Traction Coefficient Research Articles

A comparison of friction modifier performance using two laboratory test scales

This paper describes two methods, carried out at two different test scales, for assessing the friction modifier performance. Study A used the wear data from a full-scale rig test at the voestalpine Schienen GmbH and compared it with the wear data from twin disc tests using the SUROS test machine at the University of Sheffield. Study B compared the ‘retentivity’ data, from a full-scale rig at the University of Sheffield, with the data from the SUROS tests. Study A concluded that a good correlation existed between the two scales although assumptions made in the full-scale contact calculation introduce a large spread into the results. There was a greater correlation between the two data sets at more severe contact conditions. Study B showed a different baseline coefficient of traction between the two scales and that a longer test length is required to fully evaluate the ‘retention’ of the friction modifier on the full-scale rig. The paper expands on a previous conference presentation on the same subject. Additional information on the test procedure and test rigs is included here. Surface and subsurface analyses of the SUROS test samples have also been added. The analyses have shown that applying the friction modifier leads to a similar wear mechanism as for the dry contact, but the wear is less severe and there is less subsurface deformation. A discussion describing the differences in test scales and comparing lab tests to field operation is also included.

A review on recent wheel/rail interface friction management

Traction is influenced greatly by the climatic conditions, especially under wet conditions induced by rain or snowfall which could cause the loss of adhesion and could reduce the coefficient of traction (COT) to a dangerous level. This is why good adhesion is needed to ensure safe and good traction of train. Poor adhesion between wheel and rail can cause various problems and it could be fatal. Friction modifiers are applied to the contact interface to control adhesion, whether to enhance friction or lubricates, to ensure safe running of the rolling stock. In handling poor adhesion issues, sand is the most popular and the main adhesion enhancer used globally on railway networks. Sand particles enhance adhesion dramatically in contaminated wheel/rail contacts such as rail covered by autumn leaf layers or oil stain, and in wet rails. In this review paper, a compilation of some of the research of over the last decade is presented. The topics that will be presented in this paper are the adhesion characteristics of wheel/rail contacts under wet and dry condition, and the influence of sanding on rails covered with leaf layers.

High‐precision control of tracked field robots in the presence of unknown traction coefficients

AbstractAccurate steering through crop rows that avoids crop damage is one of the most important tasks for agricultural robots utilized in various field operations, such as monitoring, mechanical weeding, or spraying. In practice, varying soil conditions can result in off‐track navigation due to unknown traction coefficients so that it can cause crop damage. To address this problem, this paper presents the development, application, and experimental results of a real‐time receding horizon estimation and control (RHEC) framework applied to a fully autonomous mobile robotic platform to increase its steering accuracy. Recent advances in cheap and fast microprocessors, as well as advances in solution methods for nonlinear optimization problems, have made nonlinear receding horizon control (RHC) and receding horizon estimation (RHE) methods suitable for field robots that require high‐frequency (milliseconds) updates. A real‐time RHEC framework is developed and applied to a fully autonomous mobile robotic platform designed by the authors for in‐field phenotyping applications in sorghum fields. Nonlinear RHE is used to estimate constrained states and parameters, and nonlinear RHC is designed based on an adaptive system model that contains time‐varying parameters. The capabilities of the real‐time RHEC framework are verified experimentally, and the results show an accurate tracking performance on a bumpy and wet soil field. The mean values of the Euclidean error and required computation time of the RHEC framework are equal to 0.0423 m and 0.88 ms, respectively.

