Note: Descriptions are shown in the official language in which they were submitted.
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Backqround of the Invention
The present invention relates to a press blanket for a pressing
device intended for extracting liquid from a web of matted fibers
passing through a press nip of a papermaking machine, comprising
a layer of an elastomeric material, in which axial threads,
extending transversely to the direction of movement of the web,
are embedded as reinforcing threads.
The invention further relates to a method for producing a press
blanket for a pressing device intended for extracting liquid
from a web passing through a press nip of a papermaking machine,
wherein a layer consisting of an elastomeric material is poured
onto a casting mold which latter is provided with axial threads
that serve as reinforcing threads and are mounted in axial
direction along the circumference of the mold.
Related Prior Art
U.S. patent 5,320,702 discloses a press blanket and a method
for producing the same, wherein a layer consisting of an ela-
stomeric material is poured onto a cylindrical casting mold,
along whose circumference axial threads are mounted to extend
in axial direction, while at the same time reinforcing threads
extending along the circumferential direction of the casting
mold are wound into the compound. One thus obtains a press
blanket consistingofan elastomericmaterialwhich is reinforced
by axial threads embedded in the compound and circumferential
threads extending on top thereof.
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Press blankets of this type are normally provided with grooves
or blind holes in their surfaces facing the web in order to
enable the liquid extracted during the pressing process to be
absorbed.
It has, however, been found that during operation the high
pressing forces may in the course of time give rise to the
formation of fissures, which due to the notch forces mainly
start out from the grooves or blind holes. These fissures may
enlarge over time and may end up by rendering the press blanket
unserviceable.
U.S. patent 5,302,251 discloses a similar press blanket also
comprising a layer of elastomeric material which is reinforced
by axial and circumferential threads. Alternatively, the
circumferential threads may be replaced by a fabric layer. The
press blanket is designed to develop a wavy inner surface which
allows for a better lubrication during service. To accomplish
this, the axial threads and/or the fabric may be made from a
resilient material, this leading to the development of a wavy
inner surface during service underhigh pressure. However, also
this design suffers from the formation of fissures during service
life.
Therefore, it is an object of the present invention to provide
a press blanket and a method for producing the same, which
provides for extended service life. It is a further object of
the invention to provide a press blanket and a method for
producing the same, which develops only few fissures or reduces
the propagation of fissures during service.
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These and other objectives are achieved according to the
invention by providing a continuous fabric layer, extending
substantially in parallel to the surface of the press blanket,
being embedded in the elastomeric material layer of a press
blanket and extending over the circumferential threads wound
around the axial threads. This fabric layer comprises at least
one fabric tape preferably wound spirally around said axial
threads and over said circumferential threads.
According to the invention, one succeeds with the aid of this
continuous layer in preventing any fissures, beginning at the
surface of the press blanket or at grooves or blind bores, from
propagating beyond the fabric layer and in avoiding any further
enlargement of the fissures and, thus, serious damage to the
press blanket. As normally fissures will propagate in a direction
perpendicular to the surface of the press blanket, their
propagation is now limited bythe fabric layer because the notch
forces are removed at this point by the fabric layer, the latter
extending substantially in parallel to the surface.
Thus, the invention has the effect to considerably extend the
service life of a press blanket. In addition, the strength of
the press blanket is increased by the embedded fabric layer.
According to a preferred further improvement of the invention,
the fabric tape extends substantially in the direction of
movement of the web.
21 61 486
One thus arrives at greater strength of the press blanket and
removal of the notch forces that act transversely to the blanket
surface. In addition, this allows the press blanket to be
produced in a simple and advantageous way~
The invention can be employed with particular advantage in cases
where a press blanket according to the invention is provided
with grooves or blind holes, as in these cases higher notch
stresses must be expected.
According to a further embodiment of the invention, the fabric
tape comprises approximately 60 to 90 % fibers extending in
the circumferential direction, and approximately 40 to 10 %
fibers extending in the axial direction of the press blanket.
This has the effect to give the fabric tape increased tensile
strength, especially in the circumferential direction~ which
significantly increases the strength of the press blanket.
