Slipping and sliding
The Railway Magazine’s chief correspondent ‘gets to grips’ with the reasons behind one of the railways' most controversial and irksome problems
WHILE many people will connect ‘adhesion’ with glues of one sort or another, the Concise Oxford Dictionary gives, as a fourth definition, ‘the maintenance of contact between the wheels of a vehicle and the road’. Even this does not really embrace our use of the word, as it is the flanges which ensure that railway vehicles' wheels stay on the track. In the railway industry, the word means that, at the point where the wheels are in contact with the rail, the two surfaces are not moving relative to each other in the direction of travel.
Loss of adhesion can take two forms: when an axle is being driven, it is possible for the wheels to spin round more rapidly than the vehicle is travelling. Alternatively, when braking, the wheels can ‘lock up’ and skid along the track instead of rotating.
Neither is good news for the railway operator.
In this article, I shall describe the factors affecting adhesion and the problems it can cause with the haulage of trains, although, with modern technology, controlled loss of adhesion can these days be used advantageously under certain conditions.
Friction is generated whenever one flat surface is moved over another. Under a given set of conditions, there is a ‘coefficient of friction’ for every pair of materials which can be measured in the way shown diagrammatically in the figure at the foot of this page. Weights are added to the pan on the right until movement between the two materials starts. The coefficient, usually referred to as u (‘mu’) is defined as the ratio of the force required to make the upper surface move (w) divided by that pressing the two of them together (W). If W equals 100 and w 25, the coefficient becomes 25/100. In the laboratory this is usually expressed in decimal form – 0.25 in the example shown – while in railway parlance it
