In the separation of papermakers'stock from impurities or for fractionating suspensions of paper pulp in a slurry, it is common practice to provide a pressure screen which has multiple wedgewires oriented cross-wise to the process flow. The wedgewires which form the pressure screen are typically drawn or extruded so as to have a preferred profile. Typically, the wedgewires have relatively flat upstream ends narrowing to downstream ends so as to have a wedge-like shape.

The wedgewires are oriented parallel to each other with the separation distance between the upstream ends defining fluid flow passages therethrough.

Typically, the pressure screening apparatus is of the type having an annular cylindrical screen having its longitudinal axis oriented vertically, such as is described in U. S. Pat. No. 4,267,035 issued May 12,1981 to Martin. A suspension of liquid and paper fibers, which may contain undesirable rejects or contaminate particles, is supplied to the inlet of the apparatus. Typically, the paper stock suspension is fed to the interior of the screen cylinder through one axial end of the screen, as at the upper end, and the rejects are withdrawn from the other or lower end of the screen, while the accepts pass through the slots in the screen and are collected at a location radially outwardly of the screen. Rotating foils or vanes are positioned usually on or adjacent the inside of the screen surface, in close relation to the surface, to provide rotational speed to the paper stock, impart shear force to the pulp mat along the screen surface and reduce plugging of the screen slots.

The above apparatus uses a cylindrical screen in which wedgewires and the screen slots therebetween are oriented substantially circumferentially. An alternative form of screening cylinder is disclosed in U. S. Pat. No. 5,472,095 issued Dec. 5,1995 to Malm

One manner of forming a screen cylinder is to extrude or draw wedgewires having the required shape from a metal material, such as stainless steel, provide machined racks which have been welded to grooved reinforcing rings, and position the downstream ends of the wedgewires in the corresponding grooves or valleys of the racks. Shims are placed between each of the upstream ends of the wedgewires to assist in spacing the wedgewires to maintain the required separation distance between them. The racks, rings, wedgewires, and shims are clamped in place around a weld fixture. The wedgewires are then welded to the racks and the shims removed to form a barrel-like structure. End rings, formed from bar stock, are rolled, welded at their respective ends, and then attached by welding to the axial ends of the wedgewires. Further, additional reinforcing rings, if desired, also formed from bar stock, may be attached by welding to longitudinally spaced locations on the outer surface of the screen cylinder.

However, the procedure described above allows for a great deal of inaccuracy in maintaining equidistant openings between wedgewires due to motion that occurs when assembling, manipulating, and welding the various screen elements.

Such inaccuracies are not acceptable considering the fact that it is desired to keep the variation in spacing between wedgewires to less than . 0005 inches. Further, the prior art procedure requires the use of a great many shims, thereby adding to the complexity and cost of the manufacturing process.

Accordingly there is a need for improved wedgewire pressure screen designs which provide dimensionally precise fluid passages while being relatively simple to manufacture.

Summary of the Invention Applicants have developed a procedure to overcome the inaccuracies of conventional pressure screen manufacturing methods while lowering the associated cost. What has been developed is a method of placing equally spaced bumps or integral spacing elements on a drawn, cut wedgewire. These wedgewires are then grouped together side by side in a flat arrangement and welded together to form a panel. These panels are then bent around a series of reinforcing rings to form a barrel which is in turn used for filtering. Alternatively, the panels can be bent

around a mandrel and the reinforcing rings welded on to the outside of the wedgewires.

The bumps or integral spacing elements are placed on a wedgewire by means of a specially designed stamping tool via a mechanical or hydraulic press.

This stamping tool captures or confines the material moved, forming it into an extremely accurate raised area. The raised areas are used in place of discrete shims to form even and equal spacing between wedgewires. Raised areas (bumps or spacing elements) have been manufactured in this manner having dimensional tolerances of no more than . 0005 inches.

A series of wedgewires having the raised areas or bumps on them are squarely placed into a substantially planar aluminum weld fixture with the bumps or spacing elements aligned in rows. The bump or raised area from one wedgewire bears against an adjacent wedgewire in the array to thus serve as a spacer, accurately controlling the distance (or space) between adjacent wedgewires. The weld fixture is also specially designed to keep each wedgewire equally spaced apart while ensuring that each raised area or spacing element is in contact with the wedgewire next to it.

