The invention also relates to a screen cylinder for cleaning or sorting a pulp mixture, the screen cylinder comprising screen wires arranged parallel to the axis of the screen cylinder at predetermined intervals to provide a cylindrical screen surface, said screen wires being attached onto the inner surface of support hoops arranged around them.

Screen cylinders are used, for example, for cleaning and sorting a pulp mixture. They are manufactured by attaching parallel screen wires that provide a screen surface side by side in a cylindrical shape so that there remains a slot of a desired size between them. The most generally, this is performed by providing grooves in separate support wires at even intervals, and then arranging the screen wires in the grooves at even intervals.

Subsequently, the screen wires are usually attached to the support wires by welding or brazing, and the so formed sheet-like unit is then bent to form a cylinder, and the ends of the support wires are joined by welding so that they form support hoops. The screen cylinders, however, are difficult and slow to manufacture in the known manner. In addition, the welding produces burrs, spatters and the like where the fibres adhere to and then detach from as larger masses, thereby causing problems in the steps that follow.

Further, EP 0 705 649 discloses a method of making a screen cylinder based on an interlocking fit. In the solution of the publication, the profile of the screen wires, and the mounting grooves provided in the support hoops are such that the rods inserted in the grooves stay in the grooves when the support hoops are bent. The profile of the screen wires, and the grooves of the support hoop, however, are expensive to make. Any deflections caused by inaccuracy in the manufacture easily impair the attachment, which in turn produces vibration that is harmful to the operation and durability of the screen cylinder.

The object of the present invention is to provide a better method of making a screen cylinder, and a screen cylinder, eliminating the drawbacks of the prior art.

The method of the-invention is characterized in that the screen wires are attached to the support hoop by welding them thereto from the side of the support hoop facing away from the screen wire in such a way that the weld extends to the screen wire, thereby firmly attaching each screen wire to the support hoop.

Further, the screen cylinder of the invention is characterized in that the screen wires are attached to the support hoops by welding them thereto from the side of the support hoop facing away from the screen wire in such a way that the weld extends to the screen wire, the visible section of the weld being arranged to remain on the outside of the screen cylinder.

The essential idea of the invention is that the screen cylinder comprises at least screen wires and support hoops that are attached to each other to form a planar unit. Mounting grooves are preferably provided for the screen wires at suitable intervals on the inner surface of the support hoop, i. e. on the surface that is on the inside in a finished screen cylinder. The screen wires are inserted into the grooves, after which they are attached to the support hoop by a weld provided from the outside of the support hoop, i. e. from the side facing away from the mounting grooves. After the attachment, a cylindrical structure is formed by bending the support hoops. The essential idea of an embodiment of the invention is that the weld joining a screen wire and the support hoop is provided from the outside of the support hoop by using a welding method with sufficiently deep penetration, so that the weld extends over a distance through the support hoop into the screen wire. The essential idea of a second embodiment is that a welding groove extending over a distance toward the mounting grooves is provided on the outer surface of the support hoop, and that the welding can be performed at said welding groove. The essential idea of a third embodiment is that the mounting grooves on the inside of the support hoop and the welding groove on the outside intersect over a distance so that a screen wire arranged in the mounting groove is visible from the side of the welding groove and can be attached at said groove by a weld. The welding is thus not performed through the support hoop.

The advantage of the invention is that the welding is performed on that side of the support hoop which will be on the outer surface of the screen cylinder. Consequently, no rough points, bulges or the like where fibres or other particles could adhere to and start to accumulate with time are formed on the inside of the screen cylinder on account of the attachment. Also, the weld does not necessarily need a finish, which means that a slow and expensive step can be disposed of. Further, the advantage of an embodiment of the invention is that the support hoop can be fairly broad and thereby rigid on account of the groove provided on the outer surface of the support hoop.

Due to the welding groove, the weld can be provided such that the penetration of the welding is sufficient to firmly attach a screen wire to the support hoop.

