STRAINERS 155
will naturally sink to the bottom, or if they come in low down they will be
inclined to stay down. A deep channel is provided in the bottom of the vat
in which these things collect, and a cock is usually provided, which can always
be kept partly open to allow them to be carried away to the auxiliary strainer.
Further, in some strainers an adjustable sluice is provided at about the
normal level of the stuff in the vat, and this can be so raised or lowered as to
allow the top scum, in which most of the rubber floats, to run off into the
auxiliary strainer. Thus two means are provided for getting rid of some of
the various causes of blemishes in the paper.
The principle of working of most of the inward-flow strainers is much the
same. The water in which the fibres are suspended flows through the slits
to the lower level inside, carrying fibres with it, but in order to prevent a film
of waterleaf being formed on the outside of the drum, the reflex action already
referred to has to be set up. This is done in one type of strainer by vibrating
the flexible drum itself, and so causing waves and ripples to ebb and flow
through the slits. More fibres and water will pass through than will be passed
back again, owing to gravity and the flow of water to its own level. In
the Reinicke and Jaspar, and Banning and Seybold, types of inward-flow
strainers the reflex action is caused by die oscillating of flappers, or large per-
forated plates fixed to a strong shaft. This shaft is placed immediately under
the lowest part of the revolving drum, and it moves backwards and forwards,
causing the plates to flap like wings in the stuff and send it in waves up against
the slits of the drum.
The motion is very effective in passing the stuff through, but the mechanical
action is very drastic and, being also erratic, it entails die use of very heavy
shafting and strong metal plates, which consume a large amount of power and
lubricating oil and cause a good deal of wear.
Another excellent type of inward-flow revolving strainer is that made by
James Bertram and Sons, and known as the Leith Walk Full-Drum Strainer
(Fig. 48). This is the simplest inward-flow strainer we have seen, and it has
many advantages to recommend it It is noiseless, simple in construction,
easily maintained and efficient in working. It consists of the usual cast-iron
vat lined with glazed tiles, and the drum may be either driven by a ratchet
mechanism, to give an intermittent motion, or it may revolve slowly and
continuously.
Tbe method used to produce the reflex action, to induce the passage of the
stti££ is much the same as that used in the later models of the flat strabjer;
a vibrating brass diaphragm is situated immediately below the drum, and is
actuated by two anus fixed to the vibrator and driven by a completely enclosed
driving geat below* The stroke given to the vibrator may be varied at wffl