WO2001051168A1 - Wedge wire screen cylinder and method of manufacturing the same - Google Patents

Abstract

The wedge wire screen of the present invention accomplishes economical manufacture of a durable cylindrically shaped screen, for use in closed pressure screens for the precessing of wood fibers or secondary fibers into paper pulp stock, by attaching an array of wedge wires to integral ribs prior to rolling the array and ribs into a cylindrical shape. The screen of the present invention is capable of withstanding the high stresses introduced by the alternating pushing and pulling of the rotatable rotors or beaters employed within the associated closed pressure screening apparatus without addition of structural supports after formation of the array into a cylindrical shape.

Description

WEDGE WIRE SCREEN CYLINDER AND METHOD OF MANUFACTURING THE SAME

BACKGROUND OF THE INVENTION

This invention relates to screen cylinders specifically designed for use in closed pressure screens for the processing of wood fibers or secondary fibers into paper pulp stock, in which rotatable rotors or beaters are employed within a generally cylindrical screen which alternatively push and pull fibers through the screen openings thereby subjecting the screen to high stresses.

Closed pressure screening apparatus as mentioned above is shown in U.S. Patent of Chupka et al. 4,155.841 issued May 22, 1979, for the preparation of such pulp stock for paper manufacture or for the cleaning of pulp stock otherwise containing contaminates. A screen cylinder used in such apparatus may, on the one hand, be formed by cutting or machining the required slots or grooves on the respective surfaces of a flat metal plate, usually a flat plate of stainless steel material. After the machining and cutting operations, forming the desired profiles and slots, the flat plate is rolled into the shape of a cylinder and welded at the abutting ends to form a screen cylinder. Thereafter, one or more encircling rings are fitted over the cylinder and welded in place to provide the necessary strength required to support the cylinder against the radial stresses induced by the processing of the pulp suspension in a closed pressure screen system as described.

The prior art contains a number of instructive references illustrating screens of this general manufacture and, at the same time, describing the steps and difficulties involved in the constructions of such cylinders. Frejborg U.S. Patent No. 5,718,826 issued February 17, 1998 discloses a screen cylinder for the above described purpose that has been formed flat then rolled into a cylinder. Thereafter, contoured re-enforcing rings are positioned over the cylinder, precisely located, and welded in place to form a permanent attachment to the cylinder along prescribed land areas. The rings may be formed as a loop from straight bar stock or specially formed for this purpose. The rings are then accurately machined so as to form a close fit with the outer circumference of the cylinder. Finally, the rings are attached such as by circumferential welding, to the cylindrically shaped wedge wire array.

Another and more elaborate arrangement for supporting a rolled screen plate cylinder comprises mounting the same within a second surrounding cylinder having access or flow openings therein, such as by heat shrinking the two cylinders together while maintaining registration of the openings and parts, as shown in Frejborg et al. U.S. Patent No. 5,200,072 issued April 6, 1993.

Another type of screen uses extruded or drawn wire, known as a wedge wire in view of its generally wedge-like shape, usually drawn from stainless steel round stock and cut into lengths and arranged on supporting elements, such as machined racks, for supporting the wedge wires in a particularly predetermined spaced configuration. The wedge wires may be formed with protrusions to define spacing, or may be otherwise spaced such as by temporary shims. The assembly is then rolled into a cylinder and welded at the abutting ends.

Such an assembly of accurately positioned wedge wires can define extremely efficient screening slots as narrow as 0.002 inches in slot spacing but the wires themselves are inherently weak and must be reinforced against the pulsating and radial bending forces experienced within a closed pressure screening apparatus. For this purpose, a plurality of encircling rings must be formed and then applied to the outlet surface of the screen cylinder, such as by welding. Conventional end rings may then be installed.

In another attempt to provide ultrafine slots, less than 0.004 inches in width, individual wedge wires or bars are extruded to shape and then finished or cut to a particular configuration and assembled into cylindrical retaining rings which are then bolted together by tie rods. Alternatively, toroidal supports are welded in place in surrounding relation to the wedge wires. Both methods are shown in the several examples of Gero et al. U.S. Patent No. 5,791,495 of August 11, 1998.

