Surface Initiated Rolling Contact Fatigue Research Articles

The Effects of the Lubricant Properties and Surface Finish Characteristics on the Tribology of High-Speed Gears for EV Transmissions

Electric vehicle (EV) transmissions operate at high speeds. High-speed operation puts higher demands on bearings, seals, and gears. Bearings in EV transmissions are prone to electrically induced bearing damage and may exhibit signs of pitting and fluting. Surface-initiated rolling contact fatigue is another common problem gaining increased attention lately. Most EV transmissions require a coupling between an oil-lubricated gearbox to an electrical motor that runs with minimal lubrication at very high rpm. The high mechanical and thermal stresses the seals are exposed to under starved lubrication conditions have a detrimental impact on their service life. Hence, proper lubrication is critical. In general, EV transmission fluids call for a somewhat different spectrum of properties compared to conventional ATFs. Gear tribology simulations open new ways to the design and optimization of lubrication for EV transmissions. Additionally, such simulations can also provide valuable insights into the effects of different oil properties on cooling and lubrication efficiencies, thereby helping in matching the lubricant and hardware characteristics for optimal performance. In the present communication, we demonstrate the effects of different lubricants and surface finishing technologies on the tribology of high-speed gears using tribological tests and advanced thermal elastohydrodynamic (TEHD) simulations. The important roles of lubricity additives and surface finish optimization are highlighted in conjunction with a move towards ultralow viscosity EV transmission fluids.

Numerical study on fatigue crack propagation behaviors in lubricated rolling contact

The surface-initiated Rolling Contact Fatigue (RCF) including pitting and micro-pitting is one of the key issues affecting the reliability of tribological components such as gears and bearings used in various devices. In this work, a surface-initiated crack Finite Element (FE) model which considers the effect of lubricant on crack faces was developed to investigate surface-initiated RCF using an automatic crack propagating Python script. Different lubricating states, initial crack parameters and loading conditions were simulated to analyze the evolution of crack propagation and the Stress Intensity Factors (SIFs). The RCF crack propagation path and life were predicted by employing the Maximum Tangential Stress (MTS) criterion coupled with the Paris’s law. A typical RCF failure is predicted in the numerical simulation. Results reveal that the lubricating pressurization dominates the surface-initiated RCF. In addition, the initial crack angle has a significant effect on the RCF crack propagation path and the fatigue life.

Failure Analysis of Ball Bearing of Conveyor: Overusage

Rolling bearings are machine elements found in a wide field of applications. They are reliable even under the toughest conditions. When bearings fail to meet their expected life, the consequences are increased down time, loss of revenue and missed delivery. Hence, it is important to know the bearing theoretical fatigue life. However, unfortunately, it sometimes happens that a bearing does not attain its calculated rating life. There may be many reasons for this—heavier loading than has been anticipated, inadequate or unsuitable lubrication, careless handling, ineffective sealing, improper mounting and improper clearance. This article describes the failure of a conveyor bearing in rolling contact fatigue. The investigation consists of visual observation, chemical analysis, characterization of microstructures using optical microscopes and hardness test. The study also calculates the bearing fatigue life (L10) under these operating conditions. Surface-initiated rolling contact fatigue, leading to spalling, is a life-limiting failure mode in bearings. Ferrography test, discussed in this paper, indicates fatigue spalling. Analysis suggested that the conveyor bearing failed near its calculated fatigue life and the failure occurred in rolling contact fatigue. Periodic schedule, after calculating L10 life, for changing the bearing and scheduled ferrography checks can eliminate this type of unplanned breakdown.

