194

MODERN PAPER-MAKING

lead away the water squeezed from the web. The bottom roll is driven and
the pressure of the two rolls pulls the felt round. Before the felt and paper
enter the nip there are often air pockets between them. A small suction box
with perforated holes on the top is arranged at this point and extracts the air
so that no wrinkles are formed in the paper. Sometimes, a roll of 2 to 3 inches
diameter, called a "blow roll', is used to raise the paper from the felt and allow
the air to escape.

Of kte years, press rolls of granite have become popular. The cold, crystal-
line surface does not pick up 'greasy' or fine stuff as easily as the brass shell

FIG. 71.—WEUL-AORANGED PRESSES ON A. MACHINE K>R MAKING FINE TISSUES

[Miner

does. For this reason they are valuable for use with esparto, straw, and mechani-
cal fibres, which have very little strength or length to carry them over the
leadline. They are usually run with non-metal doctors to obviate 'pencilling*.
The top press roll may be made of iron, brass, granite, or wood. The
bottom roll is usually rubber-covered. Sometimes two brass rolls are met
widb, bat tibese are very destructive to felts, since a small lump passing through
will cause die felt to be damaged. The rubber covering of the bottom roll
lessens das dangqr comiderably. With a rubber-covered roll there is a less
serae nip of the felt and paper, but even so quite satisfactory removal
of water is effected and longer life obtained from the wet felts. The