Trapped fluid in contact interface

We study the mechanical contact between a deformable body with a wavy surface and a rigid flat taking into account pressurized fluid trapped in the interface. A finite element model is formulated for a general problem of trapped fluid for frictionless and frictional contact. Using this model we investigate the evolution of the real contact area, maximal frictional traction and global coefficient of friction under increasing external pressure. Elastic and elasto-plastic material models, compressible and incompressible fluid models and different geometrical characteristics of the wavy surface are used. We show that in case of incompressible fluid, due to its pressurization, the real contact area and the global coefficient of friction decrease monotonically with the increasing external pressure. Eventually, the contact opens and the fluid occupies the entire interface resulting in vanishing of static friction. An asymptotic analytical result for the critical trap-opening pressure is found and shown to be independent of the surface slope if it is small. In case of compressible fluids with pressure-dependent bulk modulus we demonstrate a non-monotonous behaviour of the global coefficient of friction due to a competition between non-linear evolution of the contact area and of the fluid pressure. However, for realistic compressibility of solids and fluids, contact-opening cannot be reached at reasonable pressures. On the other hand, in case of elastic-perfectly plastic materials, we again observe fluid permeation into the contact interface. Finally, we study the distribution of frictional tractions during the depletion of the contact area under increasing external pressure. This process leads to emergence of singularity-like peaks in tangential tractions (bounded by the Coulomb’s limit) near the contact edges. We point out the similarity between the processes of trap opening and interfacial crack propagation, and estimate the complex stress intensity factor in the framework of linear elastic fracture mechanics.

Numerical investigation of crack initiation in rails and wheels affected by martensite spots

White etching layers (WELs) in the form of thermally induced martensite spots are often associated to so-called stud defects on rail surfaces and so-called rolling contact fatigue clusters on wheel treads. These defects might promote further material deterioration of rails and wheels and it is thus of importance to deepen the knowledge regarding their initiation mechanisms. The aim of this study is to gain insight into the problem by making qualitative assessments of different operational scenarios, involving various axle loads and amount of friction. To this end, this paper considers crack initiation, quantified through the Jiang–Sehitoglu low cycle fatigue criterion, in the vicinity of pre-existing WELs (in the form of martensite spots) subjected to varying contact load conditions. Three-dimensional finite element analyses are conducted to model phase transformations as well as the resulting residual stresses. It is seen that the contact pressure magnitude and WEL thickness affect the results only moderately, while the traction coefficient has a significant detrimental influence. It is furthermore seen that occasional wheel passages that are off-set in the lateral direction with respect to the WEL spot’s centre might be responsible for crack initiation on the gauge corner side of the rail surface.

Modeling of rolling contact fatigue in rails at the microstructural level

A micromechanical-based finite element framework is developed to investigate the rolling contact fatigue (RCF) in rails. The microstructure of a representative part of the rail in contact with the wheel during wheel passage is modeled using Voronoi tessellation algorithm. The geometry explicitly represents microstructural grain and grain interface features that represent actual microstructure of rail steel. The grain interface is modeled using cohesive elements. For higher computational efficiency, moving Hertzian load is applied to the rail surface instead of moving the wheel explicitly on top of the rail. The jump-in-cycles approach is employed to efficiently simulate a large number of loading cycles, while the material degradation at the grain interface is represented by an accurate damage evolution law. Additionally, the model is used to evaluate the effects of temperature change, traction coefficient, and grain size on initiation and progression of RCF. The results indicate that colder temperatures increase the progression of RCF, while warmer temperatures do not have any significant effects. Large traction forces at the wheel-rail interface significantly accelerate RCF growth, mainly through migration of subsurface micro-cracks to the surface of the rail. The surface cracks grow into larger ones that lead to loss of rail material. Controlling the grain size can have a positive effect on both the initiation and migration of sub-surface cracks. Changing yield strength of material with the Hall-Petch relation, the model evaluates the effect of refining the grain size. The results indicate that smaller grains can improve RCF properties of the rail. The effect of maximum contact pressure on RCF is studied, and the results show a power relation between maximum contact pressure and RCF life. Also several case studies are simulated and compared with the experimental observations. The simulation results are in close agreement with the experimental observations for the crack pattern and crack depth.

Elastohydrodynamic lubrication in point contact on the surfaces of particle-reinforced composite

Appreciable friction and serious wear are common challenges in the operation of advanced manufacturing equipment, and friction pairs may be susceptible to damage even with oil lubrication when point contact exists. In this study, a type of particle-reinforced composite material is introduced for one of the components of a heavy-load contact pair, and the performance improvement of elastohydrodynamic lubrication (EHL) is analyzed considering the rheological properties of non-Newtonian fluids. The Ree–Eyring EHL model is used considering the surface of the particle-reinforced composite, in which the film thickness includes the particle-induced elastic deformation. The problem of inclusions with different eigenstrains is solved by using Galerkin vectors. The influences of particle properties, size, burial depth, and interparticle distance on point-contact EHL are investigated. Furthermore, using several cases, the structural parameters of the particles in the composites are optimized, and an appropriate parameter range is obtained with the goal of reducing friction. Finally, the results for the EHL traction coefficient demonstrate that appropriate particle properties, size, burial depth, and interparticle distance can effectively reduce the traction coefficient in heavy-load contact.

