Improving the knowledge of electrical wheel-rail contact

Philippe DEMANCHE

SNCF (Société Nationale des Chemins de fer Français), Paris, France

Abstract

The metal on metal rail-wheel contact ensures, in addition to its role of guidance, electrical functions for the return of motors current and the track circuits shunting. In order to decrease the noise emitted by incoming trains, two solutions are gradually set up: composite brake blocks, and disk brakes. In both cases, roughness, source of the noise, is lower but in same time, the quality of the electrical contact is worse. Thus, the aim of the research project is to progress in the knowledge of the mechanical and especially electrical contact between wheel and rail, to know which are the parameters making it possible to guarantee good electrical contact. The recent theories include an element at the wheel-rail interface : the "third body". This element, representing fifteen microns thickness at the surfaces of the rail and the wheel, is composed of solid, liquid or gas elements. Under the strong constraints existing at the wheel-rail interface, this third body creeps like modeling clay. According to its total cohesion, depending on its global composition, the local contact conditions can be very different, in terms of adherence and slip. In the research project, one tries to understand how the electrical current, particularly the low currents, go through this third body barrier. The rail-wheel contact is one of the hardest mechanical problem to understand, mainly because it is not possible to instrument it. The track-circuits shunting is based on a good electrical contact between rail and wheels. In France, a huge amount of work has been done since years 50’s, to understand why certain types of rolling stock had some shunting problems. Some theoretical studies and tests have been performed. Results will also be presented. In years 2000’s, new types of rolling stock has been brought into service, equipped with disk brakes only. This type of rolling stock had some shunting difficulties, and SNCF then started a new research program to understand the reason of changing behavior. Some tests have been performed since 2003. They are presented shortly in this article, with results. SNCF has also worked a lot on theoretical approach, with two French university laboratory, having multiple knowledge (electrical, mechanical and railway dynamic). All these studies are also presented. Finally, conclusion are given on future work for this project.

Introduction

Track circuits have a high degree of reliability and profit from a long experiment. On the other hand, they have a fundamental weakness. All the installations are based on the opening or the closing of circuits. However, the detection element, i.e. the shunt, causes only a weakening of the current. That involves a great influence of parameters such as the weather and quality of wheel rail contact.

Historical work

Studies undertaken by Hertz and Boussinesq show that the contact surface between wheel and rail is an ellipse. This can be said considering a certain number of assumptions, such as the surface of contact is much smaller than dimensions of the involved bodies (wheel and rail). Hertz showed that the contact pressure is maximum in the centre of the ellipse, this pressure depending on the load of the wheel and dimensions of the ellipse of contact (this ellipse size depends on the wheel and rail geometries). Hertz also showed in the continuation of its study that the pressure decreases gradually when one moves away from the centre, and is cancelled along contours of the contact ellipse. The maximum pressure in the centre of the ellipse is varying proportionally with the cubic root of the wheel load. Hertz finally showed that the surface of contact was proportional to the square of the cubic root of the load.

However, real metal surface is a succession of "mountains" and "valleys". The real contact thus does not take place on all apparent surface (ellipse), but in points quite particular. The real surface of contact is thus made up by a few elementary surfaces. When the load is important on these little surfaces, the limits of plastic deformation of metal are reached, limits related to its hardness (H). Beyond a pressure of 0.4H, according to the work "Friction and lubrication" of Bowden and Tabor, which are the average rupture pressure, metal is crushed. One deduces from it that the real surface of contact is not related to the apparent surface of contact (Hertz ellipse), but to the wheel load. The contact thus being done in small zones, the reduction of the driving section reveals a resistance called "constriction resistance". The constriction resistance is inversely proportional to the radius of the contact area. In a model where the contact between the two metal parts is done by a cylinder of ray a , the Laplace equation gives the value of the constriction resistance :

a

R2ρ

= (1)

