Articles
Both surfaces can be described as ‘rough’ and the contact area is typically 2,000cm2 across all four wheels. This leads to excellent grip, but the disadvantage that the rolling resistance can be quite high.
By comparison, a typical rail vehicle with steel wheels rolls on steel rails. Both surfaces look smooth and the typical contact area is tiny, about 8cm2 for a typical eight-wheel carriage. Rail vehicles have low rolling resistance but, unlike road vehicles, the ‘grip’ is comparatively poor.
The contact patch between wheel and rail is not completely understood. The small contact patch resulting from steel wheels on rail is one of the keys to the success of railways but is also a cause of many of its challenges, such as rolling contact fatigue and, the subject of this article, adhesion issues. If there is not a complete understanding of the steel-to-steel contact patch, imagine the extra complication when contamination is added to the interface.
Effect of contaminants
Under ideal conditions, with clean steel surfaces in a dry atmosphere, the coefficient of friction between wheel and rail can be as high as 0.6 to 0.8, but these conditions are rare. Contaminants such as iron oxides, moisture, hydrocarbons and leaves can reduce the coefficient of friction to values as low as 0.01.
UK trains are designed to brake at deceleration rates up to 1.3m/s2, compared with a car at up to 10m/s2, and the railway timetable is designed assuming that trains brake at 0.6m/s2. To confidently deliver this braking performance the coefficient of friction should be in the order of 0.14 to 0.2. On a highly contaminated interface where the coefficient of friction is 0.01, a train will need a much longer distance to stop.
As trains have become lighter, and their dynamic performance has improved, the impact of contaminants, especially leaf fall, has become more serious. The UK has an industry-wide body called the Adhesion Research Group, which is a world leader in tackling this issue. It involves the operators, suppliers and academia in researching, developing, modelling and testing solutions. The key techniques used include:
- Managing the risk: imposing speed restrictions and lower brake rates during highest risk times
- Clearing trees from the line side
- Clearing fallen leaves from the line
- Running special trains that clean contamination from the rails using water jets at 1,000 to 1,500bar followed by the application of an adhesion-enhancing gel which resembles wallpaper paste mixed with sand
- Fitting trains with wheel-slide protection equipment (WSP, similar to a car’s anti-lock braking system) which allows the train to make the best use of the available adhesion without locking the wheels and causing flatted wheels
- Fitting sanders to the trains. Controlled by the WSP, these apply sand between the wheels and rails that roughens the contaminant and improves the braking rate.
Sanding system
The latest sanding system, which is called Double Variable Rate Sander (DVRS), can make a very significant improvement. On track that was unsanded, but set up with low adhesion, a test train took 1,200m to stop from 88km/h. With DVRS this distance was reduced to less than 400m, as good as on uncontaminated rail.
Other techniques include:
- magnetic track brakes, where electro-magnetic friction pads are clamped magnetically to the rail head, and
- eddy current brakes which act rather like a linear induction motor in braking mode, but on the rail head rather than a dedicated reaction rail.
Rail operators will have to continue to manage adhesion. This article has focused on braking, but the issue also affects acceleration.
Want the best engineering stories delivered straight to your inbox? The Professional Engineering newsletter gives you vital updates on the most cutting-edge engineering and exciting new job opportunities. To sign up, click here.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.