Standard wedge wire screens are fabricated by an assembly of shaped wires or bars which are resistance welded at their roots (bottoms) to cross support bars. The height of the wire gives the necessary section modulus to resist the pressure to which the screen is subjected. In cellulose pulp screening applications, this pressure fluctuates several times per second with high intensity, a major fatigue factor for the multiple resistance welds. Another important consideration for paper pulp pressure screens using wedge wire screen cylinders is the rotating movement of the pulp inside (or outside) the screen (typically caused by rotation of a rotor with respect to the screen surface).

This movement generates a pulsating moment on the wires because they are welded and supported only at their lower portions, again contributing to extra fatigue action on the welds, leading to premature failure.

The invention seeks to remedy the above-mentioned problems for conventional wedge wire (and/or bar) screen cylinders, plates, or cones,

particularly for paper pulp screening environments, by supporting the wires-or bars at portions above their lower portions, typically substantially at or near the tops thereof, in such a way that the harmful twisting moment, induced by the pulp slurry rotating with respect to the screen surface, is considerably reduced (e. g. at least by about 10%, preferably by about 20-300%, and all narrower ranges within this broad range). According to the invention, the wires or bars support each other to resist lateral twisting, and relive the bottom resistance welds from cyclical bending and extra fatigue. Also, by providing support substantially at or near the top of the wires or bars (spaced from the resistance welds) it is possible to maintain the width dimension of the slots/spaces between the wires or bars, which results in more uniform screening, and a better quality pulp.

According to one aspect of the invention there is provided a screen cylinder comprising: A plurality of contoured screen surface defining elements (e. g. bars, wires, or milled plates) each having a bottom connected to another element and a top remote from the bottom, and defining screening slots having a substantially uniform width of between. 002-. 24 inches; the elements formed in a screen cylinder configuration. And, a plurality of wires embedded and brazed in grooves, or weld beads, or localized cross fusions, formed in the tops of the screening elements substantially perpendicular thereto, to increase the resistance of the screening element bottoms to twisting moments by at least 20% compared to if not present.

According to another aspect of the present invention a wedge wire screen construction (in plate, screen cylinder, or cone form) is provided, comprising the following components: A plurality of substantially parallel metal support bars. A plurality of contoured metal screening wires or bars substantially parallel to each other and substantially perpendicular to the support bars. The screening wires or bars connected to the support bars at support bar connections so as to define screening slots between the screening wires or bars, the slots having a substantially uniform width of between. 002-. 024 inches (e. g.. 004-. 010 inches).

And, means remote from the support bar connections for resisting twisting moments on the screening wires or bars at the support bar connections by

reducing the twisting moments by at least 10% (e. g. about 20-300%, and-a ! ! narrower ranges within that broad range) compared to if the means were not present.

The twisting moment resisting means may comprise any conventional technique for resisting twisting moments on one portion of a wire or bar welded at another portion thereof. One particular form that the twisting moment resisting means may have comprises a plurality of embedded wires extending substantially parallel to the support bars and at a portion of the screening wires or bars substantially opposite the support bar connections, the embedded wires not extending a significant distance outwardly from the screening wires or bars.

Another form the resisting means may take comprise a plurality of weld beads extending substantially parallel to the support bars and at a portion of the screening wires or bars substantially opposite the support bar connections, the weld beads not extending a significant distance outwardly from the screening wires or bars.

The twisting moment resisting means may further comprise a plurality of localized cross fusions of the screening wires or bars extending substantially parallel to the support bars and at a portion of the screening wires or bars substantially opposite the support bar connections. Alternatively the twisting moment resisting means may comprise cross plastic deformation of the surface of the support bars at a portion of the screening wires or bars substantially opposite the support bar connections. The cross plastic deformation may be effected by mechanical pressing in a coining operation.

