Twin Disc Research Articles

Novel in-situ real-time line scan optical monitoring of wear and surface damage initiation in a laboratory twin disc test

A new optical monitoring system has been developed to photograph in-situ in real time the initiation of damage on the running surface of a rail steel twin-disc sample undergoing wear testing. The line-scan camera system has been demonstrated on the Sheffield University ROlling Sliding 2 (SUROS2) twin-disc machine. The results show the system can continuously track the development of wear flakes, with wear flake initiation and stabilisation of wear flake size observed without test interruption for the first time. Image analysis to quantify the total wear flake shadow pixel count showed a good correlation with the mass loss results, indicating the potential for the optical data to quantify rail steel wear without interruption to testing. Furthermore, in a water-lubricated test the new system enables observation of rolling contact fatigue (RCF) crack initiation through a water layer present on the specimens, without requiring test interruption. The improving knowledge of the wear and RCF performance of rail steels available from the new observation method can help improve understanding of steel performance and support to the selection of rail steel grades according to their performance.

Laboratory evaluation of a nanostructured lubricating grease for tram runflat tires

PurposeThis paper aims to develop a stable gel-type lubricant emulating commercial conditions. This encompassed rheological and tribological assessments, alongside field trials on the Medellín tram system.Design/methodology/approachThe gel-type lubricant with graphite and aluminum powder is synthesized. Rheological tests, viscosity measurements and linear viscoelastic regime assessments are conducted. Subsequently, tribological analyses encompassing four-ball and twin disc methods are executed. Finally, real-world testing is performed on the Medellín tram system.FindingsAn achieved lubricant met the stipulated criteria, yielding innovative insights into the interaction of graphite and aluminum powder additives under varying tests.Originality/valueNovel findings are unveiled regarding the interaction of graphite and aluminum powder additives in tribological, rheological and real-world trials. In addition, the wear behavior of polymers is observed, along with the potential utilization of such additives in tramway systems.

Hand operated tribometer versus twin disc dry friction characteristics measurements

Accurate friction-slip modelling is particularly important for locomotive traction and rail damage studies. This paper compares friction-slip measurement results from a hand operated (HO) tribometer and a twin disc tribomachine on AS60 rail. The results were used to calibrate traction coefficient-slip curves using the Modified Fastsim contact model. The differences between the two friction measuring instruments including scale, speed, and rail cleaning effect influenced the friction coefficient and slip value at which the friction peak occurred. The proposed equalisation technique was able to match the HO tribometer traction coefficient-slip curve with the tribomachine curve while preserving most of the Modified Fastsim contact parameters.

The influence of shot peening on gear teeth micropitting and contact fatigue failure

Case-hardened gears present a high contact fatigue load capacity in a compact and light mechanical transmission systems but sometimes, micropitting appears. This damage can lead to early contact fatigue failure called macropitting (NF ISO 10825-1). A previous study has demonstrated that an appropriate shot peening treatment could reduce the micropitting phenomenon via tests on a twin disc machine.The aim of this paper is to study the influence of shot peening on the micropitting of case-hardened gears tooth flank and its development to macropitting. Experimental tests are carried out using a back-to-back gear test rig, an interrupted test method is defined in order to monitor the micropitting initiation and its propagation on the tooth flank. Two variants of carburized spur gear specimens are tested. The reference specimen has a case-hardened ground tooth flanks which is a typical gear application. The other specimen has a case-hardened ground tooth flanks with a shot peened finishing process. The fatigue tests are run until the appearance of macropitting. The performed inspections of specimens indicate that shot peening modifies the surface roughness, the surface topography and increases the compressive residual stress. The experimental results show that micropitting occurs on both shot-peened and non-shot-peened gears after approximately the same running time. However, a difference is observed in the propagation of micropitting on the tooth flanks. In comparison to reference gears, shot peening has postponed the apparition of macropitting by more than 60%. An analytical study is also conducted to validate the prediction of micropitting using the ISO/TR 6336-22 by comparing the results with the occurrence of micropitting on the tested gears.

