space, without sliding along it. Thus, each point of the gear can move only along a circle that is perpendicular to its axis and centered on it. At any moment t, all points of the gear will be rotating around that axis with the same angular speed ?(t), in the same sense. The speed need not be constant over time.

The action surface of the gear consists of all points of its surface that, in normal operation, may contact the matching gear with positive pressure. All