‐reinforced polymers (FRPs) instead of wire and synthetic ropes in high load‐bearing and corrosive environments. Finite element model verification and parameter analysis were conducted for the rope, along with deriving theoretical equations for its tensile properties. Increasing the lay length improved model accuracy, with a maximum bearing capacity within a 10% margin of error compared to experimental results. The model length had minimal impact on calculation results, while changes in radial mesh size (0.13 and 0.2 mm) and friction coefficient (0–0.2) greatly affected maximum contact stress. The tensile strength and elastic modulus of the strand remained almost constant (approximately 1142 MPa and 44 GPa) when the friction coefficient was altered. Increasing lay length (6D–20D) enhanced the tensile properties of the strand, whereas tendon diameter (1–3 mm) had negligible impact. Arranging inner and outer strands with the same lay length was better for circular ropes than those with the same lay angle. Inverse lay direction reduced rope torque and increased strand contact stress. Rectangular ropes had higher strength and modulus conversion efficiency, with lower contact stress compared to circular ones. The accuracy of rope equations improved with longer lay lengths, and adding a correction factor for lay length can modify the equation.Highlights A flexible FRP rope with high strength and twisted form was proposed. The effect of construction factors on the tensile behaviors of ropes was clarified. Equations for calculating the tensile properties of ropes were derived.

Characterization of food emulsions and dispersions based on nonlinear friction dynamics

AbstractMany foods are emulsions or dispersions containing lipids. The friction properties of foods are evaluated because they affect food texture and processability. Here, we evaluated the friction characteristics of 55 liquid or semisolid foods using a sinusoidal motion friction evaluation system to classify them based on friction dynamics. The contact surface was made to resemble a biological surface using agar gel, which exhibited a fractal structure, and the movement of the contact probe mimicked living movement by sinusoidal motion. The change in average friction coefficient (Δμ), static friction coefficient (Δμs) in a round trip, delay time (Δδ), and friction profile depended on the condition and rheological properties. Principal component analysis showed that all the friction parameters of Δμ, Δμs, Δδ, and the appearance ratio of the profile were involved in the principal components, Z1 and Z2 which are composite variables obtained by the contraction of many friction parameters in a principal component analysis. In addition, the foods were classified into three groups by cluster analysis using Z1 and Z2. The condition of the foods, rheological properties, and the presence or absence of lipids was the factors that defined each group.

Effect of morphology on tribological properties of Fe3O4 as lubricant additive: Nanospheres, nanowires and nanosheets

This work investigates the morphologies of Fe3O4 nanoparticles how to affect the tribological properties when they are applied as nano-lubricating additives. Fe3O4 nanoparticles with different morphologies (nanospheres, nanowires and nanosheets) are synthesized by solvothermal method and added in liquid paraffin to test the tribological properties. Although Fe3O4 nanoparticles can achieve certain anti-friction and anti-wear effects under the same conditions, the Fe3O4 nanospheres shows the best tribological properties. In the optimal concentrations of Fe3O4 nanospheres, nanowires and nanosheets, the changes of the average friction coefficient are almost the same, the wear volume are decreased by 91.03%, 86.88% and 87.63%, respectively. Fe3O4 nanoparticles with different morphologies can form films in the gap between friction pairs when they are used as additives, which ensures that three different Fe3O4 nanoparticles can enhance the tribological properties of the liquid paraffin. Compared with Fe3O4 nanowires and nanosheets, besides the tribological films, Fe3O4 nanospheres play roles in self-healing and rolling friction effects between friction pairs, resulting in the best anti-friction and anti-wear.

