Note: Descriptions are shown in the official language in which they were submitted.
CA 02246677 1998-08-19 ~ ~.
P1356wo~ , . . ..
r ~ > ~
SCREEN
The present invention relates to a screen. In particular ~the
invention relates to the structure of a screen used in the
wood processing industry, providing an improvement in the
working reliability and safety of the device.
The screens used in the wood processing industry are most
often apparatus surrounded by substantially cylindrical outer
housing having connections at least for the fiber suspension
to be supplied to the apparatus, for the accepted fraction,
or the so-called accept, to be discharged from the apparatus,
and for the re3ected fraction, or the so-called reject.
Further, the apparatus may have connections for, for examplre,
plastics separated in the apparatus and for dilution or
washing liquid to be fed into the apparatus. In most cases,
the apparatus is installed in an upright position so that the
axis of the substantially cylindrical housing is vertical.
Then the top of the apparatus is easy to open or detach from
the essentially cylindrical housing and the apparatus may
easily be maintained. The shaft of the apparatus passes
through the opposite end of the apparatus, i.e. usually the
lower end, and rotates usually a so-called rotor although in
some screens the screen cylinder itself is rotatable. Inside
the cylindrical housing, there is, in addition to the rotor,
also a screen or a sieve cylinder, which in most cases is
cylindrical as is indicated also by its name, although a
number of cone-shaped screen drums are also used. Usually
the screen cylinder is located outside the rotor in the
radial direction. The screen cylinder is supported at its
both ends to the housing of the screen via intermediate
rings. The intermediate rings mentioned have been secured to
the housing of the screen and the screen cylinder is secured
to these intermediate rings via a securing flange at both
ends.
A~ENDED S'~tE~
CA 02246677 1998-08-19
P ~ 356wo~
A screen of another type is disclosed for example in U.S.
patent no. 5,326,470 which describes many different ways of
securing a screen cylinder without compressive loading. The
US patent discusses several ways of fastening a screen
cylinder at its bottom end by means of a flange extending
outwardly from the screen cyllnder and an intermediate ring
to the outer housing of the screening apparatus and at its
top end by means of a flange extending inwardly from Ithe
screen cylinder to a centrally arranged plate sealing the top
of the screen cylinder. The rotor of the screen apparatus has
been arranged outside of the screen cylinder.
Quite until recent years the problem has been the securing of
the screen cylinder to the intermediate rings in a way to
obtain maximum strength. Before the securing methods
disclosed in the U.S. patent 5,326,470 were introduced,
screen cylinders were secured to intermediate rings so that
axial compression stress was directed to the cylinders.
For instance, US patent 5,513,757 discloses a screen basket
having at its both ends a flange with a conical outer
surface. Said conical surface tapers towards the end of the
screen basket. The screen basket is attached to the
intermediate rings on the screen housing inner wall in such a
manner that the intermediate ring at the lower end of the
screen housing is provided with a conical surface matching
the conical surface at the lower end of the screen basket so
that the screen basket is lowered against the conical surface
of the intermediate ring. For the attachment of the upper end
of the screen basket an additional ring is provided. The rlng
has an inner edge provided with a conical surface mathing the
conical surface of the upper end of the screen basket and a
radially ourwardly extending flange which is provided with
holes for receiving bolts with which the additional ring is
fastened to the intermediate ring in such amanner that the
screen basket is set under compressive loading.
~E~DED S,~
CA 02246677 1998-08-19
~ ~ 3 56w~
If the screen cylinder was adequately robust and the
application stressed the screen cylinder relatively little,
no problems arose. On the other hand, when the so-called
wire screen drums become more common the mechanical endurance
of the screen cylinders became essentially weaker whereby the
mills found in particular the compression stress to cause
problems which were solved for example by the screen drum
securing methods described in the U.S. patent 5,326,470 by
which the screen was subjected to tensile stress which
- eliminated the danger of deflection of individual screen
bars. However, said US patent merely discusses cases where
the rotor is arranged outside the screen cylinder.