The use of extruded wedge wires to define screening slots has been favored by many for a number of reasons. First, the wedge wire itself can be drawn or extruded through a die with minimal handling and at favorable expense ratios as compared with a machine slotted plate. This is due in part to the need for elaborate set up and cutting into a sheet metal plate to form slots, contours, and reliefs from two sides of a plate. A slip or mistake can result in ruining an entire plate. Further, since wires can be drawn with accuracy, and since the gaps between the wires can be maintained with great accuracy using known techniques, very accurate slots with widths at 0.004 inches and lower may be readily obtained and maintained in a cylindrical screen.

As noted above, the wedge wires can be bent and distorted more readily than can segments of a solid plate. Therefore, prior wedge wire screens require supports usually in the form of accurately fabricated encircling rings which are added to the outer screen surface by welding to the wedge wires after an array of such wedge wires has been formed into a cylinder. Needless to say, the economies that flow from the creation of slots with drawn shaped wedge wires are quickly lost in the costs of material and labor in forming, positioning, and welding or securing the encircling ring members to form a complete screen cylinder.

Accordingly, there is a need for a wedge wire screen for screening paper making stock, and a method of manufacturing the same, which utilize drawn wedge wire elements for accurately defining slots and which substantially reduces the labor of assembling associated members, such as sleeves and/or rings.

SUMMARY OF THE INVENTION The present invention is directed to a wedge wire screen which is formed as a flat member and then rolled to the general form of a cylinder. Integral strengthening ribs are joined with the wedge wires prior to rolling, essentially at the outlet sides of the wires, thereby eliminating the need for installation of auxiliary strengthening rings after rolling. The necessary radial strength is achieved by the application of flat backing ribs or strips to the array of wedge wires, having a sufficient mass and thickness, to provide support for the wedge wires and then rolling the wedge wire array with ribs into a cylinder or a cylinder like screening object, accompanied by the work hardening of the ribs as they are bent into annular encircling rings. Due to the work hardening, the resulting rings are provided with enhanced strength against deformation. Such a completed assembly requires no further modification after joining or welding in the annular form except for the possible application of end rings as necessary for supporting the screen basket within the pressure screening apparatus.

The novel wedge wire screen of this invention substantially reduces manufacturing costs in the making of a screen cylinder, in the elimination of the need for fabrication of reinforcing rings and the resulting costs including labor of positioning the rings in relation to a cylindrical surface of a wedge wire array and welding the same in position. The present invention facilitates manufacture in that the reinforcing ribs are attached to a generally flat, or plate like, array of wedge wires which may be held and properly positioned in a fixture as known in the art, and properly spaced and attached by any suitable method, preferably, electron beam resistance welding. Thereafter, the plate is rolled into a cylindrical shape and welded at the axial terminal ends of the ribs.

A particular advantage to the product, manufactured according to the method of this invention, resides in the fact that substantial strength is gained by the metal, preferably stainless steel, in the cold working in rolling the flat assembled array into the general shape of a cylinder. In prior art practices, some cold working is achieved when the plate is rolled into the cylinder; when the reinforcing rings are welded in place, however, it is well known that the heat of welding the parts together causes a substantial loss in strength and hardness that had been increased by cold working, at least in the weld areas.

A further advantage of the invention resides in the fact that secondary welding steps are omitted, the only welding or bonding occurs in the manufacture of the cylindrical screen basket, when the basket is rolled into its cylindrical shape, to join the terminal ends of the ribs. Therefore, the basket may be easily inspected prior to, and after, working to be sure that all welds and seams are integral and no fractures or breaks have occurred. It can then be anticipated that the screen is capable of operating over a long period of time free of unseen defects that might cause or lead to a premature breakage. In the practice of this invention, preferably ribs that are laterally or axially narrow and radially deep are used; this permits the maximum possible open spacing between the ribs and minimizes flow restriction. At the same time, it is known that the neutral axis in the rolling or forming operation is substantially midway of the radial depth of the rib, and the extent and degree of closure in the spaces between the wedge wires may be accurately predicted. In a typical case, the original spaces between the wedge wires may be expected to be closed by between only about 5-10%, which may be readily calculated, to the end that the completed basket is provided with uniform slots of a predetermined gap.