Preferably, approximately 25 % of the individual fibers of the
fabric tape extend in axial direction~
The objects of the present invention are further achieved in
connection with a method of the before-described kind by the
fact that a fabric tape is wound into the compound in the
circumferential direction of the casting mold while the ela-
stomeric material is being poured onto the casting mold.
This enables the press blanket to be produced in a particularly
simple and advantageous way and provides at the same time a
particularly high strength, due to the good adhesion effect
between the fabric tape and the elastomeric material.
- 2 1 6 1 486
The fabric tape, just as the reinforcing threads, may in this
case consist of a high-strength material, for example of aramide
fibers.
In case that no reinforcing threads extending in the circumferen-
tial direction are used, the fabric tape, preferably, should
be wound at least in two layers so as to provide a continuous
double fabric layer offering a sufficiently high strength~
In addition, it is of course also possible to wind circumferen-
tial threads spirally over the axialthreads in circumferential
direction, while at the same time the fabric tape is spirally
wound over such circumferential threads.
During casting the casting mold, which preferably takes the
form of a casting cylinder, is rotated about is center axis
while simultaneously the elastomeric material is introduced,
which latter preferably is applied on the casting mold from
the top, through a casting nozzle.
It is understood that the features mentioned above and those
yet to be explained below can be used not only in the respective
combinations indicated, but also in other combinations or in
isolation, without leaving the scope of the present invention.
Brief Descri~tion of the Drawinqs
Further features and advantages will become apparent from the
following description of certain preferred embodiments of the
invention. In the drawings:
Fig. 1 shows a cross-sectional view of a detail of a first
embodiment of the press blanket according to the
invention;
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ig. 2 shows a modification of the embodiment according to
Fig. 1, in reduced scale;
Fig. 3 shows an enlarged cross-sectional view of a detail
of the fabric tape according to Fig. 1;
ig. 4 shows a simplified representation of a device for
producing apress blanketaccording to the invention;
and
ig. 5 shows a simplified representation of a pressing device
of a press blanket according to the invention, in
the area of the press nip.
Description of Preferred Embodiment
In Fig. 1, a press blanket according to the invention is
generally indicated by reference numeral 10.
Fig. 5 is a diagrammatic representation of the conditions
existing at a press nip 50 of a shoe press of conventional
design, where the press blanket 10 revolves between a lower
press shoe 54 and a back-up roll 52, a web 56, being constituted
by a pulp sheet containing water, being guided between the back-
up roll 52 and a felt 58 that runs above the press blanket 10.
Due to a high pressing force prevailing in the press nip 50,
the water is extracted from the web 56 in the press nip 50 and
is absorbed by the felt 58 and in part by grooves or blind holes
provided in the press blanket 10.
The press blanket 10 takes the form of an endless, hose-shaped
tape comprisingan elastomeric matrix16, preferably consisting
of polyurethane. The matrix 16 is reinforced by axial threads
22 extending in the axial direction of the press blanket 10
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and transversely to the direction of movement 60 of the web
56, as shown in Fig. 5. on the axial threads 22, there are
provided equally spaced circumferential threads 20 extending
in the circumferential direction of the hose-like press blanket,
i.e. substantially in the direction of movement 60 of the web
(Fig. 1). The circumferential threads 20 are spiral]y wound
over the axial threads 22, as will be described in more detail
further below, with reference to Fig. 4.
The axial threads 22 and the circumferential threads 20 form
together a thread structure as described for example in
US 5,320,702.
A plurality of grooves 12, extending likewise in the circumferen-
tial direction of the press blanket, i.e. in parallel to the
direction of movement 60 of the web, open toward the surface
14 of the press blanket 10 that faces the web 56. The grooves
12, which are equally spaced one to the other, serve for
absorbing the liquid being extracted from the web 56 as it passes
the nip 50. In the illustration of Fig. 1, the grooves 12 exhibit
a U-shaped cross-section 12, although a different, for example
a rectangular cross-section would be possible as well.
Between the grooves 12 and the circumferential threads 20, there
is arranged a continuous fabric layer 18, extending in parallel
to the surface 14 of the press blanket 10 and being formed by
a fabric tape 25, which is spirally wound into the compound
in the circumferential direction of the press blanket 10, above
the circumferential threads 20.
