Laboratory investigation into wheel/rail adhesion.

Wheel/rail adhesion is affected by the contamination that is present on the railhead. This can be broadly divided into three categories oil, water and solid debris. The frictional phenomena associated with each of these groups were examined on a variety of laboratory simulation rigs. The interactions between each group were then explored so that a comprehensive description of wheel/rail adhesion could be established. Concepts of boundary lubrication can be used to describe the low friction of surfaces contaminated by oil and/or water. However these concepts are shown to have their limitations when solid debris is trapped in the contact area and either significantly lowers adhesion when mixed with small quantities of water under dynamic conditions, or increases the coefficient of friction by adsorbing the oil. There is insufficient oil on most track to cause low adhesion even when the rails are wet. Under some specific circumstances debris helps form thin layers that cover the wear band and are weak enough in shear to reduce adhesion and cause wheel slip. Leaves readily form such films although the most common constituents are rust and water. Laboratory experiments have shown that low adhesion can be caused by mixtures of rust and water, and a theoretical explanation for this has been developed based on their rheological properties. It is concluded that low wheel/rail adhesion is usually caused by a viscous paste formed of solid debris and small quantities of water. It is because Britain has such a cold, damp climate that wheel/rail adhesion on BR is such a problem.