AN ARRANGEMENT IN THE DRYING SECTION OF A PAPER MACHINE

Abstract: The present invention relates to an arrangement in the drying section of a paper machine, preferably the drying section of a cylinder drying machine, for enabling the shrinkage and/or stretch, of a paper web to be controlled transversely to the direction of web travel with the aid of a number, normally two, straps (2), each of which extends or passes through the whole of the drying section or a part thereof, and is positioned adjacent to an edge part of the paper web (1) for the purpose of imparting to the web (1) a force which is oriented at right angles to the direction of web travel and which acts essentially in the edge portion of the web. The strap (2) is coated on one side thereof (4, 5) with an adhesive layer facing the paper web (1) and intended for co-action therewith, or vice versa.

Description: The present invention relates to an arrangement in the drying section of a paper machine. Although the present invention can be applied in all kinds of drying sections, it will be described here solely with reference to the drying sections of cylinder drying machines. The primary object of the invention is to enable the shrinkage and/or stretch of a paper web transversely to the direction in which the web is transported to be controlled with the aid of a number of straps, normally two, which are each intended to extend or to pass through the whole, or a part of the drying section and are positioned in the proximity of the edges of the web in a manner to impart to the paper web during the drying process a force which is oriented at right angles to said transport direction and which acts essentially in the edge regions of the web. BACKGROUND PRIOR ART The known prior art consists in the methods and arrangements described and illustrated in International Patent Application PCT/SE85/00068 published under International Publication No. WO 85/3534. These known methods and arrangements are effective in controlling shrinkage and/or stretch of a paper web during a web drying process, by subjecting the paper web to an outwardly directed force during that part of the drying process in which the web has a dry solids content of at most 75% and in which the force applied is caused to act in the edge regions of the web. This known publication proposes various methods for holding the edges of a paper web against the drying cylinders within the drying section during a web drying sequence. The publication teaches to this end an arrangement which incorporates grooved edges and an arrangement which incorporates slalom wires, or S-wires, and in which a cylinder is provided with two peripheral rings which engage in grooves provided in the wire in a manner to force the web into the grooves. The publication also teaches an arrangement in slalom wires in which the paper web is pressed into and held in two grooves with the aid of ribs, and also an arrangement in which edge straps are arranged on both sides of the web in the edge regions thereof and are adapted to hold the edges of the web against the cylinder via grooves formed therearound. The publication also teaches that the edge straps, or bands, can be held to and guided against the cylinder by means of two rings. Finally, the publication discloses an arrangement in which the edge straps are provided with longitudinally extending flanges, such that the flanges of two similarly located edge straps will co-act with and be held firmly against recesses formed in the periphery of the cylinder, preferably adjacent the end walls thereof. The present invention can be said to be a further development of this latter arrangement. DISCLOSURE OF THE INVENTION TECHNICAL PROBLEMS It will be apparent from the aforegoing that the prior International Application illustrates a plurality of different embodiments for applying oppositely directed forces to the edge portions of a paper web as it passes through the drying sections. When studying the measures taken to this end it will also be seen that a qualified technical problem resides in the provision of conditions, with the aid of simple means, which will enable the respective edge portions of a paper web to follow a pre-determined path throughout the whole of the drying section, or if so desired throughout a part of said section, and then the forwardly located part of said section, and therewith create conditions for controlling and establishing the extent of web shrinkage and/or web stretch at right angles to the direction of web travel as the web passes through the drying section, thereby realizing the technical advantages recited in the aforesaid publication. It will also be seen that a technical problem resides in the simple provision of a strap which, while effectively solving the aforesaid technical problem, is of simple construction and cheap to produce. A further technical problem resides in the provision of a strap which, within reasonable limits, is able to eliminate the risk of accidents at high web speeds. A related technical problem is one of providing