Heavy haul rail/wheel wear and RCF assessments using 3D train models and a new wear map

Wear and Rolling Contact Fatigue (RCF) on wheels and rails are influenced by relative motions between the studied rail and wheel pair, and therefore influenced by train dynamics. Wear and RCF assessments using a train dynamics approach provide more accurate and comprehensive results comparing with conventional assessments that often do not consider train dynamics. In addition, considering material property differences, currently available rail wear maps are not able to accurately describe the wear performance of materials that have significantly different properties. This paper presented a method for rail/wheel wear and RCF assessments using 3D train dynamics models. The use of train models allows the consideration of coupler forces, traction, and brakes during the assessment. Wear assessments were conducted by using an in-house rail wear map model developed from the twin disc wear machine at the Centre for Railway Engineering (CRE). The new rail wear map was developed by using the AS 60 rail material (hardness 340 HB). The results indicated that the methods proposed in this paper could be used for rail/wheel wear and RCF assessments. Traction/braking forces and coupler lateral forces had evident influences on wear and RCF assessment. Different wear maps were also shown to have significant influences on the wear rate results.

A new scaled testing methodology for the analysis of rolling contact fatigue in railway steels.

Twin disc testing is a relatively cost effective and practical method of investigating the damage resistive properties of rail steels in a scaled environment. The loading conditions and dimensions used in testing are scaled in such a way that the Rolling Contact Fatigue (RCF) generated on a twin disc rig are representative of the conditions experienced at the full-scale. Typical methodologies to generate RCF on a twin disc rig involve the application of water to the contact patch throughout the test, with perhaps a few thousand dry cycles being run beforehand. This research explores the use of weather data to reflect the real-world contact conditions more accurately between a wheel and rail. Met Office weather data collected from 1981 to 2010 shows the average annual precipitation in the UK is around 133 days of rain or snow, which is nearly 36 % of the year. In this study, weather data was used to determine the ratio of wet to dry cycles in the testing methodology.  50k dry cycles are run in intervals of 5k and then transitioned to 25k of wet cycles, which is 30% of the total cycles. Intervals were kept at 5k so attaining 36%. If RCF is not initiated after the first 75k cycles, which ends with 25k wet cycles, then the same dry and wet cycles are repeated. RCF data generated using this methodology demonstrated expected differences in the RCF resistive properties of two rail steel grades, R260 and R350HT. In test 1 the point of initiation for each grade was not captured, though cracks were visible around the expected cycles to initiation, which was verified using optical microscopy. The cracks viewed on the softer R260 steel samples, ‘’260’’ referring to the Brinell hardness value of the steel, were more developed at 85k cycles and showed signs of spalling, whilst the cracks on the harder R350HT sample were much smaller. In test 2 the point of initiation was found for each grade. Optical microscopy was also used to show the depth and direction of head check cracks to ensure the damage viewed from the head was RCF and not plastic flow or cyclic wear. This paper shows the necessity for considering the number of wet cycles applied during twin disc testing and demonstrates it is possible to run a high number of wet cycles which are more representative of real-world conditions when generating RCF on twin disc rigs.  

Fatigue crack initiation in the presence of stratified surface layers on rail wheels

This work presents a study on the fatigue crack initiation in the presence of stratified surface layers (SSL) – a combination of white and brown etching layer – on rail wheels. Reproducible SSLs comparable to field samples are created by a defined mechanical load and two consecutive laser surface treatments on discs made from two wheel materials (ER7, ER9). Fatigue testing is done by a twin disc tribometer, where the wheel discs with SSLs are driven against discs made from R260 rail material. The results show significant fatigue crack initiation and propagation in the presence of the SSLs, exclusively on the leading edge of the SSL, more pronounced for the wheel grade ER9. Based on topographical, light optical and scanning electron microscopy analysis an explanatory model is presented, describing fatigue crack morphologies and outpointing the relevance of SSLs on rail wheels to fatigue crack initiation.

