The mobile element includes a central rigid zone (2) provided with arms (6) extending radially from the central zone (2) towards a peripheral zone including teeth (8). The arms (6) are flexible to allow a small tangential and/or radial movement of the teeth (8) in order to absorb shocks. The invention is characterized in that the arms (6) are curved and bend gradually towards an orientation tangential to the rotation of the mobile element, in that the thickness of the arms gradually decreases, and finally, in that the ends of the arms form the teeth.