SCREW. In mechanics, one of the six mechanical powers, consisting of a spiral ridge or groove, winding round a cylinder, so as to cut every line on the surface parallel to the axis at the same angle. The screw may be formed either on the outside or inside of the cylinder; in the former case, it is called the exte rior or male screw ; in the latter, the in terior or female screw. The action of the screw resembles that of the wedge, or inclined plane ; but as the cylinder has always a handle attached to it, the screw is in reality a compound of the inclined plane and lever ; and if the direction of the power be parallel to the base of the cylinder, and perpendicular to its radius, an equilibrium is produced when the power is to the resistance or pressure as the interval between the adjacent threads is to the circumference described by the point to which the power is applied. Hence the mechanical advantage afforded by the screw is proportional jointly to the fineness of the threads and the small ness of the cylinder relatively to the length of the lever or handle. It is to be observed, however, that by diminish ing the distance between the threads, or by diminishing the diameter of the cyl inder, we diminish also, in both cases, the strength of the screw ; and hence there is obviously a limit to the increase of power. But the action is greatly in creased by means of the contrivance called a double screw, or, from the name of its inventor, Hunter's screw, which consists in the combination of two screws of unequal fineness of thread, one of which works within the other. In this case the power does not depend upon the interval between the threads of either intervals in the two screws, and may be increased to almost any extent.
The endless screw consists of a screw combined with a wheel and axle in such a manner that the threads of the screw work into the teeth fixed on the periphery of the wheel. Suppose the power applied to the handle of the screw, and the weight attached to the axle of the wheel, then there will be equilibrium when the power is to the weight as the distance between the threads multiplied by the radius of the axle is to the length of the lever or han dle, multiplied by the radius of the wheel.
The water screw, or screw of formed by winding a flexible tube round a cylinder in the form of a screw. If the machine, thus constructed, be placed obliquely, so as to make with the vertical an angle equal to that which the spiral makes with the lines parallel to the axis of the cylinder, there will be in each convolution of the spiral a part parallel to the horizon. If any body, then, be placed within the spiral at this part, it will remain at rest ; and if the screw be turned the body will ascend, because the part of the screw behind it becomes more inclined than the part before it, and it is consequently urged forward. This sim ple but ingenious contrivance is usually employed for the purpose of raising wa ter to a small height, but it may be em ployed to raise any substance that can pass within the tube ; and it is evident that the action may be increased by placing several tubes or spiral channels(for they may be formed of wood or iron) on the same cylinder. The principle has been recently applied to the propelling of steam-vessels.
The micrometer screw is a contrivance adapted to astronomical or optical instru ments, for the purpose of measuring an gles with great exactness. The very great space through which the lever of the screw passes in comparison of that which is described by the cylinder in the direction of its length, renders the screw of immense use in subdividing space into minute parts.
As a mechanical power, the screw has innumerable applications ; but is employ ed with most effect in all cases in which a very great pressure is required to be exerted within a small space and without intermission. Hence it is the power gen erally used for expressing juices from solid substances, for compressing cotton and other goods into hard dense masses for the convenience of carriage, for coin ing, stamping, printing, &c.