Effect of grease viscosity and thickener on the wear resistance of a class D railway wheel

Using greases offers numerous advantages to the railroad system, which include the lifespan extension of wheels and rails, fuel consumption decrease, and the reduction of the friction coefficient and component wear. However, there is still a lack of understanding regarding the relationship between the grease formulation and its performance. This study aims to investigate the impact of grease viscosity and thickener on its ability to reduce wear. Thus, a series of tests were conducted using a twin disc tribometer. Four greases were tested, formulated with an identical additive package and base oil and varying viscosities (50 cSt and 500 cSt) and thickeners (calcium and lithium). The results indicated that the lithium-thickened greases performed better than the calcium ones due to lower mass losses and crack depths, smaller plastically deformed subsurface layers, and higher retentivity. Additionally, the findings revealed that the lower (50 cSt) viscosity showed superior performances also due to lower mass losses and crack depths. However, the higher viscosity exhibited higher retentivity compared to the 50 cSt greases.

Studying the transfer mechanisms of water based top-of-rail products in a wheel/rail interaction

The railway industry uses top-of-rail products to control and manage the friction in the wheel/rail interface to help ensure efficient train operations and reduce wheel and rail damage. A product is typically applied from a wayside applicator that pumps a puddle onto the rail head where a passing wheel will pick it up and then transfer it down the track. The aim of this study was to study the transfer mechanisms of water-based top-of-rail friction modifiers (TOR-FMs) and how they are linked to the friction conditions in the wheel/rail interface. The transfer mechanisms were split into three parts: pick-up, carry-on and consumption. Pick-up looks at how the product transfers from the puddle on the rail to a wheel tread, whereas the carry-on mechanism relates to the product transfer back to the wheel. Consumption focuses on the removal rate of the product layer from the wheel or rail. A full-scale rig and twin disc machine were chosen to perform the tests because each rig could give different insights into understanding the product transfer mechanisms. Two products were tested of similar formulation. Results show that there are differences in the transfer and friction between the two products despite them being relatively similar. The test methods developed can clearly resolve differences between varying product types, which could be useful for product development studies or approvals work. The outcomes could also be used to develop a model of transfer/consumption.

Methodology to assess damage mechanisms of rail steels based on small-scale experiments

A new methodology to distinguish between the damage mechanisms wear only and RCF crack initiation in combination with wear in rolling contacts is proposed in this work. The new methodology distinguishes between the damage mechanisms by comparing the experimental crack depth against a critical crack depth which is governed by the surface roughness and by the plastic shear deformation depth in the near-surface layer. The new methodology is used to assess the results of twin disc experiments that have been conducted with three pearlitic rail steels at different normal contact pressures and different longitudinal creep values. The observed experimental results show agreement with the criterion and the threshold of the new methodology.

Corrosive effects of artificial rainwater on rolling contact fatigue on wheel and rail materials

Rolling contact fatigue (RCF) can damage and reduce the life of the wheel and rail materials due to the degradation of surfaces in contact. The complex combination of RCF with corrosion can also contribute to the rapid surface degradation of wheel and rails materials. It is a serious problem when a railway system's environmental conditions can lead to severe corrosion.Even in some cases, severe corrosion caused by rainwater was observed in some open warehouse facilities. This work aims to investigate the influence that corrosion by artificial rainwater has on the generation and propagation of cracks caused by RCF. Twin disk tests were conducted on disk specimens manufactured from wheel and rail sections with corrosion and without corrosion zones on their contact surfaces. Corrosion on the wheel and rail disk specimens was accelerated by the potentiodynamic anodic polarization (PAP) process. The results show that PAP tests can promote pits and micro-cracks formation with different morphologies and sizes in both materials. These surface defects in rail material can grow and accelerate the cracks from the surface and down deep into the subsurface. Finally, pitting and corrosion were removed by the wear action in the wheel material. However, longer and deeper cracks were observed. Results showed that corrosion significantly influences RCF development on rail material.

