SCREW. The screw is one of the most powerful and useful of the simple machines or mechanic powers. It is a modification of the inclined plane, as will easily appear to any one who reflects a little on its construction. If a triangular piece of gaper be rolled round a cylinder it will form a spiral inclined line round it, which will be not an inapt representation of the nature of the screw. The screw with the projecting thread moves within a concave spiral groove cut in interior of a hollow cylinder, which is termed the female screw or nut. The screw is generally turned by means of a lever, as represented in the annexed cut at a b; and the power obtained by the instrument i u ent s calculated by dividing the circumference of the circle described by a b by the dis tance between two successive threads of the screw. Thus, if the lever a b be thirty inches long, and the dis tance between two threads of the screw be half an inch, the circum ference described will be 94 inches; which, divided by half an inch, gives 188 as the increase of power obtained by this machine. In this case, a man who could exert a force of 10016s. would be enabled to produce a pressure equal to that of 18,800 lbs. From these remarks it will be seen that there are two ways of increasing the power of this machine; • viz. by lengthe • the lever a b, or by diminishing the distance between the threads. The former would be limited by the tmwieldiness thereby given to the machine; and the latter, by the circumstance that the threads become weaker in proportion as they are diminished, and hence a alight resistance would tear them from the cylinder. These inconveniences are obviated in a contrivance of Mr. Hunter's, in which the required strength and compactness may be carried to any extent.. This contrivance consists in the use of two screws, the threads of which may have any given strength, but which differ slightly in While the working point is urged forward by that screw whose have the greater breadth, it is drawn back by that whose threads have the less; so that during each revolution of the screw, instead of being advanced through a space equal to the breadth of either of the threads, it moves through a space equal to their difference. The power of such
a screw will be equal to that of a single screw the distance between whose threads is equal to the difference of the distances between the threads ;ust mentioned.
The great power and compactness of tow screw, as a mechanical engine, render at highly useful in the formation of presses, in which a great pressure is required. The screw is therefore usually employed in the expression of juices from solid bodies, in coining and in reducing the bulk of light and soft bodies, so as to render them convenient for exportation. (See Paess.) The screw is also used very efficiently in the measurement of very minute motions and spaces. Thus, suppose the screw to have one hundred threads in the leni of an inch, each revolution of the screw will advance the point one hundredth of an inch. Now, if the head of the screw be a circle one inch in diameter, the circumference of the head will be somewhat more than three inches; this may be easily divided into a hundred equal parts, each distinctly visible. If a fixed index be used, the hundredth part of the revolution of the screw may be observed. and this will advance the point of the screw one ten-thousandth of an inch. To observe the motion of the point of the screw, a fine wire is attached to it, which is carried across the field of view of a powerful microscope, by which its motion is made distinctly perceptible. Such a screw is called a micrometer screw, and is much used in graduated instruments, for astronomical observations. Bunter's screw may be also conveniently used for the same The mostcommon kind of screws are those used by carpenters and other mechanics, for fastening wood, or wood and metal together, and are therefore usually termed in this country wood-screws, though the Scotch name of screw nails is somewhat more appropriate, as distinguishing them from other kinds of screws.