The conflict between stability and curving has been well documented since the dawn of the railways. Advances in computer technology and deeper understanding of the complex mechanics of the wheel-rail interface have led to the study of innovative designs. This thesis outlines the need for steering systems for railway applications. A number of innovative passive solutions have been proposed to reduce the conflict between stability and curving. Comparisons of some of these solutions show that significant benefits can be obtained by using uncommon configurations.

The Gautrain Rapid Rail Link (GRRL) is a rail transit system in South Africa that links Johannesburg and Pretoria, as well as Johannesburg and the O.R. Tambo International Airport. Travelling at speeds of up to 160 km/h, the Gautrain system is the first of its kind on the African continent. This dissertation covers an investigation into the relationship between the vertical and lateral forces, speed and superelevation in a GRRL curve. The research described in this dissertation is based on an experiment which involved running a test train through a curve at various speeds, changing the cant of the curve by tamping and repeating the train runs. The cant was changed due to high wheel wear rates. The curve already had a cant deficiency, and this cant deficiency was subsequently increased by reducing the curve’s cant. Assessing the before and after tamping test data validated the existence of the expected relationships between the vertical and lateral rail forces, the speed and the cant. The change in cant had a minimal effect on the magnitude of the vertical forces, although a transfer of loading between the high and low legs did occur. The theory indicates that the 14 % reduction in cant in this curve given all of the other curve characteristics should have resulted in an increase in the lateral forces. There was however a roughly 50 % reduction in the maximum lateral forces after the cant was reduced that can be explained from a train dynamics point of view. In addition, there was an increase in safety due to a reduced derailment ratio at this curve’s normal operating speed of 85 km/h. It is not unreasonable to presume that a 50 % reduction in the maximum lateral forces could lead to a halving of the wear rate of the rail and wheels in this curve with similar results to be expected in other curves on the rail network. / Dissertation (MEng)--University of Pretoria, 2016. / The Chair in Railway Engineering at the University of Pretoria / Civil Engineering / MEng / Unrestricted

Heavy haul railway operations permit the transport of huge volumes at lower cost than other modes of transport. The low cost can only be sustained if the maintenance costs associated with such railway operations are minimised. The maintenance costs are mainly driven by wheel and rail damage in the form of wear and rolling contact fatigue (RCF). Low wear rates in the wheel-rail interface have resulted in an increase in the prevalence of rail RCF, thereby increasing rail maintenance costs. The aim of this study is to develop an approach to reduce rail RCF on South Africa’s iron ore export line by managing the worn wheel shape. This approach is developed by evaluating wheel and rail profile shapes that contribute the most to RCF initiation, studying the influence of suspension stiffness and rail profile changes as well as a redesign of the wheel profile. The influence of wheel and rail profile shape features on the initiation of rolling contact fatigue (RCF) cracks was evaluated based on the results of multibody vehicle dynamics simulations. The damage index and surface fatigue index were used as two damage parameters to assess the influence of the different features. The damage parameters showed good agreement to one another and to in-field observations. The wheel and rail profile shape features showed a correlation to the predicted RCF damage. The RCF damage proved to be most sensitive to the position of hollow wear and thus bogie tracking. RCF initiation and crack growth can be reduced by eliminating unwanted shape features through maintenance and design and by improving bogie tracking. Two potential mitigation measures had been adapted from those published in literature to reduce RCF. The mitigation measures involved changes in suspension stiffness to spread wheel wear across the tread and the use of gauge corner relief rail profiles. These mitigation measures were evaluated by means of multibody dynamics and wear maintenance costs. These mitigation measures, however, did not prove to be successful in reducing RCF initiation while maintaining a low wheel wear rate. The current operating conditions on South Africa’s iron ore line, although still not optimal overall, were found to be better in terms of their wear and RCF performance than the two proposed RCF mitigation measures. Based on the finding of the study on two RCF mitigation measures it was recommended that a conformal wheel profile be developed to spread the wheel wear across the tread to reduce the occurrence and propagation of RCF cracks, while still maintaining low wheel wear rates. A comparative study of this new wheel profile design and the current wheel profile design was therefore performed using multibody dynamics simulation together with numerical wheel wear and RCF predictions. The advantages of the conformal wheel profile design were illustrated by evaluating the worn shape and resulting kinematic behaviour of the conformal design. The conformal design had a steadier equivalent conicity progression and a smaller conicity range compared with the current wheel profile design over the wheel’s wear life. The combination of a conformal wheel profile design with 2 mm hollow wear and inadequate adherence to grinding tolerances often result in two-point contact, thereby increasing the probability of RCF initiation. The conformal wheel profile design was shown to have many wear and RCF benefits compared with the current wheel profile design. However, implementation of such a conformal wheel profile must be accompanied by improved rail grinding practices to ensure rail profile compliance. Based on these findings an approach is proposed where the conformal wheel profile design together with improved compliance of the in-service rail profiles to the target rail profile are implemented. This has the potential to reduce RCF initiation on South Africa’s iron ore export line. / Thesis (PhD)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / PhD / Unrestricted