Preferably the contoured screening wires or bars have an upstream surface having an angle of between about 70-110 degrees (e. g. about 70-95 degrees) to the flow direction of slurry to be screened flowing past the screening wires or bars, and have a downstream surface sloped over at least the majority of the length thereof and making an angle of between about 5-60 degrees (e. g. about 15-30 degrees) to the flow direction. The screening construction is preferably disposed in a pressure screen in operative association with a rotor, to screen cellulose pulp.

According to another aspect of the present invention a method of making and using a wedge wire screen construction (as described above) is provided comprising the following: (a) Placing the support bars substantially parallel to each other, and placing the screening wires or bars substantially parallel to each other and substantially perpendicular to the support bars. (b) Welding (e. g. resistance welding) the bottoms of the screening wires or bars to the support bars so as to provide welds (e. g. resistance welds) and to define screening slots between the screening wires or bars, the slots having a substantially uniform width of between. 002-. 024 inches. (c) Forming surface manifestations in the tops of the screening wires or bars to facilitate resistance to twisting moments on the screening wires or bars at the welds. And, (d) using the screen construction to screen cellulose pulp having a consistency of between about . 5%-5%.

In the method (c) may be further practiced by forming grooves in the tops of the screening wires or bars substantially perpendicular to the screening bars or wires, and embedding metal wires therein, and brazing the embedded wires in place, so as to reduce the twisting moments on the resistance welds by at least 10%. Alternatively (c) may be further practiced by forming weld beads substantially perpendicular to the screening bars or wires as the surface manifestations. As another alternative (c) may be further practiced by forming localized cross fusions as the surface manifestations, e. g. by TIG electro-fusion, laser fusion, or electron beam fusion, and so as to reduce the twisting moment on the resistance welds by about 20-300%. As another alternative (c) may be further practiced by cross plastic deformation of the tops of the screening wires or bars, e. g. by mechanical pressing against each of the screening wires or bars in a coining operation.

The method may further comprise, between (c) and (d), forming the screen construction into a screen cylinder; and (d) may be practiced in a pressure screen. Also in the method (a) through (c) and (e) may be practiced to produce a screening cylinder wherein the contoured screening wires or bars have an upstream surface having an angle of between about 70-95 degrees to the flow direction of slurry to be screened flowing past the screening wires or

bars, and have a downstream surface sloped over at least the majority-of the length thereof and making an angle of between about 5-60 degrees to the flow direction; and (d) may be practiced to cause the slurry to move in the flow direction past the upstream surface and then the downstream surface of each screening wire or bar.

According to yet another aspect of the present invention a wedge wire screen cylinder is provided comprising: A plurality of substantially parallel metal support bars. A plurality of contoured metal screening wires or bars substantially parallel to each other and substantially perpendicular to the support bars and having tops and bottoms. The screening wires or bars connected to the support bars at support bar connections at the bottoms of the screening wires or bars, so as to define screening slots between the screening wires or bars, the slots having a substantially uniform width of between. 002-. 024 inches. The support bars and screening wires or bars formed in a screen cylinder configuration. And, a plurality of wires embedded and brazed in grooves, or weld beads, or localized cross fusions, formed in the tops of the screening wires or bars, extending substantially perpendicular to the screening wires or bars. The support bar connections preferably comprise welds, and the contoured screening wires or bars have an upstream surface having an angle of between about 70-110 (e. g. about 70-95) degrees to the flow direction of slurry to be screened flowing past the screening wires or bars, and have a downstream surface sloped over at least the majority of the length thereof and making an angle of between about 5-60 (e. g. about 15-30) degrees to the flow direction.

