Former for a paper machine

A former for a paper machine: The paper machine includes a perforated forming cylinder and a suction device for drawing water inward through the perforations. An inner and an outer belt are wrapped around the forming cylinder. The outer belt meets the inner belt at the peripheral surface of the forming cylinder in a generally wedge-shaped inlet slot located at and upstream of the bottom of the forming cylinder. A pulp fiber suspension nozzle of a width that covers the axial length of the forming cylinder injects pulp into the inlet slot. A supporting device is provided at the inlet slot and includes a convexly curved surface, convexly curved around an axis generally parallel to the cylinder axis. The supporting device includes a concavely curved surface that follows the convexly curved surface downstream of the flow of pulp. A suction device communicates with the concavely curved surface beneath the outer belt. The outer belt rides over the convexly curved surface and down onto the concavely curved surface. The inner belt is a wire mesh. The outer belt is a felt strip.

The invention relates to a former for a paper machine comprising a forming 
cylinder around the peripheral surface of which are wrapped two endless 
belts and further comprising a pulp or fiber suspension nozzle of the 
width of the forming cylinder and having an outlet opening that is 
directed into a wedge-shaped inlet slot which is defined between the two 
belts at the circumference of the forming cylinder. 
BACKGROUND OF THE INVENTION 
Numerous so-called twin-wire formers of the above described type are known. 
For example, see German Offenlegungsschriften Nos. 2 534 290; 2 248 454 
and 2 442 925. Machines of this construction have recently become 
increasingly adapted for the use at high speeds. The basic principle is to 
maintain the outer belt under a given tension during its travel, thereby 
to produce in the portion of the periphery of the forming cylinder which 
is wrapped, a pressure in the normal direction which removes water from 
the injected fiber suspension or the web of fibers being formed. 
Paper webs produced on such twin-wire formers frequently exhibit many 
defects. Often, the edges of the webs have smaller thickness than their 
central regions, the fibers may be undesirably aligned in their 
longitudinal direction, and/or the web appears flaky, fluffy or cloudy 
when one looks through it. Ideally, the web should have approximately the 
same strength in both its longitudinal and its transverse directions, 
should not appear cloudy when looked through and should have a constant 
thickness profile measured over its width. 
SUMMARY OF THE INVENTION 
The object of the present invention is to create a twin-wire former which 
is able to produce paper webs that have the desired properties with regard 
to the aforementioned features. 
The present invention provides a former for a paper machine. The former 
comprise a rotatable forming cylinder. Around the surface of the cylinder 
extends an inner and an outer preferably endless belt. The outer belt is 
separated from the cylinder periphery at one region of the cylinder so as 
to define a generally wedge-shaped injection slot formed between the two 
belts at the periphery of the forming cylinder. There is a nozzle for pulp 
or fiber suspension, which nozzle has a width extending over the axial 
length of the forming cylinder, the nozzle has an outlet opening which is 
directed into the pulp injection slot. Most important, a supporting device 
that extends across the width of the machine is positioned downstream of 
the pulp nozzle and is associated with the outer belt in the pulp 
injection slot at the cylindrical surface of the forming cylinder. The 
supporting device has a supporting surface which is concave with respect 
to the belt in the direction of travel of the belt, i.e. around an axis 
parallel to the axis of the forming cylinder. In a preferred embodiment, 
upstream with respect to the flow of pulp from the pulp nozzle, the 
supporting device has a convex supporting surface that is convex with 
respect to the belt around an axis parallel to the cylinder axis. 
The invention is based upon recognition of the following ideas. The 
aforementioned defects of paper webs that have been produced on twin-wire 
formers are due to the aforementioned squeezing action which is exerted 
between the two endless belts. The thinning of the paper web at its edges 
occurs because the pulp is forced toward the edge of the machine within 
the injection or inlet slot as a result of the squeezing action, and 
because a part of the pulp in the suspension in this way leaves the 
injection or inlet slot. The cloudiness is also caused by the squeezing 
action. The jet of pulp which emerges from the pulp nozzle is impounded 
thereby, leading to flocculation. As a counter-measure against the 
flocculation, the pulp nozzle has up to now frequently imparted a higher 
velocity to the pulp than the velocity of the web-forming belts. However, 
this has led to the undesired longitudinal orientation of the fibers in 
the paper web. 
