SCREEN CYLINDER MANUFACTURING METHOD AND SCREEN CYLINDER

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method of manufacturing a screen cylinder, the method comprising fitting screen wires forming the screen surface of the screen cylinder in connection with supporting bars of the screen cylinder, into grooves formed adjacently at predetermined intervals in the supporting bars; bending the supporting bars into a ring shape; and attaching the ends of the supporting bars bent into the ring shape together in such a way that the screen wires form a cylindrical screen surface in the direction of the axis of the screen cylinder.

[0002] Further, the invention relates to a screen cylinder for purifying and screening a pulp mixture, the screen cylinder comprising screen wires which form the cylindrical screen surface of the screen cylinder and have been fitted into grooves formed at predetermined intervals in ring-shaped supporting bars of the screen cylinder.

[0003] Screen cylinders are used, for example, for purifying and screening a pulp mixture. Screen cylinders may be manufactured for instance in such a way that parallel screen wires forming a screen surface are fitted adjacent to each other in such a way that a screen slot of a desired size remains between them. Most often this is done by forming grooves at even intervals in separate supporting bars, the screen wires being fitted into the grooves. The width of the grooves is typically some hundredth parts of a millimetre wider than the base part of the screen wires. After this, the screen wires are attached to the supporting bars by, for example, laser welding or brazing. The plate-like screen panel thus formed is bent into a cylindrical shape by rolling the supporting bars into a ring shape. The ends of the supporting bars bent into the ring shape are attached together by, for example, welding. Around the supporting bars, special supporting hoops may be mounted to support the screen cylinder structure. One screen cylinder of this type is disclosed in US publication 6 589 424, for instance.

[0004] Rolling of a straight plate-like panel into a cylindrical shape does not, however, remove the clearances between the base parts of the screen wires and the fitting grooves of the screen wires. This clearance may even cause great variation in the width of the screen slot between the screen wires. A clearance of 0.03 millimetres, for example, may cause an error of up

to 0.06 millimetres in the width of the screen slot. An error as great as this is extremely significant when the screen slot has a width of 0.15 millimetres, for instance.

[0005] The clearance may also deteriorate the mechanical durability of a screen cylinder. When examined dynamically, the screen wire is not, due to the clearance, supported against the side surface of the groove in the supporting bar but is merely supported by a laser weld. Since the screen wire is not supported against the edges of the groove in the supporting bar, the force effects occurring in the use of the screen cylinder may cause the screen wire to live relative to the weld and, in the worst case, they may finally cause the screen wire to be detached from the weld.

BRIEF DESCRIPTION OF THE INVENTION

[0006] An object of this invention is to provide a new type of method for manufacturing a screen cylinder.

[0007] The method according to the invention is characterized by pressing the supporting bars at one of the manufacturing stages of the screen cylinder after having fitted the screen wires into the grooves of the supporting bars of the screen cylinder in order to remove the clearances between the screen wires and the grooves of the supporting bars.

[0008] The screen cylinder according to the invention is characterized in that the supporting bars have been pressed at one of the manufacturing stages of the screen cylinder in order to remove the clearances between the screen wires and the supporting bars.

[0009] In order to manufacture a screen cylinder, screen wires forming the screen surface of the screen cylinder are fitted in connection with the supporting bars of the screen cylinder, into grooves formed adjacently at predetermined intervals in the supporting bars. After this, the supporting bars are bent into a ring shape, and the ends of the supporting bars bent into the ring shape are attached together in such a way that the screen wires form a cylindrical screen surface in the direction of the screen cylinder axis. Further, at some point in manufacturing the screen cylinder after having fitted the screen wires in the grooves of the supporting bars of the screen cylinder, the supporting bars are pressed to remove the clearances between the screen wires and the grooves of the supporting bars.