An Iterative Numerical Approach to Evaluate the Variable Friction Coefficient of Steel AMS5643 Using Ring Compression Tests

The coefficient of friction is an important variable that must be defined to allow the accurate prediction of the forming geometry and stresses involved in metal forming processes. Literature reports have shown that the coefficient of friction does not remain constant with respect to variables including but not limited to contact pressure, sliding speed, surface roughness, and surface morphology. Ring compression tests provide a simple and efficient process by which to measure the variable coefficient of friction present in the bulk-metal process; however, the conventional interpolating method can result in a poor evaluation of the evolution of friction, especially if the coefficient of friction changes significantly during a test. In this article, a novel approach to evaluate the relationship between the coefficient of friction and contact pressure is outlined using friction calibration charts generated via iterative computation models and ring compression tests. This relationship can be programmed into a computational model to allow for the coefficient of friction to behave as a dynamic variable. This approach improves on the prediction of the computational model when compared to conventional interpolation methods.

Design and Optimization Analysis of Downhole Robot Support Mechanism Based on Finite Element Method

To increase the traction force of the telescopic downhole robot, the support mechanism of the downhole robot was optimized, and a slider-spring transmission mechanism was introduced to solve the problem of jamming. A mathematical model of the support mechanism was established, covering the relationship between the contact normal pressure and the hydraulic cylinder load. The finite element simulation model of the friction block of the support mechanism was established based on Ansys software. Stress analysis of the casing under different contact normal pressures was carried out, and the frictional characteristics of the support mechanism friction block were obtained. The results show that the support mechanism composed of four linkages and friction locking blocks proposed in this paper can adapt well to casing of different diameters. The slider spring, as a transmission mechanism, has the elasticity and buffering effect, which makes the support mechanism have a certain flexibility, alleviating the vibration impact during obstacle crossing and reducing the impact on logging instruments. It enables the robot to passively adjust during obstacle crossing, effectively improving the traction capability of the downhole robot. Through finite element simulation, it was found that the depth of the plow groove produced by the friction block under normal pressure is basically proportional to the normal pressure. The greater the normal pressure, the deeper the plow groove, and the plow groove friction coefficient increases continuously with the increase of normal pressure, but the change in the plow groove friction coefficient gradually decreases as the pressure increases.

Mechanism Study of Symmetric Mechanism-Rolling Bearing Dynamic System Considering the Influence of Temperature Factor

Due to the symmetry of their structure and the way they rotate, rolling bearings are often used at high temperatures and high speeds. When the temperature changes, the material properties of the rolling bearing change, which in turn causes the dynamic model equation of the rolling bearing to change. Given the problem in that the conventional dynamic model equation does not consider temperature changes, but because temperature changes cause changes in material performance parameters resulting in differences between the dynamic simulation signals and the actual signals, this paper fully considers three factors, namely, the thermal expansion coefficient, the hardness value, and the friction coefficient. With the influence of temperature changes, a study on the mechanism of the symmetric mechanism-rolling bearing dynamic system considering the influence of temperature factors is proposed. First, combined with the material properties of the rolling bearing, the changes in thermal expansion coefficient, hardness value, and friction coefficient caused by temperature changes were analyzed. Second, the functional relationship expressions obtained above were substituted into the existing conventional dynamic model. Finally, the comprehensive functional relationship formula was substituted into the conventional dynamic model to obtain the rolling bearing dynamic model under temperature difference changes. By studying the mechanism of the symmetric mechanism-rolling bearing dynamic system that considers the influence of temperature factors, this method can undertake a more comprehensive consideration of the dynamic analysis research into rolling bearing fault diagnosis, thereby verifying the effectiveness of the method.

Impact of variable viscosity on asymmetric fluid flow through the expanding/contracting porous channel: A thermal analysis

This study aims to look at the problem of fluid flow through a rectangular channel between two parallel and permeable plates with expansion/contraction capability from two points of view. Considering that the fluid used in this study has properties such as non-Newtonian, Casson, incompressible, with temperature-dependent viscosity and MHD, and the fluid flow has properties such as two-dimensional, laminar unsteady, small electromagnetic force and electrical conductivity, an external magnetic field with uniform intensity. The governing equations for this problem with these features are non-linear partial differential equations (PDEs), which were converted into two non-linear ordinary differential equations (ODEs) using similarity transformation and solved analytically using the hybrid analytical and numerical (HAN-method). After using the similarity transformations, the profiles of similarity velocities, F(ξ), F′(ξ) and similarity temperature, θ(ξ) and their derivatives appear next to physical parameters such as A and R. As R changes from −10 to 10, the average values of F(ξ), F′(ξ), and θ(ξ) are increased while the value of A remains constant and positive, which shows that both plates have suction at the same time or injection at the same time. On the other hand, otherwise, when the value of A changes from −8 to 8, the average value of F(ξ) only increases, but the average values of F′(ξ) and θ(ξ) decrease while the value of R is constant and equal to 10. The results obtained in this article have been compared with other previous articles for validation. The most important novelty of this article is to express the changes in parameters using 3D and 2D contours. Changes in local skin friction coefficient and local Nusselt number were also investigated.

