Semicircular key is a kind of key, its upper surface is a plane, the lower surface is a semicircular arc, the two sides are parallel, commonly known as crescent key. The working surface of the semi-circular key is two sides, and the torque is transmitted by the side. It has the same good neutrality as the flat bond. The key can swing around the center of arc curvature of the bottom surface of the groove in the shaft groove, so it can automatically adapt to the tilt of the bottom surface of the hub keyway.