Traction Coefficient Research Articles

მეცნიერები ხახუნზე და შედეგების რეალიზაციის დონე

The article presents the main indicators of friction drives of belt conveyors: traction coefficient, operating parameters of contact surfaces, drive service life, etc. Troubles of design, improvement of manufacturing and production, which are objective with by many design, production and operating companies in many countries. According to the currently available data analysis, the need to increase the thraction factor (maximum = 40) and performance capabilities have not yet been exhausted, especially since the friction coefficient in vacuum is high and at the same time stable to rheological conditions; Based on this, the possibility of obtaining a high thraction factor ( ) using a vacuum drive independent of rheological conditions is being investigated.

Wedge-loading planetary traction drive: Modeling and performance analysis

The continuous development of motors with high speed and high power density has significantly increased the demand for high-speed reducers. Traction drive has emerged as a new option for high-speed reducers due to its high transmission efficiency and accuracy in high-speed transmission. In the previous theoretical studies on traction drive, the traction coefficient was set as a fixed value, and the influence of elastohydrodynamic lubrication (EHL) on the traction coefficient was ignored. Therefore, this paper investigated the wedge-loading planetary traction drive (WL-PTD) and established a theoretical model combining the macroscopic and microscopic traction mechanisms of the traction drive based on Hertzian contact and mixed elastohydrodynamic lubrication (mixed EHL). The loading principle and transmission performance of WL-PTD were studied through numerical computations, and the effects of different design parameters on transmission performance were discussed. When the input speed of WL-PTD was 10,000 rpm, the peak value of the global sliding coefficient was approximately 1.27×10−5, and the maximum transmission efficiency reached 97.41%, indicating an excellent transmission performance of the WL-PTD. The findings of this study can provide theoretical guidance for subsequent optimization design.

Effects of hydrogen on the tribological performance of heavily loaded lubricated contacts

The objectives of this investigation were to experimentally characterize the effect of hydrogen on the tribological performance of heavily loaded lubricated contacts. In order to achieve these objectives, a ball-on-disk film thickness and traction measurement test rig and a four-ball wear tester were modified to introduce gaseous hydrogen into the lubricated contact. Using the ball-on-disk test rig, lubricant film thickness, and traction were measured operating in air, nitrogen, and hydrogen environments. The results indicate that hydrogen environment has no effect on the lubricant film thickness and traction coefficient. The wear performance of the lubricant was evaluated in air, nitrogen, and hydrogen environments using the four-ball tester according to ASTM D4172 standard. Wear scar dimensions were determined through optical profilometry and compared for each condition. The results demonstrated that the wear scar diameter is slightly higher in a hydrogen environment. Additionally, the effects of hydrogen embrittlement on the wear performance were investigated using the four-ball tester. The balls used in the four-ball tester were precharged with hydrogen in Ammonium Thiocyanate. Wear scar profilometry results showed significantly larger wear scars and excessive pitting on the worn surface of precharged specimens. Results from this investigation shows that a gaseous hydrogen environment has a negligible effect on friction and wear as compared to diffused hydrogen in the material.

Study on Traction Characteristics of Point Contact State under Oil–Air Lubrication

Oil–air lubrication technology is commonly utilized in the lubrication design of traditional components to reduce friction between contact pairs. This study focuses on testing the point-contact friction pairs of two quantitative valves using a self-made oil–air lubrication traction force machine with a G95Cr18 disk and ball as the friction pairs. The test data are analyzed using a four-parameter exponential model. Additionally, a calculation model of the oil–air lubrication flow field is established, defining boundary conditions and conducting flow field simulation analysis. The results of the ball and disk test show that the traction coefficient increases with load and decreases with entrainment speed. Furthermore, when the air-speed is 12 m/s, the traction coefficient is lower for the oil supply of 6 mL/min than for the oil supply of 1.5 mL/min. According to CFD analysis, the volume fraction of the oil phase in the contact area increases with the increase of entrainment speed when the slip–roll ratio is 0.1. The theoretical values from the four-parameter exponential model align well with the experimental results, and the fitting accuracy is higher than 0.95.

Effect of Soccer Boot Outsole Configuration on Translational Traction Across Both Natural and Artificial Playing Surfaces.