Composite Control Law for Nonlinear Systems With Mismatched Disturbances for a Ball-Ramp Dual-Clutch Transmission

The dual-clutch transmission (DCT) was developed to increase the transmission efficiency and the shift performance. However, in a DCT, due to uncertainty related to the actuator, the tie-up phenomenon can arise, in which two clutches engage together or the clutch torque control performance deteriorates. Especially, the actuator uncertainty increased when using a special mechanism to increase controllability and efficiency. Among them, the ball-ramp DCT (BR-DCT), which uses a self-energizing mechanism, can reduce the consumption of actuator energy while also reducing the tie-up effect. However, the nonlinearity of the actuator must be considered, such as friction between parts and change in friction coefficient. Among the methods by which this type of uncertainty is estimated, the nonlinear disturbance observer is most effective when used to estimate the unmodeled nonlinearity of an actuator and time-varying uncertainty. In order to execute disturbance rejection control to improve the performance of the shift controller using the estimated uncertainty, it is necessary to configure disturbance compensation input. However, because the BR-DCT powertrain includes mismatched disturbances and nonlinearity of the actuator, it is difficult to apply the existing methods on the composite control law to shift controller. Therefore, in this study, we propose a composite control law to guarantee integrated stability in nonlinear systems such as a BR-DCT powertrain. The proposed method was verified through a powertrain test bench equipped with a BR-DCT. Finally, the proposed composite control law was able to converge both the tracking error and the disturbance estimation error.

Investigation of Friction Coefficient Changes in Recycled Composite Materials under Constant Load

Investigation of Friction Coefficient Changes in Recycled Composite Materials under Constant Load

The Effect of Clamping Force and Friction Coefficient on The Clamping Clamp

In order to study the influence of clamping force and friction coefficient on the stress distribution of drill pipe joint during the buckling process, the nonlinear contact problem and elastoplastic mechanical properties of Φ139.7mm × 9.17mm drill pipe were simulated by finite element analysis method in this paper, and the stress distribution and size of drill pipe joint during the buckling process were obtained. And the effect of clamping force and friction coefficient on the joint of drill pipe. It is found that the stress of drill pipe joint is mainly concentrated in the contact position of drill pipe and the stress value of drill pipe joint shows a trend of increasing. When the friction coefficient is 0.15 and the clamping force is 600KN, although there is stress concentration in the contact position between the clamp teeth and the drill pipe joint, it does not exceed the yield limit of the drill pipe joint, and no plastic deformation occurs. Through the simulation calculation, it is found that the maximum clamping force and the maximum torque are 1200KN and 81.9KN.m respectively. When the torque is fixed, the change of friction coefficient has a great influence on the maximum clamping force of drill pipe joint. With the increase of friction coefficient, the maximum clamping force of drill pipe joint decreases continuously, and the friction coefficient is inversely proportional to the maximum clamping force. The research in this paper enriches the understanding of the buckling process of punching pliers and provides a reference for the rational buckling of pipe string.

