232 MODERN PAPER-MAKING
When ordering a new roll, the makers should be made aware of the quality
for which it is to be used, when they can produce a roll with the water-mark
spaced to come very correct.
Extra weight on the couch and press rolls will prevent shrinkage on the
drying cylinders to some extent; so also will tightening up the dry felts, but
in these cases the shrinkage is apt to take place later—/.*., on the dryer after
tub-sizing.
A laid dandy roll is more tricky to deal with than a wove. It is customary
to have a small tube roll or a soft brush set exactly under the roll. This allows
a little more water to be used after the suction box. The tube roll or brush
takes away the extra water that is pressed through the wire by the weight of
the dandy roll. But if too much water passes under the roll, it will 'lift' sections
of the sheet partly off the wire.
These sections may be even lifted off the wire and stick to the roll for a
few turns, when they come off and spoil the wire by passing through the couch
rolls. If a good water-mark cannot be got without this happening, the make
of the paper on the wire must be attended to. More shake and, if neces-
sary, more water are required, so that more suction is used on the first suction
box. This will ensure that the sheet is pulled down on the wire with more
force than the weight of the dandy roll can exert to lift it up again.
A little manipulation of the tube roll or brush may be all that is necessary,
but care must be taken not to raise the roll or brush so high that the wire is
lifted off the suction boxes, or that the paper is made to look streaky. All
dandy rolls should be allowed i-inch play between the brackets, so as to allow
for the slight movement of the shake, and to prevent them being dragged.
Dandy rolls being very delicate and expensive, too much care cannot be
expended on their use and storage. They should be carefully washed out by
means of a good jet of water when they are taken off the machine. If they are
left standing for some time, while the machineman does something else, the
frothy matters in the meshes will quickly dry up, and be very difficult to get
rid of Indeed, it is sometimes impossible to clean them without using strong
acid, which is the worst possible enemy of the dandy roll.
A machineman who neglects to wash out and thoroughly clean a dandy
roll immediately on taking it off a machine does a grave injustice to his employer
and to his shift mates, who will perhaps be compelled to take drastic means of
cleaning it before it can be used again. In any case, every machineman should
cardfefiy examine the condition of a dandy roll before he puts it over the wire,
and rcjKHt to his foreman any damage it may have received.
When the web has passed under the dandy roll the paper is made. Subse-
quent operations are simply water extraction, with the minimum of damage