Twin-disc Machine Research Articles

Extended crack propagation in nitrided and carburised steels subject to rolling contact fatigue

To predict the Rolling Contact Fatigue life of components such as rolling element bearings, it is important to understand the failure mechanisms. A rather unusual failure was observed on integrated races into a planetary gear train. The failure of the planet of this gearbox was attributed to the very long propagation, at an unusual depth, of a crack, which probably started at the surface. The present preliminary study aims to gain a better understanding of the factors that can lead to this type of failure. Firstly, a twin-disc machine is used to reproduce the contact conditions identified in the application. A second set of more severe conditions is also tested. A comparison is made between a carburised steel (16NiCrMo13) and a nitrided one (32CrMoV13). Two distinct crack patterns and spalling characteristics between the two materials are observed. Some unusual crack characteristics are observed in the carburised sample for a specific set of contact conditions.

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.

Berechnungsmethode zur Beurteilung von Verschleiß bei Stahl-Bronze-Wälzpaarungen hinsichtlich Schneckengetrieben

AbstractWorm gears are usually used in industrial applications with the material pairing of hard worm and softer worm wheel, where the failure mode wear often limits the service life. Up to now, extensive, costly and time-consuming component wear tests need to be carried out for each tribological pairing in order to calculate the amount of wear on worm wheels. Therefore, no practical and efficient experimental method is currently available to optimize material-lubricant pairings with respect to wear capacity for worm gear drives. For this reason, a model contact wear test was developed applying a twin-disk test rig. Based on this, the paper covers a new wear calculation method to characterize the wear behavior of steel-bronze rolling-sliding contacts regarding worm wheels. Therefore, the basics for the new calculation method for a model contact will be presented. Furthermore, the validation of the newly developed wear calculation method on the basis of test results of experimental investigations using the twin-disk test rig is addressed. Subsequently, results of the new wear calculation method are compared to results of the wear calculation method for worm wheels for individual tribosystems. Special focus will be given to similar trends in the calculated wear between the worm gear contact and the model contact. This new calculation method enables, in combination with wear tests on the twin-disk machine, efficient information on the wear behavior of worm gears and the development of optimized material-lubricant pairings concerning wear performance for worm gear drives.

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.

An investigation into the effect of slip rate on the traction coefficient behaviour with a laboratory replication of a locomotive wheel rolling/sliding along a railway track

This paper investigates the effect of slip rate on the traction coefficient of the wheel/rail interface using a twin-disc machine under the replicated tribological and locomotive operating conditions. The traction coefficient decreases with increasing slip rate. The interface materials of the tested samples get less time to accumulate strain hardening, resulting in a lower increase in surface hardness under a faster slip rate. The running-in period is shorter under a higher slip rate, indicating that the surface micro-geometry of the interface materials reaches a steady state faster. The wear and traction coefficient outcomes of this research could be helpful in improving locomotive traction control systems, and in increasing the safety and maintenance efficiency of railway operations.

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.

Experimental study on the influence of slip ratio on irregular wear behaviors of wheel/rail

This study aimed at investigating the effect of slip ratio on the wheel-rail polygonal wear using a twin-disc machine. The slip ratio used in the test included 0.91%, 3.83%, 4.55% and 9.43%. The profile of samples, vibration, wear and damages were analyzed. Through establishing the relationship between speed, frequency and slip ratio, the conditions for generating apparent polygons were analyzed. The results indicate that under the lower slip ratio, the wheel-rail system has no obvious main vibration during the wear process and is not easy to generate polygon wear. With the increase in the slip ratio, the wheel-rail system easily excites some polygonal characteristic frequency during the wear process. When the characteristic frequency is excited and the frequency amplitude increases continuously, the sample produces obvious polygon wear of corresponding order. When the characteristic frequency is excited but the amplitude of frequency is low, the sample will easily produce slight polygon wear. In addition, when obvious polygonal wear is generated, the damage form at the crest is rolling contact fatigue wear and a small amount of peeling pits, and the damage form at the trough is the severe peeling. Meanwhile, the hardness and plastic deformation layer thickness in the crest region is lower than in the trough region.

Wheel/rail adhesion and damage under different contact conditions and application parameters of friction modifier

Friction modifier (FM) is a newly developed third medium to control the service performance of wheel/rail. The wheel/rail performance is influenced by the wheel/rail contact conditions and FM application parameters. Therefore, the effects of FM application parameter and the wheel/rail contact parameters on the adhesion and damage behaviors of wheel and rail materials should be explored. In this study, the influences of application amount of FM, the wheel/rail contact stress and the creepage on the adhesion and damage behaviors were studied using a twin-disc machine. Results indicated that the increase in contact stress would reduce the wheel/rail minimum adhesion coefficient and effective holding time of FM. The increase in the creepage would increase the adhesion coefficient and reduce the holding time. Under the wheel/rail contact stresses of 800 and 1100 MPa, the application of FM could reduce wheel/rail wear and damage. But under 1500 MPa, FM could cause serious rail surface damages. Moreover, the surface crack propagation process of rail mainly included three stages: initial stage, transition stage and rapid propagation stage. The “oil wedge effect” was the main mechanism for aggravating the surface damages of rail under 1500 MPa.

