A screw nut assembly of a linear actuation device, having a screw, a main screw nut and a safety screw nut is disclosed. The main screw nut and the safety screw nut engaged with the screw are operative to displace linearly along an axis of the screw. The safety screw nut includes an exterior surface and the main screw nut includes a receiving portion for receiving the safety screw nut. The exterior surface of the safety screw nut and an interior surface of the receiving portion of the main screw nut further include a notch and a resilient member to be engaged with the notch.