Upon completion, conventional end rings, either at one or both ends of the resulting cylinder, may be applied to permit insertion into preexisting paper pulp pressure cleaning apparatus, which commonly require end rings for locating the screen cylinder concentric with the impulse rotor mechanism of the apparatus. However, it is within the scope of this invention to use and utilize a screen cylinder made according to this invention without end rings in apparatus specifically designed to accept and support such a screen. In other words, in practicing this invention, the end rings also may be eliminated to the extent that they are not needed or required to impart additional strength to the product.

It is accordingly an important object of this invention to provide a cylindrical shaped screen plate or screen cylinder, and the method of making the same, in which the necessity for adding reinforcing rings to a wedge wire array after formation into a cylinder is eliminated.

A further object and advantage of the present invention resides in the employment of reinforcing ribs integrally attached to an array of wedge wires when substantially flat, which is subsequently rolled into a generally cylindrical shape, thereby adding increased cold working yield strength to the material, and eliminating the need for subsequent addition of preformed reinforcing rings to the cylinder. A further object and important advantage of the invention is the reduction of costs in the materials and manufacture of a wedge wire type screen cylinder for pressure screening applications.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a section of flat screen with reinforcing rings attached to an array of wedge wires, in accordance with this invention:

Fig. 2 is an inverted end view of the flat screen section of Fig. 1 ; Fig. 3 is an end view of a screen cylinder formed from an extended length section as shown in Fig. 1; and

Fig. 4 is a fragmentary enlarged side elevation of the screen cylinder of Fig. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the figures of the drawings, which illustrate preferred embodiments and the best mode of the invention, an array 10 of wedge wires 10A forming screening members is shown. Typically, the wires 10A are wedge shaped in cross section as they are broader at the radial inner end 12 than at the radial outer end 1 1 and having a radial height which exceeds the maximum circumferential width, often by about a ratio of 2: 1. Such wires have been made for years, are well known in the art and are routinely drawn accurately to a desired shape. Typical wedge wire shapes are shown, for example, in U.S. Patent Nos. 5,255,790 issued October 26, 1993 and 5,472,095 issued December 5, 1995, as examples only. A preferred wedge wire configuration is shown in PCT application PCT/US99/06120 which is incorporated herein by reference.

Wedge wire screens have been successful in screening papermaking stock where the wires run parallel to the axis of the screen rotation, as the width of the slots may be accurately controlled. The trend toward the use of narrower slots may be accommodated at lower cost and more accurately than can corresponding slots be cut or formed by sawing or machining a flat plate. Furthermore the wires themselves can readily define or identify the profile of the slot for optimum screening performance, throughput, and removal of contaminants.

In a preferred method of manufacture of the screen according to this invention, an array 10 of individual wedge wires 10A are supported in a fixture, as known in the art, with a predetermined spacing between the wires which may be regulated by shims, by protrusions on the wires themselves, or in some other manner. A plurality of ribs 15 are then applied to the wedge wire radial outer ends 1 1 and intimately secured to the wedge wires 10A in the position of the array, such as by electron beam resistance welding. The ribs 15 preferably have a uniform height and width, generally having a radial height which is a substantial multiple of the axial width. In the example shown, the height to width ratio of the ribs 15 is about 6:1, thereby, leaving a maximum of spacing between ribs 15 for the out flow of accepted material passing through the gaps 22 between the wedge wires 10A. It will also be seen that the height of the ribs 15 is a substantial multiple of the height of the wedge wires 10A. In the case of a preferred embodiment, the height of the ribs 15 compared to the height of the wedge wires 10A is on the order of about 5: 1.

