SUCTION BOXES 187
ming or howling noise. This has the effect of upsetting all the others, as the
vibration will have the same effect as putting very excessive shake on the wire,
and the stuff will 'flood' into the couch. The boxes, after the first, should be
very close together, so as to keep up a continuous pull on the web, one box
taking it up the instant the other leaves off, and leaving little space or time for
the water to rise again to the surface of the sheet.
It is often advised in making certain papers: 'Do not use too much suction/
But no one can use too much suction without causing the wire and boxes to
vibrate. If the machine is making paper with the boxes not drawing up to
their 'maximum', it means the machine is running too slow, unless held back
by lack of drying power or other causes. It should be put up in speed so as
to be able to make the paper, and no more, with all possible suction in use.
Of course, wear on the wire is increased, but so is production, and a paper-mill
is not run to keep a wire on the machine for a long time, but to get the greatest
possible output of paper in the shortest time, and of the best quality. With
regard to the latter, the drier the stuff can be made before it goes to the couch
the more bulky and less wire-marked it will be. The couch-roll jackets and
felts will last longer and the felts will not get dirty so soon. There will be
fewer breaks at the presses and the whole work of the machine will be made
easier. Therefore in all cases work your suction boxes, after the first, to the
utmost limit of their capacity without vibrating them. Very often a good
water-mark is crushed and made dull by the stuff being too moist when going
into the couch rolls.
While dealing with suction boxes it should be mentioned that when a
suction couch is in use, it is possible, and indeed often desirable, to leave much
more water in the sheet after the last suction box than would be possible when
a top couch roll is in use. The reason for this, of course, is that with a top
couch roll, if too much water is left in the sheet, there is a risk of crashing at
the couch, With a suction roll this trouble does not arise, and the excess water
is removed satisfactorily by the vacuum of the suction couch roll.
Couch Rolls and Jackets,—The bottom couch roll is driven by the gearing
and pulls the wire round. The top couch roll gives the first pressure which
squeezes the pulp into the first semblance of a sheet of paper. The bottom
roll is strongly built with a brass shell well stiffened with spokes and ribs. The
top roll is greater in diameter and may have a brass shell or may be built of
mahogany. Both rolls are, or ought to be, slightly crowned to work a certain
weight at the spindle ends. Couch rolls are not set right on top of each other;
the top roll sits well into the wire, for the reason that pressure of the wire
against the circumference of the roll starts the pressure which culminates in
the hard nip between the two rolls.