Abstract: Gearing mechanism providing for the automatic engagement and disengagement in a radial direction between successive gears in a gear train. A first gear, shown in the form of a rack gear, has a plurality of teeth traveling in a fixed path in space. Selectively meshing with the first gear is a second gear in the form of a pinion mounted upon a driving shaft, frictionally driven by the shaft, and having a degree of radial freedom of movement with respect to the shaft which is such that when the force opposing the driving of the first gear by the second gear exceeds a predetermined desired value the second gear is automatically driven out of mesh with the first gear. The two gears are restored to their initial meshing engagement by a further rotation of the second gear in the same direction through a predetermined angle.