[0010] By removing the clearances between the screen wires and the fitting grooves formed in their supporting bars by pressing the supporting bars, these clearances can be removed in a manner which is simple but requires some force, using apparatuses with a simple structure and operation. With regard to the costs, the solution is also inexpensive because today it is not possible to competitively manufacture joints without clearances by increasing the machining accuracy of the grooves and the rolling accuracy of the screen wires.

[0011] According to one embodiment, the supporting bars are pressed inwards in the direction of the screen cylinder radius after the ends of the supporting bars bent into a ring shape have been attached together and a cylindrical screen cylinder structure has been provided. Thus, the need of pressing for the supporting bars becomes reasonable, measured as a distance, and is achievable without greater forces. Such a pressing arrangement is easily implementable, because when pressed as a ring, the structure is not particularly susceptible to buckling. Further, the possible buckling tendency is primarily directed only to the inside of the screen cylinder, which is easily preventable. Furthermore, the attaching point between the ends of the supporting bars can thus be made similar to the rest of the supporting ring with regard to the roundness, strength and other properties.

[0012] According to a second embodiment, the ends of the supporting bars are pressed together before bending the supporting bars into a ring shape. Pressing the supporting bars when they are straight allows manufacturing screen panels of screen cylinders of different sizes with simple arrangements because it is easy to arrange support structures of different sizes as a plane structure. On the other hand, large screen panels can be manufactured, out of which smaller screen panels may be mechanically worked to serve correspondingly as preforms of screen cylinders of different sizes.

BRIEF DESCRIPTION OF THE FIGURES

[0013] Embodiments of the invention are explained in more detail in the attached drawings, in which

Figure 1 shows schematically a cross-section of the basic structure of a screen cylinder, seen from the direction of the screen cylinder axis;

Figure 2 shows schematically the screen cylinder according to Figure 1 , cross-sectioned in the direction of the screen cylinder axis;

Figure 3 shows schematically the structure of a possible joint between the screen wire and the supporting bar of the screen cylinder according to Figures 1 and 2, seen from the end of the screen wire;

Figure 4 shows schematically an arrangement for manufacturing a screen cylinder, seen from the screen cylinder axis;

Figure 5 shows schematically a side view and a partial cross-section of a second arrangement for manufacturing a screen cylinder; and

Figure 6 shows schematically a top view of the arrangement according to Figure 5 for manufacturing a screen cylinder.

[0014] For the sake of clarity, embodiments of the invention are shown simplified in the figures. Similar parts are denoted with the same reference numerals in the figures.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Figure 1 shows schematically a cross-section of a screen cylinder 1 , seen from the direction of the screen cylinder axis; Figure 2 shows schematically the screen cylinder 1 according to Figure 1 , cross-sectioned in the direction of the screen cylinder axis; and Figure 3 shows schematically the structure of a possible joint between the screen wire and the supporting bar of the screen cylinder, seen from the end of the screen wire. Around the whole inner periphery of the screen cylinder 1 , there are screen wires 2 forming the screen surface of the screen cylinder 1. Between the screen wires 2, there are screen slots 3, through which liquid and a desired part of the fibres can flow to the outside of the screen cylinder 1 at the same time as sticks, too large fibres, fibre bundles and other material to be separated remain on the inner surface of the screen cylinder 1 to be removed from its other end. The screen wires 2 are typically fitted in connection with supporting bars 4 before bending the supporting bars 4 into a ring shape so as to form a screen cylinder 1 with a diameter of an appropriate size. For the sake of clarity, Figure 1 shows arrow R to indicate the radius of the screen cylinder 1. In Figure 1 , ends 4' and 4" of the supporting bars 4 have been attached together by welding at an attaching point 4a in such a way that the supporting bars 4 form a ring-like structure. There are supporting bars 4 in the direction of the axis of the screen cylinder 1 at appropriate intervals, so that the screen wires 2 remain firmly and rigidly enough in place. One example of a typical screen cylinder is one having a diameter of

1 ,000 mm, the screen wire width of 4 mm and the screen slot width of 0.25 mm, whereby the number of screen wires in the screen cylinder is 739.