The following results are obtained by using the invention. Only the impact 
surface of the concave supporting surface serves in general to support the 
outer belt. This means that at other places, the outer belt no longer 
rests on the supporting surface. It thus travels "hollow" here. A negative 
pressure is established within the wedge-shaped hollow space which is 
formed by the outer belt and the part of the supporting surface that is 
not contacted by the belt, as a result of a pumping action of the belt. 
The belt endeavors to empty the hollow space of air or of liquid which has 
penetrated. More air or liquid is removed from the hollow space than can 
be drawn into it laterally. The negative pressure developed in the hollow 
space is thus opposed to the drainage pressure prevailing between the two 
belts. This means that the drainage proceeds more gently in the very first 
part of the drainage region and that the aforementioned detrimental 
effects of impounding, flocculation, or thinning of the edges of the web 
no longer occur. The inward flow of air in the edge regions of the belts 
causes the outer belt to come closer to the inner belt at the edges than 
in the central region. As a result, the gap present between the two belts, 
through which the suspension could emerge laterally, is reduced or even 
entirely closed. 
Various embodiments of the invention are possible. In principle, the 
aforementioned effect of the invention will be more evident the less 
permeable the outer belt is. If the outer belt is a wire screen and 
therefore very permeable, water will initially flow out of the suspension 
through the wire screen. Then, however, a layer of fibers deposits on the 
wire screen, obstructing the flow of the water through it. This in its 
turn leads to a pressure gradient towards the outside so that the wire 
screen approaches the concave guide surface. However, in a preferred 
embodiment of the invention, the outer belt may also consist of felt, 
which is inherently substantially less permeable to water than a wire 
screen. Furthermore, after the formation of the web, this felt can be used 
directly as a web-guiding belt. It can carry the web of paper along with 
it and conduct it, for instance, through a subsequent press. Other 
combinations are also conceivable. For instance, the inner belt may be of 
felt and the outer belt of wire, both belts may be of felt, or both may be 
of wire. 
The dewatering or draining of the web can be effected by the forming 
cylinder if it has a cylindrical surface which is drilled or otherwise 
provided with openings. Suction zones for drawing liquid through the 
openings can then be provided in known manner inside the cylinder. 
However, the invention can also be applied to a forming cylinder which 
does not have such openings in its peripheral cylindrical wall. Water 
removal through the concave guide surface is also possible if the guide 
surface is provided with appropriate water discharge channels. 
Water discharge can be controlled by the vacuum in the water discharge 
zone. In the injection or inlet slot, the concave supporting surface is 
arranged so as to converge toward the forming cylinder, corresponding to 
the removal of water in this region. When the drainage vacuum and the 
convergence between the forming cylinder and the supporting surface are 
optimally set, no positive pressure is present in the gap between the 
belts and the differences in speed between expelled pulp and the 
travelling belts particularly the outer belt, can be reduced or 
eliminated. Lateral squeezing out of pulp is thereby effectively 
countered. 
The convergence between the supporting surface and the forming cylinder can 
also be made somewhat greater so that at the end of the supporting surface 
between the belts, there prevails approximately the same positive pressure 
as is produced in the adjoining zone as a result of the tension of the 
outer belt. 
It is advisable to arrange the above mentioned supporting surface of convex 
curvature in front of or upstream of the concave supporting surface and 
particularly to arrange the convexly curved supporting surface in the 
region of impact of the pulp emitted from the pulp nozzle. This serves the 
following purposes: (a) It seals off the space between the outer belt and 
the concave portion of the supporting surface on the outlet side, which is 
important in high speed operation. (b) As a result of the preferably small 
radius of curvature of the convexly curved supporting surface, the jet of 
pulp can be applied to the outer web without any great disturbance and 
without inclusions of air. (c) The outlet from the pulp nozzle can be 
brought very close to the web-forming zone and the well-deflocculated 
suspension which emerges from the pulp nozzle can be fixed within a very 
short time before the fibers flocculate together. 