with the aid of simple means conditions for ensuring that the edges of the paper web are firmly held to the strap. Another technical problem will be seen to exist in creating, with the aid of simple means, conditions which enable the respective edge portions of the web to adhere to the straps through an adhesive layer, preferably applied to one side thereof, and to provide with the aid of simple means conditions for controlling the orientation of the strap through the drying section. It will also be seen that a technical problem resides in the provision of conditions, with the aid of simple means, with which the adhesive force of the adhesive lager can be made so effective as to resist occurrent tensile forces, preferably forces that occur as a result of the tendency of the web to shrink and/or an intention to stretch it, but which, at the same time, will allow the mutually co-acting edge portion and strap to be separated from one another upon completion of the drying process. A further technical problem in this regard is one of providing a strap arrangement which can be adapted readily for adhesive co-action with a paper web and which is suitable for use in a drying section of the aforesaid kind. Another technical problem resides in the provision of a strap which can be incorporated readily in an existing drying section. It will also be seen that a further problem is one of creating in an existing drying section, with the aid of simple means herefor, conditions which will enable one or more adhesive layers to be applied continuously to either the strap or the edge portions of the paper web. Another problem in the present context is one of establishing how wide the adhesive layer must be in order to resist the forces that occur while, at the same time, enabling the strap to be parted readily from the web upon completion of the web drying process; a particular problem in this regard is one of enabling the adhesive layer to hold a paper web firmly to a hot elastic strap which has planar surfaces. It will also be seen that a technical problem resides in the manufacture of a strap which is suitable for the afore-said purpose. SOLUTION The present invention thus relates to an arrangement in the drying section of a paper machine, and preferably in the drying section of a cylinder drying machine, for the purpose of controlling or forced guiding the shrinkage and/or stretch of a paper web transversely to the direction of web travel in the drying section, with the aid of a number of straps, normally two, each of which is intended to extend or to pass through the whole or part of the drying section and is positioned in the vicinity of the outer edges of the web, so as to apply to the paper web during the drying process a force which is oriented at right angles to the direction of web travel and which acts essentially on respective edge portions of the web. It is proposed in accordance with the present arrangement that the strap(s) and/or the paper web is/are coated with an adhesive layer on the side(s) thereof facing the paper web or the strap(s), respectively, the intention being that the strap(s) and the paper web coact with each other. Naturally, both the web and the strap can be provided with a suitable adhesive layer, in order to obtain greater adhesion between the strap and the respective edge portion of the web. According to a further embodiment of the invention two mutually parallel straps are each provided with an adhesive layer for adhesion to a respective edge portion of the web. The width of the adhesive layers is such as to enable the adhesive bond established between strap and web to withstand the forces that occur when the web shrinks during a drying process, and to withstand the forces that occur when the web is to be stretched across its width, the first mentioned being liable to occur as the straps pass through the drying section in a controlled, optionally convergent fashion, whereas the latter may occur when the straps pass through the drying section in a divergent fashion. Advantageously, the strap is arranged to be parted from the paper web over a guide roller, therewith to separate the edge portions of the web from respective straps. The respective straps may comprise a thin section and a thicker section, where the thicker section of the strap incorporates one or more longitudinally extending projections or flanges which are intended to co-act with corresponding grooves formed in the peripheral surfaces of the drying cylinders. The thicker parts of two mutually parallel straps face away from one another while the two thinner parts thereof face towards one another. According to one advantageous embodiment of the invention the strap comprises a plurality of interjoined strap sections. According to another embodiment, however, the strap may be produced in one single piece, with both the thin and the thicker strap parts being of mutually the same material. In this latter case it is proposed that the thinner part of the strap decreases in thickness towards the centre of the