Friction, wear and tear, lubrication

    Definition

    In technology, lubrication means the use of lubricants to reduce friction and to reduce or prevent wear and tear.

    For a better understanding, we must first consider friction.

    Life on our planet would not be conceivable without friction.
    We use friction for walking (between the shoes and the ground), for driving cars (between the tyres and the road surface), for the coupling in a car, for switching at the synchroniser rings. Friction is required here for proper operation.

    There are cases, however, when we try to reduce friction to a minimum. When skiing, for example, the sliding properties of skis are improved by using specific coatings. In the case of a hovercraft, an air cushion is created under the vehicle to ensure that the lowest possible amount of energy is required for forward motion, or when moving a cabinet, people try to reduce friction, for example, by using soap as a lubricant.

    Examples of friction: Tyres
    Tyres on a car are used, among other things, to transfer the forces of acceleration, deceleration (braking) and lateral movement to the road. The drier the street and therefore the higher the friction, the better that this works. In case of fast acceleration or braking, however, or on a bend taken at high speed, tyres can squeak; dry friction is not enough here for the transfer of forces and the tyres lose traction and spin. This results in increased wear and tear on the tyres and on the road surface (lane grooves).

    On a wet road (humidity serves as a light lubricating film), wheels lose traction more easily at when setting off, braking distance increases, and the car slides off the road in bends more easily than on a dry road. This condition can be described as mixed friction.

    Friction and lubrication: Aquaplaning
    Completely liquid friction between the tyres and the road surface results in the feared aquaplaning. When this happens, on a wet road at a speed of over 80-100 km/h, a water front forms in front of the tyres, which serves as a lubricating film and the tyres float on this film. Forces of acceleration, braking and lateral motion cannot be transferred to the road, and the vehicle becomes uncontrollable. This phenomenon that occurs between the tyres and the road surface, whilst it can have severe consequences for the car and its occupants, is the ideal condition at every lubrication point of the car.

    Lubrication in the engine
    How does it work in the engine? The precision-machined metal surface (bearing cup, crankshaft, cylinder lining, piston, piston ring, tooth flank) looks like a mountainous region under the microscope. Both gliding surfaces have roughness peaks, which can catch on each other.

    To illustrate this principle, we present the three types of friction in the following for clarification.