After a desired number of wedgewires have been placed in the weld fixture, they are clamped into place to form a flat panel. Clamps are stretched from one lateral edge of the panel to the other lateral edge and run perpendicularly to the wedgewires. The clamps are located longitudinally between the rows of raised areas, allowing enough room, typically one-half inch, for welding apparatus to be used.

Preferably a thin GTAW or GMAW (1/8 equal penetration) weld bead is then placed over each row of bumps or raised areas, thus covering them. The wedgewire panel is welded on one surface only, thus yielding a flat, flexible screen panel.

Although the weld bead is preferable disposed along the row of bumps that extend transversely with respect to the longitudinal axes of the wedge wires, the weld beads may be located anywhere along the length of the wedge wires.

Preferably a plurality of spaced, transversely oriented rows of weld beads are provided along one surface of the wedge wire panel. Although a weld bead is specifically noted, other bonding means such as adhesives, epoxies, solders etc can be mentioned as being exemplary.

To form a cylindrical pressure screen of the type wherein the flow of paperstock suspension is from the exterior of the basket shaped screen to the interior of the screen, reinforcing rings are spaced longitudinally evenly over a mandrel.

Screen panels formed by the above method are bent or rolled to conform to the outer diameter of the rings so as to completely encompass the rings. The screen panels are welded to the rings by welding each individual wedgewire to the ring. Top and bottom end rings or flanges are then welded into place at the axial ends of the wedgewires. Although the pressure screen design and manufacturing method is presently used to create filter baskets for paper pulp filtering, such pressure screens are not limited solely to this use.

The dimensions of the slots or fluid passages between the wedgewires remain relatively constant after bending or rolling the screen panels due to the locations of the weld bead and the accurate spacing resulting from the raised areas or bumps. Normally, the slots or spaces between adjacent wedgewires will be on the order of about 0.001-0.008 inches. During the manufacturing process at the slurry inlet surface the raised areas act as pivot points so that bending occurs exactly at the points where the bumps meet the adjacent wedgewires. Preferably, these are also the points where the weld beads are placed. Rolling or bending of the screen panel can occur in either direction in a plane perpendicular to the plane of the screen panel and parallel to the rows of weld beads and raised areas while maintaining the dimensional tolerance of the slots between adjacent wedgewires to the required . 0005 inches.

It is accordingly an object of the invention to provide a pressure screening apparatus having accurately dimensioned slots or fluid flow passages.

Another object of the invention is to improve the precision with which wedgewires are positioned with respect to each other in a pressure screening apparatus.

A further object of the invention is the simplification of the process of manufacturing wedgewire pressure screens while maintaining the required slot area dimensional tolerances and achieving cost savings.

Yet another object of the invention is to provide a pressure screen having wedgewires including integrally formed spacing elements.

Other objects and advantages of the invention will be apparent from the following description, and the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a side elevational view of a first embodiment of a wedgewire pressure screen in which a plurality of wedgewires are in abutting relationship and separated by integral spacing elements; Fig. 2 is a section view of the wedgewire pressure screen of Fig. 1 taken along the plane represented by line 2-2 of Fig. 1; Fig. 3 is an enlarged partially cut away top plan view of one of the wedgewires of Fig. 1; Fig. 4 is a section view of the wedgewire pressure screen of Fig. 1 similar to that shown in Fig. 2 but prior to formation of the weld bead; Fig. 5 is an enlarged partially cut away top plan view of a second wedgewire; Fig. 6 is a section view of a wedgewire pressure screen formed from wedgewires made in accordance with Fig. 5 shown similar to the sectional view of Fig. 2; Fig. 7 is a side view partially in phantom showing apparatus for forming an integral spacing element in a wedgewire in accordance with the first embodiment of the invention; Fig. 8 is a front view partially cut away showing the forming apparatus of Fig. 7; Fig. 9 is a side view partially in phantom showing apparatus for forming an integral spacing element in a wedgewire in accordance with the second embodiment of the invention; Fig. 10 is a front view partially cut away of the forming apparatus of Fig. 9; Fig. 11 is a longitudinal section view of a cylindrical pressure screen in which fluid flow is from the interior of the cylindrical basket shaped screen to the exterior of the screen showing the weld beads along the upstream ends of the wedgewires aligned with the outer mounting rings;