Micro-pitting and wear characterization for different rolling bearing steels: Effect of hardness and heat treatments

The current trend of increased power density and reduction of oil viscosity in machine components is forcing engineers and scientists to engineer surfaces to endure higher contact pressures and stresses induced from rolling/sliding motion. Typically, surface-initiated rolling contact fatigue represents the main failure mode and has been gaining more and more attention lately. In this paper, the extent and morphology of the surface damage of different bearing steels in disc-on-disc contact under reduced lubrication conditions are studied. For this, rough-on-smooth contact was selected to promote surface fatigue damage on the smoother surface. Special attention was given to study both the effect of the hardness difference of the discs and the influence of different heat treatments, i.e. surface induction hardening (SIH) or through hardening (TH), on micro-pitting and wear performance. It was demonstrated through tribo-testing that the faster, rough surface undergoes only mild wear and plastic deformation of asperities. However, three different damage modes were observed and assessed on the slower, smoother surface, where the damage mode showed a strong dependence on a surface hardness difference between the smooth and the rough surface. Furthermore, it was observed that a plain medium carbon steel (DIN C56E2) showed better surface fatigue resistance than a low alloy high carbon steel (DIN 100Cr6) at similar surface hardness levels if SIH is applied on the former.

Numerical simulation of rail surface-initiated rolling contact fatigue in the switch panel of railway turnouts

Considering the complex characteristics of the track structure in railway turnouts, it is difficult and also expensive to experimentally study rail damages; therefore, numerical methods are an effective alternative. This study presents a numerical method to simulate rail surface-initiated rolling contact fatigue in the switch panel of railway turnouts. This method includes simulation of the vehicle–turnout wheel–rail dynamic interaction, analysis of the wheel–rail multipoint non-Hertzian rolling contact that considers the relative motion between the switch and stock rails, and calculation of the accumulated rail surface-initiated rolling contact fatigue. The accumulated rail surface-initiated rolling contact fatigue after the vehicles passed a turnout switch panel 80 times (the average number of vehicles running on the Chinese high-speed railway lines per day) in the through route with facing move was simulated based on this procedure. The result showed that the maximum surface-initiated rolling contact fatigue damage of the switch rail and the stock rail was 1.57 × 10−2 and 0.62 × 10−2, respectively. Surface-initiated rolling contact fatigue in the switch rail mainly occurred at the gauge angle, and in the stock rail it mainly occurred at the center of the rail. In addition, the influence of track parameters (rail inclination, track gauge, and friction coefficient) is analyzed. The friction coefficient influenced the rail surface-initiated rolling contact fatigue. When the coefficient exceeded 0.3 in particular, the rail rolling contact fatigue damage increased sharply. Hence, suitable friction control measures should be taken during rail maintenance in order to mitigate the rail surface-initiated rolling contact fatigue damage, e.g. by keeping the wheel–rail friction coefficient below 0.3.

Rolling contact fatigue behaviour of rails: Wedge model predictions in T-Gamma world

In this study, T-Gamma and Wedge models have been compared with each other for the prediction of surface-initiated rolling contact fatigue cracks on rail surfaces. Both models are able to account for different observed rolling contact fatigue-wear regimes in tracks, but with very different physical backgrounds. The T-Gamma model uses empirically determined damage functions by introducing a relationship between the wear number (T-Gamma) and the rolling contact fatigue damage increment. Different rolling contact fatigue-wear regimes are considered in this empirical approach based on the idea that initiated cracks get partially or fully removed by the wear mechanism, not accounting for the full complexity of the occurring tribological phenomena. The Wedge model represents a physical approach, where contact stresses and its impact on plastic deformations and related material anisotropy are considered. Thus, the prediction of different rolling contact fatigue-wear regimes is based on these physical relationships, where plastic shear deformations in the near-surface layer play a key role. For comparison, the wheel–rail contact data from stochastic multibody dynamics simulations of a metro vehicle with conventional bogie technology running in three curve radii have been used. While the T-Gamma model always predicts the same rolling contact fatigue damage increment for a given T-Gamma value, the Wedge model shows a scattering of the predicted rolling contact fatigue damage increments when plotting them over T-Gamma because of the explicit consideration of contact stresses. Thus, each scenario consisting, for example, of certain vehicles, curve radius, wheel–rail profile combination, friction conditions, rail material, etc. needs its own damage function in the T-Gamma world. This should be kept in mind when applying the standard T-Gamma model to scenarios which differ significantly from the scenario it has been parameterised for.

