Abstract:
A method is provided for manufacturing a screen cylinder, as well as a screen cylinder with screen wires in the axial direction of the screen cylinder set at predefined intervals into a cylindrical screen surface and fastened to support rods, with end rings are arranged at the ends of the screen cylinder. At least one end ring is mounted at one end of the screen cylinder in such a manner that the end ring is arranged to at least one support rod at the ends of the screen wires or closest to the ends of the screen wires without fastening the end ring to the screen wires. When installing the end ring, a shrink fit is formed between the end ring and support rod, wherein the end ring is arranged to press the support rod substantially perpendicular to the axis of the screen cylinder.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a method for manufacturing a screen cylinder, in which method screen wires are set at predefined intervals side by side and fastened in the axial direction of the screen cylinder to form a cylindrical screen surface in connection with ring-shaped support rods, and in which method end rings are further mounted at the ends of the screen cylinder.  
         [0002]     The invention further relates to a method for manufacturing a screen cylinder, in which method screen wires are set at predefined intervals side by side and fastened to support rods that are bent in the shape of a ring so that the screen wires in the axial direction of the screen cylinder form a cylindrical screen surface, and in which method end rings are further mounted at the ends of the screen cylinder.  
         [0003]     The invention yet further relates to a screen cylinder for cleaning or screening fibre pulp, the screen cylinder having screen wires in the axial direction of the screen cylinder set at predefined intervals to form a cylindrical screen surface and fastened to ring-shaped support rods, and the screen cylinder ends having end rings arranged thereto.  
         [0004]     Screen cylinders are used for instance to clean and screen fibre pulp. Screen cylinders are manufactured for instance by fastening parallel screen wires that form a screen surface side by side in a cylindrical form so that a slot of a desired size remains between the wires. Generally this is done by welding or brazing the screen wires to ring-shaped support wires or rods. The screen wires can be fastened to the support rods in the radial direction of the screen cylinder either inside or outside the support rods. To strengthen the structure of the screen cylinder, separate support rings can be fastened to at least a few of the ring-shaped support rods. These support rings are fastened either to the inner circumference or outer circumference of the support rods depending on the relative order of the screen wires and support rods in the radial direction of the screen cylinder. The structure of the screen cylinder is complemented by fastening end rings at the ends of the screen cylinder. When the end rings are fastened to the ends of the screen cylinder, the ends of the screen wires are welded to the end rings.  
         [0005]     However, fastening the end rings by welding causes a lot of work, first when opening the root of the weld and, after that, during the actual welding. For instance, in a screen cylinder with a diameter of 1200 mm, over 20 metres of weld joint is produced. Fastening the end rings to the screen cylinder by welding also causes welding stresses in the structure of the screen cylinder, whereby during use loads are generated due to the varying pressure inside the screen and mechanical loads, and the generated loads may make the structure of the screen cylinder break.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0006]     It is an object of the present invention to provide a screen cylinder with improved strength and a method for manufacturing it.  
         [0007]     The method of the invention is characterized by installing at least one end ring at one end of the screen cylinder in such a manner that the end ring is arranged to at least one support rod at the ends of the screen wires or closest to the ends of the screen wires, and by forming a shrink fit between the end ring and support rod, in which a substantially perpendicular force to the axis of the screen cylinder acts between the end ring and support rod, and the force, through the support rod, locks the screen surface formed by the screen wires substantially immobile in relation to the end ring.  
         [0008]     A further characteristic of the method of the invention, in which the support rods are bent in the shape of a ring only after the screen wires are fastened to the support rods, is that at least one end ring of the screen cylinder is installed to one end of the screen cylinder in such a manner that the end ring is arranged to at least one support rod at the ends of the screen wires or closest to the ends of the screen wires, and by forming a shrink fit between the end ring and support rod, in which a substantially perpendicular force to the axis of the screen cylinder acts between the end ring and support rod, and the force, through the support rod, locks the screen surface formed by the screen wires substantially immobile in relation to the end ring.  
         [0009]     Further, the screen cylinder of the invention is characterized in that at least one end ring is installed at one end of the screen cylinder in such a manner that the end ring is arranged to at least one support rod at the ends of the screen wires or closest to the ends of the screen wires without fastening the end ring to the screen wires, and that there is a shrink fit between the end ring and support rod, in which a substantially perpendicular force to the axis of the screen cylinder is arranged to act between the end ring and support rod, and the force, through the support rod, locks the screen surface formed by the screen wires substantially immobile in relation to the end ring.  
