PRESS. A machine for the compression of any article. or substances, by the application of screws, levers, wedges, &c. in a convenient manner. As the combinations of the mechanical powers are almost illimitable, it follows that there may be presses made of an almost infinite variety of forma ; but by no possible combination or arrangement of the mechanic powers, can any power be obtained: that must be derived from manual labour or some other moving force; and as no motion can take place in any machine without a loss from friction of some portion of the original force applied, that press which imparts the greatest mechanical energy, with the least proportion of friction, and in the most con venient manner, Is the best. It however happens in most cases, that friction is reduced in proportion to the excellence of workmanship or perfection of form ; and as this circumstance enhances the cost, a preference is often given to machines of rude construction, and of less convenience in form. Under the article On, we have described a variety of presses of very simple construction, but of great energy and little cost ; they are, however, for the most part, not sufficiently compact and convenient for the operations of the packer, or for the general purposes of our manufactories. Screw presses generally consist of six members or pieces ; viz. two fiat smooth tables of wood or metal ; the lower one fixed, and the other above it movable. Between the surfaces of these tables the goods to be pressed are laid, and one or more screws, worked by a lever, are made to force the movable table or board towards the immovable one, and thus produce the pressure on tho interposed body. This is the general nature of the machine, of which there are many varieties, each adapted to its own particularpurpose. The most modern screw-presses have generally but one screw, preferably made of iron, which, at its lower end, has a massive globe head, with four holes through it, for the reception of the end of the lever employed to turn the screw ; the thread of the screw passes through a nut fixed fast in the head or top of the frame of the press. The frame, in this case, con sists of a lower bed or horizontal piece, on which the matters to be pressed are laid, two upright cheeks being firmly united with it, and supporting the head, or upper horizontal pieces of the press, in which the nut of the screw is fixed; the lower point of the screw is united with the follower, or moving bed of the press, and this rests upon the substance to be pressed, and thepower of the screw forces it down upon it. A press of this kind is described under the article
HOT-PIESSING, but adapted to the latter object.
Another kind of screw-press consists of two screws, which are immovably fixed in the lower board or bed; and passing through holes in the upper board, have nuts upon them, which, being turned by a lever, draw the two boards together, and exert a pressure upon any thing placed between them. Sometimes the screws pass through the upper board, and are tapped into the lower one ; then the screws themselves are turned round by a lever put through their heads instead of turning the nuts. Presses of this kind, when accurately made, have a com munication with wheel-work, from one screw to the other, so that both shall turn round together, and cause the two boards of the press to advance parallel to each other. The bookbinder's cutting press is a modification of this, and is used by bookbinders, stationers, and others. See BOOKBINDING.
The screws for presses were formerly made of wood, with sharp threads; that is, the worm of the screw, if cut across, would make a triangular section, the base thereof abutting upon the cylinder of the screw. In this method it was necessary to have the threads very coarse, to give them sufficient strength, and then the power of the screw was not so great as in the other presses, where the screws are made of iron, and their threads not above one-third or one-fourth the distance asunder; the tenacity, hardness, and smoothness of the metal also diminishes the friction considerably. The frames of the modern presses are also made of iron, wood being found incapable of permanently resisting the great strain to which they are subject, as all the fibres, even of the hardest oak, become separated into ribands, and then break, one at a time, till the whole beam fails.