A screw jack is a mechanical device that can increase the magnitude of an effort force.
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. A screw jack consists of a strong screw shaft which extends vertically from a heavy supporting base, and is turned with a bar or lever. On the upper end of the screw is a bearing surface which supports the load. When the screw it rotated it extends, lifting the load. An alternate form of jackscrew is a threaded shaft with a collar or nut on it. When the screw shaft is rotated, the nut moves along the shaft. This type is used in devices such as scissors jacks and heavy duty actuators and positioning systems. A jackscrew amplifies force; a small rotational force (torque) on the screw shaft can exert a large linear force on a load. The smaller the pitch of the screw threads, the larger the mechanical advantage, the ratio of output to input force. 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.