Sponsor: Trafikverket/JVTC, LKAB
Researchers: Jens Hardell, Braham Prakash
The friction (also called adhesion) that occurs between the wheel and rail sets the limitations for train operation. If friction is too low it can result in wheel slippage, poor braking performance, deterioration in running stability, lateral forces, wheel climb up, derailment and rail corrugations. A friction level which is too high will, on the other hand, result in increased wear of the wheels and rail, higher energy consumption, reduction in train speed and track degradation. Some studies suggest that close to €100 million annually is lost due to inadequate friction management of the wheel-rail system.
Friction in the wheel/rail contact occurs between the wheel tread and top surface of the rail and also between the wheel flange and gauge face of the rail. It is desirable to have a relatively higher coefficient of friction between the wheel and top of rail to ensure sufficient traction for acceleration and braking. On the other hand, friction should be minimised between the wheel flange and gauge face of the rail in order to reduce wear and improve the overall efficiency.
Friction control, or friction management, involves the application of an engineered composite layer at the wheel/rail interface to optimise the friction behaviour. At the wheel tread-top of rail contact it is common to apply some polymer based compound through spraying or direct transfer from sticks to achieve a desired friction level. At the wheel flange-gauge face of the rail it is common to apply a lubricant such as oil or grease to minimize friction and wear.
Despite several studies on various friction modifiers there are still some existing knowledge gaps. Very little work has been carried out to investigate the performance of friction modifiers in cold climate and heavy haul transport. Important questions to address are e.g. the optimal friction value at low temperatures, application techniques, optimal surface topography for application of friction modifiers and techniques for measuring friction online/on-board.
There are strong incentives to optimise friction from a performance, safety and operational point of view; especially for heavy haul transport in cold climate.