Consider a square-threaded screw jack carrying a load. If the screw retains its position even after the moment is removed, it is called self-locking. This happens if the frictional force direction reverses and the reaction force acts on the other side of the thread's normal. Here, the static friction angle can be larger or equal to the lead angle. To wind a self-locking screw downwards, a moment has to be applied to the screw in the opposite direction. By applying the equilibrium equations for the forces, the magnitude of the moment can be determined. If both angles are equal, the reaction acts vertically, balancing the load such that the screw is on the verge of winding downwards. If the friction angle is smaller than the lead angle, the screw is not self-locking, and a moment in the opposite direction is applied to stop the downward winding of the screw. This moment creates a horizontal force that prevents the sliding of thread on the inclined plane. From the free-body diagram, the magnitude of this moment can be determined.