A jackscrew is a type of jack which is operated by turning a leadscrew

Leadscrew

A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...

. In the form of a screw jack it is commonly used to lift heavy weights such as the foundations of houses, or large vehicles.

Advantages

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.

Mechanical advantage

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

Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

, is
where is the force the jack exerts on the load is the rotational force exerted on the handle of the jack is the length of the jack handle, from the screw axis to where the force is applied is the pitch of the screw threads; the distance between them.

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.

Applications

A jackscrew's threads must support heavy loads. In the most heavy-duty applications, such as screw jacks, a square thread

Square thread form

The square thread form is a common thread form for leadscrews. It gets its name from the square cross-section of the thread.-Advantages & disadvantages:...

 or buttress thread

Buttress thread

The buttress thread form, also known as the breech-lock thread form, refers to two different thread profiles. One is a type of leadscrew and the other is a type of hydraulic sealing thread form...

 is used, because it has the lowest friction. In other application such as actuators, an Acme thread

Acme thread form

Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews . They offer high strength and ease of manufacture. They are typically found where large loads or high accuracy are required, as in a vise or the leadscrew of a lathe...

 is used, although it has higher friction.

The large area of sliding contact between the screw threads means jackscrews have high friction and low efficiency as power transmission linkages, around 30%–50%. So they are not often used for continuous transmission of high power, but more often in intermittent positioning applications.

The Ball screw

Ball screw

A ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with...

 is a more advanced type of leadscrew that uses a recirculating-ball nut to minimize friction and prolong the life of the screw threads. The thread profile of such screws is semicircular to properly mate with the bearing balls. The disadvantage to this type of screw is that it is not self-locking.

Jackscrews form vital components in equipment. For instance, the failure of a jackscrew on a McDonnell Douglas MD80 airliner due to a lack of grease resulted in the crash of Alaska Airlines Flight 261

Alaska Airlines Flight 261

Alaska Airlines Flight 261, a McDonnell Douglas MD-83 aircraft, experienced a fatal accident on January 31, 2000 at the Pacific Ocean about 2.7 miles north of Anacapa Island, California. The two pilots, three cabin crewmembers, and 83 passengers on board were killed and the aircraft was destroyed...

 off the coast of California in 2000.

The jackscrew figured prominently in the classic novel Robinson Crusoe

Robinson Crusoe

Robinson Crusoe is a novel by Daniel Defoe that was first published in 1719. Epistolary, confessional, and didactic in form, the book is a fictional autobiography of the title character—a castaway who spends 28 years on a remote tropical island near Trinidad, encountering cannibals, captives, and...

. It was also featured in a recent History Channel program as the saving tool of the Pilgrims' voyage – the main crossbeam, a key structural component of their small ship, cracked during a severe storm. A farmer's jackscrew secured the damage until landfall.

In electronic connectors

The term Jackscrew is also used for the removable screws that hold D-subminiature

D-subminiature

The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smaller connectors used on computer systems....

 electrical connector

Electrical connector

An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...

s together. These screws draw the two connector halves together and hold them mated, and when unscrewed serve to separate the connector halves apart for unmating. These small jackscrews may have ordinary screw heads or extended heads (also making them thumbscrews) that allow the user's fingers to turn the screws.