Patent Publication Number: US-8113358-B2

Title: Pre-tensioned sifter screen

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application, pursuant to 35 U.S.C. §119(e), claims priority to U.S. Provisional Application Ser. No. 60/910,048, filed Apr. 4, 2007. That application is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     Embodiments disclosed herein relate to shaker screens for vibratory sifters. More specifically, embodiments disclosed herein relate to shaker screens for vibratory sifters that are pre-tensioned. More specifically still, embodiments disclosed herein relate to apparatuses and methods for using gyratory sifters including pre-tensioned shaker screens including rigid external extensions. 
     2. Background Art 
     Generally, sifters include a class of vibratory devices used to separate sized particles, as well as to separate solids from liquids. Sifters are used to screen, for example, feed material, plastic resins, and powders during industrial sorting and/or manufacturing operations. 
     Because sifters may be in continuous use, repair operations, and associated downtimes need to be minimized as much as possible. Often, the filter screens of sifters, through which sized materials or liquids are separated, wear out over time and subsequently require replacement. Therefore, sifter filter screens are typically constructed to be removed and replaced. While there are numerous styles and sizes of filter screens, they generally follow similar design. 
     Typically, filter screens include a perforated plate base upon which a wire mesh, or other perforated filter overlay, is positioned. The perforated plate base generally provides structural support and allows the passage of fluids or sized material therethrough, while the wire mesh overlay defines the largest solid particle capable of passing therethrough. While many perforated plate bases are flat or slightly arched, it should be understood that perforated plate bases having a plurality of corrugated or pyramid-shaped channels extending thereacross may be used instead. The pyramid-shaped channels may provide additional surface area for the sized solids or fluid-solid separation process to take place while acting to guide solids along their length toward the end of the sifter from where they are disposed. 
     A typical sifter filter screen includes a plurality of hold-down apertures at opposite ends of the filter screen. These apertures, preferably located at the ends of the filter screen that abut walls of the sifter, allow hold down retainers of the sifter to grip and secure the filter screens in place. However, because of their proximity to the working surface of the filter screen, the hold-down apertures must be covered to prevent solids in the material passing therethrough from bypassing the filter mesh through the hold-down apertures. To prevent such bypass, an end cap assembly is placed over each end of the filter screen to cover the hold-down apertures. 
     In alternate sifters, a filter mesh may be stretched over a frame that is an integral part of the sifter body. Thus, such filter meshes may become tensioned as they are disposed within the sifter. Such methods of attaching filter mesh may involve time consuming filter mesh changing operations, as the old filter mesh must be removed, and a new filter mesh must be tensioned within the sifter body. Removing and then reattaching a filter mesh may involve substantial manual labor, and require significant sifter downtime. 
     In certain sifter screens, the screen includes a frame with a filter screen attached to the top of the screen. The screen is generally a flat screen with a plurality of location tabs that extend from the base of the frame that help guide the frame into position within the sifter. The lateral extension of the location tabs then press against the body of the sifter, thereby creating a desired tension across the surface of the screen. However, such screens are prone to design failure, because the tensioning of the frame may cause bowing and otherwise cause breaks to form in the body of the frame. Furthermore, the location tabs often wear out over time, decreasing the tension on the frame, and decreasing the sealing efficiency of the screen to the sifter. As sealing efficiency decreases, sized particles may slip though spaces along the screen edge, leading to product loss, further stress points along the screen that may lead to premature screen failure, and otherwise result in the requirement to change the filter mesh more frequently. 
     Typically, screens used with sifters are placed in a generally horizontal fashion on a substantially horizontal bed or support structure located within a basket in the sifter. The screens themselves may be flat, nearly flat, corrugated, depressed, and/or contain raised surfaces. The basket in which the screens are mounted may be inclined towards a discharge end of the sifter. The sifter imparts a rapidly reciprocating motion to the basket and the screens. A source material, from which particles are to be separated, is poured onto a back end of the vibrating screen. The material generally flows toward the discharge end of the basket. Large particles that are unable to pass through the screen remain on top of the screen, and move toward the discharge end of the basket where they are collected. Smaller particles and/or fluid pass through the screen and collect in a bed, receptacle, or pan therebeneath. 