Fig. 12 is an enlarged top section view of a wedgewire pressure screen formed into a cylindrical screen in which fluid flow is from the interior of the screen to the exterior of the cylinder showing the lateral sides of the pressure screen joined together by a key bar, the pressure screen and key bar joined at their downstream ends to an outer support ring; Fig. 13 is an enlarged top section view of a wedgewire pressure screen formed into a cylindrical screen in which fluid flow is from the exterior of the cylinder to the interior of the cylinder showing the lateral sides of the pressure screen joined together by a key bar, the pressure screen and key bar joined at their downstream ends to an inner support ring; Detailed Description of Preferred Embodiments Referring first to Figs. 1-4, a portion of a pressure screen 10 constructed in accordance with a first embodiment of the invention has a plurality of stainless steel wedgewires 12 including integral spacing elements 14 in substantially parallel abutting relationship. The integral spacing elements 14 are a series of raised areas or bumps 14 formed in the wedgewires 12 and located at evenly longitudinally spaced points along one lateral side of the wedgewires 12. In all of the drawing figures, the bumps or raised areas 14 are shown grossly exaggerated in size relative to the wedgewires 12 for purposes of illustration only. It is to be understood that the drawing figures are not to scale and that the bumps or raised areas 14 are in reality considerably smaller than shown.

The wedgewires 12 are arranged such that leading edges 16 (see Fig.

2) of the raised areas or bumps 14 contact the other lateral side of the adjacent wedgewires 12 and are aligned so as to form horizontal rows 18 of raised areas or bumps 14 that run in a direction substantially cross-wise to the length of the wedgewires 12. The wedgewires 12 are joined together by a thin weld bead 20 placed on top of two or more rows 18 of raised areas 14.

As may be seen more clearly in Figs. 2 and 4, the wedgewires 12 are so called due to their generally wedge-shaped profiles. The wedgewires 12 have a generally broader upstream or fluid inlet end 30 tapering to a narrower rounded downstream or accepts end 32. A fluid inlet surface 34 includes a flat portion 36, a

left or up-flow inclined portion 38, and a right or down-flow inclined portion 40.

The direction of flow indicated in Fig. 2 represents the direction in which a suspension including paper pulp would flow across the upper face of the screen panel 10 defined by the inlet surfaces 34 of the wedgewires 12. The fluid inlet surface 34 need not have the specific shape shown and may be shaped as necessary to achieve required performance criteria for the finished pressure screen 10. Extending from the upstream end 30 to the downstream end 32 are a left or up-flow lateral side 42 and a right or down-flow lateral side 44, each of which is contoured to achieve the desired profile.

Near the highest point of the upstream end 30, which in this case is the flat portion 36, the wedgewire 12 has a place of maximum width 46. The bump or raised area 14 extends from the left or up-flow lateral side 42 of the wedgewire 12 proximate the place of maximum width 46 and defines a spacing element 14 formed integrally with the wedgewire 12. The bump or raised area 14 cooperates with an adjacent abutting wedgewire 12 to accurately align adjacent ones of the wedgewires so that a screen slot or fluid passage 48 (Fig. 4) having an accurately controlled width is provided. Inwardly of the bump or raised area 14 in the up-flow inclined portion 38 of the inlet surface 34 is an indentation or divot 50 (Fig. 4) where the wedgewire 12 has been struck by a first striking tool (Figs. 9 and 10) to form the spacer bump 14.

Turning now to Figs. 5 and 6, a second embodiment of the invention is depicted. The wedgewires 12 have the same general construction as those of the first embodiment described above. However, the bump or raised area 14 now extends from the right or down-flow lateral side 44 of the wedgewire 12. The wedgewire 12 includes a groove or flattened area 52 in the upstream face 34 extending from the leading edge 16 of the bump 14 towards the left or up-flow side 42 of the wedgewire 12. The groove 52 has been formed by a second striking tool (Figs. 9 and 10) during formation of the bump 14. The leading edge 16 of the bump 14 extends from the widest part 46 of the wedgewire 12 to contact the adjacent wedgewire 12 at the point where the up-flow inclined portion 38 intersects the up- flow lateral side 42 of the adjacent wedgewire 12.

Referring particularly to Figs. 12 and 13, (viewed in conjunction with Fig. 4), it may be seen that in one embodiment the weld bead 20 lies partially within the grooves 50 of the wedgewires 12 so that the height of the weld bead 20 does not substantially extend beyond the uppermost flat portion 36 of the upstream surface 34.

