Modeling and measuring friction effects

Friction may well be nature's most useful phenomenon. In machinery in which it is not the driving force, however, friction is an undesirable parasitic phenomenon, generating heat and wasting energy. Large sums are spent each year on lubricants to eliminate as much friction as possible in mechanical devices. On the other hand, when friction is the source of traction and braking, it is important to keep friction at a high level. To achieve this end, much effort and funding are expended on projects such as improving tires and antilock brakes. The goals of this article are first to review some of the physical characteristics of friction, especially the need for state-space dynamic models; then to describe apparatus for measuring friction effects and to present the results achieved using this apparatus; and, finally, to discuss some of the issues relating to measurement of friction effects.

Physical Models of Friction

Friction effects can be imagined as resulting from two mountainous surfaces, one inverted above the other, as illustrated in Figure 1. The motion of one surface relative to the other causes the friction force. The height and sharpness of the mountains (asperities) represent the roughness of the surfaces. The lower mountains support the normal force (load) pressing down on the upper surface. The larger the load, the more deeply the upper surface penetrates into the lower, making it more difficult for the surfaces to move apart. This effect helps explain why the friction force depends on the load .