Investigation on Elliptical Contact Starvation for High Slide-to-Roll Ratio: Application to Gears

Starvation can occur in lubricated systems due to a lack of lubricant. Though the starvation conditions of rolling element bearings are fairly well known, fewer studies are devoted to gears. In this study, experiments are conducted to analyze starvation for contact conditions similar to those of gears; that is, for high slide-to-roll ratio. A new oil-off test procedure on a twin-disc machine is proposed. The oil-off phase is analyzed through the friction coefficient and the bulk temperatures of discs. It is highlighted that low viscosities of residual oil located on the track have a positive effect on starved conditions. Otherwise, large ellipticities worsen starvation by reducing replenishment in the contact vicinity. Finally, a new inverse method is proposed to estimate the central oil film thickness during oil-off phases. The results obtained are consistent with existing models. This method proves to be suitable for severe starvation because it takes into account the lubrication regimes and the lubricant–material interactions.

Rheological and tribological evaluation of friction modifiers for wheel‐rail applications

AbstractFriction modifiers (FM's) are substances deliberately applied to the top‐of‐the‐rail in the field in order to control the traction coefficient. In this work, two FM's, a commercial (FM1) and a custom‐made (FM2), and a gauge lubricant (CGL) were evaluated in the laboratory using rheological and tribological tests. Two rheological tests were done: oscillatory (frequency and strain sweeps) and flow tests (viscosity vs. shear rate). Tribological tests were done in a twin disk machine to obtain the traction coefficient under different slip values. Retentivity tests were also done in the same machine. Several physical–chemical tests were also done to characterise the products. The results of rheological tests showed that there are notable differences that can influence the performance of FMs. From the studied materials, the FM2 product tends to be more viscous even for high‐shear rates. The tribological tests showed that the traction coefficient is different for all the studied materials and changes from 0.15 to 0.45 depending on the creepage. The retentivity of FM2 is 1.9 times higher than FM1. The results show a good correlation between the rheological behaviour and the coefficient of traction and retentivity.

Study on wear and rolling contact fatigue behaviours of defective rail under different slip ratio and contact stress conditions

Surface defects play a vital role in wheel/rail rolling contact fatigue (RCF) damage, and materials around the defects act different under various working conditions. This study aims to investigate the effect of slip ratio and contact stress on the evolution of RCF behaviour of material around the surface defect on the rail. A wheel/rail twin disc machine was used to realise the experimental study and the defects were prepared by conical impacts. Optical microscope, scanning electron microscope and electron backscattered diffraction were used to analyse the microstructure. Results indicate that the evolution of material around the defect mainly include three parts: material flowing into and covering the defect, crack initiation and propagation, defect being worn off after the test stop and leaving a remaining crack. With the slip ratio increased, the cycling number when the defect being removed was decreased except the sudden change in the slip ratio of 12%. The slip ratio was regarded as the main factor of the change in the remaining crack angle. The cycling number when defect being removed was also decreased when the contact stress increased. Meanwhile, the change of the remaining crack depth was basically due to the contact stress. Furthermore, the most severe working condition is referred to the slip ratio of 12% with the contact stress of 1800 MPa.

Rail rolling contact fatigue formation and evolution with surface defects

Surface defects can induce serious rolling contact fatigue (RCF) damage at wheel/rail interfaces and even cause fracture failure of rail material. This study aims to explore the formation mechanism of surface defects on rails, and to trace the evolution process of RCF behavior of material around the surface defect. Experimental studies were conducted on a wheel/rail twin-disc machine considering two forms of defects: indentation defects caused by ballast impacts (IDBs) and indentation defects caused by cone penetration head impacts (IDCs). Results indicate that IDB can cause RCF cracks that propagate downward deep into the subsurface of rail due to the formation of a material hardening layer (MHL), causing severe damage. IDCs with different sizes and angles were grouped into an affected group and a non-affected group by considering a critical size dividing line and whether the MHLs existed on the defect surface or not. The evolution process of a crack in the affected group includes four main periods: fracture of the MHL, crack initiation, the rail steel matrix filling up the MHL gap and crack propagation downward. Further, the increase in both the angle and the depth of the IDC would lead to severe RCF damage.

Effects of dent size on the evolution process of rolling contact fatigue damage on defective rail

Defects such as dents on the rail surface might induce severe rolling contact fatigue (RCF) and then threaten the running safety of railway vehicles. This study aims to explore the initiation and propagation of RCF cracks around dents with different sizes through observing both surfaces and cross-sections of the defective rail. The experiments are conducted on a wheel/rail twin-disc machine. Results indicate that the microstructure evolution process around the dent can be divided into three main stages, namely crack initiation, crack propagation and dent removal. Dents in different sizes act very similarly in each stage, including the position, the crack growth path and the final appearance after the dent has been fully worn off. However, dents in different sizes cause different lifetimes of crack initiation and different lasting times of dents at each stage. For the lager dent, cracks initiate later but propagate for a longer time period, so the dent size plays an important role in the RCF evolution process.