The unification of certain terms and concepts used in the study of land transport-technological vehicles

On the basis of the analysis of terminology used in textbooks, monographs and articles, related to the theory, design and calculation of automobiles, tractors, wheeled and caterpillar transport and traction machines, the uniform terms and concepts are proposed, the most frequently used in the above mentioned disciplines and related technical documents to describe similar phenomena or processes. The coefficient of traction, tractive force on the surface adhesion, the vehicle adhesive weight, the coefficient of adhesive weight, the specific tractive force, the specific hook load, the coefficient of resistance to the rectilinear motion of the machine, the road surface resistance coefficient, the tractive force of the machine, the free engine power, the engine power losses associated with its maintenance, the parameter of rotation of the caterpillar tractor and the power factor of rotation are analyzed. In various literary sources, different terms are used to designate the same quantities, the same definitions are treated differently. It has been causing certain inconveniences and introduces confusion in the theory of land vehicles. The necessity for such unification is particularly relevant in relation to the approval of the Federal Education Standard in the speciality 23.05.01 «Land Transport-Technological Means», in which the above mentioned disciplines are included in the training of specialists, and the knowledge of the relevant technical terminology is considered at the level of competence. In this article, concrete proposals are given on the unification of certain terms and concepts used in the above mentioned theoretical disciplines. Since the field of terminology and definitions is rather painful and delicate, these proposals are submitted for a substantive discussion by professionals.

Interact of the tractor driving wheels with the soil by considering the rheological properties of soils

Abstract The work offers a mathematical model of interaction of the tractor driving wheels with the soil. A rheological model is used as a law of shear deformation of soils as the principal law of linear deformation. Calculation formulae of tangential force and traction coefficient of the driving wheels are deduced. A numerical example and graphs are used to demonstrate that the theoretical results are quite close to the experimental data. Dependence of the traction coefficient of the deformable wheel with the ground allows analyzing the impact of rheological indicators of grounds and geometric and regime parameter of a wheel rolling and using a more grounded approach to the analysis of the problematic issues of dynamics of wheeled tractors. As it can be seen from formulae the tangential traction force and traction coefficient of an elastic wheel necessary to overcome the forces of friction, shear and ground cutting depend on normal load mg, acting on the wheel, wheel parameters rheological properties of the ground and movement regime.

Experimental observation of temperature and surface roughness effects on wheel/rail adhesion in wet conditions

ABSTRACTIn this study, the relationship between the adhesion coefficient in the wheel and rail contact zone and the temperature under wet conditions was evaluated by means of a twin-disc rolling contact machine while varying the surface roughness of two rolling discs. The experimental results show that the adhesion coefficient is increased by raising the temperature. The surface roughness has a significant influence on the adhesion coefficient, but no trend of increasing the adhesion coefficient proportionally at low temperature was observed. Also, the adhesion coefficient has local maximum or nearly constant value at a threshold of surface roughness. On the other hand, re-adhesion phenomenon, which is characterized by a dramatic increase of the traction coefficient followed by a reduction in slip ratio, was found generally occurs in low temperature.

Improving the Estimation Accuracy of the AIST-SHANEL Model by Traction Coefficient Calibration of Multiple Surfactants

化学物質の全国暴露状況の把握において, 汎用的かつ高精度な暴露解析モデルの利用が不可欠である。本研究では, 暴露解析モデルの活用指針に繋がる知見を得る為, 産総研-水系暴露解析モデル (AIST-SHANEL) Ver.3.0を用いて, 5種類の界面活性剤を対象に河川水濃度の推定性能を評価した。また, 化学物質の土壌吸着性に関わる有機炭素水分配係数 (Koc) と, 化学物質の流出特性を示す水路底泥の負荷流出係数 (掃流係数) の推定性能への影響を確認した。その結果, Kocが小さい (1×103 L kg-1 オーダー) 界面活性剤は掃流係数30 m-1程度, Kocが大きい (1×105～6 L kg-1 オーダー) 界面活性剤は300～1000 m-1程度で, 良好な推定性能を示した。化学物質のKoc値に従って適切な掃流係数を設定することで, 本モデルは様々な化学物質の暴露評価に利用可能と考えられる。

