A transmission device includes an external transmission device, an internal transmission device, and a spacer ring. The external transmission device is cylindrical and has an outer wall and an inner wall. The inner wall has a plurality of inwardly extending arc-shaped protrusions. The internal transmission device is disposed in the inner wall and has an outer circumferential surface including a plurality of outwardly extending arc-shaped protrusions. The spacer ring is located between and contacts the arc-shaped protrusions of the external transmission device and the arc-shaped protrusions of the internal transmission device. When torque between the external transmission device and the internal transmission device is greater than a predetermined torque, one or more of the arc-shaped protrusions of the external transmission device or the arc-shaped protrusions of the internal transmission device deform and the external transmission device rotationally slips relative to the internal transmission device.