Soccer boots are produced with different stud patterns and configurations to provide players with extra traction on specific surface types to minimize slipping and improve player performance. Excessive traction, however, can lead to foot fixation injuries, particularly anterior cruciate ligament tears. The purpose of this study was to explore the translational traction properties of 5 different outsole configurations moving in 4 different directions across both natural grass and artificial grass (AG) playing surfaces. It was hypothesized that longer studs or studs with an asymmetric shape would yield a higher traction coefficient compared with the recommended stud configuration for the given playing surface. Descriptive laboratory study. A custom-built testing apparatus recorded the translational traction of 5 different soccer boots moving in an anterior, posterior, medial, or lateral direction on both natural grass and AG playing surfaces. A 3-way analysis of variance was performed to determine the effect of outsole configuration on the traction, and a post hoc Tukey analysis was performed to compare different outsole configurations with a control. For the natural grass playing surface, the longer and asymmetric studs yielded a significantly higher (P < .05) traction coefficient on 75% of loading scenarios, while on AG, they yielded a significantly higher traction on 50% of loading scenarios. Some soccer boots yielded higher traction values compared with the recommended configuration. The results highlight the importance of boot selection on different playing surfaces. Higher traction values could increase the injury risk for players due to excessive traction and foot fixation.

Explicit finite element simulations of dynamic low adhesion behavior between wheel and rail in the presence of high-frequency vibrations

PurposeDynamic low adhesion (DLA) has become an urgent problem for the high-speed wheel-rail system because of continuous decrease of adhesion redundancy in the past decades. This article aims to provide a simulation method to reveal the mechanism of DLA under high-frequency vibrations.Design/methodology/approachA transient wheel-rail rolling contact model is developed for a typical Chinese high-speed railway system using the explicit finite element (FE) method. Instantaneous adhesion exploitation levels are studied in the time domain, for which driving cases over corrugated rails are taken as an example. A speed up to 500 km/h is considered together with different traction coefficients and corrugation dimensions. DLA is expected when the instantaneous adhesion exploitation level reaches 1.0, that is adhesion saturates and full sliding contact occurs.FindingsThe instantaneous adhesion exploitation level can be very high in the presence of corrugation, even at low traction coefficients. DLA is found to occur as great vertical unloading takes place and causes a significant increase of creepage. An approach is further developed to determine the critical depth of corrugation over which DLA occurs.Originality/valueThis study employs the transient wheel-rail rolling contact model to predict the instantaneous adhesion exploitation level under high-frequency vibrations. The presented results reveal a mechanism of DLA being beneficial to guidelines for future railway practice.

An efficient 3D finite element procedure for simulating wheel–rail cyclic contact and ratcheting

Various models for simulating rail ratcheting behaviour were developed to study rolling contact fatigue (RCF) damage in rails. However, limitations remain in terms of the accuracy of wheel–rail contact modelling and computational efficiency of the cyclic loading simulation. This study developed an efficient 3D finite element (FE) procedure to simulate ratcheting in rails subjected to numerous load cycles. The procedure simulates a wheel rolling repeatedly over a rail section with updated stress–strain states, enabling automatically executed cyclic loading simulation given a predefined number of cycles. To ensure the accuracy of the contact modelling, the effect of meshing schemes on subsurface stress distribution was examined. In addition, the FE contact model with the selected meshing scheme, which balances accuracy and computational efficiency, was verified against the widely accepted CONTACT program. Subsequently, a non-linear kinematic hardening (NLKH) steel material was used in the FE model for ratcheting simulations with up to 100 wheel-loading cycles. The rail surface and subsurface stress states were replicated under partial-slip wheel–rail rolling contact conditions with traction coefficients of 0.10, 0.20 and 0.35, respectively. The ratcheting behaviour was extensively analysed in terms of plastic deformation, contact patch evolution, and ratcheting rates. The simulated plastic deformation was found to alter the contact geometry and thus contact stresses, which in turn affect further accumulation of plastic deformation and subsequent ratcheting strains. These findings highlighted the importance of considering the interplay between the rail ratcheting behaviour of the rail and evolving contact conditions for predicting ratcheting and RCF damage in rails.