[0016] On the inner surface of the supporting bars 4, i.e. on the surface directed towards the inside of the screen cylinder 1 , there are fitting grooves 5 or attaching grooves 5 or grooves 5 of the screen wires 2, into which the screen wires 2 are fitted. The grooves 5 have been made in the supporting bars 4 in the direction transverse to their longitudinal direction, and they have a first edge 5', a second edge 5" and a bottom 5'". The screen wire 2 may be fitted in the groove 5 in such a way that the screen wire 2 is not specially attached to the groove 5 but that between the screen wire 2 and the groove 5, a pressing effect or shape locking which attaches the screen wire 2 to the groove

5 is generated when the supporting bar 4 is bent into a ring shape. When the screen wires 2 are fitted into the grooves 5 of the supporting bars 4, the screen wires 2 may also be specially attached to the groove 5, for example by welding, such as laser welding or soldering, such as brazing or gluing the screen wire 2 by its base 2' to the bottom 5'" of the groove 5, which ensures that the screen wires 2 are not detached from the grooves 5 during the later manufacturing stages of the screen cylinder 1. For the sake of clarity, Figure 3 does not show this attachment in greater detail. The width of the groove 5, i.e. the distance between the first edge 5 ¹ and the second edge 5" of the groove 5, is originally typically somewhat wider than the base 2' of the screen wire 2, so that the screen wire 2 can be fitted into the supporting bar 4. Consequently, there remain clearances between the base 2 ¹ of the screen wire 2 and the edges 5', 5" of the groove 5, allowing movement, i.e. living, or deformation of the screen wire 2 relative to the welding, as a result of which the screen wire 2 may de detached from the welding. Figure 3 shows the screen cylinder 1 in a situation where these clearances have already been removed at one of the manufacturing stages of the screen cylinder 1 , for example in exemplary manners of manufacturing a screen cylinder, presented in the following.

[0017] Figures 1 and 2 further show schematically supporting hoops

6 arranged around the supporting bars 4, the purpose of which is to support the structure of the screen cylinder 1. The supporting hoops 6 can be arranged either around all supporting bars 4 of the screen cylinder 1 or around only some of the supporting bars 4.

[0018] Figure 4 shows schematically from the direction of the axis of the screen cylinder 1 a method and an arrangement where the clearances be-

tween the screen wires 2 and the grooves 5 of the supporting bars 4 are removed from the screen cylinder 1 after having bent the supporting bars 4 into a ring shape and having attached the ends 4 ¹ and 4" of the supporting bars 4 bent into the ring shape together by welding, for example. In the solution of Figure 4, means are arranged around each supporting bar 4 to press the supporting bar 4. In the case of Figure 4, these means comprise a roller chain 7 having rollers 8 arranged as a chain. Further, the means comprise a hydraulic cylinder 9, a first end T of the roller chain 7 being attached to its fixed end 10 and a second end 7" of the roller chain 7 being attached to its moving end 11 , i.e. piston. Subsequently, pumping of pressure fluid into hydraulic cylinders 13 begins via an operating unit 12 and required hoses 13, 14 in such a way that the pistons of the hydraulic cylinders start moving inwards into the hydraulic cylinders 9. Each hydraulic cylinder 9 may have an operating unit 12 of its own, or all hydraulic cylinders may have one common operating unit 12. When the piston of the hydraulic cylinder 9 starts moving inwards, the roller chain 7 begins to tighten around the supporting bar 4 and press the supporting bar 4 in the direction of the radius of the screen cylinder 1 in such a way that the clearances between the screen wires 2 and the grooves of the supporting bar 4 begin to disappear or become smaller, as a result of which the diameter of the screen cylinder 1 becomes slightly smaller. The rolling friction of the rollers 8 of the roller chain 7 allows the pressing force caused by the roller chain 7 to be evenly distributed around the screen cylinder 1. Substantially the same pressure can be pumped substantially simultaneously to all hydraulic cylinders 14 in such a way that all supporting bars 4 are subjected to the same kind of pressing effect substantially simultaneously. If required, a different pressure can be pumped to some of the hydraulic cylinders 14, in which case a given part of the hydraulic cylinders 1 obtain a different pressing effect. If considered necessary, the interior of the screen cylinder 1 may, for the duration of the shrinking of the screen cylinder 1 , be provided with a cylinder which does not restrict the diameter of the screen cylinder 1 but the purpose of which is to prevent the screen cylinder 1 from buckling inwards.