with ρ material resistivity. If there are N contact areas, modelled by cylinders, the total constriction resistance decreases inversely proportional to N. The electrical contact being done in very restricted surfaces, the density of current through the contact is significant, involving a local and important rise of temperature. According to the ORE report [1], the local contact temperature can reach 3200 K for a voltage of 1V considering metals used in the composition of steel, one can deduce from these considerations when a contact exists, this contact is almost as good as in the case of welded materials. Metallic oxides behave like electrical insulators. Their resistivity is variable according to the hygrometry, since they absorb the humidity of the air. To compare, the resistivity of steel is approximately 20.10-6 Ω.cm and that of Iron oxide Fe2O3 of 105 Ω.cm. Thus, the constriction resistance and an oxide contact resistance are very different (factor 106 approximately between the both). In preliminary conclusion, the wheel-rail contact resistance is sufficiently low to be negligible if contact surfaces are clean. On the other hand, with oxide, resistance is very significant (example of an oxide bridge a thickness of 0,01 µm : 1,3 kΩ for a surface of contact of 0.08 mm²). It is only after the mechanical rupture or electric breakdown (following a rather high electric field) that the current can go through the contact. There can be good contact between two peaks of metal, or electric arc and creation of a metal bridge by fusion of metals due to the local heating. These two phenomena are respectively called B-fritting, and A-fritting. According to theories of Holm, electric conduction is restored only if the continuity of the oxide coating, even very thin, is stopped. The A-fritting phenomenon can be compared to the perforation of dielectric layers. In consequence of A-fritting, one obtains very small channel sections which involves metal local fusion by local temperature rising. In all the cases, A-fritting appears only if electric field is sufficient (about 107 V/cm). To summarize, two fundamental phenomena interfere :

- structure of the physicochemical state of surface compared to the structure of the support; - reversible adsorption of gases balance in the structural interstices, the latter being able to be

identified with the "vacuums" of the porous state of oxides. The variable elements which return the resistance of an inconstant oxide coating and there passage of the irregular current are as follows:

- porous structure of the layers of spontaneous oxides of ferrous alloys, - adsorption of gases of the real medium by the "vacuums" of granular oxides (thus porous) and of

the interstices inter-crystalline and inter-crystallite, - the nature of the metal of the support or contact with the oxide coatings.

For a kinematic electrical contact (train is moving), the contact surface is renewed permanently, so that a point is found to belong to the contact during a time Tc. The duration of contact is inversely proportional to wheels rotation speed, and thus to the linear speed of the train. One can thus estimate at first approximation Tc at :

vLTc = (2)

where L is the length of the contact, and v the speed. This time is called "mechanical contact time". The contact pressure in a given point of this surface changes during the time of contact Tc, according to the repartition of the loads on the surface of contact in the direction of the movement. According to its position in the zone of contact, the pressure at the point of contact increases gradually until reaching a maximum, and then decreases until being cancelled. The direct consequence of a limited duration contact is that, some phenomena being able to be established only after one certain time Tf, these phenomena occur only in the condition as cf TT ≤ . In particular, A-fritting, where thermal phenomena and fusion are concerned, requires a time for the construction of the metal bridge through oxide. This time of establishment is about the millisecond (according to Holm). It is moreover related to the conditions of pressure and electric voltages. The metal bridges created by the phenomenon of fritting can be destroyed by mechanical vibrations : it is off-fritting. When the train is moving, the differences with the static contact are felt very quickly, even at the very reduced speeds where cf TT < . On the other hand, one can expect that there is no more notable

difference beyond a certain speed, speed for which fc TT ≈ .

Experience feedback

With this old and interesting approach, one also has elements of feedback : - there is no problem of shunting when the train is not moving (even for the light rolling stocks), - there is no problem of shunting when the train is moving along a curve, - there is no problem of shunting when the train brakes, - there is no problem of shunting when the wheel rail contact is wet (rain, …),

The current study tries to understand these phenomena in particular.

Tests performed since 2003, at SNCF

Comparison tests of two rolling stocks with respect to the aptitude for shunting

The objective of this test is to determine the influencing parameters on residual voltage recorded at the track circuit receiver. The controlled parameters are :

- Speed (5, 10, 30, 60, 90 km/h, Vmax, coasting) - Type of rolling stock (X1(good) and X2(bad))

Comparison tests of two rolling stocks with respect to the aptitude for shunting

Then there are some additional recordings or statements:

- Track geometry (recordings made the week before the tests) - Rolling stock dynamics (recordings of car body vertical and transversal accelerations) - Weather conditions (temperature, hygrometry, state of rail, rain, …) - Voltage at track circuit receiver (on 3 zones) - Quality of rail surfaces (roughness, chemical composition, video with strong enlargement) - Preceding circulations - Rail-wheel contact video

Records for two types of rolling stocks (X1 and X2)

The shape of the residual voltage curves is nominal in the case of X1 and very specific in the case of X2. In the significant case of X2. When the train moves from transmitter towards receiver, the residual voltage increases. This residual voltage seems limited by an "imaginary" curve, representing the tendency. The residual voltage curve also presents peaks locally, showing a changing of the track comportment, as the peaks are always located at the same places between the runs. Thus, to understand what these two elements were due to, investigations have been performed as mentioned above. The results are relatively poor, concerning the track point : The residual voltage is related to the distance between train and transmitter, and thus seems in connection with the power available at the contact. The local brutal variations are clearly related to the track. It could be explained by differences of:

