SCREW-PRESS. This machine offers great facilities for the appli cation of power ; as the force applied may be almost infinitely multi plied by increasing the length of the lever by which it is turned, and diminishing the distance between the threads. It is also very convenient in cases where a continued pressure is required; because, in ordinary cases, the friction of the screw is too great to allow it to run hack on the removal of the power by which it is turned. In the common screw-press the articles to be pressed are laid upon a stationary bed, forming the base of a strong frame, in the upper cross-bar or /scud of which a nut is firmly secured. The screw works up and clown in this nut, and to its lower end is attached the follwer, , or moving piece which presses on the substance operated upon. The connection between the screw and this piece is such that the follower rises and falls, but does not turn round with the point of the screw; and the steady motion of the follower is provided for by making it fit closely to the side-pieces or cheeks of the press, which therefore act as guides. At the lower end of the screw there is usually a massive globular head, pierced with two holes at right-angles with each other, which receive the end of a long iron lever, by which the screw is turned. The best screw-presses are made of iron.
The great space required for turning a long lever is a serious incon venience in some cases, and has led to the contrivance of several methods for turning the screw with great power by a more compact apparatus. lu a press invented and patented by Mr. Dunn, the screw is turned by means of a short bent lever acting upon a ratchet-wheel fixed on the lower part of the screw. In another ingenious modification of the common screw-press, invented by Mr. Pouchee, a large cogged wheel is fixed horizontally on the screw, just below the common head for receiving the end of the lever. A small pinion, having a square axis to receive a lever handle, is fixed on the platten or follower of the press, and works into the large cogged-wheel. This press is worked in the ordinary way until the screw is turned as far as the lever will cou venicntly move it. The bar is then removed, and the handle of the pinion put on, and by turning it the screw may be further depressed.
In oue form of screw-press for hot-pressing, the plates do not, as in those of the usual construction, need to be removed for heating ; they being kept at the necessary temperature by hot air introduced into four hollow columns, which serve the purpose of cheeks to the press.
While the diminution of the size of the thread affords the meaus of increasing the power of a screw-press, it is attended by the serious disadvantage of diminishing its strength. This difficulty may be avoided by the use of a double or differential screw. The press may be made of the usual form, excepting that the lower end of the screw is cut with a finer thread than the upper part. This smaller screw is received into a nut resting on the follower, and capable of turning on it. The head for receiving the lever by which the screw is turned is in this case placed above the head or top beam of the press. When in use the nut is keyed fast to the screw, so that it turns with it; and the action is exactly the same as that of the common press. When the screw has been turned as far as it will go in this way, the key that connected the nut with the screw is removed, and the nut is keyed fast to the presser. The screw being again turned, the lower thread enters the nut, so that the presser is depressed only through a space equal to the difference between the width of the upper and lower threads of the screw. By making the two parts of the screw very nearly alike in fineness, the distance traversed by the platten may be diminished, and the pressure increased almost to infinity.
Screw-presses are occasionally made with more than one screw. In one arrangement of this kind the screws are stationary ; and the nuts, which arc fitted into the follower, are turned by means of cogged wheels driven by an endless screw laid horizontally on one side of the follower, and terminating at each end in a winch-handle. Mr. Brindley's press has several screws instead of one, each having a cogged wheel working into another fixed on a plain central shaft, which is moved by a winch and bevil gear.