In an alternate embodiment, the lower surface of the ribs 15 may be formed with spaced recesses 16 for receiving the outer ends 11 of the wedge wires 10A. The ribs 15 of this configuration are referred to as racks or linear rack gears, due to the appearance of the spaced recesses 16.

The ribs 15 are selected with radial strength and spacings so as to support the wedge wire array 10 in pressure papermaking screening apparatus without the need for supplemental support in the form of supplemental encircling rings, and preferably, without the support of end rings. A particular advantage is that in rolling an array 10 of wedge wires 10A and ribs 15. as shown in Fig. 1, into a wedge wire screen cylinder 30, as shown in Fig. 3, the metal of the ribs 15 is cold worked, primarily by compressing the radial inner half of the rib 15 and stretching the radially outer half of the rib 15 about a neutral axis which lies approximately midway between the radial inner and outer ends of the rib 15. Such cold working of the ribs 15 to form encircling rings 35 creates work hardening and increased yield strength. This increased strength is maintained in the finished screen cylinder 30 since extreme heating or welding is not applied to the ribs 15 after rolling into the cylinder 30 shape. This method is distinguished from conventional practice where supplemental reinforcing rings are welded over the structural support racks for the wedge wire array 10 after formation in a cylindrical shape.

In the manufacturing process, an array 10 of wedge wires 10A and ribs 15 is connected as shown in Fig. 1 of a sufficient length as to form a screen cylinder 30 of a desired diameter by bringing the respective terminal ends 20, 21 of the individual ribs 15 together and welding the individual ribs 15 at their respective terminal ends 20, 21 (Fig. 3) to form a wedge wire screen cylinder 30 with integral encircling rings 35. There will of necessity, be a predetermined narrowing of the screening gaps or slots 22 between the wedge wires 10A. The extent of such narrowing is accurately predictable, due to the adjacent circumferential compression of the ribs 15 at the region at which the wedge wires 10A are attached; depending on the diameter of the resulting screen cylinder, such narrowing will usually be less than 10% of the initial gap width.

In an example of a wedge wire screen cylinder made according to this invention, wedge wires 10A were placed in an array 10 at 25 millimeter spacing center to center. The wedge wires 10A were 5 millimeters in thickness and 30 millimeters in height prior to rolling the array 10 into cylinder 30. A screen cylinder 30 was made of this wedge wire array 10 with an inside diameter of 24 inches with no evidence of over stressing of the ribs 15 or breakage at the points of adhesion to the wedge wires 10A.

While twenty-four inch diameter is a typical and common diameter for screens of the kind described, such screens may use cylinders 30 as small as about 18 inches in diameter up to 60 inches in diameter or more, according to this invention.

In summary, the invention relates to a wedge wire screen and method in which a flat plate like material is rolled into generally cylindrical form and including an array of wedge wires, when rolled, extending parallel to the axis of the cylinder, the plate including ribs integrally attached to a radially outer surface of the wedge wire array prior to rolling that provide all the necessary radial strength for the screen after being rolled into a cylindrical form.

A wedge wire screen and method of making a plurality of wedge wires attached to an edge of a plurality of transversely spaced ribs, in which the ribs have height which substantially exceeds the width at the region of attachment to the wedge wire array and which ribs are rolled into the shape of a cylinder while the wedge wires so attached, resulting in cold working of the material of the ribs. A screen cylinder for screening papermaker's stock in pressure screening apparatus structurally comprising a circumferential array of individual wedge wires defining screening slots therebetween, the wedge wire are integrally attached to a plurality of axially spaced encircling rings, in which the rings are first attached to the array in a flat condition and rolled into the circular condition of the screen cylinder, and in which the rings provide all or substantially all of the radial bending support for the wedge wire screen.

The wedge wire screen as described above in which the rings, when formed into the cylindrical shape, have a radial dimension which exceed the axial thickness of the ring in the order of about 5:1.

