# Observation of Branly's effect during shunting experiments on scaled wheel-rail contacts

Hugues Chollet, Frédéric Houzé, Philippe Testé, Florent Loete, Xavier

Lorang, Sébastien Debucquoi

► To cite this version:

Hugues Chollet, Frédéric Houzé, Philippe Testé, Florent Loete, Xavier Lorang, et al. Observation of Branly's effect during shunting experiments on scaled wheel-rail contacts. 9th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems, Aug 2012, Chengdu, China. pp.111-114. hal-00778780

HAL Id: hal-00778780

https://centralesupelec.hal.science/hal-00778780v1

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# OBSERVATION OF BRANLY'S EFFECT DURING SHUNTING
## EXPERIMENTS ON SCALED WHEEL-RAIL CONTACTS

H. CHOLLET ¹)*, F. HOUZÉ ²), P. TESTÉ ²), F. LOËTE ²), X LORANG ³), S. DEBUCQUOI ¹)

1) IFSTTAR, Université Paris Est,
Boulevard Newton, Champs sur Marne, F-77447 Marne la Vallée Cedex 2, France.

2) LGEP, UMR CNRS-Supélec 8507, Universités Paris-Sud 11 et UPMC,
11 rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France

3) SNCF, Direction de l'Innovation et de la Recherche, Département Physique Ferroviaire et Confort,
40 Avenue des Terroirs de France, F-75611 Paris Cedex 12, France.

* hugues.chollet@ifsttar.fr

## ABSTRACT

At the demand of SNCF, an experimental study of electrical circuit shunting phenomenon has been led together by LGEP-Supelec and IFSTTAR on a scaled rolling test bench. The electrical measurements have shown a non linear behavior identified as a Branly effect.

## 1 INTRODUCTION

The shunting principle is used in track circuits to detect the presence of the trains. This principle is applied in many countries including Japan and France. The quality of the electrical contact between the wheels and the rails is necessary to safely detect the trains. ORE [1] and SNCF are doing a regular survey of this safety component. Field experiments are made systematically on new rolling stocks, whose behavior is controlled at full scale, because for noise considerations, these modern rolling stocks are designed to reduce the wear of the contact area, thus reducing the cleaning effect of creepages and the surface damage due to roughness. In some extreme cases, on poorly frequented sections, and on straight lines where the creepages do not clean the surfaces by abrasion, a very regular voltage drop is observed looking like a diod behavior. It seems strange that several real dirt contacts could behave like a finely elaborated solid state silicium diod, consequently this phenomenon led to the present research.

In this paper, it is intended to verify if a scaled laboratory experiment could be useful for the

fundamental understanding of this particular shunting behavior.

INRETS (now IFSTTAR) used to scale mechanical experiments from a long time [2] but here it is combined with electrical considerations. The paper will present the basis of the scaling strategy, then an observation of some non linear electrical results, a comparison to field experiments, leading to the confirmation of a Branly effect in the rolling contact, and a conclusion.

## 2 SCALED EXPERIMENTS

### 2.1 The similarity laws

The similarity laws applied in this study were proposed at the same time by Zobory [3] and Giovachini - Pascal, presented later by H. Chollet in his PhD [2]. They are firstly based on dimensional analysis with a constraint on materials : as far as rolling contact behavior is considered, it is particularly important that pressure, creepages and structural stiffnesses are scaled in a controlled way. This is assumed by the use of identical materials at both sides
of the contact : wheel and rail steels.

<table><caption>Table 1. Initial Scaling Factors</caption><thead><tr><th>parameter</th><th>(unit)</th><th>scaling factor</th></tr></thead><tbody><tr><td>material</td><td></td><td>identical</td></tr><tr><td>stress</td><td>N/m<sup>2</sup></td><td>1/1</td></tr><tr><td>pressure</td><td>N/m<sup>2</sup></td><td>1/1</td></tr><tr><td>length</td><td>m</td><td>1/4</td></tr><tr><td>angle</td><td>[]</td><td>1/1</td></tr></tbody></table>

Similarity laws. Initial conditions on pertinent parameters.

The main scaled factor is the length, reduced here by a factor ¼ chosen as a good compromise between contact size reduction and bench cost reduction.

From the conservation of the stresses, and from the reduction of volumes with the same material density, it comes down a condition on time [N] = [kg m s⁻²]. The time is reduced by a factor 4. This means, even if it is not used there, that the frequencies of the vibrations are amplified by a factor 4, a consideration that can be also made on the dynamic model of a harmonic oscillator [2].