Investigation of the process of controlled starting of the open-pit locomotive for ensuring the maximum adhesion coefficient at the starting

In order to control the calculated coefficient of adhesion of the locomotive bandages with the rail, a controlled starting of the career locomotive is suggested, in which the relative traction coefficient varies over the time according to the exponential dependence. To realize a rational starting process the tractive effort during the ¼ of the starting period should be ¾ of the maximum which is recommended by the traction conditions for a given section of the track, and for the remaining time, i.e. in the interval of the subsequent three-fourths of the starting time, the tractive effort must be smoothly brought to the required level, acceleration during the same period must also be changed (decreased) and at the end of the starting process, when the train reaches the working speed at this track section, it must be made equal to zero. It is proposed a relationship for determining of the starting time of the locomotive, depending on the initial parameters and actual operating conditions, which makes it possible to develop technical requirements for the automatic launch system of a open pit locomotive, which ensure the implementation of the maximum possible values of adhesion coefficients closed to optimal after the starting process.

Formation the driving force of wheeled and caterpillar machine-tractor units

On the basis of existing dependencies describing the traction-coupling properties of wheeled and caterpillar machine-tractor units, theoretical and experimental investigations, a model of the formation of the driving traction force of units depending on the weight of the tractor, coupling properties of the propulsion unit, load distribution and propulsion slipping is obtained. The analysis of calculated coefficients shows a considerable variation of values, which causes difficulties in using in the calculations. Therefore it’s necessary to know their values at each interaction of the tractor propulsion unit with the soil. The same values of slipping can be obtained with different values of the coefficients. The task is simplified if data of this dependence have the same coefficient. During field investigations an active experiment was developed i.e. the levels of factors variation changed purposefully depending on the conditions of conducting each experiment. Composite B-plans (Box-design) of the second order were realized. The following parameters were chosen as the variable factors: the coefficient of the using coupling weight, the coefficient of traction and the weight of the tractor. The works were conducted according to the plan of the three-factor experiment matrix. As the agricultural field were a freshly plowed field, a stubble field and a wet meadow, with the coefficients of traction of 0,50, 0,65 and 0,80 respectively. The value of resistance to movement was fixed by a dynamometer during towing tractors MTZ-82, MTZ-2022 and MTZ-3022 DC.1 in neutral on the various agricultural fields. Efforts on the hook were changed by shifting gears in the gearbox of a towed tractor whose engine operated in the compressor mode. The coefficient of traction was determined with completely braked wheels of the tractor and towing it with a towing vehicle during simultaneously measuring the force on the coupling with the help of a dynamometer. The obtained model of driving force formation for wheeled and tracked machine-tractor units takes into account the weight of the tractor, coupling properties of propulsion units, load distribution, slipping and has a minimum number of experimental coefficients, which makes it more accessible and easily used for engineering calculations.

Numerical Study of Cage Dynamics Focused on Hydrodynamic Effects of Guidance Land Clearances for Different Ball-Pocket Clearances in Cryogenic Environments

This study presents the dynamic motion of a ball bearing cage submerged in a cryogenic fluid under high-speed conditions. The dynamic motion of the cage has been studied as a function of the race land-cage and ball-cage pocket clearances for different inner race rotation speeds under light load conditions. In addition, this study conducted computational fluid dynamics (CFD) analysis using commercial software to analyze the fluid dynamic forces on the cage. The hydraulic force obtained from the CFD analysis was coded in commercial ball bearing analysis software as a function of the eccentricity ratio and rotation speed of the cage. Finally, the dynamic motion of the ball bearing cage considering the effects of fluid dynamic forces has been studied. The results include the cage whirling amplitude, fluctuation of cage whirling speed, and cage wear for various cage clearances and rotation speeds. The cage whirling amplitude decreases as the outer guidance clearance decreases, and it decreases as the rotation speed increases up to 11,000 rpm because of the increasing hydrodynamic force of the liquid nitrogen (LN2). However, the probability density function curves indicate that an increase in the rotor speed increases the standard deviation in the cage whirling frequency. The wear loss of the cage was greatest for the largest race land-cage and the smallest ball-cage pocket clearances. Consequently, the analysis results for various operating conditions (inner race rotation speeds, cage clearances, traction coefficients, etc.) are in good agreement with the reference results.

