Patent ID: 8568264

Claim:
A speed reducer comprising: a ring gear which is formed with a plurality of gear teeth on the inner circumference thereof; a revolving gear which is disposed to be eccentric with respect to the ring gear, is formed with a plurality of gear teeth on the outer circumference thereof and is engaged with the ring gear; a circular cam which is disposed to be able to rotate with respect to the revolving gear in a central position of the revolving gear; a first rotational shaft which is installed in the circular cam, is disposed on the central axis of the ring gear, and rotates the circular cam around the central axis to revolve the revolving gear around the central axis; a penetration pin which is inserted in a through hole formed in the revolving gear; and a second rotational shaft which is disposed on the central axis of the ring gear, is connected to the penetration pin, and outputs its rotation according to the rotation of the revolving gear, wherein the ring gear includes a first ring gear and a second ring gear which overlap with each other in a thickness direction of the gear teeth and are able to rotate in reverse directions around the central axis, wherein the revolving gear includes a first revolving gear which is engaged with the first ring gear and a second revolving gear which is engaged with the second ring gear, wherein the circular cam includes a first circular cam which is disposed to be able to rotate with respect to the first revolving gear and a second circular cam which is disposed to be able to rotate with respect to the second revolving gear, and wherein a biasing member is disposed between the first ring gear and the second ring gear and biases the first ring gear and the second ring gear in reverse directions around the central axis.