It is the primary object of the present invention to provide an enhanced life wedge wire screen construction, which may be used to practice a method of screening cellulose pulp. This and other objects of the invention will become clear from an inspection of the detailed description of invention and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS- FIGURE 1 is a top perspective schematic view of an exemplary wedge wire screen plate (e. g. before bending into a screen cylinder); FIGURE 2 is a side view of the screen plate of FIGURE 1; FIGURE 3 is a view like that of FIGURE 1 only utilizing the present invention; FIGURE 4 is a view like that of FIGURE 3 showing the plate bent into a screen cylinder configuration (only a portion of the screen cylinder being shown); FIGURE 5 is a view like that of FIGURE 3 showing another embodiment of the invention; and FIGURE 6 is a view like that of FIGURE 4 showing yet another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS FIGURE 1 shows an exemplary wedge wire screen construction 10 which includes a plurality of substantially parallel metal (e. g. support) bars 11 and a plurality of contoured metal screening wires or bars 12 substantially parallel to each other and substantially perpendicular to the support bars 11. The screening wires or bars 12 are connected to the support bar 11 at the root or bottom 13 of each, e. g. by resistance welds 14 (seen most clearly in FIGURE 2).

The connection is made so as to define screening slots 15 between screening wires or bars 12, having a substantially uniform width of between. 002-. 024 inches, typically between. 004-. 010 inches.

The screen construction wires or bars 12 preferably have a high debris efficiency removal screen cylinder contour, such as shown in U. S. patents 880,540, 5,000,842,5,255,790,5,073,254, and 5,607,589, the disclosures of which are incorporated by reference herein. Commercial versions of these screen cylinder contours include those sold by CAE ScreenPlates Inc. of Glens Falls, New York and Lennoxville, Quebec under the trademark"PROFILE@". Such contours-

one of which is schematically illustrated in FIGURE 2--are formed-by-an upstream (with respect to the tangential direction of pulp flow F in FIGURE 2) surface 17, and a downstream surface 18. The upstream surface 17 is substantially perpendicular to the flow direction (e. g. preferably has an angle a of between about 70-110° (e. g. between about 70-95°, such as about 85°)), and the downstream surface 18 is sloped over at least a majority of the extent thereof, preferably having an angle 3 of between about 5-60° (e. g. about 15°- 30°). A screening slot 15 is defined between the upstream and downstream surfaces 17,18, opening up into an enlarged opening providing communication between the top and bottom surfaces as seen in FIGURE 2. The screening slots 15 have the width thereof as the critical dimension, that is a dimension parallel to the general flow path F. Typical widths for the slots 15 are between. 002-. 024 inches (and all narrow ranges within that broad range), e. g. between. 004-. 010 inches.

According to the invention, as illustrated in the embodiments in FIGURES 3 through 6, all of the components of the prior art screen 10 illustrated in FIGURES 1 and 2 are utilized, in addition components are provided.

According to the invention extra support is provided at or near the tops 20 of the wires or bars 12 spaced from the resistance welds 14. The supports resist the harmful twisting moment that results when the screen 10 is used in screening cellulose pulp at about. 5-5% consistency. The extra support provides a support in such a way that the harmful twisting moment is considerably reduced, e. g. by at least about 10%, preferably by about 20-300%, and all narrower ranges within that broad range.

One way that extra support may be provided substantially at or near the top 20 of the wires or bars 12 (spaced from the resistance welds 14) is-as illustrated in FIGURES 3 and 4-to join the tops 20 by the addition of a cross weld metal bead 22, or a small diameter or cross-sectional area metal (e. g steel) wire, rod, or bar 23. A small groove 24 may be machined, cast, or otherwise formed, in the screening metal wires or bars 12 forming the screen surface (as seen in FIGURES 3 & 4) to allow easy deposit of the cross weld bead 22, wire (including rods or bars) 23, so that the bead 22, wire, rod, or bar 23 is

substantially within the upper contour of the screen cylinder 25 or plate 26 (that is does not significantly extend outwardly from the top, screening, surface of the cylinder 25 or plate 26). The elements 23 may be brazed in place.