Despite the fact that one problem in the securing of a screen
cylinder was, at least partially, eliminated a number of new
problems were revealed which were previously hidden behind
the more serious problems discussed above. It has been found
out now that it is very difficult to screw on the screen
cylinder securing bolts so tight that they do not come off
gradually for example as a consequence of vibration of the
screen. When conventional securing methods were used, a
small axial clearance was common at the ùpper end of the
screen drum between the screen drum flange and the
intermediate ring attached to the housing of the screen
which was due to the many tolerances of the manufacturing
technique and often even differences in tolerances of
different manufacturers. In many cases, the screen drum is
not delivered by the same manufacturer who originally
manufactured the screen. However, the clearance mentioned
allows the bolt securing to "live" , for example accordinglto
the temperatures or other stresses, so that the securing
bolts of the drum are loosened quite easily.
A preferred embodiment of the invention brings a simple and
advantageous solution to this problem. An essential feature
~M~ND~D S~ T
CA 02246677 1998-08-19
P1356wo~ ' a , '
-
A preferred embodiment of the invention brings a simple and
advantageous solution to this problem. An essential feature
of the approach according to the invention is that the
securing bolts of the screen drum are locked non- turnable by
a means locking several bolts at a time, preferably all the
securing bolts of the screen drum.
An other drawback of the securing methods based on the
tensile stress of the screen drum described in the U.S.
patent discussed is their complexity. In order to secure the
drum in a way creating tensile stress, according to the
embodiment of the patent mentioned illustrated in figures 4
to 9 and 11, the drum must be secured by bolts at its lower
end and even inside the drum which means that in practise the
rotor must be detached from the screen before the drum can be
replaced. From the figures of the patent mentioned, only one
allows bolting the screen cylinder at its upper end, only,
and even then the rotor must be removed from its place in
order to detach the screen cylinder because the inner
diameter of the securing flange of the lower end of the
screen cylinder is smaller than the outer diameter of the
rotor. In other words, it seems clear that a securing method
allowing tensile stress presupposes the use of a flange
extending inside the screen cylinder which in turn means that
the rotor must be detached for the replacement or maintenance
of the screen cylinder.
International patent application PCT/SE94/00013 discloses an
approach in which the screen cylinder is in a way floating so
that there is at both ends of the screen cylinder a radial
slot and at the slot there is a groove for for example an o
ring which seals the slot and at the same time supports the
screen cylinder radially in its place. In the axial direction
the screen drum is supported at least at one of its ends by
pins extending through the drum to the body of the screen so
that the drum cannot move axially or be turned. A problem
~AEN~
CA 02246677 1998-08-19
WO 97130211 PCT/FI97/00104
International patent application PCT/SE94/00013 discloses
an approach in which the screen cylinder is in a way
floating so that there is at both ends of the screen
cylinder a radial slot and at the slot there is a groove
for for example an O ring-which seals the slot and at the
same time supports the screen cylinder radially in its
place. In the axial direction the screen drum is supported
at least at one of its ends by pins extending through the
drum to the body of the screen so that the drum cannot
move axially or be turned. A problem is, however, that
when a drum is left ~loating in the radial direction only
supported by rubber rings, pressure pulses always vibrate
the drum to some extent and even a small movement wears
out both the sealing members and the pins mentioned.
An approach according to an other preferred embodiment o~
the invention eliminates these problems. In other words,
it both allows replacing the cylinder without detaching
the rotor, and it supports the cylinder in its place so
that pressure pulses cannot move the screen cylinder.
The characteristic features of the screen according to the
invention are disclosed in the appended patent claims.
The screen according to the invention is described more in
detail below with reference to accompanying drawing
figures of which
Fig. 1 is a general illustration of a screen used in the
wood processing industry;
Figs. 2a and 2b illustrate, partly in section, a screen
detail according to a preferred embodiment of the
invention;
Fig. 2c illustrates an alternative embodiment the function
of which corresponds to that of Fig. 2b;
,. . . ...
CA 02246677 1998-08-19
WC~ 971302~1 PCT/F197/00104
Fig . 3 illustrates an other preferred embodiment which in
principle corresponds to that of Fig. 2a;
Fig. 4 illustrates, partly in section, a screen detail
~ according to an other pre~erred embodiment o~ the
invention;
Figs. 5a and 5b illustrate, partly in section, a screen
detail according to a third preferred embodiment of the
invention;
Fig. 6 illustrates, partly in section, a screen detail
according to a fourth preferred embodiment of the
invention;
Fig. 7 illustrates, partly in section, a screen detail
according to a fifth preferred embodiment of the
invention;
Fig. 8 illustrates, partly in section, a screen detail
according to a sixth preferred embodiment of the
invention; and
Fig. 9 illustrates, partly in section, a screen detail
according to a seventh preferred embodiment of the
invention.
