Patent Publication Number: US-6702924-B2

Title: Main roll for an air press of a papermaking machine

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Prime Contract No. DE-FC36-01GO10622 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to air press assemblies of papermaking machines, and, more particularly, to rolls used in such air press assemblies. 
     2. Description of the Related Art 
     For many years attempts have been made to use external air pressure to force water out of a paper web. Rather than compress a sheet at a press nip to the point where hydraulic pressure drives water out, as is the case in normal wet pressing, it was reasoned that more water could be removed, and sheet bulk could be maintained, if air pressure could be applied to supplement roll nip generated hydraulic pressures. One such attempt involves providing a multi-roll or other structure forming an air press having a closed chamber, wherein air is circulated through the chamber to convect moisture out of the paper web. 
     One significant challenge that exists in designing an effective multi-roll air press assembly is in minimizing the leakage of air from the assembly adjacent each set of roll ends, each set of roll ends defining a respective chamber end. Typically, a respective seal assembly is held in tension against each chamber end. However, the opportunity for air leakage from the nip region between respective rolls exists, especially in the end portions of each roll not in contact with the paper web and at least one web transfer fabric being fed therethrough. 
     Another challenge that exists in designing an effective multi-roll air press assembly is in minimizing the cost, labor and down time associated with the replacement of a worn roll, especially large-diameter main rolls. Even if some type of a surface cover is used in such an instance, replacement involves removing the surface cover from the entire roll length and then placing on a new surface cover. The expense can be increased if the entire roll should instead require replacement. 
     What is needed in the art is a roll for use in an air press assembly of a papermaking machine that results in decreased air leakage from multi-roll air presses adjacent nip ends thereof, and has readily accessible and replaceable portions which tend to wear for a certain amount of time before any significant wear begins to occur on the primary part of the roll. 
     SUMMARY OF THE INVENTION 
     The present invention provides a roll for use in an air press of a papermaking machine which has a hard central section, in order to promote paper web processing, and soft, elastomeric ends, in order to reduce air leakage from the air press. 
     The invention comprises, in one form thereof, a roll for use in an air press assembly of a papermaking machine, the roll having a pair of ends associated therewith. The roll includes a pair of edge portions with each edge portion extending to one of the pair of ends. Each edge portion has an edge surface portion composed of a first material, the first material having a first hardness. The roll further includes a middle portion located between the pair of edge portions, the middle portion having a middle surface portion composed of a second material. The second material has a second hardness, the second material being harder than the first material. 
     The invention comprises, in another form thereof, a method of pressing a fiber web using an air press. The method includes the step of providing an air press for pressing a fiber web. The air press includes at least one cap roll including a first cap roll; and at least one main roll including a first main roll, the first cap roll and the first main roll forming a first nip therebetween, each main roll having a pair of ends associated therewith. Each main roll has a pair of edge portions, each edge portion extending to one of the pair of ends. Each edge portion has an edge surface portion composed of a first material, the first material having a first hardness. Each main roll also has a middle portion located between the pair of edge portions, the middle portion having a middle surface portion composed of a second material. The second material has a second hardness, the second material being harder than the first material. The method further includes the steps of forming a seal between the pair of edge portions of the first main roll and the first cap roll and feeding the fiber web through the first nip into the air press. 
     An advantage of the present invention is that the soft edge portions promote sealing of the respective nips of the plurality of rolls in an air press assembly, thereby reducing air leakage and increasing the effectiveness of the air press assembly. 
     Another advantage is the soft edge portions, which are replaceable, have a larger average diameter than and thus tend to wear out before the central hard portion, which is not as readily replaced as the soft edge portions. As such, with frequent enough replacement of the soft edge portions, the need for replacing the central hard portion is greatly reduced. 
     Yet another advantage is that the size of the soft edge portions can be chosen such that each soft edge portion is limited to the area where sealing is required and such that each does not interfere with the conveyance of a fiber web through the air press assembly. 
     An even farther advantage is that by placing the soft rubber on the edges of each of the larger-diameter main rolls instead of the smaller-diameter cap rolls, the soft rubber has an increased chance for a longer life as there is more chance for cooling, during a rotation, and less nip cycles per minute. Furthermore, more rubber is exposed to metal in each main roll, allowing greater heat transfer and thus better cooling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic, side view of an embodiment of a papermaking machine of the present invention; and 
     FIG. 2 is a side, partial sectional view of a main roll and an adjoining cap roll shown in FIG. 1; and 
     FIG. 3 is a fragmentary, sectional view of a main roll and an adjoining cap roll shown in FIG.  1 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, there is shown a papermaking machine  10  for processing a web  12  which generally includes an air press  14  and a plurality of conveyor rolls  16 , web  12  typically being a fiber web. 
     Air press  14  includes a pair of main rolls  18  and a pair of cap rolls  20  juxtaposed thereto, thereby forming a plurality of nips  22  therebetween. At the very edges of main rolls  18  and cap rolls  20  are mounted bevel plates  24  and cap seal rings  26 , respectively. Bevel plates  24  and cap seal rings  26  are provided with a beveled notch  28  (FIGS. 2 and 3) and a beveled key  30 , respectively, to permit each set of adjoining bevel plates  24  and cap seal rings  26  to matingly seal and thereby interlock main rolls  18  and cap rolls  20  in a cross-machine direction  32 . Cap seal ring  26  further has an orthogonal extension  34  that has a beveled extension end  36  configured to mate with adjoining cap roll  20 . 
