Estudios de aceros

29

Wear at the wheel/rail interface when sanding is used to increase adhesion
R Lewisà and R S Dwyer-Joyce Department of Mechanical Engineering, University of Sheffield, Sheffield, UK The manuscript was received on 9 May 2005 and was accepted for publication on 31 August 2005. DOI: 10.1243/095440905X33260

Abstract: Sanding is used in train operation to improve adhesion at the wheel/railinterface during both braking and traction. An experimental study has been carried out to determine the effect of sanding on the wear of wheel and rail materials. Static tests were performed using actual wheel and rail sections. Dynamic tests were carried out with and without sand on a twin disc machine, where wheel and rail steel discs are loaded together and driven under controlled conditions ofrolling and sliding. Sand was fed into the disc contact through a standard compressed air sanding valve. In both static and dynamic tests, sand caused severe surface damage. During the dynamic tests, because of the application of sand, wear increased by factors between 2 and 10. The wheel steel wear rates showed the largest increases. Wear in wet conditions was higher than that in dry conditionsbecause the wet discs entrained a larger amount of sand through the contact that otherwise was ejected when the discs were run dry. The mechanisms of sanding wear have been investigated. Severe plastic flow as well as a high material removal rate and surface corrugation takes place. The rail discs are scored by sand particles that embed and stick in the softer wheel disc. As well as wear by abrasion,the discs were subject to a rapid fatigue process and large chunks of material fractured from the surface. A simple abrasive wear model has been developed to predict wear of rail material caused by sand in the wheel/rail contact, which shows good correlation with test results. There are a number of idealizations inherent in the test simulations that lead to increased severity over the actualwheel/rail contact. These include the amount of sand entering the contacts in both types of test and the disc geometry and motion in the dynamic tests. While the twin disc specimens have been scaled down, the sand is as used on the railway network, which may lead to surface damage appearing more severe than it would be in an actual wheel/rail contact. Results are therefore only to be taken as a guide towhat happens in the full size wheel/rail interface. Keywords: wheel/rail interface, sanding, wear

1

INTRODUCTION

Loss of adhesion between a railway wheel and a track has implications for both braking and traction. Poor adhesion in braking is a safety issue as it leads to extended stopping distances. In traction, however, where it may lead to delays, it is a performance issue. Loss ofadhesion may occur as a result of dew, early morning frost, or leaves on the line [1, 2].

Ã

Corresponding author: Department of Mechanical Engineering,

University of Sheffield, Mappin Street, Sheffield S1 3JD, UK.

Sanding is currently used in train operation to improve adhesion during both braking and traction. In braking, it is used to ensure that the train stops at the shortest possibledistance and occurs automatically when wheel slide is detected and the driver has selected a high brake demand or an emergency brake. Sanding in traction is, however, a manual process. The driver must decide when to apply sand and how much to use. The sand is normally supplied from a hopper mounted under the train. Compressed air is used to blow the sand out of a nozzle attached to the bogie anddirected at the wheel/rail interface (Fig. 1). The
Proc. IMechE Vol. 220 Part F: J. Rail and Rapid Transit

F01905 # IMechE 2006

30

R Lewis and R S Dwyer-Joyce

Fig. 2

Experimental set-ups for (a) static tests and (b) twin disc tests

Fig. 1

Sanding apparatus

sand is usually blown at a constant flow rate, although variable rate sanders are also used. The potential for damage...