Adhesion and damage characteristics of wheel/rail using different mineral particles as adhesion enhancers

Mineral particles are used to improve adhesion between wheel and rail, but they might lead to severe damages of wheel and rail surfaces. To investigate the adhesion enhancement phenomena and damages of both wheel and rail induced by adhesion enhancers, the crushing strengths of four types of mineral particles (zinc oxide, sand, spinel and alumina) were firstly explored on a uniaxial compression tester. Then the adhesion, wear and damage of wheel/rail were studied on a twin-disc machine under the wet condition. The results show that the characteristic crushing strength was 24 MPa for zinc oxide, 40 MPa for sand, 51 MPa for spinel and 73 MPa for alumina, respectively. For particle with the lowest crushing strength (zinc oxide), the adhesion coefficient was the lowest (around 0.20). With the increase in the crushing strength to 40 MPa, the adhesion coefficient was improved to around 0.28. The particles with higher crushing strengths induced larger wheel/rail wear rates and severer rolling contact fatigue (RCF) damages on wheel and rail. The RCF cracks were large in lengths but small in angles, which finally developed into large pieces of material peeling off from the surface. Sand was the most suitable adhesion enhancer considering its good adhesion enhancement and the relatively mild damage on wheel/rail materials.

Experimental evaluation of tangential contact force characteristics between wheel and rail under wet condition using twin-disk rolling machine

Experimental evaluation of tangential contact force characteristics between wheel and rail under wet condition using twin-disk rolling machine

Scale and contact geometry effects on friction in thermal EHL: twin-disc versus ball-on-disc

Plint (IMechE 182 (1):300–306,1967) reported a reduced coefficient of friction with increasing roller sizes in EHL traction measurements. In this study, a similar scale effect has been observed when comparing measured traction curves at the same operating conditions between a ball-on-disc rig and a twin-disc machine of different geometrical sizes. This scale effect has been studied numerically for point contacts of different radii of curvature Rx based on thermal EHL simulations. Results show that the reduced friction for large Rx is caused by an increase in film thickness and the enhanced thermal effects. The mechanisms are: (1) heat is hard to conduct across a thicker EHL film due to bad thermal conductivity; (2) shear is mainly localized in the middle film.

Influence of surface integrity on fatigue life of bearing rings finished by precision hard turning and grinding

Machined surface integrity of bearing rings is often the main concern for the bearing manufacturers as it affects their fatigue life. The present study investigates the surface integrity of AISI 52100 bearing rings finished by precision hard turning and grinding and its influence on fatigue life. A twin-disk machine is used to evaluate the RCF life. As main results, precision hard turning can achieve low surface roughness 0.1 μm Ra with respect to the required 0.2 μm Ra obtained by grinding. Both processes introduce microstructural changes. At subsurface, precision hard turning induces subsurface compressive and maximum residual stresses at 10–50 μm depth, corresponding to the transition zone formed after a thin white layer (<1 μm); however, grinding induces tensile residual stresses from 15 μm depth, corresponding to the bulk material. The residual stresses measured during RCF test (after running-in process and after spalling) of ground ring specimens exhibit peak compressive value in subsurface from 140 μm depth. The ring specimens machined by precision hard turning have fatigue life four times higher than those machined by grinding. This enhancement of RCF life is due to the low roughness reached and also to the subsurface compressive residual stresses at shallow depth before RCF test.

Influence of temperature on adhesion coefficient and bonding strength of leaf films: A twin disc study

In the autumn, train operations in the UK are likely to be unstable due to the low adhesion coefficient between the wheel and rail, and fallen leaves on the line have been known as the main cause of this problem. In this study, the temperature effects on the adhesion coefficient as well as the bonding strength of the leaf film were investigated using a twin disc machine, to develop a potential prevention and mitigation method. The high surface temperature of the disc seemed to improve the adhesion when the leaf powder suspension was present, forming a linear relationship between the surface temperature and adhesion coefficient. The surface temperature around 240 °C could be enough to attain the required level of traction. The high temperature could decrease the bonding energy between the leaf film and rail, possibly decelerating the chemical reaction for the leaf film formation. Continuous drag braking in the autumn was proposed as a countermeasure. Two effects can be expected for this method: prevention of leaf contamination by removing the leaf residue on the wheel surface and improvement of the adhesion level by braking heat.

Evaluation of the coefficient of friction of rail in the field and laboratory using several devices

ABSTRACTAccurate friction measurement is vital to apply appropriate friction management techniques to the wheel/rail interface. This work analyses different friction measurement techniques under a variety of conditions in the laboratory and the field. Tests have been carried out using a pendulum tester, hand-push tribometer, twin-disc machine and full-scale rig in the UK and Colombia for a variety of interfacial conditions and rail hardness. The pendulum has been found to be more sensitive to different conditions than the hand-push tribometer. This is due to the area that the pendulum sweeps being smaller, and so it can be more carefully controlled and therefore measure the surface condition being tested. This is in contrast to the push tribometer which needs a long section of rail to take a measurement. Twin-disc and full-scale rig creep curves show good agreement between each other.