         [0010]     The essential idea of the invention is that in a screen cylinder intended for cleaning or screening fibre pulp, in which screen wires are set in the axial direction of the screen cylinder at predefined intervals to form a cylindrical screen surface and fastened to support rods and in which end rings are arranged at the ends of the screen cylinder, at least one end ring is installed at one end of the screen cylinder in such a manner that the end ring is arranged to at least one support rod at the ends of the screen wires or closest to the ends of the screen wires, and a shrink fit is formed between the end ring and support rod, in which a substantially perpendicular force to the axis of the screen cylinder is arranged to act between the end ring and support rod, and the force, through the support rod, locks the screen surface formed by the screen wires substantially immobile in relation to the end ring.  
         [0011]     The invention provides the advantage that the screen wires are not welded to the end ring, whereby stress from the welding and directed to the weld joint are avoided. The slow and expensive manufacturing stage of welding the screen wires and end ring is then also left out. By fastening the end ring and support rod to each other either by separate locking means extending in the radial direction of the screen cylinder through the end ring to the support rod and/or one or more weld joints between the end ring and support rod, it is possible to further ensure that the screen surface of the screen cylinder does not rotate relative to the end rings and the entire body of the screen. Owing to the invention, the end rings can be re-used when replacing the screen cylinder. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]     The invention will now be described in greater detail by means of preferred embodiments and with reference to the attached drawings, in which  
         [0013]      FIG. 1  is a schematic cross-sectional view of a screen cylinder in the axial direction of the screen cylinder,  
         [0014]      FIG. 2  is a schematic cross-sectional view of the screen cylinder of  FIG. 1  as seen from the end of the screen cylinder,  
         [0015]      FIG. 3  is a schematic cross-sectional view of a second screen cylinder in the axial direction of the screen cylinder,  
         [0016]      FIG. 4  is a schematic cross-sectional view of a third screen cylinder in the axial direction of the screen cylinder,  
         [0017]      FIG. 5  is a schematic cross-sectional view of a fourth screen cylinder as seen from the end of the screen cylinder, and  
         [0018]      FIG. 6  is a schematic cross-sectional view of the screen cylinder of  FIG. 5  in the axial direction of the screen cylinder. 
     
    
       [0019]     In the figures, the invention is shown simplified for the sake of clarity. Similar parts are marked with the same reference numbers in the figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020]      FIG. 1  is a schematic cross-sectional view of a screen cylinder  1  as seen from the end of the screen cylinder  1 , and  FIG. 2  is a schematic cross-sectional view of the screen cylinder of  FIG. 1  in the axial direction of the screen cylinder  1 . On the inner surface of the screen cylinder  1 , there are screen wires  2  placed around the entire inner circumference of the screen cylinder  1  so that they form a screen surface. Between the screen wires  2 , there are screen slots through which liquid and a desired part of the fibres is allowed to flow outside the screen cylinder  1  while slivers and too large fibres, fibre bundles and any other material to be screened remain on the inner surface of the screen cylinder  1  to be removed at its other end. The screen wires  2  are fastened to support wires  3  or rods  3  before the support rods  3  are bent in the shape of a ring in such a manner that a screen cylinder  1  having a suitable diameter is formed. The screen cylinder  1  can also be made in such a manner that the screen wires  2  are fastened to the inner circumference of the support rods  3  that are already in advance bent in the shape of a ring. There are support rods  3  at suitable intervals in the axial direction of the screen cylinder  1  so that the screen wires  2  remain sufficiently rigidly and firmly in place. The screen wires  2  can be fastened to the support rod  3  by welding, but the fastening of the screen wires  2  is also assisted by the pressure due to the bending of the support rod  3  on the inner edge of the support rod  3 . Instead of welding, the screen wires  2  can also be fastened to the support rod  3  by a crimp joint. Support rings  4  can also be installed around the support rods  3  to support the support rods  3  and receive the forces generated by the pressure difference caused by varying pressures on different sides of the screen surface of the screen cylinder  1  and, thus, to strengthen the structure of the screen cylinder  1 .  FIG. 1  also shows arrow R in the radial direction of the screen cylinder  1  and pointing from the direction of the axis of the screen cylinder  1  to the direction of the outer circumference of the screen cylinder. Arrow R is also shown in FIGS.  2  to  6  to facilitate the reading of the figures.  
         [0021]      FIG. 2  further shows schematically the fastening of the end rings  5  of the screen cylinder  1  to the screen cylinder  1 . The end ring  5  is fastened to the screen cylinder  1  by a shrink fit, in which the end ring  5  is installed around the support rod  3  at the end of the screen cylinder  1  or closest to the end of the screen cylinder  1  and surrounding the screen wires  2 , after which a shrink fit is formed between the end ring  5  and support rod  3  so that the end ring  5  presses the support rod  3  substantially perpendicularly to the axis of the screen cylinder  1 , i.e. in the radial direction of the screen cylinder  1  toward the inside of the screen cylinder  1 .  
