The embodiment includes a hinge, a pivot shaft, a cam, and a cam follower. The pivot shaft includes an engaging portion bound by two curved surfaces and two non-rotatable surfaces. The cam is non-rotatably sleeved on the engaging portion of the pivot shaft and defines a noncircular hole and two cutouts. The noncircular hole is bound by two first contact surfaces and two second contact surfaces. The second contact surfaces are tangent to the curved surfaces of the engaging portion of the pivot shaft. The cam follower is rotatably sleeved on the engaging portion of the pivot shaft and resists the cam. The cam follower forms two peaks engaging with the cutouts in the cam.