EFFECT OF ENTRAINMENT SLIDING DIRECTION TO THE TRACTION COEFFICIENT FOR EHL ELLIPTICAL CONTACTS IN THE LINEAR ISOTHERMAL REGION

Abstract In many mechanical systems, elastohydrodynamic lubrication (EHL) is apparent in the interfaces be-tween rolling/sliding parts. Even at small slide-to-roll ratios, detailed knowledge about the oil film thickness and the traction exerted by the highly pressurized lubricant is crucial for the efficiency and reliability of such systems. In this study, we explore the impact of surface velocity direction and contact ellipticity on traction characteristics within EHL through numerical simulations using an elliptical EHL contact solver based on a fully-coupled finite-element approach. We present a novel master curve that correlates the traction coefficient slope with the lubricant entrainment direction and ellipticity, for el-liptical EHL contacts in the piezo-viscous elastic (PE) regime. We demonstrate that the master curve, shows a maximum relative difference of less than 3.2% to the numerical simulation results, proving its efficacy. This curve is particularly beneficial for multibody dynamics models, providing a reliable tool for predicting frictional forces in systems where elliptical EHL contacts, operating within the PE regime at low SRR, are prevalent.

A nonlinear dynamic analysis of skidding behavior in rolling bearings using lubricant traction coefficients and cage flexibility

Skidding is a primary cause of rolling element bearing (REB) failure, which is influenced by operating conditions, lubricant properties, and bearing design. This paper presents an improved nonlinear dynamic model of REBs to analyze the impact of lubricant characteristics on REB skidding at different speed states. The model considers time-varying traction coefficient, cage flexibility, and cage pocket clearance. The selected tested lubricants include lithium grease (LGT2), compound calcium sulfonate (CCS) grease, and 4109 Chinese aviation oil. Simulation results show that CCS grease effectively reduces cage skidding, with the lowest load limits (300 N at 2000 RPM) under constant speed conditions and the highest stable limit (2932 RPM) during acceleration. LGT2 grease exhibits minimal sensitivity to acceleration and fluctuation speed parameters change.

A biomechanical report of an acute lateral ankle sprain during a handball-specific cutting movement

ABSTRACT Biomechanical measurements of accidental ankle sprain injuries are rare but make important contributions to a more detailed understanding of the injury mechanism. In this case study, we present the kinematics and kinetics of a lateral ankle sprain of a female athlete performing handball-specific fake-and-cut manoeuvres. Three-dimensional kinematics and kinetics were recorded and six previously performed trials were used as reference. Plantarflexion, inversion, and internal rotation angles were substantially larger than the reference trials and peaked between 190 and 200 ms after initial ground contact. We observed a highly increased inversion and internal rotation moment. However, compared to the non-injury trials the data also revealed a reduction in the second dorsiflexion moment peak. Ground reaction forces were lower throughout the injury trial. Other parameters at initial ground contact including ankle and hip position, step length, and the traction coefficient indicate that a preparatory maladjustment occurred. This study adds valuable contributions to the understanding of lateral ankle sprains by building upon previously published reports and considering the shoe-surface interaction as an important factor for injury.

A Discrete Element Model of High-Pressure Torsion Test to Assess the Effect of Particle Characteristics in the Interface

Abstract Sand particles have been used since the early stages of the railway industry to increase adhesion at the wheel–rail contact. However, there is a limited understanding of how sand particle characteristics affect the tribological performance of the wheel–rail contact. In this work, the high-pressure torsion test used as a small-scale simulation of the interface is numerically modeled using the discrete element method (DEM). The DEM model is then utilized to investigate the effect of different particle characteristics on the frictional performance of wheel–rail contact which can provide more insight into micromechanical observations. The effects of various particle characteristics including their size, their number, the number of fragments the particles break into, and the parameters defining the behavior of the bonds between particle fragments on the coefficient of traction (COT) are systematically investigated. Results show that, in dry contacts, the coefficient of traction decreases when the size or number of sand particles increases. This can be attributed to the formation of weak shear bands between the fragments. Further investigation is needed for wet- and leaf-contaminated contacts. It is also found that the COT is more sensitive to the stiffness of the bond between the fragments of a broken particle compared to the strength of the bond. A limiting value for bond strength was identified, beyond which the sand particles exhibited ductile behavior rather than the expected brittle fracture. The findings from this study can be useful for future research on adhesion management in wheel–rail contact and the modeling approach can be scaled up to the full contact.

