SCREW, one of the mechanical powers (q.v.), is a modificatio»,of the inclined plane (q.v.). as may be shown by wrapping a piece of paper in the form of an inclined plane round a cylinder. In the screw, the spiral line, formed by the length or slope of the plane, is raised up in a ridge, and a lever is attached for the purpose of working it, so that the screw is really a compound machine, combining the lever and the inclined plane.
i It may be used as an instrument for penetration, as in the auger, gimlet, etc., or as a means of producing pressure, the latter being its most important application as a incchani• cal power. For this purpose it is made to work in a " female screw" or nut (2. hollow cylinder grooved on the inside:so as to correspond to the threads of the screw); the not is then firmly fixed in a massive frame, and the revolution within it of the screw causes the lower extremity of the latter to advance or recede. The principle of the Screw's application is the same as that of an inclined plane pushed further and further under a heavy body so as to raise it up. Now in the inclined plane, P, the power or force, is to IV, the weight raised or the pressure overcome, as the height of the plane to its base; that is, in the screw, as the distance between two threads is to the circumference of the cylinder. But as the twist is not applied at the circumference of the cylinder directly. lint by means of a lever, it follows that the power applied, P, is to IV, as the distance of two threads to the circumference described by P at the end of the lever. We see, then, that the power of the...screw is increased by diminishing the distance between the threads; but as this cannot be effected without weakening the instrument, there is an evident limit to the increase of power in this way. The power can also be increased by lengthen
ing the lever; but the best mode is that proposed by Mr. Hunter (in the Phil. Trans. vol. 17), in which are employed two screws of different fineness, the coarser of them hollow and grooved, to act as a nut for the other. The outer and coarser screw is the one to which the power is applied by a lever, and it is adjusted in the manner before described; the inner is so fastened as to be capable of vertical motion only. When the outer screw is turned so as to move its extremity downward, the inner screw moves upward, but not to the same amount; thus, if the outer screw have 6 threads to the inch, and the inner one 7, one turn of the outer screw depresses it a of an inch, but as the inner one rises + of an inch, the whole descent of the point which produces .pressure is only 1 1 , or 1 - 1 of an inch; hence the pressure applied is 7 times greater than could be given by the outer, 6 times greater than could be given by the inner screw, ,and equal to what would be given by a screw with 42 threads to the inch, with the same power applied. The advantage of Hunter's screw is that the threads may be any thickness, and consequently each screw any strength we please, provided the difference be small enough. The screw is one of the most powerful of the mechanical powers, but the friction generated by it amounts to about * of the force applied.