SOLUTION

When two solid surfaces are brought into contact, a finite normal force is needed to pull the two solids apart. This force is known as the force of adhesion or simply adhesion. Thus, adhesion is the phenomenon that occurs when two surfaces are pressed together either under a normal load or under a combined normal and shear load.

The coefficient of adhesion is defined as the ratio of tractive effort required to propel the wheel of a locomotive to its adhesive weight. Adhesion traction is the friction between the drive wheels and the steel rail.

Adhesion is caused by friction, with the maximum tangential force produced by a driving wheel before slipping given by:

F_max= coefficient of friction × Weight on wheel

The word “adhesion”, means the coefficient of friction required between the driving wheels and the running surface to carry the tractive effort of interest. A vehicle is said to be “motored to adhesion” when the traction motors will rate continuously all the tractive effort that the maximum available adhesion will support. The available adhesion depends on the weather and many other factors; it always decreases as speed increases. With DC. locomotives, since the dc. motors are light, it is desirable to place the whole of the locomotive weight on driving wheels to obtain the required tractive effort whereas, with A.C locomotives since the weight of electric equipment is higher, the required tractive effort can be obtained by placing only a portion of the weight on the driving wheels and supporting the rest on the trailing axels.

For rapid transit cars having every axle motored, about 5% adhesion is required for each 1 mph/see of accelerating rate. A D.C locomotive can typically attain 28-30% adhesion and an ac locomotive in the range of 33-35% adhesion on the clean, straight rail. On rubber-tired vehicles, adhesion is not a factor of concern.