Study on Cage Stability of Solid-Lubricated Angular Contact Ball Bearings in an Ultra-Low Temperature Environment

In the ultra-low temperature environment, the material properties of the bearing change, which puts forward higher requirements for the dynamic performance of the bearing cage. The bearings operating in ultra-low temperature environments commonly use solid lubricants. This study first focused on measuring the traction coefficients of molybdenum disulfide (MoS2) solid lubricant in a nitrogen atmosphere, and the Gupta fitting model is constructed to derive the traction equation. Subsequently, the dynamic differential equation of angular contact ball bearings was established, and the stability of the bearing cage in a nitrogen environment was simulated and analyzed based on the dynamic model. The accuracy of the simulation model was verified through comparison. The results show that less than 10% of errors exist between the experimental data and the traction curve fitted by the Gupta model, and the stability of the cage is closely related to operating parameters and bearing structure parameters. Cage stability increases with axial load but decreases with radial load. The cage stability is optimal when the radial internal clearance of the bearing is approximately 0.06 mm. When other conditions remain unchanged and the ratio of the cage pocket hole gap to the cage guide surface gap is 0.2, the cage stability is the best. The research results will provide a foundation for the design and application of solid-lubricated angular contact ball bearings in ultra-low temperature environments.

SCREW DRUM STRUCTURAL PARAMETERS ANALYSIS ON PROPULSION PERFORMANCE IMPACT

Screw propeller can perform engineering operations in complex environments and have a wide range of applications. To improve its operational efficiency, it is necessary to analyze the influence of the parameters of the screw drum on the performance indicators of the operation, in order to confirm the structural parameters under the optimal operational efficiency. By coupling the Discrete Element Method (DEM) with RecurDyn, the simulation of the locomotion process of a screw propulsion mechanism in a sand environment is conducted. Output results such as travel speed, maximum traction force, and maximum torque are measured. Through computation, performance indicators including slip ratio, traction coefficient, and dimensionless power can be obtained. By coupling the Discrete Element Method (DEM) with RecurDyn, the simulation of the locomotion process of a screw propulsion mechanism in a sand environment is conducted. Output results such as actual speed, maximum traction force, and maximum torque are measured. Through computation, performance indicators including slip ratio, traction coefficient, and dimensionless power can be obtained. Analyzing the numerical indicators of the propulsion system, it is determined that the optimal structural parameters for the screw propulsion mechanism to operate in a sand environment are a helix angle of 40°, aspect ratio of 4, blade height ratio of 0.15, and the number of blades of 4.

A traction coefficient formula for EHL point contacts operating in the linear isothermal region

Many mechanical systems including rolling/sliding parts, require traction data across a spectrum of operating conditions to predict their motion effectively. Numerous studies have examined the thermal effects and shear-thinning concerning the traction curve, but only a few have focused on the traction coefficient in the linear isothermal regime for low SRR. In this work, we investigate traction coefficient characteristics of EHL point contacts in the linear isothermal regime, over a wide range of operational conditions. To this end, we conduct numerical simulations utilizing a fully-coupled finite element-based model, resulting in a prediction formula for the traction coefficient slope. With this formula, the traction coefficient slope could be predicted for the operating conditions considered.

Investigation of frictional rolling contact behavior in a switch panel considering traction load

The dynamic effects involved in wheel-rail interactions can be increased by the traction force potentially causing rapid degradation of turnout rails. A precise stress/strain analysis is essential for wheel-rail frictional rolling contact and the related degradation. Therefore, an explicit finite element method is established considering dynamic effects related to the contact. Furthermore, the frictional rolling contact solutions and related degradation with different traction coefficient in turnout are investigated. The results indicate that the effect of asymmetric excitation caused by traction force and geometric irregularities will lead to yaw motion of wheelset. The width of running band on the switch rail is increased as the top surface width of the switch rail increases. In addition, the contact patch on the switch rail will gradually shift forward compared to that on the stock rail, leading to excessive shear stress and increase of microslip. Meanwhile, the adhesion-slip distribution, uneven wear and severe plastic deformation during transition are recognized.

Patternable Process-Induced Strain in 2D Monolayers and Heterobilayers.