- composition of third body = > the analysis of composition did not reveal anything particular between several zones

- thickness of third body, = > these analysis could not be made on site. Analyses were made on pieces of rail cut and taken away. Differences exist on the thickness, but also on cohesion, but it is difficult to establish a clear link to residual voltage

View of the third body at sweeping electron microscope ( © INSA Lyon, France)

- dynamics of the train, in particular the slips => the slips were not evaluated during the test, - geometry of the track = > the analyses made did not show particular correlations with residual

voltages.

- Roughness = > the 3D roughness analysis did not show significant differences between the zones. Between the trains X1 and X2, some differences exist, but it is difficult to link them to a difference of residual voltage.

3D roughness analysis for trains X1 and X2

The global variation of the residual voltage is not due to local conditions of the track, but linked to the distance between test train-set and the track circuit transmitter. This comportment is due to combination of wheel and rail pollution, and particularly metal oxides. We will see in the results of another test that non-linear phenomena are involved.

Comparison of several brake blocks in different voltage configurations

The objective of this series of tests is to determine the significant parameters influencing the rail-wheel contact impedance (in this test, wheel and rail are not moving relatively to each other). The controlled parameters are the following :

- Type of brake block (Cast iron P10, composite 1 (not suitable from shunting point of view) , composite 2 (suitable) )

- Frequency (20Hz, 2 Khz, 10 Khz, 50 Khz) - Voltage (from 0 to 4V) - Wheel load (from 10 to 100 kN)

From a new wheel, this wheel is given a surface which quality is characteristic of the brake block in test. The brake blocks are then replaced by pieces of rail, one on each side of the wheel. A electrical current then pass through the two rails via the wheel. Voltage and current are recorded. These operations make it possible to consider the impedance of two wheel-rail contacts (real and imaginary part). Each configuration of parameter is tested on a distinct contact point, in order not to influence a measurement by preceding measurement. The same test was reproduced for a wheel deposited from train X2.

Current -Voltage curves

In static, the impedance of the double rail-wheel contact oscillate between 0 and 1.5 ohms. The main difference between the “good” brake block and the “bad” one is reproducibility. Indeed, the best block (from the shunting point of view) is the cast iron. It has a low impedance, and with each test the same impedance (for a set of identical parameters). Composite 2, also suited, has an average impedance slightly more significant, and a very correct reproducibility, although slightly worse than the cast iron one. The last block (composite 1) as well as the wheel of X2 presents very variable impedances for identical sets of parameters. Moreover, for high levels of impedances, a semi-conductor comportment is observed, which has a good correlation with the line tests. Complementary tests have been performed to

understand the differences of comportment (chemical analysis, 3D roughness, metallurgic analysis). None of these analysis gave any significant results to explain differences.

In line test : track circuit parameters influence on shunting, with X2 train-set

Last test is a complementary test for the two first ones. A laboratory track circuit is installed. One controls the following parameters :

- Transmitter voltage (from 4 to 12V) - Track circuit length (400 and 700m) - Transmitter frequency (2000, 4000 and 6000 Hz) - Speed of the train : 30 km/h

To compare, fixed shunts are moved along the track circuit with following impedances (0.15, 0.25, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 5 and 10 ohms). The residual voltages for these shunts are the following :

We can see on the figure that the residual voltage is almost constant as the fixed shunt is moved along the track from the transmitter to the receiver. This is not the case for the train in test (red curve):

the two main elements are :

- In terms of impedance, the impedance of the train is not constant when the train-set moves from transmitter towards receiver. By comparing with the residual voltage records made by moving the fixed shunts along the track, the resistance of train-set can be estimated to 0.25 ohms on the transmitter side, and up to 10 ohms on the receiver side in the worst case. Actually, the train cannot be considered as a simple impedance, but like a diode (semi-conductor).

- Whatever the track circuit

adjustments (voltage,

frequency, length, …), the residual voltage at the receiver is always about the same. Interpretations : the behaviour seems to be that of two diodes in parallel heads spades, or of a relatively symmetrical Zener diode. Physically, that is not easily explainable all the same, because the track and wheel chemical compositions are relatively homogeneous, while a diode contains a junction PN, pure silicon doped N or P.