As illustrated in Fig. 1, a screen mainly comprises an
outer housing 1, connections arranged therein for the pulp
2 to be fed in, for the accepted fraction, the so-called
accept 3, and for the rejected fraction, the so-called
reject 4. Inside the outer housing, there is a screen or
sieve surface 5, and in the vicinity of the surface 5
there is provided a member 6, a so-called rotor movable in
relation to the surface 5.
Figure 2 illustrates in detail how a screen drum is
reliably secured in its place according to a preferred
embodiment of the invention. The figure illustrates only
the top end 12 of the screen drum, its securing flange 14,
an intermediate ring 16 secured/arranged to the housing of
the screen, and a so-called impurity separator 18 and the
CA 02246677 l998-08-l9
WO 97/30211 PCT/FI97/00104
actual securiny bolts 20 o~ the screen drum. As may be
seen in the figure, the intermediate ring 16 includes in
this embodiment an annular sur~ace 162 parallel with the
axis o~ the screen against which the guiding surface 142
of the securing flange 14 of the screen drum is tightly
set. Guided by these two surfaces, the screen drum 5 is
positioned centrally around the axis o~ the screen. Thus,
the hole in the securing ~lange 14 for the securing ~olt
20 is a little loose so as to allow the axial guiding o~
the screen drum 5 to take place via the surfaces 142, 162
mentioned, only. As illustrated in the figure, the screen
drum 5 has been secured to the intermediate ring through
its ~1ange 14 by means of the bolts 20. In prior art
apparatus, the same ~olts were used to secure also the so-
called impurity separator 18 which is an essentially
cylindrical ring extending above the screen drum 5 and
prevents very coarse impurities, such as ~or example
stones, nuts and other such pieces, ~rom entering the
screening zone between the screen drum and the rotor 6
(see Fig. 1). Figure 2b illustrates that the securing
~lange 14 of the screen drum 12 ~irst o~ all has an
annular guide sur~ace 19 ~or the impurity separator 18 ~or
centering the impurity separator 18 centrally around the
axis o~ the screen. The ~igure also illustrates the
securing flange 14 having a cylindrical recess 21 which
partly opens to the cylindrical guide surface 19. The
idea is that when the screen drum is secured with the
bolts 20 to the intermediate ring 16, one of the side~ of
the head of the hexagon head bolt substantially aligns
with the guide surface 19 mentioned and is parallel with
its tangent so that when the impurity separator 18 is
installed in its place against the guide sur~ace 19 it
locks the securing bolt 20 o~ screen drum 12 unrotatable
in its place.
CA 02246677 1998-08-l9
WO 97/30211 PCT~F~97~00~04
In a corresponding way the guide surface of the impurity
~ separator may be arranged outside the periphery of the
screen drum securing bolts, thus obtaining a corresponding
locking o~ the bolts.
Further, in the embodiment illustrated in figure 2c, the
lower part of an impurity separator 18, either the
securing ~lange 22 of the impurity separator 18, or the
lower part of the cylindrical piece itself, has been so
shaped that it prevents the securing bolts 20 of the
screen drum 5 from being unwound. In other words, in the
securing flange 22 or at the lower end of the cylindrical
piece there is a recess 23 which at least partly adapts to
the form of the end of the securing bolt 20 and which
locks the end of the securing bolt 20 non-turnable in its
place.
Naturally, if the screen does not include an impurity
separator the separator may be replaced by a plain ring,
which is placed by the side of or on top of the circle of
bolts securing the screen drum, depending on which of the
embodiments, i.e. securing methods described above is
chosen, and locks all the bolts non-turnable either in the
way illustrated in Figs. 2b and 2c or in some other way.
Of course, it would be advantageous in most cases to be
able to lock all the bolts with one single means, for
example by means of an impurity separator or a particular
ring, but it is possible also to provide several locking
means each of which locks several bolts at a time.
Preferably these means form an annular ring on top of or
by the side of the bolt circle but the inventive idea
covers also other applications in which several bolts are
locked in their places at a time. The impurity separator
18 itself or the ring replacing it is secured in its place
by separate securing bolts. Thus, although the total
number of securing bolts used increases the load directed
CA 02246677 1998-08-19
WO g7/30211 PCT/FI97/00104
to the 6ecuring bolts o~ the impurity separator and the
ring is negligible compared to the securing bolts 20 of
the screen drum 5 and thus the tendency of these bolt to
be unwound is also remarkably smaller.