To attract more customers and compete with other modes of transportation, railway transport needs to ensure safety, punctuality, high comfort, and low cost; wheel–rail adhesion, i.e., the transmitted tangential force in the longitudinal direction during driving and braking, plays an important role in all these aspects. Adhesion needs to be kept at a certain level for railway operation and maintenance. However, wheel−rail contact is an open system contact. Different contaminants can present between the wheel and rail surfaces, forming a third-body layer that affects the adhesion. Prediction of wheel–rail adhesion is important for railway operations and research into vehicle dynamics; however, this prediction is difficult because of the presence of contaminants. This thesis deals with wheel–rail adhesion from a tribological perspective. The five appended papers discuss wheel–rail adhesion in terms of dry conditions, lubricated conditions, leaf contamination, iron oxides, and environmental conditions. The research methodologies used are numerical modelling, scaled laboratory experiments, and field tests. The research objective is to understand the mechanisms of the adhesion loss phenomenon.  A numerical model was developed to predict wheel–rail adhesion based on real measured 3D surfaces. Computer simulation indicates that surface topography has a larger impact on lubricated than on dry contacts. Plastic deformation in asperities is found to be very important in the model. Ball-on-disc tests indicate that water can give an extremely low adhesion coefficient on smooth surfaces, possibly due to surface oxidation. Investigation of lubricated contacts at low speed indicates that oil reduces the adhesion coefficient by carrying a normal load, while adhesion loss due to water depends on the surface topography, water temperature, and surface oxidation. A field investigation indicates that leaves reduce the friction coefficient because of the chemical reaction between leaves and bulk materials. The thickness of the surface oxide layer was found to be an essential factor determining adhesion reduction. Pin-on-disc experiments found a transition in the friction coefficient with regard to the relative humidity, due to a trade-off between the water molecule film and the hematite on the surface. / <p>QC 20131031</p>

The wheel-rail contact is at the core of all research related to vehicletrackinteraction. This tiny interface governs the dynamic performanceof rail vehicles through the forces it transmits and, like any high stressconcentration zone, it is subjected to serious damage phenomena. Thus,a clear understanding of the rolling contact between wheel and rail is keyto realistic vehicle dynamics simulation and damage analysis. In a multi-body dynamics simulation, the demanding contact problemshould be evaluated at about every millisecond for several wheel-rail pairs.Hence, a rigorous treatment of the contact is highly time-consuming.Simplifying assumptions are therefore made to accelerate the simulationprocess. This gives rise to a trade-o between the accuracy and computationaleciency of the contact model in use. Conventionally, Hertz+FASTSIM is used for calculation of the contactforces thanks to its low computational cost. However, the elliptic patchand pressure distribution obtained by Hertz' theory is often not realisticin wheel-rail contact. Moreover, the use of parabolic traction bound inFASTSIM causes considerable error in the tangential stress estimation.This combination leads to inaccurate damage predictions. Fast non-elliptic contact models are proposed by others to tacklethis issue while avoiding the tedious numerical procedures. The studiesconducted in the present work show that the accuracy of these models iscase-dependent. To improve the accuracy of non-elliptic patch and pressure estimation,a new method is proposed. The method is implemented in an algorithmnamed ANALYN. Comparisons show improvements in patch and, particularly,pressure estimations using ANALYN. In addition, an alternative to the widely-used FASTSIM is developed, named FaStrip. Unlike FASTSIM, it employs an elliptic traction boundand is able to estimate the non-linear characteristic of tangential stressdistribution. Comparisons show more accurate estimation of tangentialstress and slip velocity distribution as well as creep forces with FaStrip. Ultimately, an ecient non-elliptic wheel-rail contact model consistingof ANALYN and FaStrip is proposed. The reasonable computationalcost of the model enables it to be used on-line in dynamics simulationand its accuracy can improve the damage predictions. / <p>QC 20160202</p>

