Some screws are made to move something rather than to clamp objects together. Such screws are found in vises, screw jacks, airplane landing gear, lathes, animation cranes, large valves, and many other types of machinery where a part must be moved powerfully in a straight line and accurately positioned.
The most common thread for this purpose is the Acme thread.
Square threads are also used, but they are more difficult to make.
An Acme screw and its nut typically have a much tighter fit than fastener screws. An Acme screw should never be separated from the nut that fits it; either alone is worthless.
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Last revised: 8 November 2003.