     Air press  14  further includes an air cylinder  38  and a corresponding cap seal ring pulleys  40  associated with each cap roll  20 ; and an end seal arrangement  42  associated with each of the two collective end sets of main rolls  18  and cap rolls  20 . 
     Each cap roll  20  includes a cap roll core  44 , a cap roll cover  46  positioned on cap roll core  44  and a pair of cap-end wear assemblies  48  (one of which is shown in FIG.  3 ). Cap roll core  44  is preferably made of steel or another suitable metal or alloy, and cap roll cover  46  is preferably made of rubber or another elastomeric material. Most preferably, cap roll cover  46  is made of a harder rubber in order to add to cover life and reduce heat buildup therein. 
     Proximate each end of cap roll  20 , cap roll cover  46  has a beveled cover portion  50  that mates with corresponding orthogonal extension  34  of cap seal ring  26  and beveled extension end  36  associated therewith, thereby forming a ring-cover seal  52  therebetween. Cap roll cover  46  is provided with a permeable membrane  54  thereon which may extends over a substantial portion of the length thereof, including the central portion (relative to cross-machine direction (CD direction)  32 ) thereof. However, near each ring-cover seal  52 , cap roll cover  46  instead could have an impermeable membrane  56  attached thereon, impermeable membrane  56  extending across ring-cover seal  52  and onto orthogonal extension  34  of cap seal ring  26 . Impermeable membrane  56 , when present, helps minimize the leakage through ring-cover seal  52 . A sufficient seal can still be obtained, though, even if impermeable membrane  56  is not employed. 
     Each main roll  18  has a pair of main roll ends  58  (one of which is shown in FIG. 3) and includes at least three portions, relative to cross-machine direction  32 . Specifically, each main roll  18  has a pair of edge portions  60  (one of which is shown in FIG. 3) and a middle portion  62 . Each edge portion  60  extends to one of main roll ends  58 . Middle portion  62  is located between edge portions  60 , with edge portions  60  being replaceably mounted thereagainst by bevel plates  24 . 
     Each edge portion  60  has at least an edge surface portion  64  composed of a first material, the first material having a first hardness, and middle portion  62  has a middle surface portion  66  composed of a second material, the second material having a second hardness, the second material being harder than the first. Specifically, the first material preferably is a soft, elastic rubber or other suitable elastomer and has a softness of greater than 10 P &amp; J (P &amp; J is a hardness unit based upon Pusey &amp; Jones standard measurement; plastometer (P&amp;J) readings increase with softer materials; they measure the indentation of a ⅛-inch diameter ball under one kilogram of weight for one minute (see, e.g., the website www.vailrubber.com/roller-coverings.html)) and preferably at least about 100 P &amp; J. Conversely, the second material preferably is one of a steel, a ceramic material, a hard rubber and a hard plastic and has an approximate hardness of between 0 to 20 P &amp; J. Middle surface portion  66  of middle portion  62  crowned to counteract pressure effects. 
     The size and shape of edge portion  60  and edge surface portion  64  are chosen based upon specific parameters. Edge width  68  needs to extend beyond ring-cover seal  52  in order to achieve sufficient sealing therewith or, if present, with impermeable membrane  56  at nip  22  between main roll  18  and cap roll  20  but favorably should not extend far enough to be juxtaposed to permeable membrane  54  (i.e., so as to have edge portion  60  just in the area it is needed and so as to not interfere with fiber web  12 ). Edge surface portion  64  must be made to seal the fabric edge despite the air pressure within air press  14 . Edge surface portion  64  can be made to give the best sealing, for example, by crowning edge portion  60  during forming and/or by causing edge portion  60  to bulge. As such, when mounted, edge portion  60  has a maximum edge diameter  70  (schematically shown) that is greater than a maximum middle diameter  72  (schematically shown) of middle portion  62 . Bevel plate  24  is held onto edge portion  60  by threaded fasteners  74 , and tightening thereof can be used to produce a desired bulge in edge surface portion  64 . Tightening of bevel plate  24  against edge portion  60  offers the further advantage of placing edge portion  60  under compression, thereby adding to the life of the rubber, as cracks do not tend to propagate in a material placed under compression. 
     In use, air press  14  is provided for dewatering a fiber web  12 . Air press  14  includes at least a first cap roll  20  and at least a first main roll  18 , first cap roll  20  and first main roll  18  forming one nip  22  therebetween. Each main roll  18  has a pair of main roll ends  58  associated therewith, and each main roll  18  includes a pair of edge portions  60 , each edge portion  60  extending to one of main roll ends  58 . Each edge portion  60  has an edge surface portion  64  composed of a first material, the first material having a first hardness and has a middle portion  62  located between edge portions  60 , middle portion  62  having a middle surface portion  66  composed of a second material, the second material having a second hardness, the second material being harder than the first material. Sealing between the pair of edge portions  60  of at least first main roll  18  and at least first cap roll  20  is initiated, and fiber web  12  is fed through one nip  22  into air press  14 . 
     One alternate form of the invention includes providing middle portion  62  with a thin soft side (not shown) at each side thereof and adjacent to a respective edge portion  60  in order to act as a further seal for interior edge  76  of impermeable membrane  56 . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.