An apparatus including a shaft, a wheel cylindrically mounted on the shaft, a cylinder coaxial with the wheel and having an inner wall, and at least two centrifugal braking assemblies mounted on the wheel uniformly distributed in a circumferential direction is provided. Each centrifugal braking assembly includes a brake member pivotally mounted on the wheel and having at least one curved friction surface, a cam pivotally mounted on the wheel and in slidable contact with the brake member, and a spring connected between the cam and a pin mounted on the wheel. The brake member, the cam, and the spring cooperate to rotate the curved friction surface of the brake member and in turn, contact with the inner wall of the cylinder when a rotational speed of the shaft exceeds a first predetermined value.