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Adhesion in the wheel-rail contact
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Tribologi. (KTH Railway Group)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

To attract more customers and compete with other modes of transportation, railway transport needs to ensure safety, punctuality, high comfort, and low cost; wheel–rail adhesion, i.e., the transmitted tangential force in the longitudinal direction during driving and braking, plays an important role in all these aspects. Adhesion needs to be kept at a certain level for railway operation and maintenance. However, wheel−rail contact is an open system contact. Different contaminants can present between the wheel and rail surfaces, forming a third-body layer that affects the adhesion. Prediction of wheel–rail adhesion is important for railway operations and research into vehicle dynamics; however, this prediction is difficult because of the presence of contaminants.

This thesis deals with wheel–rail adhesion from a tribological perspective. The five appended papers discuss wheel–rail adhesion in terms of dry conditions, lubricated conditions, leaf contamination, iron oxides, and environmental conditions. The research methodologies used are numerical modelling, scaled laboratory experiments, and field tests. The research objective is to understand the mechanisms of the adhesion loss phenomenon. 

A numerical model was developed to predict wheel–rail adhesion based on real measured 3D surfaces. Computer simulation indicates that surface topography has a larger impact on lubricated than on dry contacts. Plastic deformation in asperities is found to be very important in the model. Ball-on-disc tests indicate that water can give an extremely low adhesion coefficient on smooth surfaces, possibly due to surface oxidation. Investigation of lubricated contacts at low speed indicates that oil reduces the adhesion coefficient by carrying a normal load, while adhesion loss due to water depends on the surface topography, water temperature, and surface oxidation. A field investigation indicates that leaves reduce the friction coefficient because of the chemical reaction between leaves and bulk materials. The thickness of the surface oxide layer was found to be an essential factor determining adhesion reduction. Pin-on-disc experiments found a transition in the friction coefficient with regard to the relative humidity, due to a trade-off between the water molecule film and the hematite on the surface. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , 31 p.
Series
Trita-MMK, ISSN 1400-1179 ; 2013:15
Keyword [en]
Adhesion, wheel-rail contact, contaminants, tribology
National Category
Tribology
Research subject
The KTH Railway Group - Tribology
Identifiers
URN: urn:nbn:se:kth:diva-133342ISBN: 978-91-7501-896-6 (print)OAI: oai:DiVA.org:kth-133342DiVA: diva2:660774
Public defence
2013-11-22, F3, Lindstedtsvägen 26, KTH, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20131031

Available from: 2013-10-31 Created: 2013-10-30 Last updated: 2013-10-31Bibliographically approved
List of papers
1. Adhesion modeling in the wheel-rail contact under dry and lubricated conditions using measured 3D surfaces
Open this publication in new window or tab >>Adhesion modeling in the wheel-rail contact under dry and lubricated conditions using measured 3D surfaces
2013 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 61, 1-10 p.Article in journal (Refereed) Published
Abstract [en]

Adhesion between wheels and rails plays an essential role in the safe, efficient, and reliable operation of a railway network. Particularly under lubricated conditions, which can be a natural lubricant as water and an applied lubricant as rail oil, trains can experience adhesion loss. This paper presents an adhesion model constructed using the measured 3D wheel-rail surfaces. The numerical model comprises of three parts: a normally loaded contact model; an interfacial fluid model; and a rolling-sliding contact model. Simulation examples use the numerical model to investigate how water or oil contamination might affect wheel-rail adhesion in contacts with different surface roughness levels. Simulation indicates that adhesion peaks are almost at the same creep on different surfaces. The fluid load capacity is inversely proportional to the adhesion coefficient, both of which are clearly dependent on vehicle speed. Oil reduces adhesion coefficient more than water does. The adhesion coefficient on the low roughness surfaces is higher than that on the generated smooth surfaces under oil-lubricated conditions while it is the opposite for water-lubricated contact.

Keyword
wheel-rail contact, adhesion, numerical model, measured 3D surfaces
National Category
Tribology
Identifiers
urn:nbn:se:kth:diva-48770 (URN)10.1016/j.triboint.2012.11.022 (DOI)000317801400001 ()2-s2.0-84871800452 (Scopus ID)
Note

QC 20130212. Updated from submitted to published.

Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2017-12-08Bibliographically approved
2. An adhesion model for wheel-rail contact at the micro level using measured 3d surfaces
Open this publication in new window or tab >>An adhesion model for wheel-rail contact at the micro level using measured 3d surfaces
2014 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 314, no 1-2, 162-170 p.Article in journal (Refereed) Published
Abstract [en]

Railway vehicles require a certain level of wheel-rail adhesion for efficient, reliable, and economical operation. A comprehensive wheel-rail contact model is useful for optimizing the adhesion, to simulate vehicle running conditions and to predict wear and rolling contact fatigue. A new contact model using measured 3D surfaces has been developed, comprising normal contact, rolling-sliding contact, flash temperature, and local friction coefficient models. This model can predict the local contact pressure, including the plasticity, local flash temperature, local tangential stress, local friction coefficient, and global adhesion coefficient. The influence of surface topography, creep, and speed on the adhesion coefficient, real contact area, and contact temperature is discussed. Results indicate that, due to increased contact area, the adhesion coefficient decreases with increased surface roughness, although the change is small. Furthermore, increasing speed reduces the adhesion coefficient due to the increasing contact temperature.

