The Screw Pair. Most prime movers such as the internal combustion engine, turbines of various kinds and electric motors are so designed that the power which they supply is available as rotary motion. For many purposes this is quite satisfactory, but sometimes it is necessary to convert this form of motion to one of translation. There are several ways in which this can be done, for example, by linkages such as the slider crank mechanism which will give reciprocating motion, by cams, or by rack and pinion. When, however, the linear motion is required to be smooth, uniform and continuous the most suitable kinematic arrangement is often the screw pair. This comprises a screw and nut engaging so that equal angular displacements of the one relative to the other result in correspondingly equal relative axial displacements. Then if either element is fixed the other will, upon being rotated, advance axially in relation to it.
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