<table><caption>Table 2. Resulting Scaling Factors</caption><thead><tr><th>parameter</th><th>(unit)</th><th>scaling factor</th></tr></thead><tbody><tr><td>volume</td><td>m<sup>3</sup></td><td>1/64</td></tr><tr><td>mass</td><td>Kg</td><td>1/64</td></tr><tr><td>time</td><td>s</td><td>1/4</td></tr><tr><td>frequency</td><td>Hz</td><td>4</td></tr><tr><td>speed</td><td>m/s</td><td>1</td></tr><tr><td>acceleration</td><td>m/s<sup>2</sup></td><td>4</td></tr><tr><td>force</td><td>N</td><td>1/16</td></tr></tbody></table>

Similarity laws. Resulting conditions on other parameters.

This condition on the time lead itself to the conservation of the translational speeds, to the amplification of the accelerations, etc (table 2).

A particular acceleration is the gravity. As it is a field with a constant value on the earth surface, it is not scaled correctly, thus, an additional external force must be applied on the wheelset suspension system in order to complete the gravitational force. With the

mass on rail of only 32 Kg, a large part of the scaled weight must be applied by a mechanical system, visible on the top of the loading frame ( figure 1). Between the full scale and the reduced scale, the load on rails comes from 120 kN to 7500 Newtons for a typical experiment.

Different similarity laws have been proposed by other authors [4, 5], but here, where it could be important to reproduce the mechanical resistance of an expected oxide layer, it seemed the most adequate.

With this similarity strategy, it can be verified on the basis of Hertz' theory with the conservation of Young's modulus, that the contact patch dimensions are reduced in the same factor than other length, while the conservation of stresses implies the conservation of the contact pressure. a=1.7mm and b = 1.5 mm in a typical experiment.

A modification of the elliptic ratio, typically 20 % is introduced by the fact that experiments are led on a test bench, on which the rails are curved; this is not due to the similarity laws.

These mechanical similarity laws have been applied by H. Chollet for railways critical speed studies of non linear bogies. About wheel rail contact applications, considering Kalker's theory, the same similarity is observed for tangential forces resulting from the eventual creepages. Kalker's coefficients, by example, are non dimensional and are reproduced by scaled experiments, results which were presented in Cambridge at the CM conference in 1990 [5].

This scaling experimental method will be used as a mechanical basis in this study, extended to electrical parameters scaling.

## 2.2 The electrical similarity

The electrical similarity is not so clearly established. It can be based on several strategies: by example similarity of the current density for thermal considerations, or similarity of the striction resistance in the contact :

$$R= \rho /2a$$
However this expression is based on a perfectly smooth contact, while the electrical effect observed hereafter can be influenced by the roughness.

Two mechanisms are proposed by Holm [6] in order to explain conduction through oxide layers : fritting type A is a supposed fusion of metal columns through some oxide cracks; fritting type B is a supposed direct conduction by roughness summits piercing the oxide layer on some parts. Both does not lead to simple expressions that can be scaled by advance.

## 2.3 The test bench

One of IFSTTAR's test benches using this similarity theory has been used in this project. The scaled test bench is composed of a steel wheelset scaled at 1:4 ratio, applied on a roller equipped with two discs representing the two rails.

<figure><img src="image_2.png" /><figcaption>Fig. 1. View of the test bench. A vertical force is applied on the scaled wheelset in order to reproduce the contact conditions.</figcaption></figure>

The rigid wheelset used for these first experiments was simplified to 1:10 coned wheels in order to avoid flanging. The wheelset is rolling freely, auto-centered by the dicone, without significative creepages.

The roller representing the rails has been modified by insulating the discs from the rest of the structure.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Wheelset electric scheme. Principle of the four probe technique and localisation of oxidized elements.</figcaption></figure>

A voltage or current is applied by copper fiber brushes pressed against each rail-disc near the mechanical wheel/rail contact area and the current is obliged to pass through the wheelset. On both sides a pair of copper fiber brushes allows the contact voltage measurement. The current value is obtained from the voltage drop at a known resistor terminals (four probe technique, see figure 2). When one contact is isolated, the current drops down and the voltage reaches the generator maximal value, as it can be seen on figure 4.

One of the wheel-rail couples (figures 2, 3) has been corroded in order to reproduce different oxidized states known to create shunting problems. The other rail is used as a clean standard reference.

<figure><img src="image_4.png" /><figcaption>Fig. 3. View of the contacts. The left one has been oxidized by corrosion in a NaCl solution.</figcaption></figure>
# 3 RESULTS

## 3.1 Scaled experiments

Experiments have been led in static and in rolling situations at low speed. The feeding voltage effect and the frequency (continuous, low frequency, or nominal frequency) has been studied.

