Friction phenomena in rolling contacts caused by suspensions

Friction plays an important role both in nature, e.g. in avalanches and landslides, and in technology. Even in everyday life, we can hardly avoid the topic of friction. Examples of this are walking safely on various surfaces, e.g. asphalt, ice, gravel, or steering vehicles, where the friction in the tyre-asphalt contact has a significant influence on driving dynamics and therefore driving safety. Friction is also of great importance in railway service. Friction is necessary in the contacts between the wheels of railway vehicles and railway tracks both made of steel in order to transmit the required traction and braking forces. These are necessary for reliable and, above all, safe railway operation. Under normal conditions, the friction in the wheel-rail contact is sufficient for this, but there are conditions in which very low friction values can occur. This makes the aforementioned reliable traction and safe braking more difficult, which makes the additional use of technical aids, such as the introduction of sand into the wheel-rail contact, necessary. One example of the occurrence of very low coefficients of friction is autumn conditions, in which leaves fall onto the rail surface and mix with water, creating friction conditions that come very close to lubricated conditions when a railway vehicle passes over. Another example is the so-called "wet- rail" phenomenon. In this case, small wear particles from wheels and rails, iron oxides and other solid particles mix with small amounts of water to form pasty substances on rail surfaces, which can also lead to such undesirably low friction coefficients. The unpleasant thing is that, in contrast to the low friction coefficients caused by leaves, this phenomenon occurs much more randomly and is therefore much more difficult to control technically. One reason for this is, for example, the natural evaporation of the water content, as a result of which the concentration of solid particles can change permanently over time and along the track. The aim of the project is to investigate this "wet-rail" phenomenon and thus gain a better understanding of it. To this end, innovative laboratory experiments are combined with physics-based simulation models in which the dynamic interaction between solid particles and liquids between rough wheel-rail surfaces is considered. This project is based on an international co-operation. The experiments are carried out at the Brno University of Technology, while the development of the simulation models is located at Virtual Vehicle Research GmbH. The results of the project should make a significant contribution to research into complex friction effects that goes far beyond the specific application of wheel-rail contact.