EP4361345A1

EP4361345A1 - Shoe press

Info

Publication number: EP4361345A1
Application number: EP23205353.8A
Authority: EP
Inventors: Francesco Simoncini; Michael Angerilli
Original assignee: Toscotec SpA
Current assignee: Toscotec SpA
Priority date: 2022-10-27
Filing date: 2023-10-23
Publication date: 2024-05-01

Abstract

Shoe press comprising a presser (10) formed by an upper block (11) and a support (12) enslaved to an actuation unit (3) that controls its activation by applying a thrust (H) of a pre-established value on a lower surface (122) of the support (12), wherein the upper block (11) has a greater elasticity than the support (12), wherein the upper block (11) and the support (12) are joined to each other so as to form a single body, and wherein the actuation system (3) comprises a plurality of hydraulic or pneumatic pistons (30) arranged along a direction (CD) of prevailing development of the presser (10).

Description

The present invention relates to a shoe press. In particular, a shoe press in accordance with the present invention can be used for the production of systems for the treatment of web-like materials in which the formation of an extended pressure nip or "extended nip" is required. Such systems are generally used for the dehydration, by compression, of the fibrous material from which paper is obtained, in particular tissue paper.
It is known that a shoe press generally comprises a pressing element, known as "shoe", oriented transversally to the direction of the fibrous material being treated (so-called "Machine Direction" or "MD") and intended to compress the web itself in cooperation with a counter-presser. The latter is typically constituted by a roller also oriented transversely to said direction. The counter-presser can also be a heated cylinder, generally known as "Yankee". The shoe has a transversal profile adapted to define, in cooperation with the counter-presser, a compression nip extended along the MD and also suitable for determining a predefined pressure distribution, so that a prolonged compression of the fibrous material that passes through this nip is achieved, with pressure values generally varying along the MD.
Typically, such shoes are made of metal structures or deformable structures made of elastic materials, such as polyurethane, with pressurizable internal chambers.
Metal shoes have greater rigidity and offer greater resistance to deformation of the counter-presser, in particular when the latter is a Yankee, in correspondence with the nip. However, the typical rigidity of a metal shoe does not guarantee that there is always a uniform pressure distribution within the nip in the prevailing direction of development of the shoe (so-called "Cross Direction" or "CD"). This can lead to negative effects on the moisture profile of the fibrous material along the CD. Furthermore, since the fibrous material is generally supported by a felt while it passes through the pressure nip, the non-uniformity of the pressure profile along the CD causes a non-uniform compression of the felt itself, which further amplifies the non-uniformity of the pressure profile on the fibrous material along the CD. A further drawback due to the use of entirely metal shoes lies in the fact that the pressure profile along the MD can be adjusted only in part and only by using at least two lines of actuators or systems that simultaneously allow to apply a load in a radial direction and a torque around the longitudinal axis of the shoe itself. In this way, by modifying the pressure ratio between the inlet actuator line and the outlet actuator line (so-called "tilt") or by varying the torque while maintaining the radial load, it is possible to modify the pressure profile or "load profile" along the MD. Typically this system is used to modify the output pressure peak from time to time, in order to increase the compression and therefore the escape of water from the fibrous material, or vice versa to increase the width of the load footprint to safeguard the bulk and thickness of the product. Ultimately, using an entirely metallic shoe, whose shape is invariable, the adjustment of the pressure profile along the MD can be performed solely by adjusting the overall load and the tilt. Shoes made in the form of a block of elastic material, such as for example the shoe described in EP2013412 , guarantee greater adaptability of the shoes themselves to the deformed profile of the counter-presser, in particular when the counter-presser is a Yankee. The greater elasticity of shoes made of elastic material compared to entirely metallic shoes also produces a greater extension of the compression nip along the MD, thus increasing the compression time of the fibrous material. Therefore, for the same pressure peaks, shoes made of elastic material allow higher levels of dryness. Moreover, in elastic shoes of this type it is possible to form internal pressurizable chambers which allow the pressure profile to be actively modified even when the system is in operation. This results in greater adjustment possibilities unlike metal shoes whose shape is invariable. However, such a system has drawbacks due to the fact that the actuation mechanism must be physically integrated into the block of the elastic material. In fact, the softness of the elastic material makes the shoe with the actuation system integrated inside it more exposed to breakages and less resistance over time. The reliability of the internal components of a shoe press is a critical element, since a possible stop due to the need to replace a damaged component requires relatively long intervention times due to the repair procedure which inevitably includes the following operations:

moving the press to the maintenance position;
completing emptying of the internal lubricating oil;
disassembly of the polyurethane belt of the press (a particularly complex operation, given that it must be carried out with great care to avoid damaging the belt itself which is made of less resistant material and is very expensive);
replacement of the damaged component;
assembly of the polyurethane belt.

