A large steam locomotive weighs well over 100 tons and has either 3 driving axles with large wheels for passenger trains, or 5 axles with small wheels for goods trains. Nevertheless, it happens quite often that the wheels spin when the train pulls of. How can this be?
The static friction coefficient for steel on steel (dry) of 0,15 is indeed not bad at all. A clutch which has been constructed for a particularly good friction, only reaches a value of 0,25 to 0,4. This means that 25 to 40 percent of the pressure force is converted into friction force.
Why then do the wheels of heavy steam engine spin? If you look at the formula for friction force found in a clutch, you'll see that not only can you strengthen a clutch by increasing the pressure force, but also by increasing the surface areas which are pressed onto each other. This is exactly what the trains and rails don't have, this, by the way also applies to their braking.
Different from sliding friction, static friction is defined from the rest position. It is the resistance force which one body applies to oppose the possible shifting movement of another body. The pressure force, by the way is applied exactly vertical onto the contact surface. In the case of sliding friction a shifting takes place, and then the measuring of the forces occurs. Because the initial movement requires more energy, the static friction force is always greater than the sliding friction force.
The rolling friction force is even smaller. This is why our wheel-bearings are always made up of roller - (or even better) ball-bearings. Only those which are exposed to really great, or impact-like strain, e.g., the crankshaft, are laid put as roller-bearings. Actually the friction force coefficient can not be greater than 1, i.e. the friction force cannot be greater than the pressure force. Indeed, with slicks e.g., it can be higher, because these tyres literally stick to the road surface.
It should be emphasised again that the static and also the sliding friction itself, are independent of the size of the contact area unlike, for example, the friction force. The applied force (normal force) and the materials only play a role. 06/14