DEVELOPMENT OF BALLOON-BASED ORGAN RETRACTOR FOR LAPAROSCOPIC SURGERY

Effective organ retraction is essential to ensure safe and efficient surgery, in the case of laparoscopic surgery, organ retraction is particularly difficult to achieve. Currently such as cherry dissectors are used for retraction, however the small size results in excessive stress on the organ being retracted. Therefore, we are developing a balloon-based retractor that provides similar functionality as the cherry dissector, particularly being able to pass through a small port, yet being expandable to provide a much greater contact area as well as improved traction, thus providing increased operating efficiency and improved safety in organ retraction. In this paper, the relative merits and demerits of a number of currently available retractors are compared. Parameters under consideration include shape, size, effective traction (coefficient of friction), type of actuation and ease of use. While parameters such as pressure distribution and effective traction have been objectively measured, other attributes unique to the individual mechanisms are discussed, and finally feedback is provided from surgeons regarding the respective retractor’s ease of use and perceived efficacy.

Exploring mouthfeel in model wines: Sensory-to-instrumental approaches

Wine creates a group of oral-tactile stimulations not related to taste or aroma, such as astringency or fullness; better known as mouthfeel. During wine consumption, mouthfeel is affected by ethanol content, phenolic compounds and their interactions with the oral components. Mouthfeel arises through changes in the salivary film when wine is consumed. In order to understand the role of each wine component, eight different model wines with/without ethanol (8%), glycerol (10g/L) and commercial tannins (1g/L) were described using a trained panel. Descriptive analysis techniques were used to train the panel and measure the intensity of the mouthfeel attributes. Alongside, the suitability of different instrumental techniques (rheology, particle size, tribology and microstructure, using Transmission Electron Microscopy (TEM)) to measure wine mouthfeel sensation was investigated. Panelists discriminated samples based on their tactile-related components (ethanol, glycerol and tannins) at the levels found naturally in wine. Higher scores were found for all sensory attributes in the samples containing ethanol. Sensory astringency was associated mainly with the addition of tannins to the wine model and glycerol did not seem to play a discriminating role at the levels found in red wines. Visual viscosity was correlated with instrumental viscosity (R=0.815, p=0.014). Hydrodynamic diameter of saliva showed an increase in presence of tannins (almost 2.5–3-folds). However, presence of ethanol or glycerol decreased hydrodynamic diameter. These results were related with the sensory astringency and earthiness as well as with the formation of nano-complexes as observed by TEM. Rheologically, the most viscous samples were those containing glycerol or tannins. Tribology results showed that at a boundary lubrication regime, differences in traction coefficient lubrication were due by the presence of glycerol. However, no differences in traction coefficients were observed in presence/absence of tannins. It is therefore necessary to use an integrative approach that combines complementary instrumental techniques for mouthfeel perception characterization.

Influence of Lubricant Traction Coefficient on Cage's Nonlinear Dynamic Behavior in High-Speed Cylindrical Roller Bearing

In this paper, the formulas of elastohydrodynamic traction coefficients of four Chinese aviation lubricating oils, namely, 4109, 4106, 4050, and 4010, were obtained by a great number of elastohydrodynamic traction tests. The nonlinear dynamics differential equations of high-speed cylindrical roller bearing were built on the basis of dynamic theory of rolling bearings and solved by Hilber–Hughes–Taylor (HHT) integer algorithm with variable step. The influence of lubricant traction coefficient on cage's nonlinear dynamic behavior was investigated, and Poincaré map was used to analyze the influence of four types of aviation lubricating oils on the nonlinear dynamic response of cage's mass center. The period of nonlinear dynamic response of cage's mass center was used to assess cage's stability. The results of this paper provide the theoretical basis for selection of aviation lubricating oil.