Strain engineering in two-dimensional (2D) materials is a powerful but difficult to control approach to tailor material properties. Across applications, there is a need for device-compatible techniques to design strain within 2D materials. This work explores how process-induced strain engineering, commonly used by the semiconductor industry to enhance transistor performance, can be used to pattern complex strain profiles in monolayer MoS2 and 2D heterostructures. A traction-separation model is identified to predict strain profiles and extract the interfacial traction coefficient of 1.3 ± 0.7 MPa/μm and the damage initiation threshold of 16 ± 5 nm. This work demonstrates the utility to (1) spatially pattern the optical band gap with a tuning rate of 91 ± 1 meV/% strain and (2) induce interlayer heterostrain in MoS2-WSe2 heterobilayers. These results provide a CMOS-compatible approach to design complex strain patterns in 2D materials with important applications in 2D heterogeneous integration into CMOS technologies, moiré engineering, and confining quantum systems.

Semi-Analytical Model for 3D Multilayered Rolling Contact Problems With Different Creepage Combinations

Abstract In this work, a semi-analytical model replicating 3D rolling contact problems with elastic coating layers has been proposed. The dimension reduced boundary element-based model rests on the conventional half-space assumptions, with influence coefficients describing the relationship between the stress and the displacement. In this model, the influence coefficient describes the stress–strain relationship for multilayered materials, which is derived from the corresponding frequency–response functions. The governing equations for 3D rolling contact problems are solved efficiently using the conjugate gradient method with the classical fast Fourier transformation (FFT)-based technique to accelerate the calculation of the integral equation relating the stresses to the strains, and all the possible creepages, i.e., the longitudinal, lateral, and spin creepages, are considered. The results obtained with the present numerical model are compared with existing results, and good agreement is found. Some selected results are presented to show the effect of the material elasticity and the thickness of the coating, and the layering, on the traction distribution and the traction coefficient under different creepage combinations. In this way, this work provides important information related to the composition of layered coatings in rolling contact applications.

Traction Properties of Aerospace Lubricant under Low-Temperature Micro-Oil Droplet Supply Conditions

Aerospace bearings need to withstand the low-temperature environment of space, which will cause changes in the internal lubrication state of the bearings. This article aims to assess the traction properties of aerospace lubricants under low-temperature micro-oil droplet (hereinafter referred to as ‘micro-oil’) supply conditions, and provide a lubrication theoretical basis for studying the motion characteristics of aerospace bearings in a low-temperature environment. An experimental study on the low-temperature micro-oil traction properties of high-speed bearing lubricants was conducted on a specially designed aerospace bearing lubricant traction characteristic tester. A modified Herschel–Bulkley model (modified H–B model) was presented based on test data analysis, and the fitting results were compared with the Tevaarwerk–Johnson model (T–J model). The findings demonstrated that the traction coefficient of this lubricant decreased at a higher load and entrainment velocity, and decreased with a decreasing inlet oil temperature from 0 °C to −50 °C. The modified H–B model accurately fitted the test data and was suitable for the engineering traction coefficient calculation of lubricants and high viscosities at low temperatures. This paper can provide fundamental information for analyzing aerospace bearing friction torque variation.

Partitioning model study of the traction coefficient in a droplet model in a wellbore

AbstractGas wells need to face the problem of gas well outflow during production, and the main way to deal with gas well fluid accumulation is to calculate the critical liquid carrying velocity and other factors by using the prediction model of excess fluid accumulation. When the prediction results of the hydrops prediction model are biased, hydrops will be generated at the bottom of the wellbore, resulting in a decrease in gas well productivity. Aiming at the problem that the drag coefficient of the wellbore droplet movement model changes greatly in the process of natural gas production, which leads to the error of the wellbore effusion prediction, the commonly used droplet models and the common drag coefficient models are analyzed and evaluated. Compared with the model obtained by the partition and the existing drag model and the experimental value, it is found that the model can effectively reduce the average error rate between the calculated results and the experimental value. The findings of this study can help for better understanding of change of critical liquid carrying velocity of droplet in different Reynolds number region, which is of reference value for the subsequent drainage and mining process.

An approach for the creep-curve assessment using a new rail tribometer

The friction properties of the wheel-rail contact are characterized by a coefficient of traction (CoT), which can be affected by many contaminants. Numerous devices for assessing CoT are available; however, only a small number of them are capable of recording creep curves. This work introduced a rail-mounted tribometer that uses controlled changes in braking torque on a measuring wheel to induce creep in the contact. A methodology for assessing creep curve parameters was proposed, utilizing both analytical and numerical models of the contact. The experiments were performed both in the field and in the laboratory to investigate the effect of railhead conditions under various contact pressures and to track the evolution of the CoT during a measurement.