Leaf-related adhesion problems have been present in many railway networks all over the world in the last few decades. Since the early 1970s many measures have been undertaken in order to mitigate the problem. One of the measures adopted by many railway networks is the use of friction modifiers. However, the low adhesion problem still persists. Furthermore, the effectiveness of these friction modifiers has not well proven yet due to the lack of research in controlled conditions. Consequently, the rolling stock operators and infrastructure managers do not clearly understand the performance and side effects of the friction modifiers used on their networks. In this paper, an investigation of the performance of two existent friction modifiers in controlled laboratory conditions is presented. These friction modifiers have been used or tested in several railway networks. A twin-disk roller rig has been used to study their performance in leaf contaminated contacts. The adhesion characteristics of both friction modifiers are examined for different slip ratios. The constituents of the friction modifiers are identified and the solid components are analyzed. In addition, damage that these friction modifiers may cause to wheel and rail is also discussed.

Leaf contamination has been identified as the major cause of low adhesion incidents occurring on some railway networks in the last few decades. In the presence of leaf layers, the trains cannot have the required adhesion at the wheel—rail contact for adequate traction and braking operation. Under these circumstances, not only the punctuality but also the safety of the railway transportation can be threatened. In order to mitigate low adhesion problems, railway organizations have opted for different measures, particularly during the season of Autumn. The most employed measure consists of bringing sand to the wheel—rail interface, which can be performed by means of air-pumped sanders or in the form of sand-based friction modifiers. Although sand has widely been accepted as an effective adhesion improver, the effect of some sanding parameters on the adhesion improvement in leaf-contaminated contacts seems to be unclear. This hinders the possible optimized use of sand on the railway networks. In this paper, the influence of the number of sanding axles, particle size of sand, and wheel slip on the adhesion recovery in leaf-contaminated wheel—rail contacts is presented. Rolling—sliding tests under closely controlled conditions have been performed on a twin-disc roller rig. An electrical circuit has been connected to the rig for monitoring the effect of contamination on the electrical conductivity across the wheel—rail contact. The results show that the application of sand contributes to removing the leaf layers from the disc surfaces, which leads to a higher adhesion coefficient in comparison with the untreated (baseline) situation. Accordingly, the electrical conductivity across the wheel—rail contact is also improved. Furthermore, the adhesion recovery is shown to become larger and faster with the increase in sanding axles and wheel slip. Among the particle sizes tested in this work, medium particles are found to yield the most effective adhesion recovery.

No abstract

A popular practice is the application of friction modifiers to increase the adhesion level between wheel and rail under different contamination conditions.Particularly, two friction modifiers have been used or tested in several railway networks as adhesion enhancers to facilitate the traction and braking operation under poor adhesion conditions. However, the railway operators and infrastructure managers only count with practical observations that do not elucidate completely the effectiveness and side effects of these adhesion enhancers. In this paper, a twin-disk roller rig has been used to study their performance in dry and wet contacts under closely controlled laboratory conditions. The adhesion characteristics of both friction modifiers are examined for different slip ratios. The constituents of the friction modifiers are identified and the solid components are analyzed. Furthermore, the wheel and rail disks are examined after a series of dry tests to analyze the mass loss, surface damage, modification of surface hardness and roughness, and subsurface deformation caused by the friction modifiers compared to dry contacts.

Locomotives of railways worldwide have been using sand since 1838 to improve the wheel-rail adhesion during traction and braking operations. In more recent years, sanders have also been fitted to electrical and diesel multiple units in some railways to fight (often in combination with traction control or wheel slide protection systems) the low-adhesion conditions, especially encountered in autumn due to leaf contamination. In spite of the worldwide broad use of sand, different standards on sanding practice appear to be used by different railways, while there is a lack of fundamental understanding on the influence of sanding parameters, such as particle size distribution, feed rate, and number of sanding axles (among others), on the adhesion recovery, wear, and train detection. In order to gain a better understanding of these interrelationships, the authors have carried out laboratory investigations in recent years. As a continuation of that work, the influence of the sand particle size on the adhesion recovery in leaf-contaminated contacts is investigated in this article by means of traction tests of an electrical locomotive in a stabling yard. Three differently sized silica sands are used in the testing. In addition to the particle size investigations, the standard sand currently used in the Dutch railways is also tested to quantify its effectiveness against leaf contamination. Besides the immediate adhesion improvement upon sanding, the remaining friction level left for subsequent tractive wheel passages is also investigated for all sands tested. Furthermore, baseline tests (i.e. without application of sand) are also performed to obtain some quantitative insight into the impact of leaf contamination on the wheel-rail adhesion in the field.