Provided is a device for converting rotary motion into oscillatory axial motion, which device comprises: (a) a rotation element (1); (b) a base element (2); and (c) one or more bearings (3) for facilitating rotary motion of the rotation element relative to the base element; wherein the rotation element and/or the base element comprise one or more raised portions (4) and/or one or more lowered portions (5) over which portions the one or more bearings (3) pass in order to periodically increase and decrease axial distance between the rotation element (1) and the base element (2) as rotation occurs, thereby imparting an oscillatory axial motion to the rotation element (1) relative to the base element (2).