web. In accordance with a further advantageous embodiment of the invention the strap comprises a relatively thin centre part and two longitudinally extending projections which are symmetrical to the axial plane of said centre part. The thinner part of the strap in this case may have a thickness of less than 5 mm. The longitudinal projection may be given different cross-sectional shapes, although it is preferred that the projection has a trapezium shaped cross-section and that a co-acting groove of trapezium cross-section is formed in the periphery of the drying cylinder, in the vicinity of one end wall thereof, so as to guide the strap in an effective manner. The strap, or alternatively parts thereof, is (are) preferably made of a polymeric material. In order to simplify manufacture of the strap it is proposed that the thinner strap part comprises a plastics material, preferably a polyamide, polyester or polyurethane, or a combination thereof. In order to enable the thicker strap part to withstand the tension and compression forces which act on the outer surfaces thereof as it passes over guide rollers or the like, it is proposed that this thicker strap part is made of a rubber material or polyurethane suitable for this purpose. In accordance with one preferred embodiment of the invention the adhesive layer is applied to the strap and/or to the paper web immediately prior to the time at which the strap and the paper web are to be brought into co-action with one another. The adhesive may be applied by suitable spraying or roll brushing or pencilling methods. The adhesive layer may conveniently comprise a coating of starch, in which case the strap shall be hot when applying the adhesive thereto, preferably above 50°C. Polymer based adhesives may also be used. It is recommended that the dry solids content of the web is less than 75%, preferably higher than 40%, when bringing the web into bonding co-action with the strap. In accordance with one embodiment of the invention the paper web is adhered to the strap while the web is located in the free draw and in the initial part of the drying section. Furthermore, the adhesive layer is preferably positioned on that part of the strap located nearest the centre of the web. Finally, it lies within the scope of the invention to permit at least that part of the strap on which an adhesive coating is to be applied to be coated with a suitable release agent which will enable the adhesive material to relinquish its hold on the strap more readily subsequent to drying the paper web in the drying section. Alternatively, means may be provided for cutting away the edge portions of the paper web upon completion of the web drying process, and further means may be used to separate the cut paper strips from the strap. In this latter case, the paper strips will accompany the strap through some small distance during its return travel to the coating section, during which time the paper strips can be removed from respective straps and the straps returned for re-use in the paper machine. ADVANTAGES Those advantages primarily afforded by an arrangement according to the present invention reside in the creation of conditions, with the aid of simple means, with which the shrinkage and/or stretch of a paper web in a direction transversely to the direction of web travel can be controlled simply by adhering the web to at least one strap which passes or extends through the drying section and is guided by the drying cylinders thereof. The primary characteristic features of an arrangement according to the present invention are set forth in the characterizing clause of the following Claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments exhibiting characteristics significant of the present invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 illustrates schematically and in side view the forward part of a drying section incorporated in a paper machine; Figure 2 is a front view of a drying cylinder, and illustrates a flanged strap in co-action with holding recesses formed in the periphery of the cylinder, preferably in the vicinity of the end walls of the cylinder; Figure 3 is a perspective view of a strap of the kind illustrated in Figure 2; Figure 4 illustrates an alternative embodiment of the strap; and Figure 5 illustrates a further alternative embodiment of a strap according to the invention. DESCRIPTION OF EMBODIMENTS AT PRESENT PREFERRED Figure 1 illustrates schematically and in side view a forward part of the drying section of a paper machine, comprising upper drying cylinders 6 and lower drying cylinders 6′, and upper guide rolls 7 and lower guide rolls 7′. The paper web to be dried is referenced 1. The drying felt extending over the upper drying cylinder 6 and the upper guide rollers 7 is referenced 8, while the lower drying felt is referenced 8′. Thus, the present invention relates to an arrangement in the drying section of a paper machine for enabling the shrinkage and/or stretch of the paper web to be controlled and guided transversely to the direction of web