A first stamping tool 60 for forming the bumps or raised areas 14 of the first embodiment is generally depicted in Figs. 7 and 8. A wedgewire 12 is placed in a first bottom tool or die 62 where the wedgewire 12 is supported against movement. A first punch or striking tool 64 having a working edge 66 is positioned above the fluid inlet surface 34 of the wedgewire 12. The first striking tool 64 is brought into contact with the wedgewire 12 by means of a hydraulic or mechanical press (not shown) so as to swage or forge the bump or raised area 14. The bottom tool 62 cooperates with the striking tool 64 so as to precisely determine the maximum distance the leading edge 16 of the bump 14 extends from the up-flow lateral side 42 of the wedgewire 12 when the striking tool 64 displaces the wedgewire material immediately below the striking tool 64 working edge 66. The distance from the up-flow lateral side 42 at the base of the bump 14 to the leading edge 16 defines the width of the screen slots 48 in the finished screen panel 10.

A second stamping tool 70 for forming the bumps or raised areas 14 of the second embodiment is generally depicted in Figs. 9 and 10. A wedgewire 12 is placed in a second bottom tool or die 72 where the wedgewire 12 is supported against movement. A second punch or striking tool 74 having a working edge 76 is positioned above the fluid inlet surface 34 of the wedgewire 12. The second striking tool 74 is brought into contact with the wedgewire 12 by means of a hydraulic or mechanical press (not shown) so as to swage or forge the bump or raised area 14.

The bottom tool 72 cooperates with the striking tool 74 so as to precisely determine the maximum distance the leading edge 16 of the bump 14 extends from the down- flow lateral side 44 of the wedgewire 12 when the striking tool 74 displaces the wedgewire material immediately below the striking tool 74 working edge 76.

After individual wedgewires 12 have been drawn or extruded to the required profile, cut to length, and the spacing elements 14 have been integrally formed, the wedgewires 12 are assembled into flexible screen panels 10. A plurality of individual wedgewires 12 are placed into a specially designed substantially planar

aluminum weld fixture or rack (not shown). The weld fixture or rack has a plurality of parallel grooves or valleys within which the wedgewires 12 are placed. The rack keeps the wedgewires 12 spaced equally apart while maintaining the raised areas 14 of each wedgewire 12 in physical contact with adjacent wedgewires 12, thereby ensuring that the screen slots 48 have a desired spacing. The grooves or valleys of the rack may be V-shaped, rectangularly-shaped, shaped to substantially conform to the profile of the downstream ends 32 of the wedgewires 12, or shaped in any manner suitable for maintaining the wedgewires 12 in the required spatial relationship. In addition, while the rack has been described as being made from aluminum, it may be made of other suitable materials.

After the desired number of wedgewires 12 have been placed in the weld fixture, they are clamped into position so as to form a substantially flat panel with the raised areas 14 of adjacent wedgewires 12 aligned so as to form horizontally extending rows 18 (see Fig. 1). Clamps are stretched from one lateral edge of the panel to the other lateral edge and run substantially cross-wise to the parallel orientation of the wedgewires 12. The clamps are advantageously located longitudinally between the rows 18 of raised areas 14 so as to allow sufficient clearance for welding apparatus (not shown). A thin GTAW or GMAW (1/8 penetration) weld bead 20 is then placed preferably over at least two of the rows 18 of bumps or raised areas 14, thereby joining the individual wedgewires 12 together to form a wedgewire pressure screen panel 10 (a section of which is shown in Figs. 1 and 2) having the desired screen slot 48 spacing.

As shown in the drawing, the weld bead is preferably placed over the rows 18 at the location of the bumps or spacer elements 14. The skilled artisan will appreciate however, that the weld bead may be made at any other location along the fluid inlet surface of the wedgewire array as long as it is effective in flexibly securing the wedgewires together in a flat, panel-like array so that it may subsequently be rolled or bent into the desired cylindrical shape. Also, and as previously mentioned, one or more horizontally disposed (with respect to the Fig. 1 showing) weld beads may be provided across the array or panel.

The wedgewires 12 are welded on one surface only, the fluid inlet surface 34, thereby yielding a screen panel 10 that can be rolled or bent back and

forth in a plane that is perpendicular to a plane containing the screen panel 10 and that is parallel to the wedgewires 12. Although the weld bead 20 has been described as being a GTAW or GMAW weld bead, the weld bead 20 may be formed by other suitable bonding means such as, without limitation, other means of welding, brazing, or soldering, epoxies or by adhesives provided the resulting screen panel 10 retains the desired flexibility.