[0019] The pressure of the hydraulic cylinders 9 is raised so high that after depressurization there are no clearances between the screen wires 2 and the supporting bars 4. Acceleration of the increase in the hydraulic pressure indicates that the clearances between the screen wires 2 and the edges 5', 5" of the grooves 5 have been disappeared completely, in other words the

edges 5' and 5" of the grooves 5 have become attached to the bases 2 ¹ of the screen wires 2. Pressing can be continued until the supporting bar 4 and the screen wire portion fitted into the groove 5 of the supporting bar 4 become in practice completely plasticized in the pressing. The plasticized material may, due to the stopping of the plasticizing pressing, be recovered by the amount of the elastic deformation. After the plasticizing pressing, when the pressing has been stopped, the structure of the screen cylinder is nearly strain-free. Due to the elastic recovery of the portion of the screen wire 2 having remained outside the pressing, which may be smaller than the recovery of the plasticized material, there may remain a small clearance between the screen wire 2 and the supporting bar 4 nearest the unpressed portion of the screen wire 2 in the joint of the supporting bar 4 and the screen wire 2. This clearance disappears, however, after the supporting hoop 6 has been compressed around the supporting bar 4. The supporting hoops 6 are arranged around the supporting bars 4 after the pressing of the supporting bars 4 in such a way that a compression joint is formed between the supporting bars 4 and the supporting hoops 6, whereby a firm, clearance-free joint is formed between the screen wires 2 and the supporting bars 4. The joint between the screen wires 2 and the supporting bars 4, formed in this way, has a multiple static and particularly dynamic weight- carrying capacity as compared with previous pressure joints where clearances have not been removed in accordance with the presented solution. Disappearance of the clearances between the screen wires 2 and the supporting bars 4 can be checked visually by utilizing a device provided with 10-fold magnification, for instance.

[0020] In the case of the above exemplary cylinder which has 739 screen wires, and if there is a clearance of 0.03 mm in the joint of each screen wire, the shrinking need is 739 times 0.03 mm, i.e. 22.17 mm, in order for the clearances to close. In the diameter of the screen cylinder 1 , this means a shrinking need of about 7 mm. When the pressing of the supporting bars to remove the clearances is done at the stage where the supporting bars 4 have already been bent into a ring shape and their ends 4', 4" have been attached together to form a cylindrical structure, the pressing need, measured as a distance, becomes reasonable and is achievable without significant operating pressures of the hydraulic cylinders. Furthermore, the means required for the pressing are very simple.

[0021] When the screen cylinder 1 has been successfully shrunk in such a way that the clearances have disappeared, the means used for shrinking the screen cylinder 1 are dismounted. After this, the ring-shaped supporting hoops 6 mentioned above are mounted around one or more supporting bars 4 of the screen cylinder 1 by forming a compression joint between the supporting bars 4 and the supporting hoops 6. The supporting hoops 6 are mounted around the supporting bars 6 in such a way that the supporting hoops 6 are first heated, as a result of which the supporting hoops 6 expand in such a way that the diameter of the supporting hoops 6 increases. After the heating, the supporting hoops 6 are mounted around the supporting bars 4. After this, when cooling, the supporting hoops 6 shrink in such a way that the supporting hoops 6 are pressed around the supporting bars 4. When being pressed around the supporting bars 4, the supporting hoops 6 also simultaneously remove the small clearances caused by the possible recovery of the supporting bars 4 when the pressing has been stopped before the supporting bar 4 and the portion of the screen wire 2 fitted in the groove 5 of the supporting bar 4 have been plasticized in the area of the joint between the supporting bar 4 and the screen wire 2. When the pressing has been continued so long that the supporting bar 4 and the portion of the screen wire 2 fitted into the groove 5 of the supporting bar 4 have been plasticized, clearances are no longer generated after the pressing because after the plasticizing pressing the plasticized supporting bar 4 and the plasticized portion of the screen wire 2 recover equally much and the structure of the screen cylinder 1 remains, as mentioned above, nearly strain-free. The compression joint between the supporting bars 4 and the supporting hoops 6 also contributes to the uniformity of the joint 4a between the ends of the supporting bars 4 with the other part of the supporting bars 4.