Figure 3 illustrates an alternative of the embodiment of
Fig. 2a in which the bolt 20 of Fig. 2a has been replaced
by a so-called pin bolt 20~ and the means to be locked in
the embodiment of Fig. 3 is a nut 200 which corresponds to
the head of the bolt 20 in Fig. 2a. This means that the
nut may be locked non-turnable in its place by means of
the impurity separator or a corresponding means as
described above. Figure 3 further illustrates a pre~erred
but not indispensable additional alternative in which
spring washers 202 have been provided in connection with
the nut in the Fig. 3 embodiment, but in practise also
separately, by means of which the securing flange 14 is
pressed against the intermediate ring 16 of the screen.
Further, it is possible that the bolt 20~ is not a so-
called pin bolt but an ordinary bolt with a head which has
been screwed down through the intermediate ring 15 so that
the end of the bolt extends above the surface of the
intermediate ring like the end of the pin bolt 20~. In
this case the securing is carried out just as illustrated
in Fig. 3. And advantage provided by this securing way is
that when the bolt has for some reason broken off by the
nut side the bolt may be screwed out of the hole in a
conventional way after the screen drum has been removed.
During the first assembly, the head of the bolt may be
secured in its place for example by two welding spots so
that the bolt cannot, if snapped, be wound off and end up
in the accept. The welding spot mentioned is easy to
grind off when the bolt is replaced by a new one.
CA 02246677 l998-08-l9
WO 97/30211 PCT/FI97/00104
Fig. 4 illustrates a way according to a pre~erred
embodiment of the invention of supporting the screen drum
5 in its place without directing practically any
compression stress at all to the screen drum. AS already
stated, previously there were two ways of securing a
screen drum. The first way was to secure the screen drum
with bolts at its top end, only, and to press the conical
bottom end of the screen drum against the surface of the
conical intermediate ring. In other words, the screen
drum was set centrally in its place by utilizing
compression stress and thus it was easy to remove it from
its place without having to detach the rotor located
inside it. The other way, which was described in the u.S.
patent mentioned, avoided the compression stress but
brought ~he drawback of having to dismount the rotor at
first before the screen drum could be detached and
removed.
Fig. 4 shows how an intermediate ring 26 connected to the
housing of the screen has been provided with a conical
surface 262 opening upwards the angle of which is of the
order of 5 - 25, pre~erably 10 - 20, and appropriately
about 15 degrees. In a corresponding way the flange 28 of
the lower end of the screen drum 5 has been provided with
a conical surface 282 tapering downwards. An annular
groove 30 has been provided in the conical surface 282 of
the screen drum 5 flange 28, substantially at the center
region thereof, the depth of the groove being in most of
the embodiments presented pre~erably of the order of 3 -
10 mm and the length preferably of the order of 5 - 15 mm
depending on the dimensions of the ring/rings used, and
the groove in turn being preferably provided with a ring
32 made of rubber of some other corresponding flexible
material and preferably affixed. The cross section of the
ring 32 mentioned is preferably such that its surface 322,
preferably conical, placed against the conical surface 262
CA 02246677 1998-08-19
WO 97/30211 PCT/FI97/00104
o~ the intermediate ring 26 is relatively broad thus
ensuring a good sealing between the sur~aces 262 and 322.
The dimensions o~ the conical surface 262, 282 and 322 and
the ring 32 have been chosen so as to have as little axial
compression stress o~ the screen drum 5 as possible and to
provide a guiding which centers the screen drum lower end
in the radial direction as well as possible.
Figs. sa and sb illustrate a method o~ guiding the lower
end of the screen drum according to another preferred
embodiment of the invention. The starting point is
practically the same as in the embodiment of Fig. 4. The
only dIfference is that the flexible ring 42 has been
affixed in this embodiment in the conical surface 262 of
the intermediate ring 26 affixed in the housing o~ the
screen. The purpose is to cause the ~lexible ring 42 to
be bent, when the screen drum is pushed in its place, into
the groove 30 provided in the conical surface 282 o~ the
screen drum lower end flange 28 into the position
illustrated in Fig. 5b, and both to seal the space between
the conical surfaces 262 and 282 e~iciently and to center
the screen drum exactly in the correct position.