These operations normally require 6 to 12 hours of downtime, which, for a continuously operating plant like a paper mill, represents a significant loss of production.
Further drawbacks associated to the actuation system formed by hydraulic chambers integrated inside the shoe block derive from the fact that a system thus created is equivalent, from a point of view of the transmitted load, to the use of hydraulic cushions placed under a shoe. These systems typically operate with much lower pressure values than traditional hydraulic cylinder systems (5-10bar versus 50-80bar). If, on the one hand, the lower pressure can be an advantage in terms of power components of the hydraulic power units, on the other hand there is greater sensitivity to limited pressure oscillations. Furthermore, adjustments are more complex, requiring more precise components to control the supply pressure. In addition, the integration of the actuation system within the polyurethane block causes a significant increase in the overall manufacturing cost, as it is an overall more complex component to manufacture.
EP16805044 discloses a shoe press in which the pressing element, i.e. the shoe, is made up of a block of elastic material inserted into a more rigid body in which a containment chamber is formed which surrounds the elastic block on all sides except the upper side which is defined as the work surface. The elastic block can move parallel to itself inside said containment chamber and can be made up of several layers of materials with different degrees of elasticity or hardness. The shoe press disclosed in EP16805044 has an actuation system consisting of a hydraulic chamber formed below the elastic block or by hydraulic cylinders arranged below the containment chamber or by a combination of such actuation elements. The shape and the rigidity of the containment chamber of the elastic block limit the ability of the latter to correctly adapt to the deformed profile of the counter-presser in the CD direction so that the load cannot be correctly transmitted along the entire area of contact with the counter-presser.
DE3030233 discloses a shoe press in which the shoe is made up of a block of elastic material mounted on an underlying metal support that, in turn, is mounted on a hydraulic chamber constituting the actuation element. The hydraulic chamber is made up of flexible membranes anchored to a fixed beam. Even in this case the load cannot be correctly transmitted since every increase in pressure in the hydraulic chamber also causes a non-predeterminable lateral expansion of the same.
WO2022/122327 discloses a shoe press comprising an upper element and a support intended to form a rigid, non-flexible base for the upper element.
US6951824 discloses a shoe press intended to interact with a solid counter-presser, and therefore practically non-deformable under load, in which the shoe press has a concave surface with an invariable shape, compatible with the radius of the counter-presser, on which a yielding membrane is applied. The shoe press has a rigid core so that the nip defined by the shoe press/counter-presser interaction is of invariable shape.
WO2005/038130 describes a shoe press comprising an elastic upper side and an underlying rigid support. The latter, in particular, is made up of a beam of invariable shape which has a channel and two ends closing the channel itself. Said beam is structured in such a way that its shape remains stable.
The main aim of the present invention is to overcome the aforementioned drawbacks. Another object of the present invention is to propose a shoe press made according to construction criteria alternative to the known criteria.
This result has been achieved in accordance with the present invention, adopting the idea of realizing a shoe press having the features indicated in claim 1. Other features of the present invention are the subject of the dependent claims.
Thanks to the present invention, it is possible to ensure a very precise adaptation of the shoe to the deformed profile of the counter-presser in all operating conditions thanks to the flexibility of the shoe along the CD. Furthermore, it is possible to use elastic materials, such as polyurethane (PU) with a high degree of hardness in order to exploit its substantial incompressibility under the expected load conditions and the ability to maintain the shape of the upper profile of the shoe in order to obtain a sudden drop of pressure at the exit of the nip to avoid re-wetting of the sheet, which would otherwise tend to reabsorb part of the water previously expelled, and guarantee a high degree of dryness of the sheet at the exit from the nip. By contemplating that the elastic modulus of the shoe is, for example, 3-4 orders of magnitude lower than the elastic modulus of the metals used for the production of entirely metallic shoes, the flexibility of the shoe along the CD direction is favored without however provoking appreciable variations in the uniformity of the linear pressure applied along the CD.
These and further advantages and characteristics of the present invention will be more and better evident to any person skilled in the art thanks to the following description and the attached drawings, provided by way of example but not to be considered in a limiting sense, in which:

Fig. 1 schematically represents a possible use of a shoe press in accordance with the present invention;
Fig.2 represents an enlarged detail of Fig.1 in which some elements are not represented to better highlight others;
Fig.3 schematically represents the deformed profile of a Yankee on which a shoe press acts;
Fig.4 schematically represents a possible configuration of a shoe press in accordance with the present invention in the rest state;
Fig.5 schematically represents the shoe press of Fig.5 in active condition;
Fig.6 is an enlarged detail of Fig.5;
Figs.7A-7C represent possible examples of manufacturing the pressure element of a shoe press in accordance with the present invention;
Fig.8 is similar to Fig.6 but this figure also represents a possible pressure profile along the MD for a given tilt;
Fig. 9 represents a possible configuration at rest of the shoe in a shoe press in accordance with the present invention;
Fig. 10 represents a further possible configuration of the shoe in a shoe press in accordance with the present invention in a manufacturing phase.

A shoe press (1) in accordance with the present invention can be used to create a pressure nip (EN) which extends by a pre-established value (VN) along a direction (MD) traversed by a web-like material that passes through the same pressure nip to be subjected to compression, in particular to be dehydrated. For example, the shoe press (1) can be used to dehydrate, by compression, a sheet of wet fibrous material in a papermaking machine. In the example represented in Fig. 1 the shoe press (1) is combined with a counter-presser consisting of a Yankee cylinder (2): the sheet of wet fibrous material (S) passes through the pressure nip (EN) formed in cooperation by the shoe press (1) and the Yankee (2); after passing through the pressure nip (EN), the sheet (S) moves along the external surface of the Yankee, from which it receives heat, and, after its detachment by a suitable doctor blade (T), it is wrapped in form of a reel (RS) downstream of the Yankee. The Yankee (2) and the shoe press (1) are oriented along a direction (CD) orthogonal to the direction (MD) followed by the sheet (S) as it passes through the pressure nip (EN). Typically, said directions (MD) and (CD) are called "machine direction" and "cross direction" respectively. The structure and operation of the Yankees as well as the paper production techniques are known to those skilled in the art, therefore a more detailed description thereof is omitted. The shoe press (1) can assume a rest position, as schematically represented in Fig.4, and an operative position, as schematically represented in Fig. 2, Fig.5 and Fig.6. In the operative position the shoe is pushed towards the Yankee in such a way that the compression to which is subjected the wet fibrous sheet (S) passing through the pressure nip causes the expulsion of water from the sheet itself. Typically, the Yankee (2) has a hollow central part which houses inside it a system (not shown in the drawings) for the distribution of steam, through which is produced the heating of the external cylindrical surface (20), on which the sheet (S) runs downstream of the pressure nip (EN). Two side heads (21) close said central part at the sides and are provided with pins (22) for its insertion on a support structure (not shown in the drawings) and the connection to a motor member (not shown in the drawings) that controls its rotation around a central axis (A2) oriented along the machine direction (MD). Such a structure determines, when a pressure is applied along the direction (CD) on the external cylindrical surface (20) of the Yankee, the formation of a deformed profile (P2) in the pressure application area, i.e. along the pressure nip (EN). In fact, the presence of the side heads (21) determines a greater rigidity of the Yankee on its sides compared to the central part delimited by the cylindrical shell (20). The formation of the deformed profile generally implies a non-uniform transmission of the load along the CD by the presser.
A shoe press in accordance with the present invention comprises a presser (10) consisting of a block (11) of elastic material mounted on an underlying more rigid support (12) which, in turn, is slaved to an actuation system allowing to push it towards the counter-presser (2). Said block (11) constitutes the upper part of the presser (10). The presser (10) will also be called "shoe" in the continuation of this description.
The material of which the block (11) is made has a lower modulus of elasticity than the material of which the support (12) is made. For this reason, the block (11) is also called "elastic block" in this description.
Preferably, the elastic block (11) is a single block of material (for example polyurethane) having a modulus of elasticity 2-4 orders of magnitude lower than the material of the support (12), i.e. from 100 to 10000 times lower. For example, the support (12) can be made of aluminum, bronze or stainless steel, having elastic modules comprised between 60000MPa and 200000MPa.
For example, the material of which the block (11) is made has a hardness. comprised between 80 Shore A and 100 Shore A, preferably a hardness comprised between 92 and 96 Shore A and even more preferably a hardness of 95 Shore A and the modulus of elasticity normally is comprised between 10 and 100MPa.
Preferably, the support (12) is a flat metal body (width and length prevailing with respect to the thickness). For example, the support (12) can be made of aluminum, bronze or stainless steel, with a thickness comprised between 5 mm and 20 mm, preferably with a thickness comprised between 7 mm and 15 mm.
The elastic block (11) is joined to the support (12) so that these elements (11, 12) form a unitary body.
The joining between the elastic block (11) and the support (12) can be achieved by forming an anchor between the lower side of the elastic block (11) and the upper side of the support (12) which extends along the entire contact area between the block of elastic material and the underlying support. In this way, the elastic block (11) is free to deform elastically along the CD direction because the anchoring between the elastic block (11) itself and the support (12) is free of constraints acting on the heads of the block (11).