By providing a convex first section of a supporting surface succeeded by a 
second concave section, an optimum formation of web is obtained between 
two belts. If the concave supporting surface were not present, then a 
sudden opposing damming would result when the two belts are brought 
together, with all the detrimental consequences thereof. If the convex 
initial supporting surface were not arranged before the concave supporting 
surface, it would not be possible to bring a pulp nozzle with an 
adjustable lip close to the web-forming zone which is necessary in order 
to fix the suspension in the well-dispersed condition in which it leaves 
the pulp inlet. 
The convexly curved initial supporting surface can directly adjoin the 
succeeding concavely curved supporting surface or it may be independent of 
it. The convexly curved supporting surface can also be developed as a 
roller, although in this case the radius can be made as small as is 
desired only in the case of very narrow machines. 
Other objects and features of the invention will be noted from the 
following description of embodiments of the invention considered with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, the illustrated former is provided with a forming cylinder 
having a perforated peripheral cylindrical surface 1. Two belts each 
partially wrap around the periphery of the former, namely an outer felt 
belt 2 and an inner wire-screen belt 3. There is a pulp emitting nozzle 4, 
which extends the width of the forming cylinder 1. The nozzle 4 has an 
upper lip 5 and a lower lip 6. The nozzle conducts a jet 7 of pulp, which 
also is as wide as the forming cylinder 1, into the wedge-shaped injection 
or inlet slot, which is formed between the two belts 2 and 3 in the lower 
portion of the forming cylinder, just upstream of the bottom apex of the 
cylinder. Thus, upon rotation of the cylinder 1 accompanied by motion of 
the belts 2, 3, in the directions indicated by the arrows, a layer of pulp 
or fiber suspension is carried between the strips around the left-hand 
side of the cylinder in FIG. 1. 
The forming cylinder 1 is equipped with suction zones 8, 9 and 10 in its 
interior. These suction zones 8, 9 and 10 are connected to suction means, 
pump 29, for instance, by channels 11, 12 and 13 that are located in the 
hollow journal of the forming cylinder. 
A supporting device 14 is provided below the forming cylinder, just 
downstream of the outlet from the pulp nozzle 4 in the direction of travel 
of the pulp. This supporting device includes a supporting surface for the 
outer belt 2. 
The supporting device 14 is comprised of a hollow cross bar which extends 
across the entire width of the machine and thus along the entire length of 
cylinder 1. The supporting device crossbar is pivotally supported on the 
machine frame 17 by opposed pivot levers 19 located at opposite sides of 
the machine. The cross bar can be moved into the operative position and 
can be swung out of that position by a pneumatic cylinder 18 connected to 
the cross bar. The supporting device 14 can also be rotated about its own 
longitudinal axis relative to the pivot levers 19. Adjustable end stops 
(not shown), resilient in nature, define the end limits of the motion of 
the crossbar. If desired, the pivot 20 of the lever 19 may be adjustable 
in height. 
The support device 14 includes a convexly curved supporting surface 15 
which is followed by a concavely curved supporting surface 16. These 
supporting surfaces face toward the outer belt. Their curvatures are 
around axes parallel to the axis of the forming cylinder 1. The convex 
surface 15 is located just after the outlet from nozzle 4 and the emitted 
pulp first impinges on the outer belt there. The curvature of the convex 
surface is sharper than that of the concave surface 16. The lower surface 
projects up above the concave surface, to define the illustrated sloping 
arrangement. The outer wire screen belt 2 does not rest against the entire 
length (viewed along the belts) of the concavely curved supporting surface 
16. It does rest against the upstream region of that surface. But, in the 
central region and in the downstream region of the supporting surface 16, 
the outer screen belt 2 only comes close to the surface 16. 
It is believed that the previously described short-comings of paper webs 
produced on double belt formers may be traced to the compression effect 
referred to above which is exerted between the two endless belts. The 
thinning of the paper web at the edges thereof occurs because of this 
compressive effect. In the pulp inlet region, the paper material is urged 
toward the edges of the forming cylinder and a part of the pulp material, 
which is in suspension form, leaves the inlet gate in this way. The cloudy 
effect is also due to the compression. The jet of material emerging from 
the nozzle is thereby obstructed which leads to flocculation. Previous 
attempts have been made to counter this flocculation in which the 
suspension jet was provided with a higher speed than the web forming 
strips. However, this led to the undesired longitudinal orientation of the 
fibers in the paper web. 