Wedgewire pressure screen panels 10 as described above may be used in a variety of applications calling for the separating, classifying, or sorting by size of particles or fibers suspended in a fluid medium. The screen panels 10 may be used in a variety of configurations including as flat screens, curved or arcuate screens, or as cylindrical screens. One such cylindrical screen 80 is shown in Figs. 11 and 12.

Wedgewire pressure screen panels 10 as constructed above are bent or rolled around over a mandrel (not shown) so as to substantially form an annular cylinder 82 with the upstream or fluid inlet surfaces 34 of the wedgewires 12 directed radially inwardly. Reinforcing rings 84 are bent or rolled so as to conform to the outer diameter of the cylinder 82 formed by the downstream ends 32 of the wedgewires 12 directed radially outwardly. The rings 84 are spaced evenly longitudinally along the length of the screen cylinder 82. The reinforcing rings 84 are then welded into place by welding the downstream end 32 of each wedgewire 12 to the inner circumferential surface 86 of each ring 84 as shown by the welds 88 in Fig. 14. As shown, the reinforcing rings 84 may be axially aligned with the rows 18 of bumps or raised areas 14, but the rings 84 may be spaced between the rows 18 as well. Although the reinforcing rings 84 shown have flat inner circumferential surfaces 86, the rings 84 may have notches in them or otherwise be formed so as to cooperate or conform with the downstream ends 32 of the wedgewires 12.

In order to accommodate the situation in which one lateral edge 92 of a screen panel 10 does not exactly meet a corresponding screen panel 10 opposite lateral edge 94, a key bar 96 is inserted between the two lateral edges 92,94. The key bar 96 is made of the same material as the wedgewires 12, has substantially the same profile as the wedgewires 12, and includes bumps or raised areas 14 having the same dimensions as the bumps 14 of the wedgewires 12. However, the key bar 96 may vary in width up to about twice the maximum width of a wedgewire 12 as

necessary in order to close the gap between the lateral edges 92,94 of the screen panels 10. As shown, the profile of the key bar 96 need not exactly conform to the profile of the wedgewires 12 so long as the required screen slot or fluid passage 48 size is maintained between the key bar 96 and the adjacent wedgewires 12.

To finish the screen cylinder 80 of Fig. 11, mounting rings or flanges 98 are welded into place at the axial ends of the wedgewires 12 at the top and bottom of the cylinder 82. The screen cylinder 80 described is of the type in which a suspension of liquid and paper fibers enters the interior of the screen cylinder 80 through one axial end of the cylinder 80. Accepts and liquid flow radially outwardly, that is, from the upstream ends 30 to the downstream ends 32 of the wedgewires 12, through the screen slots or fluid passages 48 to be collected at a location outwardly of the cylinder 80. Rejects in the suspension are withdrawn from the other axial end of the screen cylinder 80.

In Fig. 13 is depicted a detailed view similar to Fig. 12 of a screen cylinder of the type in which fluid flows radially inwardly through the screen slots or fluid passages 48. In contrast to the cylinder screen 80 of Fig. 11, the downstream ends 32 of the wedgewires 12 are attached to an outer circumferential surface 100 of the mounting rings 84 by welds 88.

While the processes and products herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise processes and products, and that changes may be made therein without departing from the scope of the invention.

For example, while the pressure screen 10 as described above is preferably made of metal material, specifically stainless steel, it could be made of high impact strength plastic such as a polycarbonate or poly (methyl) methacrylate.

Rather than forming the integral spacing elements 14 by swaging or forging, the spacing elements 14 could be molded as part of the wedgewires 12, or formed by melting or deposited via electrodeposition, plasma spray or other deposition methods.

In addition, while the foregoing embodiments showed the bumps or raised areas 14 being formed on one side of the wedgewires 12 only, they could just as easily have been formed on both sides 42,44 of the wedgewires 12 with bumps 14

on the up-flow sides 42 either in longitudinal alignment with bumps 4 on the down- flow sides 44, or offset in longitudinal alignment so as to provide an alternating or staggered alignment. If the bumps 14 on both sides 42,44 of the wedgewires 12 are in alignment, then the spacing of the screen slots 48 is determined by the combined height of the bumps 14 in contact with each other.