     In some sifters, a fine screen cloth is used with the vibrating screen. The screen may have two or more overlying layers of screen cloth or mesh. Layers of cloth or mesh may be bonded together and placed over a support, multiple supports, a perforated plate, or an apertured plate. The frame of the vibrating screen is resiliently suspended or mounted upon a support, and is caused to vibrate by a vibrating mechanism (e.g., an unbalanced weight on a rotating shaft connected to the frame). Each screen may be vibrated to create a flow of trapped solids on top surfaces of the screen for removal and disposal thereof. The fineness or coarseness of the mesh of a screen may vary depending upon the operational requirements of a specified sifting operation. 
     Replacing un-tensioned screens and filter mesh used in sifters generally is a time consuming and inefficient process that involves removing a number of either attachment mechanisms used to hold down and tension screens, or replacing and tensioning filter mesh to an integral screen, as described above. These screen replacement operations may involve significant downtime of the sifter, which may slow the manufacturing operation. Additionally, present pre-tensioned screens are prone to structural failure do to frame breakage and loss of filter mesh tension over time. 
     Accordingly, there exists a continuing need for a pre-tensioned sifter screen that may resist structural failure and may provide for more efficient screen changes. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect, embodiments disclosed herein relate to a screen for a gyratory sifter including a frame having a plurality of cross-members and a rigid external extension. Additionally, the screen includes a filtering element securably attached to the frame. 
     In another aspect, embodiments disclosed herein relate to a method for installing a pre-tensioned screen in a gyratory sifter including inserting the pre-tensioned screen into the gyratory sifter. The pre-tensioned screen including a frame having a plurality of cross-members and a rigid external extension and a filtering element securably attached to the frame. Additionally, the method including mating the pre-tensioned screen to a ball box, wherein the mating includes placing the pre-tensioned screen against a contoured section of the ball box, and securing the pre-tensioned screen with a screen plate. 
     In another aspect, embodiments disclosed herein relate to a method of manufacturing a pre-tensioned screen for a gyratory sifter including forming a frame, wherein the frame includes a plurality of cross-members and a rigid external extension. The method further including securing a filtering element to the frame. 
     Other aspects and advantages of the disclosure will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a perspective view of a gyratory sifter screen according to embodiments of the present disclosure. 
         FIG. 2A  shows a partial cross-sectional view of a gyratory sifter including a screen according to embodiments of the present disclosure. 
         FIG. 2B  shows a break-away view of a gyratory sifter including a screen according to embodiments of the present disclosure. 
         FIGS. 3A-3C  show cross-sectional views of a screen according to embodiments of the present disclosure. 
         FIGS. 4A and 4B  show a method of installing a screen in a gyratory sifter according to methods of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, embodiments disclosed herein relate to shaker screens for vibratory sifters. More specifically, embodiments disclosed herein relate to shaker screens for vibratory sifters that are pre-tensioned. More specifically still embodiments disclosed herein relate to apparatuses and methods for using gyratory sifters that include pre-tensioned shaker screens having rigid external extensions. 
     Referring initially to  FIG. 1 , a perspective view of a gyratory sifter screen  100  according to embodiments of the present disclosure is shown. In this embodiment, sifter screen  100  includes a frame  101  having a plurality of cross-members  102  forming a plurality of relief areas  103 , thereby allowing the passage of materials therethrough. Additionally, screen  100  includes a filtering element  104  disposed on frame  101  including cross-members  102 . As illustrated, frame  101  also includes a rigid external extension  105  extruding from a basal section. 
     In embodiments of the present disclosure, frame  101  and cross-members  102  may be manufactured from, for example, carbon fiber, carbon steel, stainless steel, or other materials as known to those of ordinary skill in the art. For example, in certain embodiments, frame  101  and cross-members  102  may be formed from a single steel piece formed via pressing, bending, or stamping (i.e., a method to make the relief areas) to an appropriate geometry. However, in alternate embodiments, frame  101  and cross-members  102  may be formed from a composite mixture of metal support members surrounded by a glass/resin matrix. Those of ordinary skill in the art will appreciate that the specific manufacturing process of frame  101  and cross-members  102  is not a limitation of the present disclosure. 