The lower side (110) of the elastic block (11) can have a width (measured in the MD direction) equal to or less than the width (even measured in the MD direction) of the upper side (120) of the support (12), as schematically represented in Figs.7A-7C. For example, the width of the elastic block (11) measured in the MD direction is comprised between 50 mm and 300 mm, preferably between 70 mm and mm. Furthermore, for example, the thickness (S11) of the elastic block (11) above the support (12) is comprised between 3 mm and 50 mm, preferably between 5 mm and 25 mm.
For example, the block (11) of elastic material can be joined to the support (12) by gluing. More preferably, the aforementioned anchoring can be achieved by directly vulcanizing the block (11) onto the support (12); in this case, preferably the upper side of the support (12) is preliminarily subjected to sandblasting and degreasing to facilitate the adhesion of the elastic block (11). Furthermore, the upper side (120) of the support (12) can be provided with depressions (121) into which the material of the block (11) can be poured which, when solidifying, will give rise to the formation of corresponding appendages (111) inserted in the same depressions (121). The latter can be, for example, grooves longitudinally formed on the upper side (120) of the support (12). Preferably, said grooves are dovetail-shaped. The block (11) of elastic material can also be already preformed with the appendages (111) to be inserted into the depressions (120) of the support (12). In the latter case the joining can be achieved by gluing the two parts. An intermediate anchoring layer can also be positioned between the block of elastic material (11) and the support (12), also made of elastic material (for example polyurethane of lower hardness than that used to create the block 11) whose function is that to facilitate the different expansions between the rigid layer and the elastic layer. For example, the material of the intermediate anchoring layer has a hardness of 60 Shore A while the material of the block (11) has a hardness of 95 Shore A. This layer of anchoring material can have a very limited uniform thickness (0.5-5mm, preferably 1-2mm). Whatever the construction process chosen, preferably the elastic block (11) and the more rigid support (12) have respective flat surfaces (110, 120) which, in the assembled shoe (10) configuration, are counter-faced and adhered to each other. Furthermore, preferably, the upper face (120) of the rigid support (12) does not exhibits depressions or irregularities causing non-uniformity in the rigidity of the support itself. The block (11) preferably has a cross section made to obtain a pressure profile in the MD direction which determines a gradual increase in pressure in an entry zone of the sheet (S) in the pressure nip (EN) and, at the exit, a pressure peak. For this purpose, the block (11) preferably has an entry zone (A) defined by a relatively large radius curvature (radius of curvature between 30 mm and 100 mm, preferably between 35 mm and 55 mm), followed by a intermediate zone (B) which is flat or slightly convex towards the outside (radius of curvature between 1000 mm and 5000 mm) and an exit zone (C) defined by a connection of a relatively small radius with the intermediate zone (between 2 mm and 30 mm, preferably between 5 mm and 25 mm) and by an inclination greater than 10° (preferably greater than 13°) with respect to the underlying support (12).
The presser (10) is slaved to an actuation system (3) comprising a series of hydraulic cylinders (30) arranged below the presser and acting on the lower side (122) of the support (12). The actuation system (3) is supported by an underlying fixed beam (T3).
For example, the actuation system (30) is of the type described in EP2994569 , i.e. of the type comprising a plurality of hydraulic pistons (30) arranged in two parallel rows along the CD direction inside a block (31) in which pressurization are formed chambers (32) supplied with a hydraulic fluid through corresponding supply lines formed in a lower part (33) of the block (31). In this example, the actuation system (3) is fixed on the underlying fixed beam (34) through the lower part (33) of the block (31) and has suitable surfaces (35) for lateral guide and containment of the pistons (30).
A configuration of the actuation system which includes two parallel rows of pistons allows the tilt of the shoe, i.e. of the presser (10), to be adjusted more easily. Fig.8 represents, in a thicker line, a qualitative trend of a possible pressure profile (PP) along the MD for a given tilt. In this diagram the abscissa axis is oriented along the MD direction and the axis of the ordinates (PR) is vertical in the drawing. The peak pressure (PK) is, for example, a value of 5 MPa. It is understood that this value is illustrative and that the actual value will depend on the operating conditions chosen by the user.
In any case, the actuation system (3) allows the compression load to be applied to the sheet (S) through the shoe (10) in a direct and controllable way, both in terms of the value of the overall applied load and in terms of the tilt.
In the exemplificatory configuration shown in the drawings, the group formed by the presser (10) and the respective actuation system is inside a tubular polyurethane sock (4), known per se, rotating around an axis parallel to the CD. Under the sock (4) and near the presser (10) more nozzles can be positioned (not visible in the drawings and which are known per se), to spray lubricating oil downstream of the presser (10). In a known way, the lubricating oil, dragged by the rotating sock (4), forms a hydraulic meatus which supports the sock itself and allows the nip pressure to be applied without an actual contact between the presser and the sock, so as to avoid abnormal wear of both the sock and the presser. The shape of the inlet side (A) of the presser (10) favors the interposition of the lubricating oil between the presser and the sock.
