Jackscrew

Screw jacks are limited in their lifting capacity. Increasing load increases friction within the screw threads. A fine pitch thread, which would increase the advantage of the screw, also reduces the speed of which the jack can operate. Using a longer operating lever soon reaches the point where the lever will simply bend at its inner end.

Screw jacks have now largely been replaced by hydraulic jacks. This was encouraged in 1858 when jacks by the Tangye company to Bramah's hydraulic press concept were applied to the successful launching of Brunel's SS Great Eastern, after two failed attempts by other means. The maximum mechanical advantage possible for a hydraulic jack is not limited by the limitations on screw jacks and can be far greater. After World War II, improvements to the grinding of hydraulic rams and the use of O ring seals reduced the price of low-cost hydraulic jacks and they became widespread for use with domestic cars. Screw jacks still remain for minimal cost applications, such as the little-used tyre-changing jacks supplied with cars.

In technical application such as actuators, an Acme thread is used, although it has higher friction, because it is easy to manufacture, wear can be compensated for, it is stronger than a comparably sized square thread and it makes for smoother engagement.

The ball screw 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 approximately semicircular (commonly a "gothic arch" profile) to properly mate with the bearing balls. The disadvantage to this type of screw is that it is not self-locking. Ball screws are prevalent in powered leadscrew actuators.

A Boeing 737 uses an adjustable stabilizer, moved by a jackscrew, to provide the required pitch trim forces. Generic stabilizer illustrated.

Jackscrews are also used extensively in aviation to raise and lower horizontal stabilizers.

The failure of a jackscrew on a Yakovlev Yak-42 airliner due to design flaws resulted in the crash of Aeroflot Flight 8641 in 1982.

The failure of a jackscrew on a McDonnell Douglas MD-80, resulting from a lack of grease, brought down Alaska Airlines Flight 261 in 2000.

The destruction of a jackscrew on a Boeing 747-400F, brought down National Airlines Flight 102 in 2013. This flight had just departed Bagram Airfield when a MRAP armoured vehicle broke loose and slid through the rear bulkhead, severing hydraulics lines and most importantly, destroying the jackscrew, rendering the aircraft uncontrollable.

The term jackscrew is also used for the captive screws that draw the two parts of D-subminiature electrical connectors together and hold them mated. When unscrewed, they allow the connector halves to be taken apart. These small jackscrews may have ordinary screw heads or extended heads (also making them thumbscrews) that allow the user's fingers to turn the jackscrew. Furthermore, the head sometimes has an internal female thread, with the male externally threaded screw shaft extending from that. The threaded-head type can be used to panel-mount one connector and provide a means to attach the mating connector to the first connector.

The jackscrew figured prominently in the classic novel Robinson Crusoe. 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.