[0022] In the embodiment shown in Figure 4, roller chains, hydraulic cylinders and devices required for using hydraulic cylinders are used for removing the clearances but it is obvious that also other appropriate means may be used for removing the clearances by shrinking the screen cylinder 1.

[0023] It is also feasible to anneal the supporting bars 4, so that it becomes soft, before shrinking the screen cylinder 1 , in which case the force required for shrinking the screen cylinder 1 is reduced. At the same time, the elastic recovery of the supporting bar 4 would be reduced. Further, by making the sides of the base 2' of the screen wire 2 and the edges of the grooves 5 of

the supporting bar 4 completely straight, generation of a force pushing the screen wire 2 outwards from the groove 5 of the supporting bar 4 can be avoided.

[0024] Figures 5 and 6 show schematically a second method and arrangement for removing the clearances between the screen wires 2 and the grooves 5 of the supporting bars 4 from the screen cylinder. A screen panel being the first stage of the screen cylinder 1 , formed of the supporting bars 4 and screen wires 2 attached to them, is shown as a side view in Figure 5, seen from the point of section A-A of Figure 6, and as a top view in Figure 6, before the supporting bars 4 have been bent into a ring shape. The screen panel has been positioned on a pressing base 15. The arrangement according to Figures 5 and 6 further comprises hydraulic cylinders 16 and a rear counterpart 17 of the pressing base 15, against which one end of the screen panel is arranged, and a front counterpart 18 of the pressing base 15, against which the fixed ends of the hydraulic cylinders 16 are arranged. The arrangement further comprises an intermediate beam 19, which is positioned between the screen panel and the hydraulic cylinders 16 in such a way that pistons 20 of the hydraulic cylinders 16 have been attached either fixedly or detachably to the intermediate beam 19. When removal of clearances between the screen wires 2 and the grooves 5 of the supporting bars 4 is begun, pressure fluid is conveyed into the hydraulic cylinders 16 to move the pistons 20 of the cylinders 16 to the left in the context of Figures 5 and 6, whereby the pistons 20 begin to move the intermediate beam 19 to the left. Thus, the supporting bars 4 remain pressed between the rear counterpart 17 of the pressing base 15 and the intermediate beam 19, in other words the ends of the supporting bars 4 are pressed. When the rear counterpart 17 remains fixedly in place and the intermediate beam 19 moves further to the left in the context of Figures 5 and 6, an increasingly great pressing effect is directed at the supporting bars 4, as a result of which the clearances between the screen wires 2 and the grooves 5 of the supporting bars 4 disappear. At the same time, the length of the supporting bars 4 is reduced by the amount of the clearances being removed. When the clearances have been removed completely, the pressure needed for pressing begins to increase greatly, whereby the pressing can be stopped. The pressing can, however, be continued until the pressing pressure does not increase any longer although the pressing continues, whereby what is called a pressing- displacement curve has turned horizontal, meaning that the material loaded

