A square-threaded screw jack is a mechanical device widely used for lifting heavy loads or applying considerable force. One of the key features that can make a screw jack more effective and reliable is its self-locking capability.

A square-threaded screw jack carrying a load is considered self-locking if the screw retains its position even after the moment applied to it is removed.

This occurs when the frictional force direction reverses, and the reaction force acts on the other side of the thread's normal. In this situation, the static friction angle is larger or equal to the lead angle of the screw. To wind a self-locking screw downwards, a moment must be applied to the screw in the opposite direction.

By applying the equilibrium equations for the forces acting on the screw, the magnitude of the moment required to initiate downward winding can be determined.

If both the static friction angle and the lead angle are equal, the reaction force acts vertically, balancing the load such that the screw is on the verge of winding downwards. In this case, the screw is in a state of equilibrium, where it can maintain its position without any additional moment applied to it.

However, if the static friction angle is smaller than the lead angle, the screw is not self-locking. In this scenario, a moment in the opposite direction must be applied to stop the downward winding of the screw. This opposing moment generates a horizontal force that prevents the sliding of the thread on the inclined plane of the screw jack.

The magnitude of the required opposing moment can be determined by analyzing the free-body diagram of the screw jack under these conditions.

This information is crucial for designing and operating a screw jack effectively, as it allows engineers to select the appropriate parameters for the specific application.