travel with the aid of a plurality of straps, in the illustrated embodiment two straps, 2, 2a, each of which is intended to extend or to pass through the whole of the drying section, or through a part thereof, and is positioned adjacent a respective edge part of the paper web, in order to impart to the web a force which is oriented at right angles to the direction of web travel and which acts essentially in the respective edge parts of the web. To this end it is proposed in accordance with the invention that two substantially parallel straps 2, 2a shall be arranged to pass over a guide roller 3 and that an adhesive layer for bonding co-action with the paper web 1 is coated on the side of respective straps 2, 2a facing the paper web 1. Although the invention is described with reference to the use of two straps 2, 2a coated with an adhesive layer, it will be understood that solely one strap can be coated with an adhesive layer for co-action with one edge part of the paper web, while the other edge part of the web is arranged to co-act with a known mechanical clamping means or the like. The layer of adhesive is applied in a known manner, either with the aid of a spray nozzle or, as illustrated, with the aid of an applicator roller 4 which co-acts with a roller 5, a doctor blade 5a and a container 5b that contains a liquid adhesive substance 5c. Although in the illustrated embodiment the adhesive is applied solely to one side of the strap 2, it will be understood that adhesive can also be applied to one side of the paper web 1, and then particularly to one side of the edge portion of the web. Alternatively, both the straps 2, 2a and the edge portions of the web may be provided with such an adhesive layer. It is essential, however, that the two straps 2, 2a and the edge portions 1a of the paper web 1 are caused to adhere to one another, and it is also important that the width of the adhesive layer is sufficient to enable the bond established to resist the forces that occur when the paper web shrinks and/or stretches. The straps 2 and 2a are assigned a movement path through the drying section which is adapted to the stretching ratio desired. If it is desired to stretch the paper web across the whole of the web width present in the entrance to the drying section, the straps 2, 2a shall diverge through the desired part of the drying section, i.e. the distance between the straps is greater for each drying cylinder. If the web is only to be stretched to a slight extent, the straps 2, 2a shall converge, and then solely to a negligibly lesser extent than the natural decrease in width of a paper web on which no stretching action is taken. Figure 2 is a front view, partially in section, of a drying cylinder 6 with which a strap 2a co-acts around the cylinder periphery adjacent one end wall of the cylinder. A similar strap co-acts with the oppositely located peripheral surface of the cylinder and end wall thereof. The strap 2a comprises a thin section 9, which rests against the peripheral surface of the cylinder, and a thicker section 10. The adhesive layer 11 is made effective to hold the edge part 1a of the web 1 firmly, despite the forces acting towards the centre of the paper web during the drying process. Although the felt 8 is not shown in Figure 2, it will be understood that the felt is positioned over the web 1, and preferably also somewhat inwardly over the adhesive layer 11. Although not shown in Figure 1, it will be obvious that the two edge parts 1a of the paper web shall be arranged to part from respective straps 2a at the terminal end of the drying section. This can be effected by causing respective straps to be parted from the paper web over a guide roller, such as the guide roller 3, while permitting the paper web to continue along its movement path. The configuration of the strap 2a is shown in more detail in Figure 3, from which it can be seen that the thicker part 10 of the strap comprises two longitudinally extending projections, a projection 12 on the upper side of the strap and a projection 13 on the underside thereof, said projections being intended to co-act with grooves 14 formed in the periphery of the drying cylinder adjacent the end walls thereof. It will also be seen from Figure 2 that when two mutually parallel extending straps 2, 2a are arranged adjacent a respective edge portion of the paper web 1, the thicker parts 10 of the straps shall face away from one another while the thinner parts 9 of said straps shall face towards one another. As will be seen from Figure 3, the adhesive layer 11 shall have a width extension which occupies at least 25% and at most 75% of the total surface of the thinner strap part 9. The total width may be 30 cm and the width of the adhesive layer may be about 10 cm. The adhesive layer 11 is applied to that part of the strap or the strap surface 9, which faces towards the centre of the web 1, i.e. away from the thicker part 10. As illustrated in Figure 4, the thinner part 9 of the strap has a thickness which decreases towards the centre of the paper