A rotatable mechanism includes a U-shaped holding seat having left and right arm, a spring unit having a limiting stem received in a reception space defined between the right arm and a protection sheath extending from the left arm toward the right arm, a control shaft extending from the left arm toward the right arm and a torsion spring sleeved the control shaft to provide a torque, and a U-shaped holding frame fastened securely to the control shaft simultaneous rotation of the holding frame together with the spring unit with respect to the holding seat.