BEATERS	79
as blowings, filter-papers and thick papers required to bulk well, or they may
be broad and blunt (f inch), as for strong rag papers, thin banks, etc- The
basalt lava stone roll (Fig. 19) has no bars and does not cut the fibres. It is used
chiefly on wood-pulp furnishes for making very \vet* and highly fibrillated
stock for greaseproof and kraft papers.
Immediately below the roll, in the floor of the beater trough, is a sunken
box, or *den?, into which fits the bed-plate. This consists of a heavy baulk
of timber or a cast iron tray into which is fitted a set of metal bars or knives,
similar to the flybars of the roll. This bed-plate (Fig. 21) is fixed, and the knives
in it are so arranged that the fly bars, meet them at a slight angle, in order that
their action on the fibres may be a shearing or tearing process, and not a direct
cut or chop. This is effected by placing the knives diagonally across the box, or,
more frequently, by having diem bent into an elbow shape as shown in the
illustration. This latter arrangement of the bars, besides giving a shearing action,
also performs the important function of taking some of the fibres from the inside
to the outside of the trough and so mixing them continuously, and thus pre-
venting those on the inside from continually circulating round a much smaller
area than those on the outside. It should be noted, in this connection, however,
that the fact that the stuff near the midfeather makes a complete drculation
more frequently than the stuff round the outside edge of the trough has the
great advantage that all the fibres are not reduced to approximately the same
length, and thus a much better and closer felted paper can be made-
Just beyond the bed-plate is the 'backfall*, which is a continuation of the
trough of the beater, carried up in the form of the arc of a circle, to correspond
with the circumference of the roll, and close to it.
The backfall should be about f*to I inch away from the roll when it is
'down*, and carried well up to about 3 inches above the centre of the shaft,
tapering away from the roll at the top until it is about 2j or 3 inches away,
thus leaving no pocket of dead stuff at the top. These dimensions are clearly
shown in the accompanying illustration.
These measurements give the best circulation and fibrillation of high-grade
wood pulp and rag stock; if the distance between the roll and the top of the
backfall is great, there will be a heavy pocket of stuff" lying dead and impeding
the flow of stuff coming away from the roll. It is important that the backfall,
should be close to the roll, in order that there may be as much friction and
rubbings of the stuff as possible during its passage from the bed-plate to the
top of the backfall, where it is thrown out into the trough again.
The fact that the bed-plate bars wear down, however, makes it impossibk
to have the roll always at the same distance from the backfall, unless the rdS
is stationary and the bed-plates are hydraulic, so that the distance must be so