web, and a particular advantage is afforded when the adhesive material is applied to this surface of decreasing thickness. In the embodiment illustrated in Figures 3 and 4 the strap comprises a thin centre part and two projections 12, 13 which are symmetrical with respect to the axial centre plane of the strap. Preferably, the thinner part of the strap has a thickness beneath 5 mm, preferably a thickness of between 0.5 and 2.0 mm. As illustrated in Figure 1, the paper web 1 is arranged to be attached to the strap 2 and the strap 2a in the free draw and at the beginning of the drying section. The longitudinally extending projections of the Figures 3 and 4 embodiment have a trapezium-shaped cross-section, although it will be understood that the projections may have other suitable cross-sectional shapes. The thinner part 9 of respective straps is suitably made of a polymeric material, particularly a plastics material, such as polyamide, or polyester and/or polyurethane, while the thicker part of the strap is made of a rubber material, since the outer parts of this strap part are perforce subjected to considerable stretching and tension forces when the strap is passed over guide rollers 3 or the like. As illustrated in Figure 1, the adhesive layer is applied to a strap immediately prior to the time at which the strap shall be brought into co-action with the paper web. The strap may pass through the whole of the drying section, or through a first part thereof. The strap may also extend through all drying sections. Web guide tracks may be provided on the peripheral surfaces of the drying cylinders adjacent respective end walls thereof. These guide tracks may comprise a plurality, such as four, extruded aluminium profiles which are positioned adjacent one another. A further roller, additional to the roller 4, may be arranged for coating one edge part of the paper web. An additional roller may also be provided for the other edge part, so that adhesive layers on the edge parts of the paper web may cooperate with adhesive layers on the respective straps 2, 2a. The adhesive is preferably a starch capable of establishing a bond of sufficient strength in the direction of web travel and also capable of enabling the strap to be released from the paper web readily at the end of the drying section. The strap is preferably hot when the adhesive layer is applied, and preferably has a temperature in excess of 50°C. The paper web is brought into co-action with the strap at a location in the drying section in which the paper web has a dry solids content of about 40%, or slightly there-above. Finally, it lies within the scope of the invention to provide at least that part of the strap to which an adhesive material is applied with a substance which will enable the paper web to be separated from the strap more readily, subsequent to drying the paper web in the drying section. Alternatively, means may be provided for cutting away the edge parts of the paper web at the end of the drying section, these separated paper strips being removed from the strap with the aid of further means provided herefor. This will mean that a separated paper strip will accompany the strap during its return travel to the strap coating section, during which time the strips can be removed from the straps and the straps returned for re-use in the paper machine. The adhesive substance used may advantageously be the substance sold under the designation "Moughton Rezosol 8207". Figure 5 illustrates a further alternative embodiment of a strap constructed in accordance with the invention. With regard to this embodiment, and also with the aforedescribed embodiments, it will be noted that the part of the strap to be coated with an adhesive layer 11 comprises a material that has a tendency toward hydrogen bonds, such as polyamide. This has the advantage of a greater bond strength with increasing dry solids contents. One requisite herefor is, of course, that the adhesive used contains hydrogen for effecting the bond. When the invention is applied in a paper drying machine, the paper already has the ability to bind hydrogen. Thus, it will be seen that by pre-heating the strap, and then particularly the part 11 thereof, conditions are created for obtaining improved adhesion. The adhesive material preferably comprises a water-soluble polymer, such as polyvinyl alcohol. In Figure 5 a strap having a thicker rubber part 10 presenting trapezium shaped parts 12 and 13 is joined to a fabric material 9a. This fabric material shall have a width which is so adapted to the drying section concerned that the part 11 can be positioned on the edge part 1a of the paper web. The part 11 has a length of from 10 to 50 mm and a length which is adapted to the desired width of the adhesive bead. The web material 9a has a thicker part 9a′ adjacent the part 10, and a thinner part 9a″ adjacent the part 11″. It will be understood that the invention is not restricted to the aforedescribed embodiments, and that modifications can be made within the scope of the following claims.