     Filtering element  104  may be manufactured from steel alloys including, for example, 301 grade stainless steel. Filtering element  104  may also be made of, for example, plastics, metals, alloys, fiberglass, composites, and polytetrafluoroethylene (PTFE). In certain embodiments, the filtering element  104  may have two or more layers of the same or different filtering mesh and may be layered in any combination. Additional materials that may be used as a filtering element  104  include porous cloth, composites, and other materials known to those of skill in the art. Generally, the material of filtering element  104  will be determined based on the requirements of a specific sifting operation. For example, in an operation requiring the sifting of a relatively large material from a relatively small material, the optimal filtering element geometry may one that will only allow the relatively small material to pass therethrough. However, in certain operations, the relative size difference in filtered material may be small. In such embodiments, filtering element  104  may include a plurality of filtering layers to achieve an optimized sifting. 
     Filtering element  104  may be securably attached to frame  101  via, for example, glues, powder epoxy, or through chemical means. However, in alternate embodiments, filtering element  104  may be secured to frame  101  via mechanical fasteners, thermal staking, thermal bonding, or other means known to those of ordinary skill in the art. Furthermore, the securing of filtering element  104  to frame  101  may be determined, at least in part, by pre-tensioning requirements. For example, in a filtering element  104  requiring high tension, the securing means must be capable of withstanding such tensions. Thus, those of ordinary skill in the art will appreciate that the specific attachment means of filtering element  104  to frame  101  will be operationally dependent. 
     Relief areas  103  include the partitioned sections between, for example, cross-members  102  and other cross-members  102 , as well as the partitioned sections between cross-members  102  and frame  101 . Relief areas  103  provide an opening that allows the passage of material through filtering element  104  and through screen  100  in general. The specific geometry of relief areas  103  is not a limitation on the present disclosure. However, those of ordinary skill in the art will appreciate that providing the maximum area of relief  103  may increase the flow of materials therethrough. As such, in certain embodiments, it may be beneficial to have a lower number of cross-members  102 , thereby increasing the relative area of screening space. However, in certain embodiments, it may be necessary to decrease the area of screening space in lieu of increased rigidity of screen  100 . Examples of such embodiments that may require decreased relief area  103  include screens  100  that have high levels of pre-tension. Because the stress on frame  101  in high tension screens  100  may be relatively increased, the structure of frame  101  may require additional support. As such, increasing the size or number, and/or decreasing the malleability of cross-members  102  may further increase the rigidity of screen  100 . 
     As the rigidity of screen  100  increases, the tension of filtering element  104  stretched thereon may also be increased. Those of ordinary skill in the art will appreciate that the area of relief areas  103 , the spacing of cross-members  102 , and the thickness of, for example, frame  101  and cross-members  102  may be varied to provide an acceptably rigid structure to support the requirements of tensioning a particular screen  100 . As such, those of ordinary skill in the art will appreciate that increasing the rigidity of frame  101  and cross-member  102  may further increase the tensioning abilities of filtering element  104  of screen  100 . 
     Screen  100  also includes a rigid external extension  105 . Rigid external extension  105  may include an extruded section of frame  101  that extends laterally therefrom. Rigid External extension  105  may further include material properties coincident with frame  101 . However, in certain embodiments, rigid external extension  105  may include different material properties, or be manufactured from a different material altogether. The specific geometry of rigid external extension  105  is not a limitation of the present disclosure; however, those of ordinary skill in the art will appreciate that in one embodiment, rigid extension  105  may include an extruded portion of frame  101 . Functionally, rigid external extension  105  must be resistant to malleability due to operational forces. Said another way, rigid external extension  105  may specifically resist movement in plane A to prevent the bending of frame  101 . Thus, the rigidity of frame  101  is reinforced by rigid external extension  105 . 
