SCREEN CYLINDER STRESS CONCENTRATION RELIEF

Abstract: A screen cylinder, and a method of making the screen cylinder, take into account the stress concentrations that are formed, particularly at the inner and outer surfaces of the finished screen cylinder, so as to minimize them. The stress relief minimization is accomplished by providing a plurality of stress relief grooves (25) in land areas (121) between accept flow providing grooves (113) that are in substantially end-to-end alignement, the stress relief grooves each having a depth between about 10-80 % (e.g. between about 40-60 %) of the thickness dimension (119) of the plate (110), so as to reduce by at least 10 % (e.g. about 30-50 %) the magnitude of deformation of the screen slots (115) due to compression or tension in the surfaces of the screen cylinder as a result of cylinder formation from the plate, compared to if the relief grooves were not present.

Description: SCREEN CYLINDER STRESS CONCENTRATION RELIEF BACKGROUND AND SUMMARY OF THE INVENTION In the manufacture of slotted screen cylinders, such as for the pulp and paper industry (e.g. for screening cellulose pulp having a typical consistency of .5-5%), as shown in U.S. patents 5,718,826, 5,200,072, 4,264,438, 4,529,520, etc., a plate of steel or like material is machined to provide a plurality of grooves substantially parallel to each other and to what will be the central axis when a cylinder is made from the plate, and a plurality of slots in at least some of the grooves (normally all of them) defining a through-extending flow path of a pre-determined size perpendicular to the plane of the plate, with a land area separating grooves, and their associated slots, from other in-line grooves and slots. After the grooves and slots are machined, the plate is rolled, utilizing conventional equipment, to form a cylinder. When that rolling takes place the surface of the plate that becomes the outer surface of the cylinder is subjected to tension stresses, while the face of the plate that becomes the inner surface of the cylinder is subjected to compressive stresses. For outflow type cylinders this results in a narrowing of the width of the slot, while for in flow type cylinders this results in an opening up of the slot. In either case, the care taken to machine a slot of a desired pre-determined size (e.g. .002-.024 inches in width) is negated to at least some extent, causing the slots to lose their initial machined geometry. Also, at rolling, cracks can appear at the ends of the slots. The ends of the slots typically include stress risers, and when combined with the stress of rolling, the stress threshold of the material may be exceeded, resulting in a poor quality cylinder. According to the present invention, a method and screen cylinder are provided which reduce the stress concentration at the slot ends and allow the rolling of screen cylinders of smaller diameters without cracking of the base material at the slot ends. This also allows the production of screen cylinders with more precisely controlled slot width, which is particularly important for inflow cylinders since according to the present invention there is less widening of the slots. In the case of inflow screen cylinders, it is possible to make smaller final actual slots than earlier possible. According to the invention, stress relief grooves are provided in the land area between in-line grooves so as to typically cut away about 10-80% (e.g. about 40-60%) of the thickness of the land area (most normally about 50%), which cuts the deformation tension and compression roughly about 50%. According to one aspect of the present invention a screen cylinder is provided comprising: A screen plate having first and second surfaces and a thickness dimension provided between the first and second surfaces, a plurality of accept flow providing grooves formed in the first surface, and a plurality of slots extending between the grooves and the second surface for effecting screening of a slurry to be screened. At least some of the grooves in substantially end-to-end alignment, and the plate having land areas between the grooves in substantially end-to-end alignment. A plurality of stress relief grooves formed in the land areas each between accept flow providing grooves in substantially end-to-end alignment, the stress relief grooves each having a depth between about 10-80% of the thickness dimension of the plate. And, wherein the screen plate is in cylindrical form with one of the first and second surfaces defining an outer surface of a cylinder, and the other of the first and second surfaces defining an inner surface of a cylinder. Each of the stress relief grooves has a bottom, which is a portion of each of the stress relief groove closest to the second surface, and preferably at least a plurality of the bottoms are rounded (typically they all are). A plurality of the stress relief grooves (e.g. all) preferably have a depth between about 40-80% of the thickness dimension of the plate, e.g. about 50%. The stress relief grooves reduce by at least 10% (e.g. by about 30-50%) the magnitude of deformation of the slots due to compression or tension in the second surface as a result of cylinder formation from the plate, compared to if the relief grooves were not present. In situations where an inflow cylinder is provided, that is where the second surface comprises the outer surface of the cylinder, the slots may have a smaller actual