Claims: 1. An arrangement in the drying section of a paper machine, preferably the drying section of a cylinder drying machine, for enabling the shrinkage and/or stretch of a paper web to be controlled transversely to the direction of web travel in the drying section, with the aid of a number of, normally two, straps which extend or pass through the whole of the drying section, or a part thereof, and are positioned adjacent an edge part of the paper web so as to impart thereto a force which is oriented at right angles to the direction of web travel and which acts essentially on said edge part of the paper web, characterized in that the strap(s) and/or the paper web is/are coated with an adhesive layer on the side(s) thereof facing the paper web or the strap(s), respectively, the intention being that the strap(s) and the paper web coact with each other. 2. An arrangement according to Claim 1, characterized in that two adhesive layers on two mutually parallel extending straps are arranged to adhere to respective edge parts of the paper web, with the width of the layers being adapted so as to establish an adhesive bond which will withstand the forces that occur during shrinkage and/or stretching of the paper web. 3. An arrangement according to Claim 1 or 2, characterized by a guide roller over which the edge part of the paper web is caused to part from a respective strap upon completion of a drying process. 4. An arrangement according to any of the preceding Claims, characterized in that the strap comprises a thin part and a thicker part. 5. An arrangement according to Claim 4, characterized in that the thicker part of the strap comprises one or more longitudinally extending projections or flanges adapted for co-action with corresponding grooves formed in the peripheral surface of the drying cylinders. 6. An arrangement according to Claim 2, 4 or 5, characterized in that in the case of two mutually parallel extending straps, the thicker parts of the straps face away from one another and the thinner parts thereof face towards one another. 7. An arrangement according to any of the preceding Claims, characterized in that the adhesive layer covers at least 25% and at most 75% of the thinner strap part. 8. An arrangement according to Claim 1 or 7, characterized in that the adhesive layer is applied to that part of the strap which is located nearest the centre of the paper web. 9. An arrangement according to Claim 1 or 6, characterized in that the thin part of the strap has a thickness which decreases towards the centre of the paper web. 10. An arrangement according to Claim 1, 4 or 5, characterized in that the strap comprises a thin centre part and two projections which are symmetrical with respect to a centre plane of the strap. 11. An arrangement according to Claim 4, 9 or 10, characterized in that the thin strap part has a thickness beneath 5 mm, preferably a thickness of 0.5-2.0 mm. 12. An arrangement according to any of the preceding Claims, characterized in that the paper web is arranged to attach firmly to one or more straps in the free draw of the paper machine. 13. An arrangement according to Claim 5, characterized in that the longitudinally extending projection has a trapezium-shaped cross-section. 14. An arrangement according to Claim 1, characterized in that the strap comprises a polymeric material. 15. An arrangement according to Claim 4, 11 or 14, characterized in that the thin strap part comprises a plastics material, such as polyamide or polyester and/or polyurethane. 16. An arrangement according to Claim 4 or 14, characterized in that the thicker strap part comprises a rubber material. 17. An arrangement according to Claim 1, characterized in that the adhesive layer is applied to the strap immediately prior to the time at which the strap is intended to be brought into co-action with the paper web. 18. An arrangement according to Claim 1, characterized in that the adhesive layer comprises a starch. 19. An arrangement according to Claim 17 or 18, characterized in that the strap is hot when the adhesive layer is applied, preferably having a temperature in excess of 50°C. 20. An arrangement according to Claim 1, characterized in that the paper web is brought into co-action with the strap at a location in the drying section in which the paper web has a dry solids content in excess of 40%. 21. An arrangement according to Claim 1, characterized in that there is first applied to at least that part of the strap to which the adhesive material is to be applied a substance which will enable the paper web to be removed more readily from the strap upon the completion of the drying process. 22. An arrangement according to any of the preceding Claims, characterized by means provided at the terminal end of the drying section for cutting the edge portions away from the paper web, and by further means for removing the subsequent paper strips from the straps adhered thereto.

Inventors: HANSSON, TORBJOERN, Hansson, Torbjörn
Applicants: FLAEKT AKTIEBOLAG
Application year: 1987
Main classification: D21F5
IPC codes: D21F5, D21F1