Accurate estimation of the coefficient of friction (CoF) is essential to accurately modeling railroad dynamics, reducing maintenance costs, and increasing safety factors in rail operations. The assumption of a constant CoF is popularly used in simulation studies for ease of implementation, however many evidences demonstrated that CoF depends on various dynamic parameters and instantaneous conditions. In the real world, accurately estimating the CoF is difficult due to effects of various uncertain parameters, such as wheel and rail materials, rail roughness, contact patch, and so on. In this study, the newly developed 3-D nonlinear CoF model for the dry rail condition is introduced and the CoF variation is tested using this model with dynamic parameters estimated from the wheel-rail simulation model. In order to account for uncertain parameters, a stochastic analysis using the polynomial chaos (poly-chaos) theory is performed using the CoF and wheel-rail dynamics models. The wheel-rail system at a right traction wheel is modeled as a mass-spring-damper system to simulate the basic wheel-rail dynamics and the CoF variation. The wheel-rail model accounts for wheel-rail contact, creepage effect, and creep force, among others. Simulations are performed at train speed of 20 m/s for 4 sec using rail roughness as a unique excitation source. The dynamic simulation has been performed for the deterministic model and for the stochastic model. The dynamics results of the deterministic model provide the starting point for the uncertainty analysis. Six uncertain parameters have been studied with an assumption of 50% uncertainty, intentionally imposed for testing extreme conditions. These parameters are: the maximum amplitude of rail roughness (MARR), the wheel lateral displacement, the track stiffness and damping coefficient, the sleeper distance, and semi-elliptical contact lengths. A symmetric beta distribution is assumed for these six uncertain parameters. The PDF of the CoF has been obtained for each uncertain parameter study, for combinations of two different uncertain parameters, and also for combinations of three different uncertain parameters. The results from the deterministic model show acceptable vibration results for the body, the wheel, and the rail. The introduced CoF model demonstrates the nonlinear variation of the total CoF, the stick component, and the slip component. In addition, it captures the maximum CoF value (initial peak) successfully. The stochastic analysis results show that the total CoF PDF before 1 sec is dominantly affected by the stick phenomenon, while the slip dominantly influences the total CoF PDF after 1 sec. Although a symmetric distribution has been used for the uncertain parameters considered, the uncertainty in the response obtained displayed a skewed distribution for some of the situations investigated. The CoF PDFs obtained from simulations with combinations of two and three uncertain parameters have wider PDF ranges than those obtained for only one uncertain parameter. FFT analysis using the rail displacement has been performed for the qualitative validation of the stochastic simulation result due to the absence of the experimental data. The FFT analysis of the deterministic rail displacement and of the stochastic rail displacement with uncertainties demonstrates consistent trends commensurate with loss of tractive efficiency, such as the bandwidth broadening, peak frequency shifts, and side band occurrence. Thus, the FFT analysis validates qualitatively that the stochastic modeling with various uncertainties is well executed and is reflecting observable, real-world results. In conclusions, the development of an effective model which helps to understand the nonlinear nature of wheel-rail friction is critical to the progress of railroad component technology and rail safety. In the real world, accurate estimation of the CoF at the wheel-rail interface is very difficult since it is influenced by several uncertain parameters as illustrated in this study. Using the deterministic CoF value can cause underestimation or overestimation of CoF values leading to inaccurate decisions in the design of the wheel-rail system. Thus, the possible PDF ranges of the CoF according to key uncertain parameters must be considered in the design of the wheel-rail system. / Ph. D.

The adhesion in the wheel-rail contact can be effectively controlled by using the friction modifier. Adhesion control can reduce excessive wear of the contact bodies or noise. The essence of this diploma thesis is to determine the optimal amount of friction modifier in wheel-rail contact depending on the climatic and operating conditions. For experimental study of the behavior of the friction modifier, the twin-disc device is used. Twin-disc simulates and controls the most important operating parameters such as speed, slip, attack angle or contact pressure. On the basis of these findings, the off-board top-of-rail lubricator system was designed. This system controls the amount of friction modifier according to current climatic and operating conditions.

To investigate the wheel/rail contact impact forces at insulated rail joints (IRJs), a three-dimensional finite element model and strain gauged experiments are employed and reported in this thesis. The 3D wheel/rail contact-impact FE model adopts a two-stage analysis strategy in which the wheel-IRJ railhead contact is first established in the static analysis and the results transferred to dynamic analysis for impact simulations. The explicit FE method was employed in the dynamic analysis. The Lagrange Multiplier method and the Penalty method for contact constraint enforcement were adopted for the static and dynamic analyses respectively. The wheel/rail contact-impact in the vicinity of the end post is exhibited via numerical examples from the FE modelling. The wheel/rail contact impact mechanism is investigated. The strain gauged experiments which consist of a lab test and a field test are reported. The signature of the strain time series from the field test demonstrates a plausible record of the dynamic responses due to the wheel/rail contact impact. By using the experimental data, both the static and the dynamic FE models are validated. It is found that the stiffness discontinuity of the IRJ structure causes a running surface geometry discontinuity during the wheel passages which then causes the impact in the vicinity of the end post. Through a series of sensitivity studies of several IRJ design parameters, it is shown that the IRJ performance can be effectively improved with optimised design parameters.