with the pressing has been completely plasticized. In other words, the supporting bar 4 and the portion of the screen wire 2 fitted into the groove 5 of the supporting bar 4 have thus been plasticized in the area of the joint between the supporting bar 4 and the screen wire 2. Continuing the pressing until this plas- ticization takes place ensures complete removal of the clearances. As mentioned above, due to the stopping of the plasticizing pressing, the plasticized material may recover by the amount of elastic deformation. Due to the elastic recovery of that portion of the screen wire 2 which has remained outside the pressing, which elastic recovery may be smaller than the recovery of the plasticized material, there may remain a small clearance between the screen wire 2 and the supporting bar 4 closest to the unpressed portion of the screen wire 2 in the joint of the supporting bar 4 and the screen wire 2. This clearance is, however, removed when the supporting bars 4 are bent into a ring shape to form the cylinder structure of the screen cylinder or when a supporting hoop 6 is placed around the supporting bar 4. After the pressing has been stopped, the screen panel can be removed from the base and bent into a screen cylinder shape, and the supporting hoops 6 can be mounted in the above- mentioned manner around at least some of the supporting bars 4.

[0025] To prevent the screen panel from buckling during the pressing, the arrangement may be provided with supports 21 against sideward buckling which support the screen cylinder on both sides and which may also be arranged in such a way that they support each supporting bar 4 separately to avoid the buckling of the supporting bars 4. Further, the arrangement may be provided with supports 22 against buckling in the direction of the panel plane, supporting the panel both from above and from below.

[0026] In the case shown by Figures 5 and 6, the screen panel has, in total, five supporting bars 4, and the pressing base comprises equally many hydraulic cylinders, i.e. one hydraulic cylinder at each supporting bar 4. However, hydraulic cylinders are not necessarily needed at each supporting bar but, when manufacturing a sufficiently rigid intermediate beam, there may be for example two cylinders or for wide screen panels three or more cylinders.

[0027] Irrespective of the stage at which the supporting bars 4 are pressed to remove the clearances, the grooves 5 for the screen wires 2, to be arranged in the supporting bars 4, can be machined as a separate batch for each screen cylinder, in other words to all supporting bars 4 in one go in such a way that the supporting bars 4 of the required batch are mounted at the

same time adjacently on a machining fastener. Further, the supporting bars 4 may be mounted on the screen cylinder in the same direction as how they are in the machining. Thus, the manufacturing method enables manufacturing of almost completely identical pieces with regard to the groove widths and the distribution of the grooves. Due to the manufacturing method of the screen wires, the start and the end of an individual screen wire are also almost completely identical. Such an arrangement allows the supporting bars to be pressed evenly. The pressing is well applicable to manufacturing screen cylinders of all sizes but the best result is achieved when the profile of the supporting bar 4, i.e. the height of the supporting bar 4 in the longitudinal direction of the screen wire 2, is restricted in such a way that it is preferably at most 16 millimetres, more preferably at most 12 millimetres and most preferably at most 10 millimetres, because with greater heights of the supporting bar 4 the forces needed for bending the supporting bars 4 in connection with rolling into a cylinder are too great when directed at the screen wires, which causes detrimental deformation in the screen wires. Further, in solutions where the attachment of the screen wires 2 to the supporting bars 4 is ensured by welding these parts together via a welding groove 23 shown in Figure 2, too high a supporting bar might prevent the weld seam attaching the screen wires 2 and the supporting bars 4 together from being positioned close to the neutral axis of the supporting bar 4, whereby the weld seam would be subjected to dynamic load variation in use, which could result in screen wires to be detached. When the seam weld between the screen wires 2 and the supporting bars 4 can be positioned close to the neutral axis, the bending strain of the supporting bar 4 does not strain the seam weld, whereby a very durable joint is formed.