Third body modelling

map of potentials in the third body model

The objective of this model is comprehension of the de-shunting phenomenon. This includes the study of the flow of matter in the wheel rail interface (third body). One will thus be interested particularly in the way the electrical current can (or can not) go from one first body to another (from the wheel to the rail), through the third body, and those according to macroscopic parameters (mechanical requests: load, periods of bearing / slipping, etc...), and microscopic (mechanical and electrical particles composing the third body characteristics). We should define the electric properties locally. To go from the first “first body” to the other, it is necessary to find a way (in fact : several ways), where current can go through. When two particles are in contact, they exchange a interaction force, which in the model is related to an interpenetration (representing a "deformability", very low). We suppose that the intensity of the current which can pass between two spheres is proportional to the contact area, and thus with the interpenetration. This one thus represents a local conductivity. If one represents our third body by an electric diagram, one will regard the centres of the particles as nodes of the network, while the branches will be defined by the various contacts between these particles.

Among the particles of the third body, it is possible to define a proportion of insulating particles , the other particles being conducting. The following figure shows the effect of this proportion of insulating particles on the total resistance of the contact.

From 0 to 20% of insulating particles, the contact behaves as if all the particles were conducting (the resistance network is not modified too much). From 20 to 60 % of insulating particles on the other hand, the total resistance of the contact grows quickly. While increasing the number of insulating particles, some conducting ways for current are broken or lost. And beyond 60 % of insulating particles, the contact is overall insulating. The remaining ways for the current passing are in insufficient number.

Conclusions

Electric study of piezzo effects is going to be undertaken : under the very strong constraints in the wheel-rail contact, elements may have an electric piezzo behaviour. However, it is probable that this effect is not at the origin of the total effect observed on the residual voltage curves (exponential form). That could on the other hand explain changes of behaviour during braking or complete stop. Semi-conductor particles will be included in the model. With clearly defined laws of conduction, initially, conduction is null until a threshold, then linear increasing beyond that threshold. setting the percentages of semi-conductor particles and thresholds, one must be able to obtain curves of impedance function of the tension applied. These data will be used in input of a track circuit model. Simulations will also make it possible in the long term to explain macroscopic effects :

- explanation of the variations of residual voltage according to the static wheel load - evaluation of the effect of the roughness of wheel and rail - Simulation of braking (slip) - under test, a reduction of residual voltage is observed during the

braking, which can be explained by an impedance decrease. There are two possible explanations : the relative mass increases, or the relative wheel-rail slipping is more significant.

- Simulation of complete stop - under tests, at the time of a complete stop, the residual voltage is null, which shows that impedance tends towards 0. (it is necessary to know if the reason comes from a mechanical or electrical effect)

- Study the influence of the wheel diameter . Generally, all the studies made show that the wheel rail contact and more generally shunting is a very difficult problem to understand, even with the modern means of study. Indeed, if variable behaviours are

observable during various tests, the complementary investigations that have been carried out ( roughness, physicochemical or others ) show with difficulty significant differences between the cases. It is why the granular model should in the coming months make it possible to improve the knowledge of the electrical and mechanical wheel rail contact, One should however know that model remains a digital model, and consequently not completely physical.

References

[1] Rapport ORE – Question A4 : Sensibilité au shuntage des circuits de voie : Principes scientifiques du contact électrique entre deux corps solides comportant des surfaces oxydées. (1960)

[2] Rapport ORE – Question A4 : Sensibilité au shuntage des circuits de voie : Contact électrique entre

rail et roue en rotation sur des surfaces oxydées et Impédance des essieux en fonction de la fréquence. (1960)

[3] Rapport ORE – Question A4 : Sensibilité au shuntage des circuits de voie : Rapport final. (1960) [4] IORDANOFF I. Modélisation du comportement tribologique des troisièmes corps – Interaction avec

les premiers corps et modélisation des contacts. Habilitation à Diriger les Recherches, INSA Lyon, Villeurbanne, décembre 2004.

[5] FILLOT N. Etude mécanique de l’usure – Modélisation par éléments discrets des débits de troisième

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influence of adhesion and particle size on the macroscopic behavior of the contact. ASME Journal of Tribology, 2002, vol. 124, pp. 530-538.

[7] IORDANOFF I. , KHONSARI M.M. Granular lubrication: towards an understanding of the transition

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ASME Journal of Tribology, 2004, vol. 126, n°3, pp. 606-614. [9] FILLOT N., IORDANOFF I., BERTHIER Y. Simulation of wear through mass balance in a dry contact.

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