Instead of being pumped into the regulating-box, in some mills, the pulp flows into a small vessel below the stun-chest, and is infect on to the sand tables by means of buckets fastened on the circumference of a wheel.
The pulp, after leaving tbe sand-tables, passes on to the strainers. These consist of strong brass or composi tion plates, having a large number of very fine V-shaped slits cut in them, the narrowest end being on the out side.
The strainers are for the purpose of removing from the pulp all lumps formed by the intertwining of the fibres, and all pieces of unboiled fibre, which, if allowed to pass on, would show in the paper as inequalities in the surface, or as dark specks. The slits are made narrow at the top, and gradually increasing in width, so as to prevent them from getting choked up. These slits allow only the separated individual fibres to pass through, and their width varies according to the quality of the paper. They are put at distances of about I in. apart. Several plates, each containing about 500 slits, are bolted together, and form a strainer. The whole strainer receives a violent shaking motion, to assist the passage of the fibres through the slits. In the machine represented, two of these strainers are shown at C (Figs. 1051 and 1052). The shaking motion is produced by the ratchet-wheel or cams a acting on the hammer b (Fig. 1052). An improved form, called the " revolving strainer," has of late years been introduced, and is often Used in addition to the ordinary ones. The pulp gene rally passes first through one of these, and then through the ordinary or " flat " strainers, as they are called. A revolving strainer is shown at D (Figs. 1051 and 1052). It consists of a rectangular box, the sides of which are plates perforated with slits. Inside this box, a slight vacuum is formed by means of an indiarubber bellows worked by the crank d (Fig. 1051). The vacuum is intended to serve the purpose of the shake in the ordi nary form. The box revolves slowly inside a vat con taining the pulp, and the strained pulp flows into the box D' (Figs. 1051 and 1052), and thence on to the flat strainers. Varions patents have been taken out from time to time for fiat strainers worked by means of a vacuum underneath tbe plates caused by the motion of discs of indiarubber or thin metal. The same principle has also been applied to the revolving strainers. After a time, the slits in the plates get too large, owing to the plate having been worn away by the constant friction of the fibres, and as they are very expensive, various attempts bave been made to invent plans for partially closing them again. Hammering will effect this, but it is liable to break the plates. Annandale of Beltonford
has introduced a method of closing the plates, by means of heavy pressure acting on small steel rollers moving on each side of the slit, in which is placed a small sheet of metal the exact thickness of the width desired. The knots and impurities which collect on the outsides of the strainers must be from time to time removed, other wise the slits would get choked up. In the case of revolving strainers, all that cannot pass through the slits falls to the bottom of the vat, in connection with which it is the custom now to have an auxiliary strainer, or "patent knotter," as it is called, shown at E (Fig. 1051). All fibre that passes through this one which is of the ordinary flat kind with shaking motion, goes into a box near E' (Fig. 1051), called the " low box " for " save-all " water (see farthor on).
The pulp, after passing through the strainers, should be perfectly free from knots and im purities, and in a fit condition for making paper. In the machine shown, it passes from the last strainer directly on to the wire, its flow being regulated by a movable gate e (Fig. 1052). In some oases, however, it first flows into a small vat, in the centre of which revolves a rod carrying paddles, with the object of keeping the pulp well stirred up. It is carried right on to the wire by means of the apron, a piece of canvas, oil-cloth, or sheet rubber, one end of which is fastened to the breast board e' (Figs. 1051 and 1052), the other end resting on, and covering, the wire to the extent of about 15 in. The edges of the apron are rolled up to prevent the pulp from overflowing. After leaving the apron, it passes under a gate, or "slicer," as it is sometimes called, made of two pieces of brass, overlapping each other in the centre, and bolted together. It is made thus to enable it to be lengthened or shortenepl according to the width of the paper ; its height from the wire-cloth can be altered by means of screws, and should be equal at all points, in order to ensure a uniformly thick sheet of paper. The ends of the two pieces forming the slicer are fastened to the frame f (Fig. 1052), or " deckle," as it is called, and this again is carried by two or more rods stretching right across the wire, and fastened by small upright supports on both sides to the frame g (Figs. 1051 and 1052). The deckle-frame also carries the grooved pulleys h (Fig. 1052), along which the deckle-straps endless square bands of indiarubber, move. The deckle-straps rest on the wire, and move with it, the width of the paper depending on their position, which can be altered by shifting the decide-frame along the rods mentioned.