[0028] Figure 2 shows schematically how supporting hoops 6 have been arranged around the supporting bars 4, the purpose of the supporting hoops being to support the structure of the screen cylinder 1. In the case shown in Figure 2, the side of the supporting bars 4 which is directed outwards from the screen cylinder 1 has a welding groove 23 parallel to the supporting bar 4 and extending around the whole supporting bar 4, whereby the supporting bar 4 has a cross-section which substantially resembles letter U. Via the welding groove 23, the screen wires 2 and the supporting bars 4 can be welded together in the above-mentioned manner, which contributes to increasing the weight-carrying capacity between the screen wires 2 and the supporting bars 4. On the other hand, flexible material may be placed in the welding

groove 23, for instance in the manner shown at the top of Figure 2, where an O-ring 24 made of rubber has been placed in the groove, the diameter of the ring being dimensioned such that the supporting hoop 6 presses it, in other words compressive stress is transmitted via the O-ring to the space between the supporting bar 4 and the supporting hoop 6. This O-ring evens out stresses and, in addition, attenuates vibrations, increasing thus the service life of the screen cylinder. In this way, it becomes possible to avoid both too loose a fit between the supporting bar 4 and the supporting hoop 6, caused by dimensional variations in the diameters of the supporting hoops 6, in which case the supporting hoop 6 would not support the screen mesh properly, and, on the other hand, too great a pressing force between the supporting bar 4 and the supporting hoop 6, which may lead to the breaking of the screen mesh.

[0029] A hollow space remaining between the U-shaped supporting bar 4 and the supporting hoop 6 may be filled with a high-viscosity flexible material 25, which can be extruded into this empty space at a desired pressure via one or more conduits 26 formed in this supporting hoop 6, the conduit being shown schematically at the bottom of Figure 2. The high-viscosity flexible material may be, for example, some type of plastic, which cures when cooling down and leaves the supporting hoop 6 tight. If the viscosity of this material is high, no special tightening between the supporting hoop 6 and the supporting bar 4 is needed. The material may also be swelling, whereby, when expanding in the closed space after the extrusion, it tightens the supporting hoop 6 around the supporting bar 4.

[0030] When the supporting bars 4 are pressed axially after the mounting of the screen wires 2 in the shape of a screen panel in such a way that the clearances in the fitting grooves 5 of the supporting bars 4 between the screen wires 2 and the supporting bars 4 disappear, and when subsequently the supporting bars 4 are bent into a ring shape in such a way that a screen cylinder shape is generated, a tenfold contact force is generated in the case of the example between the screen wires 2 and the supporting bars 4, compared with a case where the supporting bars 4 are pressed in the direction of the periphery after the supporting bars 4 have been bent into a ring shape. The above-mentioned great contact force is based on a state of great tensile stress being generated on the outer periphery of the supporting bar 4 as a result of the bending, compared with a case where supporting bars 4 are pressed in the direction of the periphery after the supporting bars 4 have been

bent into a ring shape, whereby the plasticizing of the pressing stage removes stresses from the structure of the screen cylinder in such a way that after the pressing only a small compressive stress remains in the fitting grooves 5 between the screen wires 2 and the supporting rings 4 due to the elastic recovery of the portion of the screen wire 2 remaining outside the pressing, and correspondingly, a small tensile stress remains in the supporting bar 4. When the ends of the supporting bars 4 are pressed at first, as shown in Figures 5 and 6, and the supporting bars 4 are bent into a ring shape only after that, it is sufficient to generate only small compression between the supporting hoops 6 and the supporting bars 4 when mounting the supporting hoops 6. Thus, it is also preferable to generate pressing between the supporting hoops 6 and the supporting bars 4 by using O-rings or high-viscosity material, as shown in Figure 2. In a solution where the supporting bars 4 are bent into a ring shape at first and where the supporting bars 4 are pressed to remove the clearances only after that, it may be necessary to generate greater compression between the supporting hoops 6 and the supporting bars 4 when mounting the supporting hoops 6.

[0031] In some cases, features described in this application may be used as such, irrespective of other features. On the other hand, features described in this application may be, if required, combined to form different combinations.

[0032] The drawings and the related specification are only intended to illustrate the idea of the invention. Details of the invention may vary within the scope of the claims.