into storage tanks or stuff-chests, fitted with revolving arms or
agitators; from these the pulp is pumped into a long upright supply
box at a higher level, called the stuff box, which communicates
with the sand trap or table by means of a regulating valve. With
the pulp a certain amount of water is allowed to flow on to the
sand trap so as to dilute it sufficiently to form on the wire-cloth of
Fig. 12.—Paper-Making Machine.
the paper-machine. The sand trap consists of an elevated table in which is sunk a shallow serpentine channel lined on the bottom with rough felt and divided throughout its length by a number of small strips of wood, behind which the impurities collect as the pulp flows over them on its way to the strainers.
The strainers are made of plates of brass or some hard and durable composition with fine parallel slits cut in them, through which the fibres pass, all knots and improperly divided portions remaining behind; the pulp is made to pass through them by the rapid vibration of the plates themselves or by a strong suction underneath them, or sometimes by a combination of the two. From the strainers the pulp flows into a long wooden box or trough, of the same width as the paper machine, called the "breast-box," and thence on to the wire-cloth. The wire consists of a continuous woven brass cloth, supported horizontally by small brass rolls, called "tube-rolls," carried on a frame; it is usually 40 to 50 ft. long and is stretched tight over two rolls, one at each end of the frame, called respectively the "breast-roll" and the "lower-couch roll." The ordinary gauge for the wire-cloth is 66 meshes to the inch for writings and printings; finer wires are sometimes used, however, up to 80 to the inch; for lower grades the mesh is coarser. The water, mixed with the pulp, flows from the wire-cloth by gravitation along the lines of contact between it and the tube-rolls; this water, which contains a considerable percentage of fibre, especially from finely beaten pulps, drops into a flat copper or wooden tray, from which it flows into a tank and is pumped up with the water for diluting the pulp so that none of it shall be wasted. From the tube-rolls the wire conveys the pulp over a pair of suction-boxes for extracting the remaining water from the web. The width of the web of paper is determined by two continuous straps of vulcanized rubber about i in. square, one on each side of the wire, called the "deckel-straps"; the distance between these straps can be increased or diminished; they serve to guide the pulp from the
Fig. 13.—Dandy-roll.
moment it spreads on the wire until it arrives at the first suction box, where the web is sufficiently dry to retain its edges. The Shake frame of the machine from the breast-roll to the first suction-box is hung on a pair of strong hinges, and is capable of a slight horizontal motion imparted by a horizontal connecting-rod, one end of which is eccentrically keyed on to the face of a rapidly-revolving disk driven by a pair of speed-cones, so that the speed of the shake can be altered. The object of this shake is to interlace the fibres together, but it also assists in keeping the water from passing through the wire too rapidly before the paper has been properly formed. Most machines have two suction boxes with the “dandy-roll” revolving between them on the top of the pulp (so called because it can be made to give to the paperWater-marking and Couching. any desired water-marking). The “dandy-roll” (fig. 13) is a light skeleton cylinder covered with wire-cloth on which small pieces of wire are soldered representing the watermark to be reproduced in the paper. From the last suction box the half-dried sheet of pulp passes between the "couch-rolls," so called from the corresponding operation of couching in hand-made paper, which, by pressing out most of the remaining moisture, impart sufficient consistency to the paper to enable it to leave the wire; both rolls are covered with a felt jacket, and the top one is provided with levers and weights to increase or diminish the pressure on the web. The paper is now fully formed, and is next carried by means of endless felts between two and sometimes three pairs of press-rolls to extract the remaining moisture, and to obliterate as much as possible the impression of the wire-cloth from the under-side of the web. The web of paper is finally dried by passing it over a scries of hollow steam-heated drying cylinders driven one from the other by gearing. The slower and more gradual the drying process the better, as the change on the fibres of the web due to "the rapid contraction in drying is thereby not so excessive, and the heat required at one time is not so great nor so likely to damage the quality of the paper; the heating surface should therefore be as large as possible, and a great number of cylinders is required now that the machines are driven at high speeds. The cylinders are so placed that both surfaces of the web are alternately in contact with the heating surface. All the cylinders, except the first two or three with which the moist paper comes in contact and where the greatest evaporation occurs, are encased by continuous travelling felts. The drying cylinders are generally divided into two sets between which is placed a pair of highly polished chilled iron rolls heated by steam, called "nip-rolls," or "smoother's," the purpose of which is to flatten or smooth the surface of the paper while in a partially dry condition. Before being reeled up at the end of the machine the web of paper is passed through two or more sets of "calenders," according to the degree surface or smoothness required. These calenders consist of a vertical sack of chilled iron rolls, generally five in number, revolving one upon another, and one or more of which are bored and heated by steam; pressure can be applied to the stack as required by means of levers and screws. The web of paper is now wound up in long reels at the end of the machine.
Paper-machines are now usually driven by two separate steam engines. The first, running at a constant speed, drives the strainers, pumps, shake motion, &c., while the second, working the paper machine, varies in speed according to the rate at which it requires