Impact of wheel profile evolution on wheel-rail dynamic interaction and surface initiated rolling contact fatigue in turnouts

Hollow wear is a common type of wheel profile evolution in high-speed railway, causing special contact characteristics in turnouts. Taking into account the flexibility of wheelsets and rails in turnout as well as complex constraint conditions, the vehicle-turnout coupling model is built and wheel-rail dynamic interaction in turnouts is analyzed. After calculating the wheel-rail non-Hertz contact behavior with wheel profile evolution, the accumulated surface initiated rolling contact fatigue (RCF) of turnouts are evaluated based on the shakedown theory. Results indicate that the wheel transition position is shifted backwards with wheel profile evolution. The vertical wheel-rail force is contributed by wheelset bending modes in the switch panel, and its maximum value decreases by 22% due to the 87% reduction in vertical contact irregularity in the crossing panel. The severe contact between the false flange and wing rail leads to the aggravation of lateral axle box acceleration (ABA) while the positions of its characteristic frequencies are changed from the point rail to wing rail. When the degree of hollowing becomes medium, wheel profile evolution improves the conformal contact that decreases the accumulated surface initiated RCF in the switch panel. As the degree of hollowing becomes severe, the false flange's contact with the wing rail is prone to RCF in the crossing panel.

Assessment of non-Hertzian wheel-rail contact models for numerical simulation of rail damages in switch panel of railway turnout

Rail damages in railway turnout switch panels are closely related to the complicated wheel-rail contact conditions in the turnouts. In order to assess the applicability of different non-Hertzian contact models for simulating rail damages in railway turnout switch panel, three engineering approaches, i.e. Kik-Piotrowski, Ayasse-Chollet and Sichani, are adopted to simulate rail wear and surface initiated rolling contact fatigue (RCF) damages by studying a Chinese CN60-1100-1:18 high-speed turnout switch panel. A comparison is then carried out taking into account the results calculated by Kalker’s three-dimensional (3D) theory and an elastic-plastic FEM (Finite Element Method) model. The analyzed results show that the computational efficiencies of the three engineering approaches are all quite high for simulating rail damages in switch panel. Under elastic conditions, the calculation accuracy of the Sichani approach is found to be the highest, meaning that this would appear to be the preferred method for simulating rail damages in railway turnout switch panel. The research findings can be taken as theoretical guidance for selecting wheel-rail contact models and analyzing these types of damages in railway turnout switch panels.

Model based detection and diagnosis of bearing defects

In this paper we present the Amplitude Spectral Density of Spectrogram [1] (ASDS) and how its output is post processed for determining the size of defects in bearings using acceleration signals. Fatigue modeling of the bearing is then used to define the moment to stop the operation and replace the bearing. Initially, small defects in the order of a 0.1 to 1 millimeter may arise in the running surface of the bearing. This is the moment when vibrations start to be excited by the rolling contact forces and can be observed by accelerometers on the structure of the machine. At this stage most machines are still operating well. The rolling bearing modeling work shows that characteristic bandwidth of the force impulse for defects smaller than the Hertzian length approximates the speed rolling divided by the sum of the length of the defect and length of the Hertzian contact. This simple result has been proven experimentally by running a bearing with a small defect and varying the running speed, and by varying the size of the Hertzian contact by changing the load [1]. When the small defect grows, the amplitude of the acceleration is growing with defect size but it’s bandwidth is hardly changing until the Hertzian contact is completely disrupted. At that point the force impulse’s bandwidth starts going down as the defect length becomes the dominant factor. This property is used to calculate the actual size of the defect in order to avoid operational disruption and the decision to stop is based on fatigue modeling. Eventually when the defect is about 20 percent of the roller spacing in the bearing, the bandwidth starts to become similar to the repetition frequency and the ASDS transform will no longer be able to estimate it. Experimental work with artificial defects proves the size quantification. The diagnosis with defect size quantification makes future prognosis possible. For small defects, we have proof that the defect’s rate of growth is slow [2], [3]. When the defect passes a certain size stage which depends on the load, the erosion process will exponentially increase the area damaged in the bearing . Using rolling contact fatigue modeling, under the condition of defects in a rolling bearings, it is possible to predict the likely defect growth rate [2],[4]. By fitting the estimated defect size to the exponential growth rate curve, the decision to replace the defective bearing can be deferred to the right moment without much risk of immediate failure. [1] Mol, H.A., van Nijen, G.C., “Method for Analysing Regularly Recurring Mechanical Vibrations”, Patent US5698788. Published December 16, 1997. [2] Morales-Espejel G.E., Gabelli A., “The progression of surface rolling contact fatigue damage of rolling bearings with artificial dents”. Tribology Trans. (2015); 58: pp. 418–31. [3] Rycerz et al: Propagation of surface initiated rolling contact fatigue cracks in bearing steel, International Journal of Fatigue 97 (2017) 29-38. [4] Gabelli A., Morales-Espejel G.E., “Improved Fatigue Life Analysis of Pre-Dented Raceways Used in Bearing Material Testing”, Bearing Steel Technologies 11th Volume, “Progress in Steel Technologies and Bearing Steel Quality Assurance”, ASTM STP1600, 2017, pp. 167–191 [5] A.V. Oppenheim and R.W. Schafer, "Digital Signal Processing", Englewood Cliffs, NJ: Prentice Hall, 1975, pp 358 ff. [6] D.A. Rice, V. Venkatachalam, M.J. Wegmann: "A simple envelope detector". IEEE transactions on instrumentation and measurement, vol 37, no. 2, June 1988, pp 223-226.