Further, the advantage of the third embodiment is that normal welding methods can be used, i. e. a continuous weld is provided at the bottom of the welding groove, whereby the weld need not be aligned separately at each screen wire. Also, it is then not necessary to form a separate welding groove.

The weld is thus quick to make, and the action can be easily automated.

The invention will be described in greater detail in the attached drawings, in which Fig. 1 shows a schematic cross-sectional view of a screen cylinder of the invention, seen in the axial direction of the screen cylinder, Figs. 2a to 2c show a schematic view of how the screen wires of the screen cylinder can be attached to a support hoop in the invention, Fig. 3 shows a schematic view of a second way of attaching the screen wires to the support hoops in the invention, Fig. 4 shows a schematic view of a third way of attaching the screen wires to the support hoops in the invention, and Fig. 5 shows a schematic view of a fourth embodiment of the invention.

Fig. 1 is a schematic cross-sectional view of a screen cylinder of the invention, seen in the axial direction of the screen cylinder. The figure shows a screen cylinder 1 with screen wires 2 on its inner surface, along the entire inner circle of the screen cylinder 1, so that they provide a screen surface.

Between the screen wires 2 there are screen slots 3, through which the liquid and the desired amount of fibre can flow out from the screen cylinder. Any sticks, too large fibres, fibre bundles and other material to be separated will remain on the inner surface of the screen cylinder 1. The separated material is

then removed through one end of the screen cylinder 1. The screen cylinder 1 is made by attaching the screen wires 2 to support hoops 4 before the support hoops 4 are bent to form a ring of a desired size. The screen wires 2 are attached before the support. hoops 4 are bent, since the manufacture of a planar unit is quicker and more easy to automate. The support hoops 4 are arranged at suitable intervals parallel to the axis of the screen cylinder 1 so that the screen wires 2 stay sufficiently rigidly and firmly in place. The screen wires 2 are attached to the support hoop 4 by welding, but the attachment is also enhanced by compression on the inner surface of the support hoop 4, caused by the bending of the support hoop 4. In the figure, the ends of the support hoops are joined by welding at point 4a so that a ring-shaped structure is formed. Further, the distance between the screen wires is rather long and the number of screen wires is rather small, although in reality the number of wires in a screen cylinder is rather large and the distance between the wires is very small.

Fig. 2a shows a simplified view of a screen wire 2 and a support hoop 4 before they are attached to each other. The cross-section of the screen wire 2 may vary in shape: it need not have the simplified rectangular shape shown in the figure. The edge that comes into contact with the support hoop 4 is preferably rounded, and so is the bottom of the mounting groove 5 in the support hoop 4, whereby the notch effect, which impairs durability, can be avoided. With regard to the manufacture, it is advantageous if the mounting grooves have straight walls, so that they are easy to ream or flame-cut. The essential point is that mounting grooves 5 corresponding to the cross-section of the screen wire 2 are provided on the inner surface 4a of the support hoop 4, and the screen wires 2 are inserted into the grooves in the manner shown in Fig. 2b. The mounting grooves 5 and the screen wires 2 are preferabiy in firm contact with each other at their lateral and counter surfaces so that the screen wire 2 cannot vibrate when the pressure changes. Fig. 2c shows how the screen wire 2 is attached to the support hoop 4 by welding: a weld with deep penetration and extending from the outer surface 4b of the support hoop 4 is provided. Each screen wire 2 is attached by a different weld. The welding methods that can be used include, for example, arc welding, resistance welding and beam welding. The penetration of the weld can naturally be improved by providing a welding groove, such as a notch or a hole, on the outer surface 4b of the support hoop. Fig. 2c shows a weld 6 extending

through the support hoop 4 from the outer surface 4b of the hoop to the screen wire 2. The visible sections of the weld 6 will then be on the outer surface of the screen cylinder 1, and readily adhering particles are then not easily attached to the weld from the pulp that is filtered. Further, the weld 6 is protected by the support hoop 4 from the sorted pulp flow coming from the screen cylinder. The weld 6 need thus not be finished by grinding, and the quality and appearance of the surface are not relevant. The main point is that the weld 6 ensures that the screen wire 2 is firmly attached.