Another way that extra support may be provided substantially at or near the top 20 of the wires or bars 12 (spaced from the resistance welds 14) is-as illustrated in FIGURE 5-is to provide support to the top of the wires by using a localized cross fusion 28 of the wire or bar surface. The cross fusion 28 can be achieved by using many different conventional techniques, such as TIG electrode, laser fusion, electron beam, etc., techniques.

Another way that extra support may be provided substantially at or near the top 20 of the wires or bars 12 (spaced from the resistance welds 14) is-as illustrated in FIGURE 6-by using cross plastic deformation 30 of the surface of the screening wires or bars 12 forming the screen surface. The plastic deformation 30 can be achieved by mechanical pressing against each of the wires or bars 12, such as is effected in conventional coining operations.

The basic principles of the invention are also applicable to conventional milled screen cylinders for paper pulp screening, allowing the cylinders to be made of lighter construction. The invention is applicable to screen cylinders, plates, and cones, and in atmospheric screening machines, although especially suitable for use in pressure screens, such as shown in U. S. patent 5,524,770.

Any conventional equipment and materials may be used for the construction of the screen cylinders, plates, and cones according to the invention.

The invention also relates to a method of making and using a wedge wire screen construction 25,26 using a plurality of metal support bars 11, and a plurality of metal screening wires or bars 12 each having a top 20 and a bottom 13, comprising: (a) Placing the support bars 11 substantially parallel to each other, and placing the screening wires or bars 12 substantially parallel to each other and substantially perpendicular to the support bars 11. (b) Resistance welding the bottoms of the screening wires or bars 12 to the support bars 11 so as to provide resistance welds 14 and to define screening slots 15 between said screening wires or bars, the slots 15 having a substantially uniform width of between. 002-. 024 inches. (c) Forming surface manifestations (e. g. grooves 24,

cross fusions 28, etc.) in the tops of the screening wires or bars 12 to facilitate resistance to twisting moments on said screening wires or bars 12 at the resistance welds 14. And, (d) using the screen construction 25,26 to screen cellulose pulp having a consistency of between about. 5%-5%.

In the method (c) may be further practiced by forming grooves 24 in the tops of the screening wires or bars 12 substantially perpendicular to said screening bars or wires, and embedding metal wires 23 therein, and brazing the embedded wires 23 in place, so as to reduce the twisting moments on the resistance welds 14 by at least 10%. Alternatively (c) may be further practiced by forming weld beads 22 substantially perpendicular to the screening bars or wires 12 as the surface manifestations, or by forming localized cross fusions 28 as the surface manifestations, or by cross plastic deformation (30 in FIGURE 6) of the tops of the screening wires or bars 12.

The cross plastic deformation may be further practiced by mechanical pressing against each of the screening wires or bars 12 in a coining operation, and so as to reduce the twisting moments on the resistance welds by at least 10%.

The method may further comprise, between (c) and (d), forming the screen construction 25,26 into a screen cylinder (as in FIGURES 4 and 6); and (d) may be practiced in a conventional pressure screen.

Where the surface manifestations are cross fusions, (c) may be practiced by TIG electrode fusion, laser fusion, or electron beam fusion, and so as to reduce the twisting moments on the resistance welds by at least 10%. Also, (a)- (c) and (e) may be practiced to produce a screening cylinder wherein the contoured screening wires or bars have an upstream surface having an angle of between about 70-110 degrees to the flow direction of slurry to be screened flowing past the screening wires or bars, and have a downstream surface sloped over at least the majority of the length thereof and making an angle of between about 5-60 degrees to the flow direction; and (d) may be practiced to cause the slurry to move in the flow direction past the upstream surface and then the downstream surface of each screening wire, or bar.

While the invention has been shown in particular desirable embodiments, it is to be understood that many modifications may be made thereof within the scope of the invention, including using any other conventional techniques for resisting twisting moments on one portion of a wire or bar welded at another portion thereof; therefore the invention is to be accorded the broadest interpretation possible consistent with the prior art, to encompass all equivalent constructions and methods.