A transmission device (104, 804), comprising: an external transmission device (116, 816), the external transmission device (116, 816) being of a cylindrical shape, the external transmission device (116, 816) having an outer wall and an inner wall, and the inner wall being provided with a plurality of inwardly extending arc-shaped protrusions; an internal transmission device (120, 820), the internal transmission device (120, 820) being disposed in the inner wall of the external transmission device (116, 816), the internal transmission device (120, 820) having an outer circumferential surface, and the outer circumferential surface being provided with a plurality of outwardly extending arc-shaped protrusions; and a spacer ring (218, 818), the spacer ring (218, 818) being located between the arc-shaped protrusions of the external transmission device (116, 816) and the arc-shaped protrusions of the internal transmission device (120, 820), and the spacer ring (218, 818) being capable of being simultaneously in contact with the arc-shaped protrusions of the external transmission device (116, 816) and the arc-shaped protrusions of the internal transmission device (120, 820); characterized in that when the rotation driving torque between the external transmission device (116, 816) and the internal transmission device (120, 820) is greater than a predetermined torque, one or more of the arc-shaped protrusions of the external transmission device (116, 816) or the arc-shaped protrusions of the internal transmission device (120, 820) deform to enable the external transmission device (116, 816) to slip and rotate relative to the internal transmission device (120, 820). an external transmission device (116, 816), the external transmission device (116, 816) being of a cylindrical shape, the external transmission device (116, 816) having an outer wall and an inner wall, and the inner wall being provided with a plurality of inwardly extending arc-shaped protrusions; an internal transmission device (120, 820), the internal transmission device (120, 820) being disposed in the inner wall of the external transmission device (116, 816), the internal transmission device (120, 820) having an outer circumferential surface, and the outer circumferential surface being provided with a plurality of outwardly extending arc-shaped protrusions; and a spacer ring (218, 818), the spacer ring (218, 818) being located between the arc-shaped protrusions of the external transmission device (116, 816) and the arc-shaped protrusions of the internal transmission device (120, 820), and the spacer ring (218, 818) being capable of being simultaneously in contact with the arc-shaped protrusions of the external transmission device (116, 816) and the arc-shaped protrusions of the internal transmission device (120, 820); characterized in that when the rotation driving torque between the external transmission device (116, 816) and the internal transmission device (120, 820) is greater than a predetermined torque, one or more of the arc-shaped protrusions of the external transmission device (116, 816) or the arc-shaped protrusions of the internal transmission device (120, 820) deform to enable the external transmission device (116, 816) to slip and rotate relative to the internal transmission device (120, 820).