Jackscrew
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A jackscrew mechanism uses a long threaded rod used to position a matching nut at any position along its length. Unlike hydraulic actuators that require continual pressure to remain in a locked position, forces acting on the nut along the length of the rod do not appreciably affect the rotation of the rod so the mechanism self-locks.
Jackscrews commonly use an Acme thread along the threaded rod. This pattern is very strong and can resist the large loads imposed on most jackscrews while not being dramatically weakened by wear over many rotations. Most jackscrews are lubricated with grease. More-sophisticated 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 considered self-locking.
As shown in the photo, jackscrews are commonly used in car-jacks.
[edit] 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 (of metal or plastic) that allow the user's fingers to turn the screws.
