2. Controlling friction

Depending on the situation, the aim is either to increase friction, reduce it or maintain it within a given range of values. For example, tire adhesion to the road surface must be sufficient to prevent skidding under braking and loss of control of the vehicle by the driver. Similarly, the slight friction of the sole of a shoe on a patch of ice can cause a walker to fall. Conversely, in hydrodynamic bearings, it is important to limit the friction between the shaft and the bearing during start-up or shut-down phases, when the speed is not sufficient to generate a continuous lubricating film separating the opposing parts. Finally, in sheet metal rolling, the friction between the sheet and the rolls must be sufficient for the rolls to drive the sheet into the nip where they reduce its thickness, but it is important to limit it to a reasonable level to minimize rolling torque, force and energy....