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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
- 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 lead of the screw.
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.
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 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.
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 off the coast of California in 2000.
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.
In electronic connectors
The term jackscrew is also used for the removable screws that hold D-subminiature electrical connectors together. These screws draw the two connector halves together and hold them mated, and when unscrewed 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.