Abstract: A driving element is divided into two coaxial toothed members elastically attached to one another. A first of the toothed members is fixed to a shaft. The second of the toothed members is freely mounted on this shaft. The second toothed member has a planar face, opposite and facing away from the first toothed member. That face has seats arranged around the shaft to receive first respective ends of deflection rods. The rods extend, with the shaft, outside of the toothed member. A flange has an axial passage for receiving an end of the shaft. The flange has seats for receiving second respective ends of the deflection rods. A device for clamping the flange on the end of the shaft and in an angular position around the shaft fitted to apply a deflection stress to the rods.