In some cases it is possible to introduce oil sprays upstream of the presser. The aim is to introduce an excess of lubricant immediately upstream of the pressure nip, in order to avoid possible dry rubbing between shoe and sock. These sprays are useful above all during the transitory phases (starting and stopping) of the press, when the variation of the operating parameters such as rotation speed of the sock and nip load could cause a shortage of oil and, consequently, a contact between the shoe and sock with possible abnormal wear of these components.
The shoe made according to the invention can be pushed directly against the rotating polyurethane sock which isolates the external side of the press from the external environment, or a layer (5) of material with a low friction coefficient can be provided between the shoe itself and the polyurethane sock. This second solution consists in appropriately mounting, for example through a jaw system or through the interposition of the low friction coefficient layer between two metal surfaces (50, 51) placed against each other and tightened with a plurality of screws (52). The layer in question, in addition to guaranteeing a low friction coefficient, has a high resistance to wear. The layer (5) is intended to avoid any possible contact between the rotating polyurethane sock and the elastic material which constitutes the upper part (11) of the shoe (10), even in the worst conceivable operating conditions. This measure may be convenient when the risk of incorrect lubrication could bring the polyurethane of the sock and the elastic material of the upper layer of the shoe into direct dry contact: in fact, the possible similar composition of the material that forms the sock and the material of the layer upper part of the sock, in the event of contact not mediated by a film of lubricating oil, could cause high friction with consequent localized overheating and damage to the components. For example, the layer (5) can be made of a technopolymer with a reduced friction coefficient and high resistance to contact with synthetic lubricating oils. A possible example is Thordon ^®, that is often used in the production of plain bearings and is also available in very thin thicknesses.
Since the lower part (12) of the shoe (10) is sufficiently rigid (being metallic, for example in aluminium, bronze or stainless steel as indicated before) the latter can be placed in direct contact with the pistons of the actuation system, ensuring the absence of areas with high pressure concentration due to the precise application of force by the individual cylinders. If required, a cushion layer (6) can be placed under the rigid support (12) which can contribute to further standardize the distribution of the thrust provided by the actuation system. For example, the cushion layer (6) can be a layer of polyurethane with a thickness between 5 mm and 20 mm. The usefulness of this additional layer is due not only to the ability to further distribute the thrust over a larger surface, but also to the fact that, by avoiding a direct contact between the metal surfaces of the support (12) and the heads of the pistons, which are not mutually locked, it is avoided that, following even minimal relative movements between these components, caused for example by possible angular movements spontaneous movements of the shoe, the point of force transmission shifts.
The use of the more rigid support (12) also allows the tilt to be adjusted more effectively, allowing, in fact, the use of two rows of pistons to exert a differentiated load at the entry and respectively the exit of the pressure nip.
The material used to realize the elastic block (11), despite the lower elastic modulus, maintains the ability to be worked on top, assuming and maintaining a defined profile. Furthermore, since polyurethane is a substantially incompressible material (in the sense that it can deform, but if adequately constrained, it does not allow its volume to be modified) it is possible to provide the upper layer of the block (11) with a specific and optimized profile, having the certainty that, when subjected to compression, it will maintain the ability to exert an optimized pressure MD profile on the paper directly dependent on the optimized geometric profile created during manufacturing. In essence, the incompressibility of the block (11) allows maintaining the stability of the geometric profile, i.e. maintaining the efficiency in the dehydration action due to the use of a specific profile (a typical feature of totally metallic shoes), allowing simultaneously and significantly to increase the elasticity in the CD and MD direction of the entire shoe, considered composed of the blocks (12) and (11) joined together.
In other words, the elastic block (11) made integral with the underlying more rigid support (12) so as to form a single body and the actuation system (3) which transmits the load to the rigid support (12) in a direct and controllable way cooperate synergistically with each other to overcome the previously mentioned drawbacks.
The different nature of the elastic material of the block (11) compared to the metallic material of the support (12) also determines a different value of thermal expansion. Typically, vulcanization is carried out at a temperature higher than the normal operating temperature of the shoe or, in any case, of the rom temperature (vulcanization normally occurs at temperatures above 150°C, while the peak temperatures of a shoe press in operating conditions hardly exceed 70°-80°C). Therefore, following cooling, the layer of elastic material tends to shrink more than the underlying metal layer to which the elastic material is joined. For this reason, at the room temperature or even at the operating temperature of the shoe press, the elastic block (11) can assume a tensile load state that, due to the high flexibility of the underlying metal layer due to its limited thickness, can determine a concave deformity of the shoe with the concavity facing the counter-presser as schematically represented in Fig.9. However, this deformity does not represent a problem from a functional point of view, since the loads necessary to bring the shoe back to a horizontal configuration (cancelling elastically the concavity) are considerably lower than those exerted by the actuation system of the shoe itself (in fact, a shoe elastically deformed due to the tensioning of the elastic layer can be brought back to the flat geometry canceling the concavity simply by manually pressing its ends, with a load of 20-30N, while the overall loads exerted by the actuation system normally exceeds 250,000 N).