Wear transitions in the wheel–rail contact are of increasing interest since the general trend in railway traffic is toward increased velocities and axle loads. Curving increases the risk of flanging, causing the contact to change from an almost pure rolling wheel tread–rail head contact to more of a sliding wheel flange–rail gauge contact on the high rail in curves. Under wheel flange–rail gauge contact conditions, wear transitions to severe or catastrophic wear will occur if the contact is improperly lubricated. Such a transition is the most undesirable transition in the wheel–rail contact, as it represents a very expensive operating condition for railway companies. The contact conditions responsible for this transition are very severe as regards sliding velocity and contact pressure, and thus place high demands on both the lubricant and the wheel and rail materials. The focus of this thesis is on the transitions between different wear regimes in a wheel–rail contact. Wear is discussed both in traditional tribological terms and in terms of the categories used in the railway business, namely mild, severe and catastrophic wear. Most of the work was experimental and was performed at the Royal Institute of Technology (KTH), Department of Machine Design. The effects of contact pressure, sliding velocity, and type of lubricant have been investigated, producing results that resemble those of other studies presented in the literature. The absence of research relating to the wheel flange–rail gauge contact is addressed, and it is concluded that a lubricant film must be present on rails in curves to prevent severe or catastrophic wear. The formulation of this lubricant can further increase its wear- and seizure-preventing properties. To obtain a deeper understanding of wear transitions, methods such as airborne particle measurement and electron microscopy have been used. Paper A presents the test methodology used to detect seizure and discusses the wear-reducing influence of free carbon in highly loaded contacts. Paper B presents the testing of seizure-initiating conditions for a range of environmentally adapted lubricants applied to wheel and rail materials; a transient pin-on-disc test methodology was used for the testing. Paper C presents the use of pin-on-disc methodology to study the wear-reducing effects of a wide range of lubricants. The best performing lubricant was a mineral oil containing EP and AW additives. Paper D relates wear rates and transitions to airborne particles generated by an experimentally simulated wheel–rail contact. The airborne particles generated varied in size distribution and amount with wear rate and mechanism. Paper E relates additional analysis techniques, such as FIB sectioning, ESCA analysis, airborne particle measurements, and SEM imaging of airborne wear particles, to the contact temperature. / QC 20100721 / Samba 6

<p>Numerical procedures for reliable wheel and rail wear prediction are rare. Recent development of simulation techniques and computer power together with tribological knowledge do however suggest computer aided wear prediction as possible. The present objective is to devise a numerical procedure able to simulate profile evolution due to uniform wear sufficiently accurate for application to vehicle dynamics simulation. Such a tool should be useful for maintenance planning, optimisation of the railway system and its components as well as trouble-shooting. More specifically, the field of application may include estimation of reprofiling frequency, optimisation of wheel – rail profile match, optimisation of running gear suspension parameters, and recognition of unfavourable profile evolution influencing the dynamic response of the vehicle.</p><p>The research contribution accounted for in this thesis includes, besides a literature review, modelling of the wheel – rail interface, benchmarking against traditional methods, and validation with respect to full-scale measurements.</p><p>The first part addresses wheel – rail contact conditions in the context of wear simulation as well as tribological environment and tractive forces. The current approach includes Archard’s wear model with associated wear maps, vehicle dynamics simulation, and railway network definition. One objective is to be able to include variations in operation conditions in the set of simulations instead of using scaling factors. In particular the influence of disc braking and varying lubrication conditions have been investigated. Both environmental factors like moist and contamination and deliberate lubrication need to be considered. As part of the associated contact analysis the influence of tangential elastic deformation of the contacting surfaces has been investigated and found to be essential in case of partial slip contact conditions. The influence on the calculated wear of replacing the Hertzian contact by a non-elliptic semi-Hertzian method has been investigated, showing relocation of material loss towards increased profile curvature.</p><p>In the second part comparisons have been carried out with traditional methods, where the material loss is assumed to be directly related to the energy dissipated in the contact. Attention has been paid to the understanding of the principle differences between the investigated methods, comparing the distribution of friction energy, sliding velocity, and wear depth. As a prerequisite, contact conditions with dependence on wheelset guidance and curving performance as well as influence of tractive forces have been investigated.</p><p>In the final part validation of the developments related to wheel wear simulation is addressed. Disc braking has been included and a wear map for moist contact conditions based on recent tests has been drafted. Good agreement with measurements from the reference operation, is achieved. Further a procedure for simulation of rail wear and corresponding profile evolution has been formulated. A simulation set is selected defining the vehicles running on the track to be investigated, their operating conditions, and contact parameters. Trial calculations of a few curves show qualitatively good results in terms of profile shape development and difference in wear mechanisms between gauge corner and rail head. The wear rates related to traffic tonnage are however overestimated. The impact of the model improvements accounted for in the first part of the thesis has been investigated, indicating directions for further development.</p>