width impossible for an otherwise identical cylinder without the relief grooves, e.g. an actual width of between about .002-.004 inches. In most situations the thickness dimension of the plate is between about 2-20 millimeters and the slots have an actual width of between about .002-.024 inches. The stress relief grooves may have the width that is the same as that of the accept flow providing grooves substantially in end-to-end alignment adjacent thereto, but the width may also be different; for example the width of the stress relief grooves may be between about 40-90% of the width of the accept flow providing grooves. According to another aspect of the present invention a method of making a screen cylinder from a screen plate having first and second surfaces and a thickness dimension provided between the first and second surfaces, is provided. The method comprises: (a) Forming a plurality of accept flow providing grooves in the first surface with land areas between the grooves, some of the grooves in substantially end-to-end alignment, (b) Forming a plurality of slots extending between the grooves and the second surface for effecting screening of a slurry to be screened, (c) Forming a plurality of stress relief grooves in the land areas between accept flow providing grooves in substantially end-to-end alignment, so that the stress relief grooves each have a depth between about 10-80% of the thickness dimension of the plate. And then, (d) forming the screen plate into cylindrical form with one of the first and second surfaces defining an outer surface of a screen cylinder, and the other of the first and second surfaces defining an inner surface of a screen cylinder. The procedures (a) through (c) may be practiced in essentially any order; oftentimes, however, (a) and (c) may be practiced at approximately the same time, and before (b). Typically (c) is practiced to provide the stress relief grooves with bottom portions, closest to the second surface, that are rounded; and (c) is also practiced to provide at least a plurality of the relief grooves with a depth between about 40-60% (e.g. about 50%) of the thickness dimension of the plate. For example (c) is practiced to reduce by at least 10% (e.g. about 30-50%) the magnitude of deformation of the slots due to compression or tension in the second surface as a result of cylinder formation during (d) compared to if the relief grooves were not present. Also (c) may be practiced so that the stress relief grooves have a width that is about the same as (that is about 100% of) the width of the accept flow providing grooves, but alternatively (c) may be practiced so that at least a plurality of the stress relief grooves have a width of between about 40-90% of the width of the accept flow providing grooves substantially in end-to-end alignment adjacent thereto. In the method typically (a)-(d) are practiced to provide an effectively operable screen cylinder having a diameter smaller than the diameter of an otherwise identically produced screen cylinder produced by a method not including (c). Also (a)-(d) may be practiced so as to provide the second surface as the outer surface of the screen cylinder, and so that the slots have a smaller actual width than possible if producing an otherwise identical cylinder without the stress relief grooves. The method further preferably comprises using the screen cylinder to screen a cellulose pulp slurry having a consistency of between about .5-5%. According to still another aspect of the present invention a screen cylinder is provided comprising: A screen plate having first and second surfaces and a thickness dimension provided between the first and second surfaces, a plurality of accept flow providing grooves formed in the first surface, and a plurality of slots extending between the grooves and the second surface for effecting screening of a slurry to be screened. At least some of the grooves in substantially end-to-end alignment, and the plate having land areas between the grooves in substantially end-to-end alignment. A plurality of stress relief grooves formed in the land areas each between accept flow providing grooves in substantially end-to-end alignment, the stress relief grooves reducing by at least 10% the magnitude of deformation of the slots due to compression or tension in the second surface as a result of cylinder formation from the plate, compared to if the relief grooves were not present. And, wherein the screen plate is in cylindrical form with one of the first and second surfaces defining an outer surface of a cylinder, and the other of the first and second surfaces defining an inner surface of a cylinder, and reinforcing elements provided on the outer surface. It is the primary object of the present invention to release the stress concentration during the manufacture of screen cylinders from slotted screen plates, particularly for screening pulp in the pulp and paper industry. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a side greatly enlarged cross-sectional view of an exemplary slotted screen cylinder plate according to the prior art, taken along lines 1-1 of FIGURE 3; FIGURE 2 is an enlarged schematic cross-sectional view taken along lines 2-2 of FIGURE 1; FIGURE 3 is a side schematic view of an exemplary prior art screen cylinder; FIGURE 4A is an end view of a conventional prior art screen cylinder; FIGURE 4B is an enlarged view of a portion of the cylinder of FIGURE 4A; FIGURE 5 is a view like that of FIGURE 1, taken along lines 5-5 of FIGURE 7, of a slotted screen cylinder plate according to the present invention; FIGURE 6 is a schematic cross-sectional view taken along lines 6-6 of FIGURE 5; SUBSTΓΓUTΈ SHEET (RULE 26) FIGURE 7 is a side schematic view of an exemplary screen cylinder according to the present invention; FIGURE 8A is an end view of an exemplary screen cylinder according to the invention; and FIGURE 8B is an enlarged detail cross-sectional view of a portion of the screen cylinder of FIGURE 8A. DETAILED DESCRIPTION OF THE DRAWINGS FIGURES 1-4B illustrate the prior art over which the invention is an improvement. FIGURE 1 illustrates a flat plate 10 which is rolled into a screen cylinder 11 (see FIGURES 3 and 4), as is conventional. The view in FIGURE 1 is parallel to the central axis 12 (see FIGURES 3 and 4A) of the screen cylinder 11 ultimately formed, taken along lines 1-1 of FIGURE 3; while FIGURE 2 shows a cross-sectional view of the plate 10 perpendicular to the axis 12, taken along lines 2-2 of FIGURE 1. In the conventional plate 10, an accept flow providing groove 13, which is large and does not function to effect screening, is formed in the surface 14 of the plate 10, while fine slots 15 — which do effect screening — are machined in the face 16 opposite the face 14. The slots 15 control the cleanliness of the accepts passing through the screen cylinder 11. The widths of the slots 15 are particularly important for control of accepts cleanliness. The width 18 (see FIGURE 2) of each slot 15 is typically between .002-.024 inches, while the plate 10 typically has a thickness 19 (see FIGURE 1) of between about 2-20 mm. FIGURES 3, 4 A and 4B show an outflow screen cylinder 11 produced from the plate 10, in which the surface 14 becomes the outer surface of the screen cylinder 11, and the surface 16 becomes the inner surface. If an inflow screen cylinder is constructed from plate 10, then the surface 14 is the inner surface and the surface 16 the outer one. The rolling action during cylinder production causes the outflow cylinder, as seen in FIGURES 3, 4A and 4B, to have a narrower slot 15 width, and can cause stress cracking at the ends 20 of the slots 15 for both outflow and inflow cylinders, while formation of an inflow cylinder can cause the slots 15 to "open up" and thereby become larger and thereby allow a larger size material (debris) to pass therethrough than desired. In the conventional plates 10 and associated cylinders 11 of FIGURES 1 through 4B, the grooves 13, and the slots 15, are disposed in rows, the grooves 13 and slots 15 in any one row being substantially in line with each other along a dimension parallel to the central axis 12 of the screen cylinder 11 formed thereby, as indicated by the double arrow 12 in FIGURE 1. The solid material of the plate 10 between the ends of in-line grooves 13, shown generally by reference numeral 21 in FIGURES 1 and 3, is called the "land area". FIGURES 5 through 8B are views comparable to those of FIGURES 1 through 4B, only showing the practice of the method according to the present invention, to produce a cylinder according to the present invention. In FIGURES 5 through 8B structures comparable to those in FIGURES 1 through 4B are shown by the same reference numeral only preceded by a "1". The only major difference between the invention of FIGURES 5 through 8B and the prior art of FIGURES 1 through 4B is the provision of a stress relief groove 25 in the land areas 121 between the in-line ends of the accept flow providing grooves 113. The amount of material removed to form the relief groove 25, which is a partial continuation of the grooves 113, is typically between about 10-80% of the thickness 119 of the plate 110, e.g. about 40-60%, preferably about 50% as illustrated in FIGURES 5 through 8B. That is the land area 121 is machined to have a thickness 26 (see FIGURE 5) which is between about 20-90% (e.g. about 40-60%, preferably about 50%) of the thickness 119 of the plate 110. Thus if the plate thickness 119 is 10 mm, the stress relief groove 25 has a height of about 5 mm, and the land area 121 has a thickness 26 of about 5 mm. This is of course only one example, and a wide variety of modifications are possible, the goal being to reduce significantly (by at least 10%, and typically between about 30-50%, e.g. about 50%) the magnitude of deformation due to compression or tension of the surfaces 114, 116. By cutting the stress relief grooves 25 the neutral axis of the solid section 121 of the cylinder 111 is displaced toward the slot 115, and thus upon rolling deformation is reduced or minimized. The stress relief grooves 25 are preferably about the same width as the grooves 113, but can be less. For example the width of a stress relief groove 25 may be about 40-100%, e.g. between about 40-90%, of the width of the grooves 113 on either end thereof. In FIGURES 5, 6, and 8B the stress relief grooves 25 have about 100% of the width of the grooves 113, the groove 25 bottom may be rounded, as seen in FIGURES 6 and 8B, but also may be flat, or have a wide variety of other configurations. Although not visible in the drawings, typically the screen cylinders 111, 11, are contoured. That is contoured accept flow providing grooves 113 are provided in the inlet side of the screen cylinders 111, the contoured grooves having an upstream side surface, a bottom, and a downstream side surface. The contoured