Parameters Studies on Surface Initiated Rolling Contact Fatigue of Turnout Rails by Three-Level Unreplicated Saturated Factorial Design

Surface initiated rolling contact fatigue (RCF), mainly characterized by cracks and material stripping, is a common type of damage to turnout rails, which can not only shorten service life of turnout but also lead to poor running safety of vehicle. The rail surface initiated RCF of turnouts is caused by a long-term accumulation, the size and distribution of which are related to the dynamic parameters of the complicated vehicle-turnout system. In order to simulate the accumulation of rail damage, some random samples of dynamic parameters significantly influencing it should be input. Based on the three-level unreplicated saturated factorial design, according to the evaluation methods of H, P and B statistic values, six dynamic parameters that influence the rail surface initiated RCF in turnouts, namely running speed of vehicle, axle load, wheel-rail profiles, integral vertical track stiffness and wheel-rail friction coefficient, are obtained by selecting 13 dynamic parameters significantly influencing the dynamic vehicle-turnout interaction as the analysis factors, considering four dynamic response results, i.e., the normal wheel-rail contact force, longitudinal creep force, lateral creep force and wheel-rail contact patch area as the observed parameters. In addition, the rail surface initiated RCF behavior in turnouts under different wheel-rail creep conditions is analyzed, considering the relative motion of stock/switch rails. The results show that the rail surface initiated RCF is mainly caused by the tangential stress being high under small creep conditions, the normal and tangential stresses being high under large creep conditions, and the normal stress being high under pure spin creep conditions.

Contact fatigue initiation and tensile surface stresses at a point asperity which passes an elastohydrodynamic contact

Abstract Contact mechanics and tribology was combined with fundamental fatigue and fracture mechanics to form a new mechanism for surface initiated rolling contact fatigue. Following, fatigue was investigated numerically for single asperities and craters in lubricated rolling contact surfaces. The hypothesis suggests that asperity point contacts can create sufficiently large tensile stresses for fatigue. The investigated case corresponded to a heavily loaded truck gear with ground surfaces. Reynolds equation resolved the elastohydrodynamic effect of the asperity in the transient three dimensional contacts. The Findley critical plane criterion was used for multiaxial and non-proportional fatigue evaluation. The simulations confirmed the new mechanism for rolling contact fatigue and showed how asperities can create contact fatigue in the lubricated contacts even without slip.