Figure 4 presents an electrical response showing the dependence to the location on the rail disc. After a loading phasis, the rotation is activated. Periodic variations occurs, mainly following the rail disc revolution.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Reduced scaled measurements. Typical temporal evolution of voltage drop measured on clean wheel-rail contact members (light grey) and rusted ones (darker grey); the left part of the graph corresponds to the static load application, the rest to rolling conditions</figcaption></figure>

On the clean side where it is conducting correctly, the behavior is linear and resistive, with a low resistance (the flat curve on figure 5 bottom).

As the left rail is strongly corroded, insulation occurs a large part of the time, leading to the saturated voltage which can frequently be seen on figure 4.

In intermediate conditions, it has been found that the oxidized rail was showing in some cases a non linear behavior with a voltage offset around 0.4 to 0.7 Volt for the single contact. It can be seen in figure 5 with a

square tooth signal (+ / - 0.4 Volts) on the temporal graph.

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Details of an intermediate conducting phasis; up : function of time, voltage alternances at clean and corroded sides.; down : voltage/current graph for the same extract of an experiment.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Typical plot of the density of occurence probability of V-I values for the rusted wheel-rail contact during a rolling test at a normal frequency, for an other experiment (+ / - 1 A).</figcaption></figure>

For a complete experiment of several seconds, a density representation is necessary (figure 6) showing a dark line, representative of a mean linear behavior, and a grey area, whose higher border is representing the occurrences shown in figure 5.
## 3.2 Comparison to full scale experiments

Field experiments are led on an autorail equipped with a current feeding system (SNCF design, figure 7) applied near the rolling surface.

<figure><img src="image_9.png" /><figcaption>Fig. 7. Full scaled measurements on one side. The experiments are led on special tracks with light circulation.</figcaption></figure>

Scaled experiments seems to be very close to some full scaled results. In figure 8, where the density of points is presented, the two electric behaviors of figure 5-6 can be found : a part of the time, the resistance is low, consequence of clean surfaces, while an other significative part of the time, the voltage drops to 0.4 / 0.5 Volts before the current could be established, close to the corroded situation leading to figures 5 and 6 on the test bench.

Consequently, the same electric effect is observed on scaled and full scale experiments. Scaling does not seems to have an effect by itself, however, it must be recalled that the scaling strategy implies the conservation of materials and pressures.

<figure><img src="image_10.png" /><figcaption>Fig. 8. Full scaled measurements. Typical plot of the density of occurrence probability of V-I.</figcaption></figure>

This intermediate behavior is identified as being a Branly effect, on oxide layers of over 100 Å. The French scientist Branly discovered this non linear behavior of metallic oxidized powders at the end of the 19th century and used it in semi-conductor elements named "Branly's coherers" before the discovering of the more efficient transistor.

The effect has then been forgotten for a long period before being recently reconsidered. Falcon and Castaing [7] made an experiment with oxidized steel balls, and found that the voltage needed to establish the current was proportional to the number of successive contacts.

It can be deduced that on field, consisting of two interfaces, a full scale wheelset will lead to a 0.8 to 1.5 Volt hysteresis if both wheel/rail contacts are simultaneously oxidized, which could be the case.

A last consideration could be done on the oxide layer thickness and its generation. Of course, this phenomenon is observed on lowly frequented lines, and more frequently in areas with an oceanic climate. The characterization of the oxide thickness and growth has been done using different techniques [8]; it could be the next step of the present research.

## 4 CONCLUSION

In certain conditions, track circuit may be prone to a thin oxide layer leading to a particular non linear electric behavior. This phenomenon is similar to Branly's observations on oxidized metal powders at the end of 19th century and recently reproduced on linear chains of steel balls. Consequently, knowing this regular voltage drop, it is possible to deduce the presence of a wheelset on the track circuit even with this increase of the voltage.

Scaled experiments proved to be pertinent for the laboratory observation of this electrical phenomenon. The mechanical consideration taken into account by the similarity laws applied on the test bench are giving a better confidence on the mechanical
solutions which can be found to reduce this thin layer
in order to come back to a fully conducting system.

ACKNOWLEDGEMENTS

This research, in cooperation between
SUPELEC-LGEP, SNCF and INRETS, has been
actively supported by SNCF.

REFERENCES

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« Principes scientifiques du contact électrique entre
deux corps solides comportant des surfaces oxydées »,
1960

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effort tangents au contact roue-rail", PhD thesis,
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[3] I. Zobory, A. Szabo, "Small scale Test of the Force
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[4] A. Jaschinski & all, "The application of Roller
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[5] Y. Suda, T. Miyamoto, "Active Controlled Rail
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[6] R. Holm, "Electric Contacts – Theory and
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[7] E. Falcon, B. Castaing, “Electrical conductivity in
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experiment”, Am. J. Phys. 73 (4), pp. 302-307, 2005

[8] Nakahara & all, "Relationship between surface
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