Fig. 3 in turn illustrates an embodiment in which a welding groove 7 extending over the entire length of the support hoop 4 is formed on the outer surface 4b of the support hoop, and in which a weld joining the screen wire 2 and the support hoop 4 can be provided by welding through the bottom 7a of the groove. The structure allows for larger dimensions for the support hoop 4 in the radial direction of the screen cylinder, which enables a more rigid structure. The thickness of material between the bottom 7a of the welding groove and the bottom of the mounting groove 5 can be arranged to be such that the penetration of the welding method used is sufficient. The weld will here be protected in the welding groove 7. When the support hoops 4 have been bent, an additional hoop with a protrusion corresponding to the groove can be arranged in the groove. The additional hoop further improves the durability of the structure.

Fig. 4 shows another way of attaching the screen wires 2. The mounting grooves 5 on the inner surface 4a and the welding groove 7 on the outer surface 4b of the support hoop are formed such that they intersect and are thus in contact with each other. When the screen wire 2 is in the mounting groove 5, the edge of the wire on the side of the support hoop 4 extends over the bottom 7a of the welding groove or aligns with the bottom 7a of the welding groove and can thus be seen from the outer surface 4b of the support hoop. The screen wire 2 is thus not welded in the embodiment such that a weld extending through the support hoop 2 would be provided; instead, for example, simply a continuous weld extending over the entire length of the support hoop 4 can be used. The welding is then quick to perform, and it can be easily automated.

Fig. 5 is a schematic view showing how a separate additional hoop can be easily arranged in the embodiments of the invention illustrated in Figs.

3 and 4. In this embodiment, the additional hoop 8 comprises a protrusion 9

extending to the groove 7 of the support hoop 4, the protrusion locking the additional hoop 8 in place in the axial direction of the screen cylinder. The breadth of the additional hoop 8 can, for example, equal the breadth of the support hoop 4 in the axial direction of the screen cylinder, or it can be even broader as shown in the figure, whereby there are bevels 10 at the edge of the additional hoop 8 facing the screen cylinder, the bevels joining the edge of the screen cylinder that faces the additional hoop 8 smoothly to the support hoop 4, and the additional hoop being broader at the end facing away from the screen cylinder. Fig. 5 also shows how a weld 6 is provided in the support hoop 4 when the welding is performed through the bottom of the welding groove 7. Although it would here also be possible to weld each wire separately as described in Fig. 2c, it is simpler to use continuous welding, whereby a weld is also provided in those parts of the support hoop where there are no screen wires. In the solution of Fig. 4, a weld is provided at the bottom of the welding groove 7 of the support hoop 4 and passes through the edges of the screen wires 2 that are on the side of the welding groove 7.

The drawings and the description thereof are only intended to illustrate the idea of the invention. The invention can vary in its details within the scope of the claims. The welding groove of the support hoop is thus naturally designed so that the attachment suits the welding method used. Also, in the embodiments that involve welding through the support hoop, the portion to be penetrated is naturally designed so that the penetration of the welding method used is sufficient and the joint is thus firm. If, due to bending and inaccuracy in the manufacture, there remain slots between the screen wire and the mounting groove provided in the support hoop, the entire screen cylinder can be chromed, if desired, or coated in some other way, whereby all slots are filled with the coating and there is no risk that the screen wires would vibrate. It is also not compulsory to provide mounting grooves in the support hoop: the screen wires can be arranged at suitable intervals and welded from the outside of the support hoop through the support hoop so that a weld with deep penetration is provided. The manufacture of the support hoop is then naturally simple and also cost-effective. A screen structure iike this is naturally not as firm as one provided using grooves and welding, but all the same it can be used successfully in certain applications.