The fabric tape 25 is wound in partially overlapping arrangement
so that single-layer areas 26 alternate with double-layer areas
24 where the fabric tape overlies part of the neighboring layer.
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When the press blanket 10 according to the invention is used
in a pressing device for extracting water from a web 56 in a
press nip 50 of a papermaking machine, the high forces acting
in the nip may in the course of time lead to the formation of
fissures which, due to the notch stresses caused by the grooves
12, will in most of the cases propagate from the grooves 12
in the radial direction of the press blanket 10, i.e. in a
direction toward the axial threads22. During further operation
of the press blanket 10 additional such fissures may be formed,
and the fissures may enlarge. But due to the presence of the
continuous fabric layer 18 such fissures are prevented from
propagating beyond the layer 18, which means that any further
enlargement of the fissures and, thus, possible leakage and
unserviceability of the press blanket 10 will be avoided. ~11
in all, the service life and operating safety of the press
blanket 10 is, thus, significantly improved.
Preferably, the axial threads 20 and the circumferential threads
22 consist of high-strength plastic fibers, for example aramide
fibers. The fabric tape 18 may consist of a textile fabric,
for example a non-woven fabric, and may be less strong than
the reinforcing threads 20, 22, the dimensional stability of
the press blanket 10 being already ensured by the reinforcing
threads 20, 22.
Given the fact that the fabric tape 18 extends in the circum-
ferential direction of the press blanket 10, i.e. in the
direction of movement 60 of the web, it is sometimes even
possible to do without the circumferentially extending rein-
forcing threads 20, as indicated in Fig. 2. The press blanket
illustrated in Fig. 2 is designated generally by reference
numeral 30. Those parts that correspond to Fig~ 1 are indicated
by the same reference numerals as in Fig~ 1. Modified elements
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are also designated by the same reference numerals, followed
only by an apostrophe. The fabric layer 18' consists again of
a fabric tape 25', that is spirally wound -in the circumferential
direction of the press blanket 10 - into the press blanket area
between the grooves 12 and the axial threads 22. In order to
arrive at a higher strength, it is ensured, by having the fabric
tape 25' fully overlap during winding, that a continuous double
fabric layer 18' is obtained so that each area 24' of the fabric
tape 25' overlaps an underlying area 26' of the fabric tape.
In order to further increase the strength of the press blanket
10, the fabric tape 25' may also consist of a high-strength
material, for example of glass fibers or aramide fibers.
The structure of the fabric tape 18 is illustrated schematically
in Fig. 3. It is apparent that the fabric tape 18 is formed
predominantly of threads 19 that extend in the longitudinal
direction of the fabric tape and that are held together by one
or more threads 21 extending transversely thereto. Preferably,
approximately 75 % of the threads of the fabric tape are oriented
in the circumferential direction, while 25 % of the threads
are oriented in axial direction.
A device for producing a press blanket according to the invention
is illustrated in Fig. 4 and is indicated generally by reference
numeral 40. The device comprises a cylindrical casting mold
42, between the two end faces of which the axial threads are
mounted in mutually parallel arrangement at a predetermined
distance from the surface. Polyurethane is fed via a line 48
and applied from the top through a casting nozzle 49, while
the casting mold 42 is simultaneously rotated about its center
axis 44, in the direction indicated by arrow 46, and the casting
nozzle 49 is advanced by means of a carriage in a direction
parallel to the center axis 44 so that a cylindrical circumferen-
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tial surface will gradually form during the continuous castingoperation. During this process, the compound passes through
the axial threads 22 until it comes to deposit on the casting
mold 42, thereby forming the elastomeric matrix 16 of the press
blanket 10.
At the same time, the circumferential threads 20 and, on top
of the latter, the fabric tape 18 are spirally wound into the
compound, which provides a particularly good adhesion effect
between the matrix and the reinforcing threads 20, 22 and the
fabric tape 18, respectively.
If a press blanket according to Fig. 2 is to be produced, no
circumferential threads 20 are used so that only the fabric
tape is wound into the compound, the degree of overlapping being
preferably selected in this case - as mentioned before -in such
a way that a double fabric layer is obtained.