grooves — and the screen cylinders utilizing them — may be provided as shown in U.S. patents 4,529,520, 4,836,915, 4,880,540, 5,000,842, or in other ways known in the art. Such contoured cylinders are commercially available, for example, under the trademark "PROFILE" from CAE ScreenPlates, Inc. of Lennoxville, Quebec, and Glens Falls, New York. When the screen cylinders 11, 111 are constructed they also will have reinforcing rings shown schematically at 23, 123 in FIGURES 3 and 7, respectively. However the reinforcing rings 23, 123 are typically applied after cylinder formation and in any event are not significantly related to the invention. The rings 23, 123 may be provided as shown in U.S. patent 5,718,826, or the prior art illustrated or discussed therein. The method and cylinders according to the invention are suitable for screen cylinder 111 diameters from two inches to 120 inches, for slot 115 widths 118 of from .002-.024 inches, for plate 110 thicknesses 119 of 2-20 mm, and for a wide variety of plate materials including steel, stainless steel, non-ferrous alloy composites, and others. By practicing the invention it is possible to reduce stress concentrations (e.g. by about 50%) at the slot 115 ends 120, and allow rolling of cylinders 111 of smaller diameters without cracking of the material of the plate 110 at the slot ends 120, for both inflow and outflow cylinders. The method also allows rolling of inflow type cylinders with much less widening of the slots 115 so that smaller final actual slots may be obtained than in prior art machined inflow screen cylinders, and more precise control over the slot 118 width for both outflow and inflow cylinders. According to the present invention a method of making a screen cylinder 111 from a screen plate having first and second surfaces 114, 116, respectively, and a thickness dimension 119 provided between the surfaces 114, 116 comprises, (a) Forming a plurality of accept flow providing grooves 113 in the first surface 114 with land areas between the grooves 113, some of the grooves 113 in substantially end-to-end alignment (as seen in FIGURES 5 and 7 in particular), (b) Forming a plurality of slots 115 between the grooves 113 and the second surface 116 for effecting screening of a slurry to be screened (e.g. for a cellulose pulp slurry having a consistency between about .5-5%). (c) Forming a plurality of stress relief grooves 25 in the land areas 121 between the grooves 113 so that the stress relief grooves 25 each have a depth between about 10-80% of the thickness dimension 119 of the plate; and then (d) forming the screen plate into a cylindrical form 111 (FIGURES 7, 8 A and 8B) with one of the surfaces defining an outer surface of the screen cylinder (the surface 114 in the embodiment of FIGURE 8B) and the other the first and second surfaces defining an inner surface of the screen cylinder (116 in FIGURE 8B). The procedures (a)-(c) may be practiced in any order, and are practiced using entirely conventional techniques of milling, machining, etc. The procedure (d) is also practiced utilizing entirely conventional techniques. In the disclosure all broader ranges also specifically include all specific narrower ranges within the broad range. For example, 30-50% includes 31-49%, 35-48%, 40-41%, and all other narrower ranges. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims necessary to cover equivalent methods and products.

Claims: WHAT IS CLAIMED IS: 1. A screen cylinder comprising: a screen plate having first and second surfaces and a thickness dimension provided between said first and second surfaces, a plurality of accept flow providing grooves formed in said first surface, and a plurality of slots extending between said grooves and said second surface for effecting screening of a slurry to be screened; at least some of said grooves in substantially end-to-end alignment, and said plate having land areas between said grooves in substantially end-to-end alignment; a plurality of stress relief grooves formed in said land areas each between accept flow providing grooves in substantially end-to-end alignment, said stress relief grooves each having a depth between about 10-80% of said thickness dimension of said plate; and wherein said screen plate is in cylindrical form with one of said first and second surfaces defining an outer surface of a cylinder, and the other of said first and second surfaces defining an inner surface of a cylinder. 2. A screen cylinder as recited in claim 1 wherein each of said relief grooves has a bottom, which is a portion of each said relief groove closest to said second surface, and wherein at least a plurality of said bottoms are rounded. 3. A screen cylinder as recited in claim 1 wherein a plurality of said relief grooves have a depth between about 40-60% of said thickness dimension of said plate. 4. A screen cylinder as recited in claim 1 wherein said relief grooves reduce by at least 10% the magnitude of deformation of said slots due to compression or tension in said second surface as a result of cylinder formation from said plate, compared to if said relief grooves were not present. 5. A screen cylinder as recited in claim 3 wherein said relief grooves reduce by about 30-50% the magnitude of deformation of said slots due to compression or tension in said second surface as a result of cylinder formation from said plate, compared to if said relief grooves were not present. 