SUCTION PRESSES

195

top press roll is fitted with a doctor, either iron or composition, the
purpose of which is to clean the roll and also to hold up the stuff which
may stick to the roll, and prevent the sheet from going round if a
break occurs.

Suction Presses (Fig. 73).—The tendency is nowadays for the ordinary press
to give way to the suction press. This press consists of a bottom roll containing
a suction box and perforated shell, wrhich is the same as the suction couch roll,
with the exception that it is usual for this suction roll to be rubber covered
over the bronze shell. Where suction presses are used, it is not generally found

FIG. 73.—Ruw»-CQ?BsnD SUCTION PIBSS ROLLS

necessary to have such a heavy top roll, or to use such pressure, as was often
found necessary with the ordinary press. Some paper-makers maintain that
if they had the option of having a suction couch or a suction press, they would
prefer the suction press. The reason for this is that all felt troubles are prac-
tically eliminated, very much longer life is obtained from the wet felts, very
even drying of the paper results, and a much more uniform sheet is obtained.
It is usual to have 14 to 24 inches of vacuum in the suction roll, and the hard-
ness of the rubber used varies between 30 P-J. and 50 P-J., the first bong
comparatively hard, and the second much softer.