Railway rolling -mechanical aspect of traction

All means of transport are based on a physical phenomenon: gravity and buoyancy for navigation, air lift for flight, friction between two bodies for land transport. Human and animal footsteps on the ground are a case in point. Transporting on land involves generating movement along a desired trajectory, thanks to the effort exerted by the lower limbs, balanced by the friction of the feet on the ground.

The ability of humans and animals to carry a load is limited. The invention of the wheel made it possible to build a vehicle capable of considerably increasing this load, thanks to the low friction of the wheels on the ground. This was the development of overland transport. However, this wheel-ground contact does not guarantee the desired trajectory: if the friction force becomes too low, transverse forces, due to gravity or centrifugal force, can cause the vehicle to deviate from its trajectory: this is known as skidding. In addition, if the vertical load on the wheel is too great in relation to the reaction of the ground, the wheel will sink: this is called bogging. Mitigating this risk goes back to Antiquity: streets were carved with "ruts" between the stone slabs, through which the wheels were guided. As early as the Middle Ages, and especially in the 16th century, "rolling and guiding tracks" were developed to distribute rolling loads on the ground and mark out their trajectory. The first metal rails were introduced in England by Richard Reynolds in 1763. The wheels were fitted with a shoulder, called a "flange", to ensure guidance. This made it possible to couple several vehicles together, forming a "train" (from the verb traîner), each of which was guided independently. The loads towed can be very substantial: ore trains weighing over 20,000 tonnes are in operation around the world.

This technology is the basis of the "railway system". We analyze its physical characteristics: bearing and guideway geometry, nature of wheel-rail contact, forces in the presence of adhesion: traction and braking conditions.

In certain special cases of urban transport, the wheel is fitted with a pneumatic tire. Their friction and guiding characteristics are analyzed in the second part.

Wheels are integrated into a rolling stock assembly. The architecture of these rolling elements, which enable traction and braking forces to be exerted, is studied in the third part.

Certain traffic conditions, due to the profile of the line, no longer allow the friction forces resulting from simple wheel-rail contact. In such cases, other solutions are implemented, such as rack and pinion or funicular railways. This is the subject of Part Four.

All railway system components are based on rolling bearings....