A wedge wire screen as described, in which the axial spacing of the rings from each other, center to center, approximately equals the radial thickness of the rings. The wedge wire screen as described, in which the screen is free of any other structural support for maintaining the array of the wedge wires on the ribs.

A wedge wire screen basket in which a wedge wire array is attached to a plurality of individual encircling ribs which ribs are spaced axially along the length of the wedge wire array in which the ribs with attached web wires is formed from a flat condition to a cylindrical condition with the wedge wires attached, and in which the screen basket is otherwise free of encircling ribs attached by welds.

A method of making a cylindrical wedge wire screen basked for use in closed pressure screening apparatus for screening paper making pulp that includes the steps of applying an array of wedge wires to a plurality of generally parallel linearly extending ribs, which ribs are spaced from each other and which have a height substantially greater than their width, and thereafter rolling a flat section of said ribs with said array attached into a generally cylindrical form with there being sufficient strength in the ribs for radial support of the wedge wire array during screening free of additional support structure. What is claimed is:

Claims

1. A wedge wire screen cylinder comprising: a plurality of axially extending wedge wires and a plurality of circumferentially extending encircling rings; said wedge wires being connected to said encircling rings with said encircling rings being axially spaced from one another and extending perpendicular to, and surrounding, said wedge wires; said wedge wire screen cylinder further comprising screening gaps separating adjacent wedge wires.

2. A wedge wire screen cylinder as in Claim 1, wherein; said encircling rings have an radial height which substantially exceeds their axial width.

3. A wedge wire screen cylinder as in Claim 1, wherein; said encircling rings have a radial height which exceeds their axial width on the order substantially five-to-one.

4. A wedge wire screen cylinder as in Claim 1 , wherein; said encircling rings are axially spaced from one another, center to center, substantially equal to the radial height of said encircling rings.

5. A wedge wire screen cylinder as in Claim 1, wherein; the radial height of said encircling rings exceeds the radial height of said wedge wires substantially on the order of five to one.

6. A wedge wire screen cylinder comprising: a plurality of axially extending wedge wires and a plurality of circumferentially extending encircling rings; said encircling rings comprising spaced recesses and said wedge wires comprising outer ends; said wedge wires being connected to said encircling rings, with said outer ends of said wedge wires within the respective recess of said encircling ring, with said encircling rings being axially spaced from one another and extending perpendicular to, and surrounding, said wedge wires; said wedge wire screen cylinder further comprising axially extending screening gaps separating adjacent wedge wires.

7. A wedge wire screen cylinder as in Claim 6, wherein; said encircling rings have an radial height which substantially exceeds their axial width.

8. A wedge wire screen cylinder as in Claim 6, wherein; said encircling rings have a radial height which exceeds their axial width on the order substantially lϊve-to-one.

9. A wedge wire screen cylinder as in Claim 6, wherein; said encircling rings are axially spaced from one another, center to center, substantially equal to the radial height of said encircling rings.

10. A wedge wire screen cylinder as in Claim 6, wherein; the radial height of said encircling rings exceeds the radial height of said wedge wires substantially on the order of five to one.

11. A method of manufacturing a wedge wire screen cylinder comprising the steps of: a. connecting a plurality of wedge wires to a plurality of transversely spaced ribs, with said wedge wires separated from adjacent wedge wires by screening gaps and with said wedge wires extending perpendicular to said ribs, to form a flat array of wedge wires; b. rolling said flat array of wedge wires into a wedge wire screen cylinder, with said ribs forming encircling rings circumferentially surrounding said wedge wires; and c. individually connecting the first terminal end of each said rib to the second terminal end of the respective rib.

12. The method of manufacturing a wedge wire screen cylinder as in Claim 11, wherein step a comprises connecting said plurality of wedge wires to said plurality of transversely spaced ribs via electron beam resistance welding.

13. The method of manufacturing a wedge wire screen cylinder as in Claim 11 , wherein step c comprises connecting the first terminal end of each said rib to the second terminal end of the respective rib via electron beam resistance welding.