Kalorimetrische Messung der Ver- zahnungsverlustleistung im Bereich hoher Umfangsgeschwindigkeiten

Gear damage, e.g. scuffing, reacts sensitively to the local dissipation of frictional heat during a full gear mesh. Several empirical friction models have been established in relevant literature. These can be used to calculate the gear friction locally or averaged over a mesh. For calibration purpose, experimental data from twin disc machines with stationary contact conditions are mostly used. However, the assignment of the model as well as the consideration of topographical, rheological and geometrical effects is often subject to uncertainties. In this paper, a method is presented to measure the power losses of high-speed spur gears up to vt = 100 m/s. The measurement principle uses the calorimetric properties of the circulating lubricant. The implementation of the measurement setup in a highspeed gear test rig reduces thermal interferences from sealings and bearings. Analysis regarding the measurement quality show that the measurement method delivers reproducible results with a high measurement resolution over a wide range of operating conditions.

Analysis of Traction Coefficient Subject to Rail Cleaning Effect Based on Tribomachine Measurements

The proper representation of friction contact conditions between each wheel and the rail is necessary to accurately model the behaviour of a heavy haul locomotive since friction conditions at the wheel-rail interface affect the locomotive’s dynamic performance under traction and braking conditions. In normal operations, a phenomenon commonly known as rail cleaning effect occurs. The rail cleaning effect causes increased friction coefficients between the following wheel treads and the rail head. The wheel-rail interaction causes the third body layer to be partly or wholly eliminated from the surfaces in contact and generates new layer. An experimental analysis of the changes in friction coefficients under simulated locomotive wheel-rail contact conditions, in terms of contact pressure and slip, is presented in this paper. For this study, data processing equations are presented to obtain the experimental traction coefficient and slip. Furthermore, the rail cleaning effect is examined under different slip conditions. The experiment shows the traction coefficient increases for a given number of cycles until reaching a steady value, demonstrating that the rail cleaning effect is measurable in various slip conditions on a twin disc machine.

Different responses of wheel–rail interface adhesion and wheel surface damage induced by an out–of–round wheel tread

The out–of–roundness of wheel treads are an inherent degradation characteristic of the wheels of railway vehicles, and the interface behaviour of the wheel and rail caused by out–of–roundness features has essential research value in high–speed train running safety and quality. Sliding–rolling contact wear testing was conducted to evaluate the effect of the out–of–round wheel on the wheel–rail damage behaviour, by using a twin–disc setup. The effects of four wheels, three wheels of different eccentricities and one normal wheel were taken as a control group. The characteristics of the wheel–rail interface adhesion, wheel surface damage and compressive residual stress (CRS) distribution were compared and analysed. The results showed a dramatic increase in the fluctuation amplitude of the wheel–rail vertical force of the eccentric wheels, and the adhesion coefficient of the wheel–rail interface significantly decreased. Furthermore, eccentric wheels dramatically accelerate the wear loss of the rail used as a counterpart during wheel–rail rolling contact. The surface damage behaviour of the eccentric wheels is obviously different along the circumferential direction, and non–uniform wear was observed. The damage mechanism of the worn surface presented varying characteristics, which could be divided into a severely damaged zone (located at the maximum of the rolling radius rmax) and a slightly damaged zone (located at the minimum of the rolling radius rmin). The surface contact stress of the severely damaged zone was high, this finding can be explained by the extremely rough worn surface caused by the severe fatigue delamination characteristics and the initiation and propagation of fatigue cracks in the severely damaged zone. Furthermore, the values of surface hardness and residual stress were relatively low. By contrary, an adhesion layer was observed on the worn surface of the slightly damaged zone. The adhesion layer protects worn surface in the slightly damaged zone was relatively flat, and the CRS was relatively high.

Using twin disc for applications in the railway: a systematic review

Using twin disc for applications in the railway: a systematic review

Effect of normal load on the wear and rolling contact fatigue behaviour of AAR class B wheel against R350HT rail in a twin disc simulator

The normal load at the wheel-rail contact interface plays a significant role in wear and rolling contact fatigue (RCF) performance of both wheel and rail steels. In recent years, there has been an increase in axle loads due to increased demand for railway services. This study investigated the effect of normal load on the wear and RCF behaviour of the AAR class B wheel versus R350HT rail using a twin disc wear simulator. The mass loss, surface damage and depth of deformation were used to assess wear and RCF, respectively. From the mass loss results, wear maps were obtained to identify different wear regimes being mild, severe and catastrophic at different contact loads and slip ratios. Scanning electron microscopy was also used to assess the surface damage of wheel and rail steels in order to identify the wear regimes with surface cracking and delamination being observed at higher contact loads indicating severe and catastrophic wear. Furthermore, the RCF crack density was found to be higher on specimens evaluated at higher normal loads with more cracks evident and crack branching forming.