         [0022]     The end ring  5  can be installed on the end of the screen cylinder  1  for instance in such a manner that it is heated during the installation so that the structure of the end ring  5  expands due to the heat. When the structure of the end ring  5  is suitably expanded, the end ring  5  is installed around the end of the screen cylinder  1  in such a manner that the ends of the screen wires  2  and the support rod  3  at or close to the ends of the screen wires  2  remain inside the inner circumference  6  of the end ring  5  or a part  6 ′ thereof. The outer circumference of the end ring  5  is marked with reference number  8 . After this, the end ring  5  is allowed to cool or it is cooled, and as the end ring  5  cools, its structure is normalized and causes pressure between the support rod  3  and end ring  5 , i.e. a shrink fit is created between the support rod  3  and end ring  5 , in which the active force is directed from the direction of the end ring  5  to the direction of the support rod  3 .  
         [0023]     The shrink fit between the support rod  3  and end ring  5  is also achieved by tightening a tightening rod around the screen cylinder close to the end of the screen cylinder  1  in such a manner that the screen cylinder  1  is pressed together in the radial direction. After this, the end ring  5  is arranged around the end of the screen cylinder  1  in such a manner that the ends of the screen wires  2  and the support rod  3  at or close to the ends of the screen wires  2  remain inside the inner circumference  6  of the end ring  5  or a part  6 ′ thereof. The tightening rod around the screen cylinder  1  is then removed and the structure of the screen cylinder  1  returns to its original form and, at the same time, pressure is generated between the end ring  5  and support rod  3 , in which the active force is directed from the direction of the support rod  3  to the direction of the end ring  5 .  
         [0024]     The shrink fit between the end ring  5  and support rod  3  is thus generally achieved either by expanding the structure of the end ring  5  before it is installed around the support rod  3 , or by pressing the structure of the screen cylinder  1  together using a force acting in the radial direction of the screen cylinder  1  before the end ring  5  is installed around the support rod  3 , or by using both of these method together.  
         [0025]      FIG. 2  shows schematically a possible cross-section of the end ring  5  when using a shrink fit. The shape of the outline formed by the outer dimensions of the cross-section of the end ring  5  of  FIG. 2  essentially resembles a square or rectangle, the inner circumference of which lacks material at the section that will be located around the screen wires  2  and support rod  3  so that the cross-sectional shape of the end ring  5  resembles the letter L. The part  6 ′ of the inner circumference of the end ring  5  is then formed by a surface A in the axial direction of the screen cylinder  1 , which settles against the support rod  3  in the shrink fit described above. At rightangles to the surface A in the axial direction of the screen cylinder  1  and forming the part  6 ′ of the inner circumference of the end ring  5 , there is a surface B, which is thus a surface perpendicular to the axis of the screen cylinder  1 . The length of the surface A in the axial direction of the screen cylinder  1  is dimensioned in such a manner that the screen wires  2  and support rod  3  surrounding the screen wires  2  at the ends of the screen wires  2  or close thereto remain within the length of the surface A in the axial direction of the screen cylinder  1 . The length of the surface B perpendicular to the axis of the screen cylinder  1  is designed in such a manner for instance that the screen wires  2  and the support rod  3  surrounding the screen wires  2  remain within the length of the surface B perpendicular to the axis of the screen cylinder  1 . On the surface A in the end ring  5 , a small edge  7  or notch  7  can be left to support the shrink fit in the axial direction of the screen cylinder  1  so that the end ring  5  will not slide away from the support rod  3  in the axial direction of the screen cylinder  1 . Action corresponding to the mechanic locking action of the notch  7  is provided or the locking action can also be increased by making a groove on the surface A of the inner circumference part  6 ′ of the end ring  5 , the shape of the groove matching the shape of the support rod  3  and into which groove the support rod  3  is partially inserted.  
         [0026]     Thus, a weld joint between the end ring  5  and the screen wires  2  is no longer used in fastening the end ring  5 , because it may cause welding stress in the structure of the screen cylinder  1  and, consequently, stress generated during the use of the screen may make the weld joint break. Due to the abandoning of the weld joint, the work phases related to welding, i.e. opening the weld root and the actual welding, are also left out. When using a shrink fit, the end rings  5  can, if desired, be re-used when the screen cylinders  1  are replaced, because, due to the missing weld joints, the end ring  5  is detachable from the screen cylinder  1  in its original condition with relatively little work. This re-usability of the end rings  5  thus saves material and costs when the screen cylinders  1  are replaced.  
