Screw

Screw may be regarded as an inclined plane or wedge wrapped round a cylinder. If a screw has n threads per inch, it is clear that a nut which fits it will move relatively to it through a distance of 1/n of an inch, if one is rotated through a complete turn with regard to the other. A lever of some kind (such as a screwdriver or spanner) is usually used to turn the screw or nut; and, if the turning force be so applied at a radius r 7inches, this force will act through a distance of 2 pi r while the nut or screw moves through a distance 1/n; the mechanical advantage will, therefore, be 2 pi r n, In practice about 1/3 of the applied force is lost in friction. The screw is employed in various mechanisms, such as the screw-press, screwjack, etc.; in such cases the relative motion of the screw and its nut is arranged to produce the desired effect. A fine-threaded screw is often used for measuring small distances. [MICROMETER, SPHEROMETER.]