In any case, this deformation of the shoe in rest conditions can also be reduced or canceled (in case it creates problems of interference with the sock or with other parts of the shoe press, when in rest configuration) by providing a curvature of the rigid support (12) in the opposite direction to the concavity resulting from the vulcanization of the elastic block. Since the stiffness of the metal support (12) is much greater than that of the elastic block (11), a limited pre-deformation is sufficient, as schematically represented in Fig. 10, to allow the metal support to completely compensate for the traction induced by the different expansion of the elastic block during vulcanization; in this way it is possible, case by case, knowing the elasticity of the materials used and the geometry of the shoe, to completely compensate for the deformation resulting in the unloaded press conditions.
Given that this deformation does not represent an obstacle to the functioning of the shoe, it is preferable to tolerate it knowing that, in the operative phase, the disturbing effect resulting from the deformation, in terms of non-uniformity induced on the linear pressure exerted on the sheet (S), is negligible.
A shoe press in accordance with the present invention offers numerous technical advantages.
The overall flexibility of the shoe is significantly increased compared to the typical flexibility of a shoe made entirely of metallic material or of a shoe made up of an elastic element mounted on a substantially non-deformable support. This provides the advantage of a greater ability of the shoe to "copy" the deformed profile of the Yankee in correspondence with the pressure zone. In this way, the shoe, adapting itself to the profile of the Yankee and, consequently, deforming, reacts elastically, exerting a reaction which disturbs the uniformity of the ideal pressure profile; but this reaction, although present, is much smaller compared to the linear pressure exerted against the Yankee. In fact, halving the thickness of the metal support layer compared to the thickness of an original metal shoe (whose thickness cannot be reduced beyond certain limits, to allow maintaining the material necessary for the creation of the pressure profile) thanks to the possibility of creating the desired geometric profile (112) on the elastic material, allows the overall elastic reaction exerted by the shoe following a deformation to be reduced by 8 times (neglecting the contribution of block 11 whose modulus of elasticity is 2-4 orders of magnitude lower than that of the metal substrate 12).
The presence of a rigid metal substrate stably connected to the upper layer made of a more elastic material allows the application of the loads necessary for the operation of the shoe press (which can reach very high values, even higher than 150 N/mm linear of shoe) to be managed more easily. Systems made entirely of elastic material that require the use of pressurized hydraulic chambers arranged underneath or inside the shoe are particularly unreliable in continuous use, tending to deteriorate quickly up to breaking. The costs associated with repairing such a component, in terms of production loss, can be very high. On the contrary, the presence of a metal substrate allows the loads necessary for the operation of the shoe press to be applied by means of hydraulic actuators applied against the metal substrate directly or through a possible further layer of elastic material, not rigidly connected to the shoe, whose function is to compensate for the differences in instantaneous positioning of the piston heads in the transient phases (loading or unloading) and to increase the width of the so-called load distribution cone on the shoe by increasing the distance between the point of application of the load by the pistons and the upper profile of the block 11 (this is to avoid the presence of areas of pressure concentration due to a eventuale lack of distribution of the load applied by the single cylinder). The use of hydraulic actuators, much more robust components than pressurized air chambers made of elastic materials, increases the reliability of the system and avoids frequent breakages with consequent long repair times and machine downtime.
The shoe made with the layer of elastic material on its upper part allows to take advantage of the greater adaptability of the shoe profile in the advancement direction of the paper (machine direction). In this way it is possible to increase the pressure footprint (area of the nip actually affected by the contact between the presser and the counter-presser), consequently increasing the time during which the paper remains in the pressure nip. This is particularly useful for producing paper with high bulk values (a measurement equal to the inverse of the density and representative of the thickness of the paper produced, indicative, in turn, of the drying characteristics of the sheet), renouncing to concentrate the pressure profile in the machine direction over a less extensive area (useful for increasing water removal at the expense of the bulk), but distributing the pressure more correctly along the entire pressure footprint in the machine direction .
In practice, the execution details can however vary in an equivalent way as regards the individual elements described and illustrated, without thereby departing from the protection offered by this patent in accordance with the following claims.

