A jackscrew is a type of jack which is operated by turning a leadscrew. In the form of a screw jack it is commonly used to lift heavy weights such as the foundations of houses, or large vehicles.

An advantage of jackscrews over some other types of jack is that they are self-locking, which means when the rotational force on the screw is removed, it will remain motionless where it was left and will not rotate backwards, regardless of how much load it is supporting. This makes them inherently safer than hydraulic jacks, for example, which will move backwards under load if the force on the hydraulic actuator is accidentally released.

The mechanical advantage of a screw jack, the ratio of the force the jack exerts on the load to the input force on the lever, ignoring friction, is

where

However, most screw jacks have large amounts of friction which increase the input force necessary, so the actual mechanical advantage is often only 30% to 50% of this figure.

A jackscrew's threads must support heavy loads. In the most heavy-duty applications, such as screw jacks, a square thread or buttress thread is used, because it has the lowest friction. In other application such as actuators, an Acme thread is used, although it has higher friction.