Elsevier

Wear

Volume 268, Issues 5–6, 11 February 2010, Pages 784-789
Wear

Tribological study of Fe–Cu–Cr–graphite alloy and cast iron railway brake shoes by pin-on-disc technique

https://doi.org/10.1016/j.wear.2009.12.014Get rights and content

Abstract

A new class of materials is being installed in railway brake blocks to substitute classic cast iron in order to reduce the rolling noise produced by the roughness of the tread-wheel surface. The tribological properties of cast iron and Fe–Cu–Cr–graphite sintered alloy brake shoes were analyzed. Kinetic friction coefficient (μ) and wear were monitored by means of a pin-on-disc technique. The sintered alloy brake showed an increase in μ at higher braking velocities while the cast iron brake exhibited a decrease in μ. Wear was greater on the sintered alloy, explained by its low shear strength which decreased due to its low thermal conductivity. The roughness produced by the sintered brake shoes in wheel-tread surface was 10 times lower than that produced by cast iron.

Introduction

Noise elimination is an issue that is gaining importance on railways. At present, an estimate of one million people in Europe has to be protected from railway noise, by noise barriers and by noise-insulated buildings [1], [2]. One of the main sources of railway noise is rolling noise arising from the contact between the wheels and the tracks. Rolling noise is the dominant source of noise at speeds between 60 km/h and 200–250 km/h [3], [4]. Rough wheels and rough tracks increase the noise emission. Thus, the rougher the wheel surface, the greater the noise produced. Other sources of noise such as aerodynamic friction are only important when high velocities are reached (more than 300 km/h).

The simplest way to reduce the velocity of a railway in motion is through the friction produced between a brake shoe and the wagons’ wheel tread. Cast iron (usually as a gray cast iron) has been the most used material in brake shoes [5]. Cast iron brake shoes have the disadvantage of making the wheel-tread surface rougher during braking [6]. Thus, a reduction in noise should be achievable through the substitution of cast iron brake shoes by other material leading to lower wheel surface roughness.

In fact, over the last years, cast iron shoes have been replaced by composite synthetic brake shoes. It has been demonstrated that cast iron brake shoes make the wheel surface much rougher than a similar product made with a composite material. Replacing the cast iron shoes with a synthetic product can therefore substantially reduce the wagon noise emissions, for example, by about 10 dB for a 100 km/h freight train [6]. Nevertheless, previous experiences using a polymeric composite brake demonstrated that the organic material can be burned, coating the wheel-tread with an organic film. This film diminishes the friction between the wheel tread and rail or brake shoe [7], [8]. Other sintered alloys have been used as an Fe–Cu–Cr–Sn–graphite alloy [9]. These brake shoes exhibited a better braking behavior with less wear on the tread wheel. However, premature wear has been observed in this material, being less cost-effective.

The aim of this work is to study and compare the tribological properties of a gray cast iron brake and a composite sintered alloy brake (Fe–Cu–Cr–Sn–graphite). A pin-on-disc technique is proposed to analyze comparatively the friction coefficient of both materials. Other mechanical analysis and photothermal techniques were performed to discuss the wear phenomena.

Section snippets

Experimental set-up

The samples used in this work were extracted directly from the commercial brake shoes of a locomotive train. Microstructure and chemical composition were analyzed by standard metallographic and energy dispersive X-ray analysis (EDX) on a SEM respectively. The sintered alloy sample showed a composition of 38% Cu, 34% Fe, 3% Cr, 15% C, 4% Sn, 4% O and 2% Al. Graphite, bronze and α-alumina were detected. The cast iron brake shoe showed a gray cast iron structure with graphite sheets in a perlite

Tribological results

The friction coefficient μ were recorded for both brake materials sliding above the steel plate. Fig. 2A shows the μ for cast iron and Fig. 2B shows the μ for sintered alloy. Cast iron curves showed that μ increased during the experiments regardless of the load conditions and velocities studied, unlike sintered alloy, which exhibited a more stable coefficient.

Braking pressures of 0.2 MPa and 0.4 MPa and velocities of 0.31 ms−1 and 1.03 ms−1 were tested. The average friction coefficients for these

Discussion

Cast iron kinetic friction coefficient curves showed that lower friction was measured for higher speeds and forces. Also, for this material, friction depended on the brake distance. However, the brake wear was higher in the sintered alloy. A μ loss measured when the force was increased, was observed in both shoe materials. Our experience in railway brake systems showed us that, in the range of pressures applied, friction force, Fc, shows a linear increase until a value at which the curve is

Conclusions

Pin-on-disc has been a useful technique for the measurement of the coefficient friction, μ, for one brake shoe made with cast iron and another made with a sintered copper alloy in contact with a standard railway wheel material. Although technique did not represent the actual conditions of the wheel-brake systems, the results obtained showed interesting different behavior of both materials studied. These differences were resumed as:

  • 1.-

    Sintered alloy has a constant and 80% higher friction

Acknowledgements

We would like to thank the R&D&I Linguistic Assistance Office, Universidad Politécnica de Valencia (Spain), for translating this paper.

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