The low adhesion problem: The effect of environmental conditions on adhesion in rolling-sliding contact
Introduction
Low adhesion between rail head and wheel tread is one of the major problems for railways in many countries all over the world. This phenomenon has a negative impact on cost, performance, and safety. The term “low adhesion” or “poor adhesion” is usually associated with the autumn season when a slippery layer from crushed fallen leaves is formed on the track. Both laboratory [[1], [2], [3]] and field research [4,5] revealed that this layer can result in the coefficient of adhesion (CoA) lower than 0.15, in some critical cases even lower than 0.05 [6]. Although it is a well-known fact that leaf contamination causes serious problems in railways all over the world; it must be emphasized that fallen leaves are not the only cause of low adhesion incidents. Besides fallen leaves, there are other causes of low adhesion which are mainly related to environmental conditions.
Water can be considered as one of the most common contaminants influencing adhesion in the wheel-rail contact. Water in the field can be found in various forms such as morning dew, fog, and light or heavy rain. These different forms may lead to different adhesion levels. In the case of bulk water, CoA can take values between 0.05 and 0.5 [[7], [8], [9], [10], [11], [12]] depending on speed, roughness, and other parameters. More significant adhesion drop can be expected for slightly wet conditions, which usually occurs due to dew or light rain. Beagley et al. [13] observed that a small amount of water from condensation decreased CoA to 0.22. Even more critical case was found when the rail was not free of solid particles. This combination of a small amount of water and solid particles (such e.g. wear debris) led to the formation of a viscous paste, which provided low (<0.15) [14] or even very low CoA (<0.05) [15].
In the case of weather conditions, daytime evolution of relative humidity (RH) and temperature can have a substantial impact on CoA. Beagley et al. [13] showed that CoA was reduced from 0.55 to 0.22 with increasing RH, while the effect of temperature was rather negligible during these tests. Similar findings were reported by Olofsson et al. [16] where the effect of RH on CoA was studied for dry and leaf contaminated contact using the pin-on-disc apparatus. It was found that the coefficient of friction (CoF) was reduced to 0.37 (dry) and 0.27 (leaf) when RH reached 95%. A pin-on-disc device was used also by Zhu et al. [17,18] who investigated the effect of RH and temperature on CoF for clean and rusted specimens. These complex studies showed that rusted discs generally led to lower CoF than found for clean discs; however, the lowest observed adhesion was still 0.4 or higher for both disc types. The lowest values of CoF was observed when RH reached 70%. A subsequent increase in RH did not lead to a further decrease in CoF.
Previous research works have shown that adhesion/friction is very variable depending on contaminants and current weather conditions. It means that low adhesion problem can happen anytime and anywhere. Although a decrease of CoA/CoF was observed in all above-mentioned studies, low CoA or CoF (<0.15) was found predominantly when contact was contaminated with leaves. However, White et al. [19] reported that many of low adhesion incidents in a real operation were not associated only with leaf contamination but there were other factors leading to low adhesion incidents. These authors suggested several reasons why these low adhesion incidents happened, such as due to the presence of water, moisture or not detectable leaf layer. The conditions and mechanisms leading to low adhesion phenomenon are not fully understood.
The main objective of this study is to reveal conditions when low adhesion (μ≤0.15) and very low adhesion conditions (μ≤0.05) can be expected in the wheel-rail contact. Special attention is paid to the effect of RH, temperature and leaf contamination. For this purpose, a ball-on-disc tribometer with a climate chamber is employed. This contact configuration is chosen because it enables to set typical rolling-sliding conditions occurring in the wheel-rail interface. Based on the previous studies, it is assumed that pure sliding configuration, where a pin is in permanent contact with the counterpart, is not sufficiently representative in terms of the formation and action of the third-body layer. The presence of this layer is important to study low adhesion problem.
Section snippets
Test setup and specimens
Adhesion measurements were conducted on the ball-on-disc tribometer (Mini–traction–Machine, PCS Instruments) where a 19.05 mm steel ball and 46 mm diameter flat steel disc was loaded against each other as is depicted in Fig. 1. Both these specimens were independently driven, thus a rolling-sliding contact (where the slip was accurately controlled) can be achieved. The slip is defined in Eq. (1), where wball and wdisc are the angular speeds of the ball and the disc respectively and rball and r
Reference tests under dry and wet conditions
To obtain reference values of CoA, adhesion characteristics under dry and wet (fully-flooded) conditions were measured for four different mean speeds (Fig. 2 and Fig. 3).
Fig. 2 shows data from the tests ran in dry conditions where CoA reached the typical values for non-lubricated rolling-sliding contact operating in laboratory conditions [9,20]. The results also indicate that there was almost no significant change in CoA as the speed increased; however, the tested speed range used in this study
Conclusions
In this work, the ball-on-disc tribometer with the climate chamber was used to identify conditions when low adhesion incidents can be expected in operation due to weather and season changes. The performed tests investigated the effect of several factors influencing adhesion. Unlike previously published articles dealing with the effect of environmental conditions on adhesion, the low and even very low adhesion conditions have been found for several contact conditions. These low and very low
CRediT authorship contribution statement
Radovan Galas: Conceptualization, Methodology, Investigation, Data curation, Writing - original draft. Milan Omasta: Conceptualization, Investigation, Methodology, Writing - review & editing, Project administration. Lu-bing Shi: Data curation, Writing - review & editing, Visualization. Haohao Ding: Writing - review & editing, Visualization. Wen-jian Wang: Conceptualization, Data curation, Writing - review & editing, Project administration. Ivan Krupka: Data curation, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are sincerely grateful to European Commission for the financial sponsorship of the H2020-MSCA-RISE Project No. 691135 “RISEN: Rail Infrastructure Systems Engineering Network,” which enables a global research network that tackles the grand challenge in railway infrastructure resilience and advanced sensing under extreme environments. The authors also thank the support of National Key R&D Program Intergovernmental Key Items for International Scientific and Technological Innovation
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