Numeric simulation of steel twin disc system under rolling-sliding contact

Various mechanical parts come into high-load rolling and sliding contact at the contact surface. Even with technological advancements, mechanical failures still occur. Rolling-sliding mechanical contact issues are the primary cause of over 90 % of surface and subsurface metallic failures and they are only becoming worse. Using discretised continuum 2D finite element methods (FEM), this research investigates the parametric contact effect of a steel twin disc system subjected to rolling-sliding contact at varied surface friction and comprehensive load condition. The equivalent von Mises stress distribution, contact pressure distribution and shift position of maximum subsurface stress on the contour region are all influenced by changes in compressive load and coefficient of friction, according to a numerical surface-to-surface contact simulation performed with Abaqus at mean Hertzian pressure. The maximal equivalent stress, for a given load, reaches a peak in the subsurface and moves farther away from the surface when the coefficient of friction decreases and comes close to the contact surface when the coefficient of friction increases. Consistency is shown by the analytical and numerical results.

Twin disc anomaly – A rare case of optic disc pit with disc coloboma

Twin disc anomaly – A rare case of optic disc pit with disc coloboma

The SPHERE view of three interacting twin disc systems in polarized light

ABSTRACT Dense stellar environments as hosts of ongoing star formation increase the probability of gravitational encounters among stellar systems during the early stages of evolution. Stellar interaction may occur through non-recurring, hyperbolic, or parabolic passages (a so-called ‘fly-by’), through secular binary evolution, or through binary capture. In all three scenarios, the strong gravitational perturbation is expected to manifest itself in the disc structures around the individual stars. Here, we present near-infrared polarized light observations that were taken with the SPHERE/IRDIS instrument of three known interacting twin-disc systems: AS 205, EM* SR 24, and FU Orionis. The scattered light exposes spirals likely caused by the gravitational interaction. On a larger scale, we observe connecting filaments between the stars. We analyse their very complex polarized intensity and put particular attention to the presence of multiple light sources in these systems. The local angle of linear polarization indicates the source whose light dominates the scattering process from the bridging region between the two stars. Further, we show that the polarized intensity from scattering with multiple relevant light sources results from an incoherent summation of the individuals’ contribution. This can produce nulls of polarized intensity in an image, as potentially observed in AS 205. We discuss the geometry and content of the systems by comparing the polarized light observations with other data at similar resolution, namely with ALMA continuum and gas emission. Collective observational data can constrain the systems’ geometry and stellar trajectories, with the important potential to differentiate between dynamical scenarios of stellar interaction.

Wear of driving versus driven discs in a twin disc rolling-sliding test

Theoretically, the stress states in wheel and rail will be different during braking (rail driving) and acceleration (wheel driving), and this will lead to different wear mechanisms and rates in each. In order to reveal the wear mechanism of wheel and rail materials under these different conditions, existing twin disc work on the wear and damage of wheel and rail where they were both considered was analysed. Some trends emerged, but there was little consistency in the way the different tests had been conducted. To avoid hardness/microstructure variables, new tests were then outlined where the same material pairs were used. The results indicated that the wear rate of the driving disc (faster one) was significantly higher than that of the driven one. The driven discs mainly experienced fatigue and abrasion mechanisms, while the driving one more typically suffered tribo-chemical reactions and fatigue. Finally, the potential causes for the difference in wear behaviours between the driving and driven discs were identified: the hardness; the formation mechanism of metal debris; the stress state; the material properties and the varying work conditions. This work has provided some important new data on wear trends at different driving conditions that will be helpful in the subsequent modelling of wheel and rail wear.