be produced with an over centre snap action this arrangement has to be moulded in a cranked condition i e offset to one side not central and so it will have an uneven bias towards one of the two extreme radially positions although this should not significantly affect its function in any event the device will function in absence of any spring biasing means in the embodiment shown in fig4 to 7 the drive elements are fixed to the shaft 10 in the form of radially outwardly extending teeth 40 each having a driving surface 42 and a cam surface 44 the outer annular member 2 is provided with radially movable radially inwardly projecting teeth 4 each having a driving surface 6 and a cam surface 8 the teeth 4 are integrally formed with a wedging element 46 having a wedging surface 48 capable of engaging a mating surface 50 formed within the annular member 2 the wedging elements 46 are integrally formed with the annular member and attached to the interior surface by flexible tails 52 which allow radial movement of the wedging elements 46 fig6 illustrates the driving mode the shaft 10 is rotated clockwise causing engagement of a driving surface 42 of a tooth 40 on the shaft with a driving surface 6 of a tooth 4 of the annular member 2 the engagement forces the wedging element 46 to move until the wedging surface 48 abuts the mating surface 50 the tails 52 are almost fully extended and the wedging elements 46 have moved up to their mating surfaces 50 to ensure full engagement of the driving surfaces 42 6 this allows very high torque to be transmitted because the higher the input load the harder the.