     In one embodiment, rigid external extension  105  may generally include an extension around the entire periphery of frame  101 . Because the extension extends around the entire periphery, the structural integrity, and thus the frames resistance to bowing from pre-tensioning, may increase. Those of ordinary skill in the art will appreciate that the size of rigid external extension  105  may vary according to the tensioning requirements of a specific screen  100 , however, in certain embodiments, the length of extension may be several centimeters. Likewise, the width of rigid external extension  105  may also vary according to specific screen  100  requirements, but generally, the wider the extension, the greater the screens resistance to bowing. As such, for higher pre-tensioning, those of ordinary skill in the art will appreciate that the geometry of rigid external extension  105  may be adjusted accordingly. 
     Furthermore, those of ordinary skill in the art will appreciate that pre-tensioning of filtering element  104  necessarily biases the bending of frame  101  toward point B. However, embodiments of the present disclosure neutralize the biasing force of pre-tensioning screen  100 . Thus, the bending of frame  101  may be decreased and wear on screen  100  may be reduced, thereby preserving the integrity of screen  100 . 
     Referring to  FIG. 2A , a cross-sectional view of a gyratory sifter  206  including a screen  200  according to embodiments of the present disclosure is shown. In this embodiment, gyratory sifter  206  includes screen  200 , screen gasket  207 , ball box  208 , a screen locating device  209 , and a screen plate  212 . Screen  200  includes structural elements (as described above relative to screen  100 ), including, for example, filtering element  204 , rigid external extension  205 , and frame  201 . Features described in  FIG. 1  may be incorporated as features in  FIG. 2 . 
     As illustrated, screen  200  is disposed within gyratory sifter  206 . A screen gasket  207  is disposed on rigid external extension  205 . Screen gasket  207  may be formed from, for example, rubber, polyurethane, or other materials known to those of ordinary skill in the art. Furthermore, screen gasket  207  may be disposed on a select side or all sides of frame  201 , according to the requirements of a specific sifter. Those of ordinary skill in the art will appreciate that seal gasket  207  may provide a more effective seal if seal gasket  207  surrounds the entire periphery of screen  200 . 
     Frame  201  also includes rigid external extension  205  extruding from the periphery of screen  200 . The interface area between frame  201  and rigid external extension  205  defines an included angle Θ. Included angle Θ is defined by the intersection of frame  201  and the protrusion of rigid external extension  205 . As illustrated, included angle Θ is substantially 90°, however, those of ordinary skill in the art will appreciate that included angle Θ could include angles of less than or greater than 90°. In certain embodiments, included angle Θ may be obtuse prior to installation of filtering element  204  onto frame  201 . The installation of filtering element  204  may then decrease included angle Θ to, for example, to a 90° or otherwise acute angle. In other embodiments, the rigidity of frame  201  and rigid external extension  205  may be such that the tensioning of filtering element  204  may not cause included angle Θ to change. 
     Generally, rigid external extension  205  may be manufactured to form a substantially continuous curve having included angle Θ at its apex. Those of ordinary skill in the art will appreciate that such a substantially continuous curve may be inclusive of a substantially 90° included angle because the manufacturing of frame  201  may provide for the curvature. Such curvature may further provide for increased frame  201  rigidity, thereby increasing the resistance of frame  201  to bending. 
     Screen  200  is disposed in gyratory sifter  206  on top of ball box  208 . Ball box  208  includes a rigid structure having contours that match a bottom surface  211  of screen  200 . As such, as screen  200  is disposed on top of ball box  208 , screen  200  may be secured in place. Examples of methods to secure screen  200  to ball box  208  may include mechanical fasteners, such as clasps, latches, or screws. However, those of ordinary skill in the art will appreciate that in certain embodiments, additional fasteners may not be necessary, as the weight and/or design dynamics of screen  200  may provide effective securing. 