As already stated, the concavely curved support surface 16 serves generally 
only at its upstream end to support the outer belt 2, while in the central 
and downstream areas of the concave surface, there is a gap. Thus the belt 
runs "hollow". In the generally wedge like cavity which is defined by the 
outer belt and that part of the concave support surface 16 which is not 
touched by the outer belt, there occurs an underpressure, due to the 
motion of the belt. The outer belt endeavors to empty the aforementioned 
cavity (be this of air or of liquid which has penetrated in). More air or 
liquid is conveyed out of the cavity than can be sucked into it laterally. 
Thus, the normal pressure which prevails between the two belts 2, 3 is at 
least partially countered in the inlet region by the underpressure in this 
cavity. This means that the normal pressure between the belts 2 and 3 is 
lower in the very first part of the draining area over the surface 16 and 
the aforementioned disadvantageous effects, i.e. obstructions, 
flocculation and thinning of the edges can be substantially reduced. The 
flowing in of air in the edge region means that the outer belt is closer 
to the inner belt at the edges than at their centers. Consequently, the 
gap present between the two belts through which the fiber suspension of 
pulp material could escape laterally is reduced in size or even closed up 
entirely. 
The convexly curved surface portion 15 has a number of functions. It seals 
off the area between the outer belt 2 and the concave part of the support 
surface 16 at the upstream end, which is important at high speeds. With a 
small radius of curvature for the surface 15, the pulp nozzle 4 can be 
applied to the outer belt 2 without much disturbance and without 
inclusions of air. The nozzle 4 can be brought quite close to the web 
forming zone and the deflocculated suspension emerging from the nozzle can 
be fixed in a very short length of time before flocculation of the pulp 
fibers might occur. 
The juxtaposition of a convex first section 15 of a support surface and a 
second concave section 16 produces an optimum web formation between two 
belts. If the concave guide surface were not provided, a sudden 
counter-obstruction with all the disadvantageous consequences would occur 
when the two belts were brought together. If the convex guide surface were 
not provided, a pulp nozzle with an adjustable lip could not be brought as 
close to the web forming area as would be necessary in order to fix the 
suspension in the well dispersed state in which it leaves the nozzle. 
There is a guide roller 21 above the forming cylinder 1. The roller 21 can 
be developed as a suction roller (not shown). The roller 21 can be 
arranged, as desired, either at a slight distance from the forming 
cylinder 1 or it can be pressed against that cylinder. There is a water 
collection trough 22 provided for the forming cylinder 1 and located just 
after the belts 2, 3 separate at roller 21. 
In the embodiment shown, the web of paper 23 that is formed is carried 
along by the outer felt screen 2 and is subsequently delivered by the 
latter. Instead of this, however, the inner belt 3 (of wire or felt) can 
also serve to convey the web of paper. 
If desired, the concave support surface 16 may be provided with openings 30 
for connection to external suction means 31, M for pressure reduction 
between the concave support surface and the outer strip 2. Where 
measurement or regulation of the pressure are required, suitable 
connections 32 and 33 may be made to the surface. In the apparatus of FIG. 
1, the concave surface 16 is provided with surface openings connected to 
the ducts 32 and 33 which may be used for drainage, suction, measurement 
or regulation. 
FIGS. 2 and 3 show alternate embodiments of formers in which the inner belt 
is a wire-screen casing which completely encircles the forming cylinder. 
In these two embodiments, so-called cylinder machines, only the main 
elements have been provided with reference numbers, namely the same 
reference numbers used in FIG. 1. Here the outer belt is a felt strip. It 
travels from one forming cylinder to the other. The outer belt thus must 
fulfill two functions: on the one hand, it serves as a web-forming element 
(as in the embodiment of FIG. 1), while on the other hand, it transports 
the individual webs that were formed from one forming cylinder to the 
other. 
Although the present invention has been described in connection with a 
plurality of preferred embodiments thereof, many variations and 
modifications will now become apparent to those skilled in the art. It is 
preferred, therefore, that the present invention be limited not by the 
specific disclosure herein, but only by the appended claims.