Figure 6 illustrates a guiding method of the lower end o~
the screen drum 5 according to a third pre~erred
embodiment o~ the invention. More precisely expressed,
the flexible ring 52 is a ring which is mainly made o~ for
example rubber but has for example a metal ring 524
disposed in the middle o~ it, which remarkably stiffens
the structure of the ring 52. This kind o~ a ring 52 need
not necessarily be a~ixed in the counter sur~aces at all
but the ring 52 stays in its place pressed by the metal
ring 524, only.
CA 02246677 1998-08-19
wb 97/3U211 PCT/FI97/00104
Figure 7 illustrates a guiding method o~ the lower end o~
a screen drum 5 according to a fourth pre~erred embodiment
o~ the invention. In this embodiment, a ~lexible ring 62
replaces the ~lange o~ the lower end o~ the screen drum 5.
Preferably the flexible ring 62 is placed in an annular
groove (60) machined in the screen drum outer surface. In
this embodiment it is advantageous to have two annular
bands 624 and 626 (~or example steel bands) arranged
inside the ~lexible ring 62 as illustrated in the ~igure.
The function of the bands 624 and 626 is to stiffen the
ring 62 particularly in the radial direction so that the
ring 62, ~irstly, stays ~irmly in its groove 60 and,
secondly, supports the screen drum 5 centrally in its
place as well as possible.
Figure 8 illustrates a guiding method of the lower end of
a screen drum 5 according to still a ~ifth pre~erred
embodiment o~ the invention. In this embodiment, a ring
78 sliding in the axial direction has been provided at the
lower end of the screen drum 5 the conical surface 782 of
which is supported by the conical surface 262 o~ the
intermediate ring 26. A ~lexible ring or several ~lexible
rings 72, pre~erably large O rings, have been provided
between the screen drum 5 and the sliding ring 78, the
rings pushing the sliding ring 78 downwards and causing it
to be set tightly against the conical sur~ace 262 o~ the
intermediate ring 26.
Figure 9 illustrates a guiding method o~ the lower end o~
a screen drum 5 according to still a sixth preferred
embodiment o~ the invention. In this embodiment, an
annular groove 30 having a bottom sur~ace 302 has been
arranged in the conical sur~ace 882 o~ a ring 88 at the
lower end o~ the screen drum 5. The bottom surface 302 is
conical. The sur~ace 302 is pre~erably not parallel with
the conical sur~ace 882 but tapers downwards ~aster. The
CA 02246677 1998-08-l9
WO 97t30211 PCT/FI97/00104
conical surface 882 in turn preferably tapers downwards a
little faster than the conical surface 262 of the
intermediate ring 26 although these surface may be
parallel, also. The groove 30 has been provided with an
O ring 82, preferably two O rings, preferably of rubber,
so that when the screen drum is pushed downwards the
ring/rings mentioned roll upwards along the bottom surface
302 of the groove 30 so that they are wedged between the
bottom surface 302 of the groove and the conical surface
262 of the intermediate ring 26 and both seal the lower
end of the screen drum 5 and center the drum exactly in
its central place.
An advantage provided by the conical forms, in which the
coning angle of the conical surface 262 of the
intermediate ring 26 is smaller than the coning angle of
the lower end 28 of the screen drum or the coning angle of
the bottom of the groove provided at the lower end,
illustrated in most of the figures mentioned above is that
there is a wedge-like space tapering towards the accept
space provided between the surfaces mentioned. When
considering the pressure circumstances inside the screen,
i.e. the fact that the pressure is the highest inside the
screen drum and the lowest in the accept space, the
pressure inside the screen drum presses the member between
the conical surfaces mentioned towards the more tapered
end of the wedge-like space and thus the sealing ability
of the member improves. In other words, it is typical of
a preferred embodiment of the invention that the sealing
and support of the lower end of the screen drum is taken
care of by two conical members so that a space is arranged
between the two surfaces which space tapers in a wedge-
like fashion towards the lower pressure, and a member
sealing and supporting the lower end of the screen drum
has been arranged in the space mentioned.
CA 02246677 1998-08-19
WO 97/30211 PCT/F1~7/00104
As may be understood ~rom the above, a structure o~ a new
type has been developed which eliminates the drawbacks and
problems o~ prior art apparatus. However, it should be
remembered that only a ~ew most pre~erred embodiments o~
the structure according to the invention have been
presented above which in no way intend to limit the scope
o~ protection o~ the invention ~rom what has been de~ined
in the appended patent claims.