     In one embodiment, ball box  208  may be manufactured from steels, such as 300 series stainless steel, carbon steel, plastics, or composites, such as those discussed with respect to frame  201  above. Referring briefly to  FIG. 2B , a break-away perspective view of screen  200  relative to ball box  208 , and a screen plate  212  is shown. In this embodiment, screen  200  has a screen gasket  207  on one side, however those or ordinary skill in the art will appreciate that in some embodiments, screen gasket  207  may extend around the complete periphery of screen  200 . Those of ordinary skill in the art will appreciate that in certain embodiments ball box  208  may be a contoured surface area on which a screen  200  may be disposed. However, in some embodiments, ball box  208  may be configured to include a plurality of balls or discs that may move against screen  200  during operation to further assist sifting. 
     Ball box  208  includes a plurality of box support members  213  that provide structural integrity, as well as a contoured surface to provide a resting surface for screen  200 . In this embodiment, ball box  208  also includes a mating surface  214 , on which screen  200  may rest when installed. As illustrated, mating surface  214  may be a recessed portion of ball box  208  that provides a support surface for rigid external extension  205 . Such a mating surface  214  may thereby provide a male/female installation configuration, so that the lateral movement of screen  200  may be restricted. 
     In this embodiment, plate screen  212  is shown disposed above screen  200 . During installation, plate screen  212  may be lowered into contact with screen  200  to limit vertical movement of screen  200 . Plate screen  212  may be locked into place via a mechanical fastener, as described above, or may otherwise be held in place by other components of the gyratory sifter. Plate screen  212  may be manufactured and/or formed from, for example, steel alloys, such as 300 series stainless steel, composites, carbon steels, or other materials known to those of skill in the art. 
     Referring to  FIGS. 2A and 2B  together, mating surface  214  is shown disposed over a screen locating device  209 . Screen locating device  209  may include airbags, such as those used in pneumatic screen changes, mechanical actuators, or other methods of locating, securing, and otherwise changing screens on gyratory sifters. In this embodiment, screen locating device  209  is movable in direction C to provide a force against screen gasket  207 . As illustrated, screen  200  is disposed between screen plate  212  and ball box  208 , such that a seal is formed by screen gasket  207  between mating surface  214  and screen plate  212 . In certain embodiments, screen locating device  209  may include slidable rails that provide a force in direction C to further increase the sealing ability of seal gasket  207 . Moreover, in some embodiments, screen locating device  209  may be used to automate or otherwise facilitate screen changes. 
     Referring to  FIG. 3A , a cross-section of a screen  300  according to embodiments of the present disclosure is shown. In this embodiment, screen  300  includes a frame  301  having a top surface  315  and a bottom surface  311 . Screen  300  also includes a rigid external extension  305  forming an included angle Θ, as described above. In this embodiment rigid external extension  305  is a protrusion of frame  301  that is flush with and forms an integral part of bottom surface  311 . However, in alternate embodiments, rigid external extension  305  may form varied geometries. Referring briefly to  FIG. 3B , rigid external extension  305  may be disposed in the center of frame  301 . In this embodiment, included angle Θ is still substantially 90°, however, rigid external extension  305  does not from a part of bottom surface  311 . Referring briefly to  FIG. 3C , in still another embodiment, external extension  305  may be substantially angled, as described above, such that included angle Θ is obtuse. Those of ordinary skill in the art will appreciate that the specific geometry of rigid external extension  305  is not a limitation on the scope of the present disclosure. Rigid external extension  305  may thus be any geometry that prevents reduces a bending of frame  301  due to pre-tensioning, as described above. 
     Referring to  FIGS. 3A-3C  together, filtering element  304  may be secured to frame  301  by any method known to those of skill in the art. Exemplary methods may include gluing, heat staking, thermal bonding, welding, epoxies, or via mechanical fasteners, all generally represented at  316 . Thus, pre-tensioning methods  316  may be used to secure filtering element  304  to frame  301  such that a desired tension may be achieved. Those of ordinary skill in the art will appreciate that the precise tension of filtering element  304  will vary depending on the requirements of the screen  300  or the sifting operation. 