Characterization and mapping of rolling contact fatigue in rail-axle bearings

Abstract This paper presents a new stage of on-going research to fully characterize the rolling contact fatigue (RCF) damage observed in rail axle Compact Tapered roller Bearing Units (CTBUs). The bearings examined in this work have been removed from service, following the identification of degradation using on-board condition monitoring techniques. The running surfaces of the bearings were examined and the damage fully characterized using a mixture of metallographic, surface profilometry and high-resolution micro-Computed Tomography (μ-CT) techniques. In this manner the RCF was categorised by initiation mechanism: sub-surface or surface. This work has led to an increased understanding of the propagation of sub-surface and surface initiated RCF and its implications for life and condition of the bearing once failure has initiated. It was determined that the sub-surface initiated RCF damage was the first to occur in service and produced large craters on the bearing running surface. Once material had been lost in this primary manner, further secondary surface-initiating RCF grew from the edges of the large craters. It was deemed that the deep primary sub-surface RCF craters were more life limiting than the secondary surface damage. Therefore maps of the damage were created allowing the two mechanisms to be examined separately, both visually and by quantitative parameters such as volume loss, area, depth, roughness etc.

Propagation of surface initiated rolling contact fatigue cracks in bearing steel

Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms. A triple-contact disc machine was used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface was monitored and crack propagation rates extracted. The morphology of the generated cracks was observed by preparing sections of cracked specimens at the end of the test. It was found that crack initiation occurred very early in total life, which was attributed to high asperity stresses due to mixed lubrication regime. Total life to pitting was dominated by crack propagation. Results provide direct evidence of two distinct stages of crack growth in rolling contact fatigue: stage 1, within which cracks grow at a slow and relatively steady rate, consumed most of the total life; and stage 2, reached at a critical crack length, within which the propagation rate rapidly increases. Contact pressure and crack size were shown to be the main parameters controlling the propagation rate. Results show that crack propagation under rolling contact fatigue follows similar trends to those known to occur in classical fatigue. A log-log plot of measured crack growth rates against the product of maximum contact pressure and the square root of crack length, a parameter describing the applied stress intensity, produces a straight line for stage 2 propagation. This provides the first evidence that growth of hereby-identified stage 2 rolling contact fatigue cracks can be described by a Paris-type power law, where the rate of crack growth across the surface is proportional to the contact pressure raised to a power of approximately 7.5.

Prediction of the effects of friction control on top-of-rail cracks

Rolling contact fatigue is a major problem connected with railway tracks, especially in curves, since it leads to higher maintenance costs. By optimising the top-of-rail friction, the wear and cracks on the top of the rail can eventually be reduced without causing very long braking distances. There are several research articles available on crack prediction, but most of the research is focused either on rail without a friction modifier or on wheels with and without friction control. In the present study, in order to predict the formation of surface-initiated rolling contact fatigue, a range of friction coefficients with different Kalker’s reduction factors has been assumed. Kalker’s reduction factor takes care of the basic tendency of creepage as a function of the traction forces at lower creepage. The assumed range covers possible friction values from those for non-lubricated rail to those for rail with a minimum measured friction control on the top of the rail using a friction modifier. A fatigue index model based on the shakedown theory was used to predict the generation of surface-initiated rolling contact fatigue. Simulations were performed using multi-body simulation, for which inputs were taken from the Iron Ore line in the north of Sweden. The effect of friction control was studied for different curve radii, ranging from 200 m to 3000 m, and for different axle loads from 30 to 40 tonnes at a constant train speed of 60 km/h. One example of a result is that a maximum friction coefficient (µ) of 0.2 with a Kalker’s reduction factor of 15% is needed in the case of trains with a heavy axle load to avoid crack formation.

Switch panel design based on simulation of accumulated rail damage in a railway turnout

A methodology for numerical prediction of accumulated rail damage in railway turnouts is presented. Based on simulation of dynamic vehicle-track interaction followed by a discretisation of the conditions in each wheel-rail contact, distributions of rail wear are calculated by the Archard model of sliding wear, while surface initiated rolling contact fatigue (RCF) damage is evaluated by the Palmgren–Miner rule and an index building on shakedown theory. Partial slip in the wheel-rail contacts and variable amplitude loading are considered. For freight traffic in the diverging route, the influence of rail inclination and switch rail elevation on damage in the switch panel is investigated in a demonstration example. Two-point contact situations with one contact on the switch rail and one on top of the stock rail induce relative motion and slip between wheel and rail leading to high energy dissipation. In agreement with field observations, it is concluded that wear is the dominating damage mechanism on the gauge side of the switch rail while the risk for RCF is higher on the crowns of the switch and stock rails. For accurate prediction of rail life for given combinations of wheel/rail materials and traffic conditions, the methodology needs to be calibrated by field measurements.