6. A screen cylinder as recited in claim 1 wherein said second surface comprises said outer surface of said cylinder, and wherein said slots have a smaller actual width than possible for an otherwise identical cylinder without said relief grooves. 7. A screen cylinder as recited in claim 1 wherein said second surface comprises said outer surface of said cylinder, and wherein said slots have an actual width of between about .002-.004 inches. 8. A screen cylinder as recited in claim 4 wherein said second surface comprises said outer surface of said cylinder, and wherein said slots have an actual width of about .002 inches. 9. A screen cylinder as recited in claim 1 wherein said thickness dimension of said plate is between about 2-20 mm and said slots have an actual width of between about .002-.024 inches. 10. A screen cylinder as recited in claim 9 wherein at least a plurality of said stress relief grooves have a width of between about 40-90% of the width of said accept flow providing grooves substantially in end-to-end alignment adjacent thereto. SUBSTTTUTE SHEET (RULE 26) 11. A method of making a screen cylinder from a screen plate having first and second surfaces and a thickness dimension provided between the first and second surfaces, comprising: (a) forming a plurality of accept flow providing grooves in the first surface with land areas between the grooves, some of the grooves in substantially end-to-end alignment; (b) forming a plurality of slots extending between the grooves and the second surface for effecting screening of a slurry to be screened; (c) forming a plurality of stress relief grooves in the land areas between accept flow providing grooves in substantially end-to-end alignment, so that the stress relief grooves each have a depth between about 10-80% of the thickness dimension of the plate; and then (d) forming the screen plate into cylindrical form with one of the first and second surfaces defining an outer surface of a screen cylinder, and the other of the first and second surfaces defining an inner surface of a screen cylinder. 12. A method as recited in claim 11 wherein (a) and (c) are practiced at approximately the same time, and before (b). 13. A method as recited in claim 12 wherein (c) is practiced to provide the stress relief grooves with bottom portions, closest to the second surface, that are rounded. 14. A method as recited in claim 11 wherein (a)-(d) are practiced to provide an effectively operable screen cylinder having a diameter smaller than the diameter of an otherwise identically produced screen cylinder produced by a method not including (c). 15. A method as recited in claim 11 wherein (c) is practiced to provide at least a plurality of the relief grooves with a depth between about 40-60% of the thickness dimension of the plate. SUBSTTTUTE SHEET (RULE 26) 16. A method as recited in claim 11 wherein (c) is practiced to reduce by at least 10% the magnitude of deformation of the slots due to compression or tension in the second surface as a result of cylinder formation during (d) compared to if said relief grooves were not present. 17. A method as recited in claim 15 wherein (c) is practiced to reduce by about 30-50% the magnitude of deformation of the slots due to compression or tension in the second surface as a result of cylinder formation during (d) compared to if said relief grooves were not present. 18. A method as recited in claim 11 wherein (c) is practiced so that at least a plurality of the stress relief grooves have a width of between about 40-90% of the width of said accept flow providing grooves substantially in end-to-end alignment adjacent thereto. 19. A method as recited in claim 11 wherein (a)-(d) are practiced so as to provide the second surface as outer surface of the screen cylinder, and so that the slots have a smaller actual width than possible if producing an otherwise identical cylinder without the stress relief grooves. 20. A method as recited in claim 11 further comprising using the screen cylinder to screen a cellulose pulp slurry having a consistency of between about .5-5%. 21. A screen cylinder comprising: a screen plate having first and second surfaces and a thickness dimension provided between said first and second surfaces, a plurality of accept flow providing grooves formed in said first surface, and a plurality of slots extending between said grooves and said second surface for effecting screening of a slurry to be screened; at least some of said grooves in substantially end-to-end alignment, and said plate having land areas between said grooves in substantially end-to-end alignment; a plurality of stress relief grooves formed in said land areas each between accept flow providing grooves in substantially end-to-end alignment, said stress relief grooves reducing by at least 10% the magnitude of deformation of said slots due to compression or tension in said second surface as a result of cylinder formation from said plate, compared to if said stress relief grooves were not present; and wherein said screen plate is in cylindrical form with one of said first and second surfaces defining an outer surface of a cylinder, and the other of said first and second surfaces defining an inner surface of a cylinder, and reinforcing elements provided on said outer surface.

Inventors: TURCOTTE SERGE, TURCOTTE, SERGE
Applicants: CAE SCREENPLATES INC, CAE SCREENPLATES INC.
Application year: 1999
Main classification: D21D5
IPC codes: B01D29, B01D39, D21D5