US Patent 8370994 Automatically opening hinge assembly for portable electronic devices

A hinge assembly includes a main shaft, a fixing seat, a follower, a cam and a first elastic element. The fixing seat is fixed with the main shaft. The follower is placed around the shaft, and the fixing seat selectively locks with the follower. The cam is placed around the shaft, and engages with the follower. The first elastic element provides an elastic force to make the cam and the follower resist each other and provides a force to make the cam and the follower rotate together.