Stress gradient effects in surface initiated rolling contact fatigue of rails and wheels

The paper investigates gradient effects, which relate to how highly stressed regions should be dealt with in fatigue design analyses. In particular stress gradients in rolling contact are investigated with a focus on differences in response between full and partial slip conditions. To this end the multiaxial state of stress beneath a wheel–rail contact featuring full or partial slip is quantified using a multiaxial equivalent stress criterion. A comparative study shows that the significant differences in peak interfacial shear stress magnitudes between full and partial slip conditions are significantly reduced when translated to equivalent stress magnitudes. An innovative procedure to quantify the gradient effects by comparing the multiaxial contact stress field to uniaxial conditions is developed and employed. For the studied cases stress gradients beneath the frictional contact were found to be similar to stress gradients outside a uniaxially loaded large plate featuring a small hole with a radius in the order of 0.5–0.7mm. The study concludes that the use of local magnitudes of interfacial shear stress in the analysis of surface initiated rolling contact fatigue under partial slip conditions is conservative. The analysis framework established in the current study can be used to estimate the level of conservativeness.

Temperature-dependent evolution of the cyclic yield stress of railway wheel steels

The evolution of the cyclic yield stress for a railway wheel steel (UIC ER7T) during cyclic plastic straining has been characterized at different temperatures from −60 to 600°C. Different constant strain amplitude levels were examined and for temperatures above 200°C, hold periods were included to study stress relaxation during constant compressive strain. The results are of use in predicting material deformation and damage. This is demonstrated by the application to improve a criterion for surface initiated rolling contact fatigue damage.

Numerical investigation of the spall opening angle of surface initiated rolling contact fatigue

The spall opening angle was studied for surface initiated rolling contact fatigue with the purpose to allow assessment of the volume of detached material. The influence of friction and the crack inclination angle on the damage spread in the contact surface was investigated with the asperity point load mechanism and a simplified three-dimensional rolling contact fatigue load. Crack arrest due to crack closure was proposed as explaining mechanism for the spall opening angle of the typical v-shaped or arrowhead crack configurations. A new three-dimensional crack geometry was presented allowing the study of the spalling surface morphology in a gear application. Stress intensity factors along the crack front were computed using the eXtended Finite Element Method (XFEM) implemented in Abaqus (6.12). Both low crack inclination angles and increased friction resulted in larger spall opening angles. For cracks with small inclination angles the effects of increased friction on the spall opening angle appeared however very little. The findings increase understanding of the surface morphology and the damage process and further motivate the asperity point load mechanism as an important source for surface initiated RCF damage.

Wheel/rail rolling contact fatigue – Probe, predict, prevent

With the aim of putting prediction and prevention of rolling contact fatigue of wheels and rails in context, the paper sets out with a description of the phenomena. This is followed by a discussion on predictive models with emphasis on influencing factors needed to be accounted for and how these can be incorporated in a predictive model of surface and subsurface initiated rolling contact fatigue (RCF). The focus is on predictive models fast enough to be integrated with multibody dynamic simulations of train–track interaction. In particular an attempt is made to highlight how current predictive models of surface initiated RCF can be extended to incorporate all affecting phenomena. The discussion on influencing factors then forms the basis for a discussion on suitable and efficient measures to maintain RCF under control. Finally the paper provides a brief summary on similarities and differences between wheel and rail damage, and an outlook of expected future trends in terms of expected damage patterns and possibilities to meet these.

Modelling of Surface Initiated Rolling Contact Fatigue Damage

A parametric study was performed to investigate the contribution of surface roughness, friction and a constant residual surface stress to the rolling contact fatigue damage process. The effects on initiation, crack path and fatigue life were examined for a gear application. The asperity point load model could predict effects on rolling contact fatigue that are observed with experiments. The study of a ring/cone crack subjected to a rolling contact fatigue load allowed to explain the v-shaped cracks typical for surface initiated spalling. The results further support the asperity point load mechanism as a source behind surface initiated rolling contact fatigue.