         [0027]     The force acting in the shrink fit between the end ring  5  and the support rod  3  at the end of the screen cylinder  1  is so strong that it prevents the rotation of the screen cylinder  1  relative to the end ring  5  and the entire body of the screen when the screen is used. This prevention of rotation can be ensured even further by fastening the end ring  5  with locking elements, such as locking screws  9 , to the support rod  3 , as shown in  FIG. 3 , or by welding the end ring  5  with partial welds  10  to the support rod  3 , as shown in  FIG. 4 . When using locking screws  9 , a hole extending from the outer circumference of the end ring  5  to the support rod  3  is drilled and a locking screw  9  is tightened to the hole to mechanically lock the end ring  5  to the support rod  3  and, thus, to the entire screen cylinder  1 . Using locking screws  9  also makes it possible to easily re-use the end rings  5 . In order to prevent the rotation of the screen cylinder  1 , the end rings  5  can, instead of the locking screws  9  or even in addition to them, be fastened with partial welds  10  to the support rod  3 , whereby short weld joints are formed between the end ring  5  and support rod  3 , preferably at several points along the length of the joint between the end ring  5  and support rod  3 .  
         [0028]     FIGS.  1  to  4  show a screen cylinder  1 , in which the screen wires  2  are inside the support rods  3 .  FIGS. 5 and 6 , in turn, show a screen cylinder  1 , in which the screen wires  2  are outside the support rods  3 . Such a screen cylinder  1  is manufactured either by fastening the screen wires  2  to the support rods  3  bent in advance into the shape of a ring or by fastening the screen wires  2  first to the support rods  3 , after which the support rods  3  are bent in the shape of a ring so that a screen cylinder  1  with a suitable diameter is formed, in which the screen surface formed by the screen wires  2  remains outside the support rods  3 . In  FIG. 6 , arrow R is arranged to point into the direction of the outer surface of the screen cylinder  1 .  
         [0029]      FIG. 6  is a schematic representation of fastening the end ring  5  to the screen cylinder  1 . In this case, too, the end ring  5  can be fastened to the screen cylinder  1  with a shrink fit. In this case, the end ring  5  is, however, mounted inside the support rod  3  at the end of the screen cylinder  1  or closest to the end of the screen cylinder  1  and inside the screen wires  2 , after which, a shrink fit is formed between the end ring  5  and support rod  3 .  
         [0030]     In the embodiment of  FIG. 6 , the end ring  5  can be mounted at the end of the screen cylinder  1  for instance by heating the screen cylinder  1  to expand its structure by the heat in the radial direction of the screen cylinder  1 . When the structure of the screen cylinder is suitably expanded, the end ring  5  is mounted inside the end of the screen cylinder  1  in such a manner that the ends of the screen wires  2  and the support rod  3  at the ends of the screen wires  2  or close to the ends of the screen wires  2  remain outside the outer circumference  8  of the end rind  5  or the part  8 ′ of the outer circumference  8 . After this, the screen cylinder  1  is allowed to cool or it is specifically cooled, whereby when the screen cylinder  1  cools, its structure returns to normal and causes pressure between the support rod  3  on the inner surface of the screen cylinder  1  and the end ring  5 , i.e. a shrink fit is formed between the support rod  3  and end ring  5 , in which the acting force is directed substantially perpendicular to the screen cylinder  1  axis from the direction of the support rod  3  to the direction of the end ring  5 . In the embodiment of  FIG. 6 , the structure of the screen cylinder  1  is thus expanded in the radial direction of the screen cylinder  1  for the purpose of mounting the end ring  5 .  
         [0031]      FIG. 6  is a schematic representation of a possible crosssection of the end ring  5  when using a shrink fit, the shape of the cross-section resembling the end ring  5  shown in FIGS.  2  to  4 . The shape of the outline formed by the outer dimensions of the cross-section of the end ring  5  shown in  FIG. 6  also substantially resembles a square or rectangle, but material is missing from the outer circumference  8  of the end ring at the section surrounding the screen wires  2  and support rod  3  so that the shape of the cross-section of the end ring  5  resembles the letter L. The surface A in the axial direction of the screen cylinder  1  and settling against the support rod  3  then forms the part  8 ′ of the outer circumference  8  of the end ring  5 . The surface B, which is the surface perpendicular to the axis of the screen cylinder  1 , is at right angles to the surface A in the axial direction of the screen cylinder  1  and forming the part  8 ′ of the circumference  8  of the end ring  5 . The dimensioning of the surfaces A and B can be done as in FIGS.  2  to  4 .  
         [0032]     The embodiment shown by  FIG. 6  can also use a notch  7  or a groove made on the surface A to form a mechanical joint between the end ring  5  and support rod  3 . Further, the joint between the end ring  5  and support rod  3  can be strengthened by means of a locking element and/or partial welds as shown in  FIGS. 3 and 4 .  
         [0033]     The drawings and the related description are only intended to illustrate the idea of the invention. The invention may vary in detail within the scope of the claims.