NEWSPRINT 273
A dandy roll is usually run, even on the fastest news machines in the country.
It closes the sheet and gives a more uniform look-through. Incidentally, a
dandy roll enables an identifying water-mark to be applied.
The couch on a modern news machine is essentially of the suction type
(see Fig. 69 and p. 192), otherwise economical speeds cannot be attained.
Occasionally a light rubber-covered top couch roll is used as well, but opinion
is divided as to whether this is really helpful. The vacuum at the couch is
usually about 17 inches of mercury.
The Press Part.—During recent years quite a number of proposals have been
made with a view to improving the pressing of newsprint.
Some years ago it was quite common practice to have two plain presses
followed by a reversing third press, all with felts. Nowadays die first two
presses are almost invariably suction presses; the third press, in the development
of modern practice, was first deprived of its felt, then, by putting the brass
roll at the bottom and the granite roll on top, it became a straight-through
smoothing press. Finally, tests showed that it had very little, if any, effect at
all on the finish of the sheet, so that, in present-day installations, there is usually
no third press at all.
The bottom rolls of both first and second presses may be rubber-covered
or just plain brass. Rubber-covered rolls have the advantage that they widen
the contact at the nip, thus giving less drastic treatment to the paper. The
rubber covering is also kinder to the felts, which are prevented from directly
contacting the perforated brass roll. In some installations, notably on better-
class paper machines, running at slower speeds than news machines, important
increases in wet felt life have been obtained, but for newsprint made at high
speeds, rubber-covered press rolls are not invariably chosen, because improve-
ment in felt life has not in all cases been established, and also because the extra
length of the drilled holes makes quite an appreciable difference to the capacity
of the vacuum pump required.
Two other important developments in. pressing, to which reference
must be made, are the stacked press and the dual press. In the stacked
press three rolls are arranged vertically above one another. The two
bottom rolls are perforated suction rolls, each having a felt, and the top
roll is granite. The sheet is transferred from the bottom roll to the middle
roll by a suction box, and there is, in addition, a suction box at each of the
two nips for removing water. Pressure is applied in the conventional way by
lever systems.
In the dual press (see p. 196) there are again three rolls together, but in
this case they are horizontal. The solid roll is in the centre, and the two
perforated rolls are on each side. The dual press makes the arranging of tie