A new method for DEM simulation of dust-bearing granular flow – Modeling the effect of dust on the change in granule friction coefficient

The simulation of dust-bearing granular accumulation flow in the theoretical and practical aspects of the engineering field poses a significant challenge due to the massive computational requirements associated with cross-scale simulations. In this study, a novel approach is proposed to address this issue by developing a model that accounts for the impact of dust on granular flow through changes in the granule friction coefficient. Initially, the theoretical analysis was conducted to examine the effects of dust on granular flow. Subsequently, a method was developed to measure the equivalent friction coefficient of granules by combining theoretical analysis and experimental work. The proposed method was then employed to simulate the stability of a spherical granule pile in a drum influenced by dust content and compared against experimental results. The simulation results demonstrate agreement with the experimental results, indicating the feasibility of modeling the effect of dust on dust-bearing granular flow through the granule equivalent friction coefficient. The dust effect can be considered equivalent to a change in the granule friction coefficient, which facilitates the reduction of the computational load generated by the Discrete Element Method (DEM) method and enables cross-scale simulations.

Study on the Tribological Properties of Micro-Al2O3 Modified Carbon Fiber Hybrid-Reinforced Polymer

Micro-sized Al2O3-modified carbon fiber hybrid reinforced polymer composites (CFHRP) were prepared using the vacuum-assisted resin transfer molding (VARTM) process, and friction and wear tests were performed on an MRTR-1 multifunctional friction and wear tester to study the effect of carbon fiber surface doping with trace amounts of Al2O3 and its content on the tribological properties of carbon fiber reinforced polymer composites. The surface morphology of the composite material was characterized using a three-dimensional confocal laser scanning microscope (LSM). The results showed that under dry friction conditions, the wear behavior of the composite material with micro-Al2O3 added was mainly abrasive wear and adhesive wear. Under the condition of water lubrication, the friction coefficient of the composite material with micro-Al2O3 added was higher than that of the carbon fiber/epoxy resin-based composite material without Al2O3, and the abrasive wear and adhesive wear were significantly reduced. In addition, under the condition of water lubrication, the cooling and boundary lubrication of water played a dominant role in the tribological behavior, so the influence of transfer film on the tribological behavior of the composite material was smaller than that under dry friction conditions. When the micro alumina content is 6%, the friction and wear reduction of CFHRP composite material are improved under water containing conditions. Under dry friction conditions, the content of micrometer alumina has a minimal effect on the change in friction coefficient.

Tribological Performance and In Situ-Generated Oxidative Layer of Cobalt-Based Alloy From 25 °C to 800 °C

Abstract The friction and wear performances of Haynes 25 alloy sliding against Si3N4 in a ball-on-disk configuration are investigated from room temperature (RT) to 800 °C. The friction-induced changes of morphology and chemical composition in the surface and subsurface regions of the wear tracks were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The results show that the friction coefficient (COF) decreases considerably via temperature, while the wear-rate increases and then decreases with temperature. During dry-sliding contact, the spontaneous formation of multiple metal oxides on the rubbing surface varies as a function of the test temperature, being responsible for either wear loss and/or a change of friction coefficient. The results of this study indicate that the presence of the glazed surface of Haynes 25 helps to lower friction. At 800 °C, a glaze layer with a multilayer structure appears on the rubbing surfaces, leading to the lowest friction coefficient and wear-rate.

A Detecting Method for “Weak” Friction-Induced Vibration Based on Cross-Correlation Analysis between Vibration and Sound Signals

The “weak” friction-induced vibration can help to monitor the change in friction and wear state of friction pairs and detect the failure of surface damage. However, they are typically submerged in complex interference components during friction. Therefore, detecting accurate “weak” friction-induced vibration is key to using it entirely. A method based on the correlation between friction-induced vibration and sound signals was proposed to achieve this goal. The ball–disk wear experiments were conducted under oil lubrication using a wear tester. Vibration and sound pressure signals generated during the experiments were recorded. By the spectrum analysis of the cross-correlation function calculated from the two types of signals, the “weak” FIV components submerged in the original signals were detected. The experimental results showed that the root mean square change of the “weak” friction-induced vibration detected in the vibration and sound pressure signals was highly consistent with the friction coefficient change. It could effectively characterize the transition of the wear stage from running-in to stable wear of the friction pairs. Therefore, the cross-correlation analysis of vibration and sound signals could be a reliable tool for detecting the “weak” friction-induced vibration.