Claims

Shoe press for forming a pressure nip (EN) in cooperation with a counter-presser (2), in which the counter-presser (2) assumes a deformed profile (P2) in correspondence with the pressure nip (EN) as a consequence of a load applied by the shoe press on the counter-presser, wherein the shoe press comprises a presser (10) formed by an upper block (11) and a respective support (12) enslaved to an actuation unit (3) which controls its activation by applying a thrust (H) of a predetermined value on a lower surface (122) of the support (12), wherein the upper block (11) has a greater elasticity than the support (12), wherein the upper block (11) and the support (12) are joined together to form a single body, wherein the actuation system (3) comprises a plurality of pistons (30) arranged along a direction (CD) of prevalent development of the presser (10), wherein the support (12) is a flat metal element, and wherein the upper block (11) and the support (12) have respective counter-faced flat surfaces (110, 120), such that the block (11) and the support (12) are deformable as a whole so as to copy the deformed profile (P2) of the counter-presser (2).
Shoe press according to claim 1, wherein the upper block (11) is glued to the support (12).
Shoe press according to claim 1, wherein the upper block (11) is vulcanized onto the support (12).
Shoe press according to any of claims 1-3, wherein the support (12) has depressions (121) of predefined shape on an upper surface (120) and the upper block (11) exhibits appendages (111) on a lower surface (110) having a shape corresponding to that of said depressions (121) and the appendages (111) of the upper block (11) are inserted in the depressions (121) of the support (12).
Shoe press according to claim 4, wherein said depressions (121) are grooves formed on the upper surface (120) of the support (12) oriented along the prevailing development direction (CD) of the presser (10).
Shoe press according to any of the previous claims, wherein the upper block (11) has a hardness between 80 Shore A and 100 Shore A, preferably a hardness between 92 and 96 Shore A and even more preferably a hardness of 95 Shore A.
Shoe press according to claim 1, wherein the support (12) is made of aluminum, bronze or stainless steel.
Shoe press according to claim 1, wherein the support (12) is a flat metal body with a thickness comprised between 5 mm and 20 mm, preferably with a thickness comprised between 7 mm and 13 mm.
Shoe press according to any of the previous claims, wherein the upper block (11) is made of polyurethane.
Shoe press according to any of the previous claims, wherein said pistons (30) are arranged in two rows parallel to each other and the pistons of each row are controlled independently of the pistons of the other row.
Shoe press according to claim 1, wherein a layer of elastic material (6) is interposed between the pistons of the actuation system (3) and the lower surface (122) of the support (12).