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Screw

a variation of the inclined plant

a screw is basically an inclined plane wrapped around a cylinderpitch is the distance between the threadsA screw can be imagined as a cylinder with an inclined plane wrapped around it (left). The triangle's hypotenuse becomes the inclined plane, or threads, of the screw. The cylinder or cone itself is called the body. The center line of the body is called the axis.

A screw finds its mechanical advantage in the ratio of two dimensions: the length of the lever that turns it and the distance between threads (pitch). When the lever arm in the diagram at right makes a full turn in the direction of the arrow, the screw is elevated a distance equal to the pitch.

The screw can function in two principal ways: it can raise weights, and it can press or fasten objects. In the former role, it converts rotary motion into straight-line motion, allowing jackscrews to raise heavy buildings and hold heavy weights in position, and propellors to move ships through water and airplanes through air. The screw's ability to move weight is also the principal behind the drill bit, in which the threads are not only driven into the material but also serve to carry the freed material out of the hole being created. In the latter role, the friction caused by the screw going through the surface of the objects to be fastened helps hold it in place, thus holding the objects together. This principle also allows a vise to be opened or closed; and, when clamped onto an object, the screw helps the vise hold onto that object. The same principle makes a monkey wrench work, as well as the corkscrew, with the screw providing the resistance necessary to make each tool do the job for which it is intended.

turning the lever arm one full circle raises the screw the distance of one thread

SOURCES
Robert O'Brien Machines New York: Time Incorporated, 1964
The World Book Encyclopedia Chicago: World Book-Childcraft International, 1979

SEE ALSO
Inclined Plane

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The Robinson Library >> Technology >> Mechanical Engineering >> Machines In General

This page was last updated on June 17, 2017.