Patent ID: 7503868

Claim:
An automatic speed changing device, comprising: a hub unit including a hub shaft with both ends supported by a frame and with an outer gear at an intermediate portion thereof, and a hub shell surrounding the outer gear and having a third ratchet ring and a second key protrusion formed on an inner peripheral surface of one side of the hub shell rotatably secured on the hub shaft; a first travel unit including a driver rotatably supported by the hub shaft while closing the other side of the hub shell and formed with second pawls on an outer peripheral surface of one side thereof, and a driven sprocket for rotating the driver by receiving power from a driving sprocket; a second travel unit provided within the hub shell, and including, at one side thereof, third pawls for selectively rotating the hub shell while being engaged with the third ratchet ring, and a carrier mounted with a plurality of planetary gears engaged and rotated with the outer gear; and a conversion unit interposed between the first and second travel units, and including a ring gear having an inner gear engaged with the planetary gears at one side thereof and a first key protrusion at the other side thereof, and a bush provided between the first and second key protrusions, thereby selectively transmitting the rotational force from the driver to the carrier or the hub shell.