Keyword
Adhesion, Contact temperature, Local friction coefficient, Measured 3D surfaces, Rolling-sliding contact, Wheel-rail
National Category
Tribology
Identifiers
urn:nbn:se:kth:diva-133357 (URN)10.1016/j.wear.2013.11.031 (DOI)000337018100022 ()2-s2.0-84899104604 (Scopus ID)
Note

QC 20140520. Updated from manuscript to article.

Available from: 2013-10-31 Created: 2013-10-31 Last updated: 2017-12-06Bibliographically approved
3. Investigation of factors influencing wheel-rail adhesion using a mini-traction machine
Open this publication in new window or tab >>Investigation of factors influencing wheel-rail adhesion using a mini-traction machine
2012 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 292/293, 218-231 p.Article in journal (Refereed) Published
Abstract [en]

Adhesion in the wheel-rail contact is a key factor determining stable running conditions and safety during train driving and braking. This paper presents an experiment performed in a mini-traction machine to simulate the problems of low adhesion in the wheel-rail contact. Tests were conducted under dry conditions and using water or oil as lubricants to study the influence of surface roughness on the adhesion coefficient. The results indicate that the adhesion coefficient can be reduced to as low as 0.02 for smooth surfaces lubricated with water. For rougher contact surfaces, the water-lubricated tests indicate a higher adhesion coefficient than do oil-lubricated ones, but also a clear dependence on water temperature. The oil-lubricated tests indicate a very slight dependence of the adhesion coefficient on variation in rolling speed, temperature, and surface roughness.

Keyword
wheel rail, adhesion, surface roughness, rolling-sliding contact
National Category
Tribology
Identifiers
urn:nbn:se:kth:diva-48800 (URN)10.1016/j.wear.2012.05.006 (DOI)000308628400024 ()2-s2.0-84864324384 (Scopus ID)
Note

QC 20121031. Updated from submitted to published.

Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2017-12-08Bibliographically approved
4. A field test study of leaf contamination on railhead surfaces
Open this publication in new window or tab >>A field test study of leaf contamination on railhead surfaces
2014 (English)In: Proceedings of the Institution of mechanical engineers. Part F, journal of rail and rapid transit, ISSN 0954-4097, E-ISSN 2041-3017, Vol. 228, no 1, 71-84 p.Article in journal (Refereed) Published
Abstract [en]

Leaves on railway tracks affect the level of adhesion between the wheel and rail, especially in autumn. When crushed by wheels, leaves form a tarnished, low level of adhesion layer that sticks to the railhead and often requires mechanical removal. A Stockholm local traffic track with a long history of adhesion problems was subjected to field tests on railhead contamination. On five occasions under different conditions, spaced over a year, the friction coefficient was measured using a tribometer and samples of the rail were taken. The techniques of electron spectroscopy for chemical analysis and glow discharge optical emission spectrometry were conducted to determine the composition of the top layer of rail contaminants and hardness was measured using the nano-indentation technique. The tarnished layer contains much higher contents of calcium, carbon and nitrogen than do leaf residue layers and uncontaminated samples. These high element contents are generated from the leaf material, which chemically reacts with the bulk material. The hardness of the tarnished layer is one-fifth that of the non-tarnished layer of the same running band. A chemical reaction occurs from the surface to a depth of several microns. The thickness of the friction-reducing oxide layer can be used to predict the friction coefficient and extent of leaf contamination.

Keyword
leaf, blackish layer, wheel/rail, field test, surface analysis
National Category
Tribology
Identifiers
urn:nbn:se:kth:diva-48804 (URN)10.1177/0954409712464860 (DOI)000328825400006 ()2-s2.0-84891056604 (Scopus ID)
Conference
The first International conference on Railway Technology: Research, Development and Maintenance, Las Palmas de Gran Canaria, Spain18-20 April 2012
Note

QC 20131202

Available from: 2011-11-23 Created: 2011-11-23 Last updated: 2017-12-08Bibliographically approved
5. Friction Between Wheel and Rail: A Pin-On-Disc Study of Environmental Conditions and Iron Oxides
Open this publication in new window or tab >>Friction Between Wheel and Rail: A Pin-On-Disc Study of Environmental Conditions and Iron Oxides
2013 (English)In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 52, no 2, 327-339 p.Article in journal (Refereed) Published
Abstract [en]

The coefficient of friction between railway wheels and rails is crucial to railway operation and maintenance. Since the wheel-rail system is an open system, environmental conditions, such as humidity and temperature, affect the friction coefficient. Pin-on-disc testing was conducted to study the influence of environmental conditions and iron oxides on the coefficient of friction between the wheel and rail. The iron oxides were pre-created in a climate chamber. The surfaces of the tested samples were analysed using X-ray diffraction, scanning electron/focused ion beam microscopy, and Raman spectroscopy. Results indicate that the coefficient of friction decreases with increasing relative humidity (RH) up to a saturation level. Above this level, the coefficient of friction remains low and stable even when the RH increases. In particular, when the temperature is low, a small increase in the amount of water (i.e., absolute humidity) in the air can significantly reduce the coefficient of friction. At high humidity levels, a water molecule film can keep the generated haematite on the surfaces, counterbalancing the effect of rising humidity.

Keyword
Friction, Wheel-rail contact, Iron oxide, Environmental conditions, Surface analysis
National Category
Tribology
Research subject
The KTH Railway Group - Tribology
Identifiers
urn:nbn:se:kth:diva-133355 (URN)10.1007/s11249-013-0220-0 (DOI)000326081700014 ()2-s2.0-84890067519 (Scopus ID)
Note

QC 20131031

Available from: 2013-10-31 Created: 2013-10-31 Last updated: 2017-12-06Bibliographically approved

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