jackscrew

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A jackscrew operates this automotive scissors-jack.

Antique locomotive screw jack.

Damaged jackscrew recovered from Alaska Airlines Flight 261

A jackscrew is a type of jack which is operated by turning a leadscrew. It is also known as a screw jack, and are commonly used as car-jacks.

A jackscrew's compressive force is obtained through the tension force applied by its leadscrew. An Acme thread is most often used, as this thread is very strong and can resist the large loads imposed on most jackscrews while not being dramatically weakened by wear over many rotations. These types are self-locking, which makes them more intrinsically safe than other jack technologies like hydraulic actuators which require continual pressure to remain in a locked position. Most jackscrews are lubricated with grease.

Advanced screw mechanisms may use a recirculating-ball nut to minimize friction and prolong the life of the screw threads, but such jackscrews are usually not self-locking. The thread profile of such screws is semicircular, not trapezoidal as in an Acme thread.

Jackscrews form vital components in equipment. For instance, the failure of a jackscrew on a McDonnell Douglas MD80 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.

Smaller scale

On a much smaller scale, the screws that join D-subminiature connectors are also referred to as jackscrews. In a similar fashion as their larger brethren, these screws draw the two connector halves together and hold them mated or jack the two 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.

See also

• Ball screw

References