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 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.
Fig. 12.—Paper-Making Machine.
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
Straining.
Forming
the Sheet.
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.
Fig. 13.—Dandy-roll.
The width of
the web of paper is determined by two continuous straps of vulcanized
rubber about 1 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
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.
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 paper
any desired water-marking). The “dandy-roll” (fig. 13) is a light
skeleton cylinder covered with wire-cloth on which small
Water-marking and Couching.
Pressing
and Drying.}}
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
series 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 twoSurfacing. 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