     Referring to  FIGS. 4A and 4B  together, a method of installing and/or changing a screen  400  according to embodiments of the present disclosure is shown. In this embodiment, a gyratory sifter  406  is illustrated including a plate deck  412 , a ball box  408 , and a screen locating device  409 . During screen removal and installation, screen locating device  409  may be actuated to lower ball box  408 . A screen may then be detached from ball box  408  and removed through a door opening  417  in gyratory sifter  406 . A new pre-tensioned screen may then inserted through door opening  406  and mated to ball box  408 , as described above. Screen locating device  409  may then be actuated to secure the pre-tensioned screen with the plate deck  412 . 
     In other embodiments wherein gyratory sifter  406  does not include door opening  417 , plate deck  412  may be manually removed by, for example, lifting plate deck  412  off of a screen (not shown). The screen may then be unmated from ball box  408  by, for example, unfastening mechanical fasteners, and removed from gyratory separator  406 . A new pre-tensioned screen may then be inserted onto ball box  408  and mated therewith. After mating the new pre-tensioned screen with the ball box  408 , the pre-tensioned screen may be secured in place by lowering plate deck  412 . Those of ordinary skill in the art will appreciate that a number of methods of removing, changing, and installing pre-tensioned screens in gyratory sifters fall within the scope of the present disclosure. In certain embodiments, the steps of mating and securing the new pre-tensioned screen in the gyratory sifter may include actuating seal locating devices, inserting male/female connectors and fastening mechanical devices. However, in other embodiments, mating and securing may otherwise include placing the pre-tensioned screen against or within a support structure. 
     Embodiments of the present disclosure, as described above may be manufactured according to various methods. For brevity, only one such method is described in detail below. In one embodiment, a screen, including a frame may be formed from, for example, a steel by pressing the steel into a desired geometry. The geometry will generally include a plurality of cross-members and a rigid external extension. In this embodiment, the rigid external extension may form a substantially continuous curve having an included angle of substantially 90°. 
     Once the frame is formed, a filtering element may be secured to the frame by applying a powder epoxy to the surface of the frame and mounting the filtering element thereto. The filtering element may be held in place at a desired tension until the filtering element is sufficiently secured to the frame. After the filtering element is secured to the frame, a screen gasket may be installed along the rigid external extension. Installing the screen gasket may include gluing or placing the screen gasket around the rigid external extension. 
     Those of ordinary skill in the art will appreciate that the above description of manufacturing screens according to embodiments disclosed herein is merely illustrative. Methods of such manufacturing steps may vary according to the materials being used, and as such, the example described is not meant as a limitation on the scope of the present disclosure. For example, in certain embodiments, forming a frame may include bending a metal, mixing a resin to form a composite, or welding frame components together. Additionally, securing a filtering element may include any method of attaching a filtering element to a frame known to those of ordinary skill in the art. 
     Advantageously, embodiments of the present disclosure may provide pre-tensioned screens that may be less likely to suffer structural integrity failure during use. Because the screens disclosed herein have a geometry that may prevent the frame from bowing due to pre-tensioning, the frames may have a longer operational lifespan. Moreover, because the screens may be less likely to bend, the filtering elements may remain more taut, thereby assisting in efficiency of sifting operations. 
     Also advantageously, pre-tensioned screens according to embodiments disclosed herein may ease the installation process associated with screen changes in sifting operations. For example, because the screens are pre-tensioned, the installation operations may take less time, require less labor, or become sufficiently automated to streamline the process. Furthermore, because rigid external extensions may have seal gaskets disposed thereon, a tighter seal may be realized between the frame and the gyratory sifter. A tighter seal may further increase the lifespan of screens disclosed herein, because the vibrations of the screen within the gyratory sifter may be restricted. Restricting vibrations of the screen against, for example, the plate screen, may reduce the wear on the filtering element that may otherwise become loose and experience reduced tautness over time. As such, the lifespan on the screen, along with the efficiency with which associated screens may operate, may thereby be further increased. 
     While the present disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of the present disclosure will appreciate that other embodiments may be devised which do not depart from the scope of the disclosure described herein. Accordingly, the scope of the disclosure should be limited only by the claims appended hereto.