Technical characteristics and agrotechnical indices of work of tillage machines

The study presents the results of analysis of the characteristics of modern tractors and tillage tools. The data obtained make it possible to assess the energy, economic, technical and shunting indices of the relevant aggregates, and can be used as a starting point in calculating the rational composition of the ma-chine and tractor fleet of agrarian enterprises. It is established that the dependencies and masses of trac-tors and the power of their engines on traction can be described with high accuracy by linear equations. On average, the mass of tracked tractors is 11 %, and the engine power is 27 % smaller than that of the cor-responding wheeled tractors. However, with a higher traction coefficient and low pressure on the soil, caterpillar tractors are less maneuverable, universal and, most importantly, less ergonomic, which hampers their wide application. With rational aggregation, such characteristics of the work of soil-cultivating units as working speed, specific fuel consumption, the utilization factor of the shift time do not actually depend on the traction class of the tractor. With an increase in the traction class of the tractor, the specific metal consumption of the implements being mounted with it increases. The difference is especially noticeable in the case of combined guns and «craters», for which a hypothetical transition from a class 1.4 tractor to a class 8 tractor led to an increase in the estimated specific material consumption by 3,7 and 3,4 times, respectively. The increase in productivity with the increase in the tractor class in most cases occurs with a delay. This allows us to conclude that the only advantage of introducing wide-angle aggregates is the reduction of the required number of operators. For the rest of the parameters, the machine with a wide-cut implement has no advantage over a group of machines with tools of identical total width.

Lubricant Selection for Minimum Traction in Elastohydrodynamic Lubrication Line Contacts During Accelerated Motion—A Numerical Approach

Transient thermal elastohydrodynamic lubrication (EHL) line contact simulations are carried out to study the traction behavior during accelerated motion considering realistic shear-thinning behavior. Using three lubricants with different inlet viscosity and shear-thinning parameters, the application of present analysis for lubricant selection is demonstrated. Owing to squeeze film action, the film evolution is delayed, and EHL traction during acceleration is found to increase much above the designed value. This effect decreases with increasing starting speed. The most shear-thinning test oil considered here yields the lowest traction coefficient with minimum variation in its value desirable for smooth and vibration-free operation.

GENERALIZED TRACTION INDICATORS OF AGRICULTURAL TRACTORS

In 1972 G.G. Kolobov and A.P. Parfenov published the monograph «Traction characteristics of tractors». The monograph generalized numerous results of field tests of domestic wheeled and caterpillar tractors of that time, conducted by machine testing stations in various soil and climatic zones of the USSR. This had an impact on formulation and study of the issue of determining of tractors slippage, and in general, the study of their traction characteristics. Averaging of the experimental data was carried out according to the formulas and coefficients developed by the authors of the monograph. The obtained dependencies, hereinafter referred to as traction characteristics of tractors, establish the dependence of the coefficient of usable traction (specific traction force on the hook) on the slipping of tractors of various types on different soils. By now, the concept of the role of the studies in research of the interaction of a wheel and track with the surface has changed. The standards have been introduced to limit slipping of agricultural tractors, considering the negative impact of their propulsors on the soil. There is a need to clarify the terminology used earlier in order to unify the same meaning terms used in different, but related technical disciplines. Objectives of work are: rethinking of previous research results and assessing their relevance; distinction between the physical meaning and concepts of the traction coefficient and the coefficient of usable traction (specific traction force on the hook); determination of traction capabilities of wheeled and caterpillar tractors on natural agrotechnical and asphalt-concrete surfaces, considering current slippage limitations. It was proposed for the purposes of engineering calculations in assessing traction performance of tractors to use as a traction coefficient of various propulsors its value corresponding to the maximum slipping on the appropriate agro-technical background or dry concrete. It is also found that the limitations on slipping of tractors on metal caterpillars are overstated on all soil backgrounds, since they rather characterize the «stop» mode of the tractor, than the beginning of the unstable operation of the engine. The parameters of traction characteristics of 4x4 wheeled tractors require correction based on statistical replenishment with the initial data on traction tests of such tractors having a layout that considers the increased role of the front drive axle in creating the tractor pulling power. For 4x2 tractors traction characteristics are at the same time characteristics of traction performance of individual driving wheels (tires). Therefore, having such data on all available driving tires of different dimensions, it is possible, at the development stage, to determine traction properties of newly created different variants of machines, including all-wheel drive.