Abstract:
A “triple layer” fabric having upper and lower systems of interwoven yarns interconnected into an integral fabric by a system of stitching yarns. Preferably, the fabric is a forming fabric having upper and lower layers of uniformly sized machine direction (MD) yarns. Upper and lower layer cross machine direction (CMD) yarns weave, respectively, with the upper and lower MD yarns to define the upper and lower yarn systems. The lower layer CMD yarns are preferably larger in diameter than the MD yarns. The lower layer CMD yarns weave in a repeat pattern with respect to the lower layer MD yarns such that the lower layer CMD yarns weave in a repeat defined by floats under groups of at least three lower MD yarns and over pairs of adjacent lower MD yarns. Each lower layer MD yarn is in at least two different pairs relative to at least two different lower layer CMD yarns within the repeat which maintains a substantial degree of stacking of the lower layer MD yarns below respective upper layer MD yarns. In an alternative embodiment the stitching yarns are incorporated in the upper layer as upper layer MD yarns where less than one half of the upper layer MD yarns also serve as stitching yarns.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/150,275 filed on Aug. 23, 1999. 
    
    
     BACKGROUND 
     The present invention relates to papermakers fabrics and, in particular, fabrics intended to facilitate the initial formation of an aqueous paper web in the manufacture of paper. 
     Papermaking machines generally are comprised of three sections: forming, press, and drying. Papermakers fabrics are employed to transport a continuous paper sheet through the papermaking equipment as it is being manufactured. The direction of travel of the fabric on the papermaking equipment is commonly referred to as the machine direction. The requirements and desirable characteristics of papermakers fabrics vary in accordance with the particular section of the machine where the respective fabrics are utilized. 
     In particular, in the forming section of papermaking equipment, forming fabrics are utilized to initially create an aqueous paper sheet or web from a pulp slurry. Typically, the pulp slurry is deposited on the top or paper carrying side of the moving forming fabric which transports the slurry over suction boxes or other means to form the paper web. The surface characteristics and drainage characteristics of the forming fabric play an important role in the initial formation of the aqueous paper web. 
     Multilayer forming fabrics are known in the art. For example, U.S. Pat. No. 5,025,839 discloses a dual layer forming fabric for use in the papermaking process which includes single system of zig zagging machine direction (MD) yarns which interweave with two layers of cross machine direction (CMD) yarns. 
     U.S. Pat. Nos. 5,013,330; 4,515,853; and 3,885,603 disclose forming and other types of papermakers fabrics having distinct top and bottom interwoven fabric layers which are interconnected by binding or stitching yarns. Such fabrics are generally known as “triple layer” fabrics, the “third” layer being the layer of binding or stitching yarns which interconnects the upper yarn system with the lower yarn system. 
     Early triple layer fabrics utilized different size machine direction (MD) yarns in the upper and lower fabric layers; the upper MD yarns being roughly one half the diameter than the lower MD yarns with twice as many upper MD yarns than lower MD yarns. Common weave patterns for the bottom or machine side layer were plain weave and 1:3 broken twill. 
     One of the problems with such triple layer fabrics was the relatively high caliper of the fabric. The high caliper allowed water drained from the aqueous paper web to be carried inside the fabric and caused problems of rewetting of the sheet during forming. Another draw back was that the bottom MD yarns tended to arrange themselves between the top layer MD yarns restricting straight through drainage and the ability to easily clean the fabrics. Also if the bottom MD yarns were not evenly spaced and correctly positioned they could cause “shadow marking” in the paper web formed on the fabric. Non-uniform spacing caused non-uniform drainage which attributed to undesired marking. 
     To decrease the caliper of triple layer fabrics, the same size MD yarns began to be used in both the top and bottom layers of such fabrics. A draw back to this construction was that in order to maintain the fine mesh top layer, much smaller lower MD yarns were employed which required comparably smaller lower CMD yarns which tended to shorten fabric life. To permit the use of larger CMD bottom yarns for better wear, it was possible to weave the lower MD yarns in pairs which, however, resulted in the reduction of straight through drainage in the fabric since the paired lower MD yarns would block the spaces between the upper MD yarns. It would be desirable to provide a triple layer fabric which permits the use of uniformly sized upper and lower MD yarns in conjunction with larger lower CMD yarns without substantially impairing straight through drainage of the fabric. 
     SUMMARY 
     A “triple layer” fabric is provided having upper and lower systems of interwoven yarns interconnected into an integral fabric by a system of stitching yarns. Preferably, the fabric is a forming fabric having upper and lower layers of uniformly sized machine direction (MD) yarns. Upper and lower layer cross machine direction (CMD) yarns weave, respectively, with the upper and lower MD yarns to define the upper and lower yarn systems. A stitching layer of CMD yarns binds the upper and lower yarn system together. 
     The lower layer CMD yarns are preferably larger in diameter than the MD yarns, the upper layer CMD yarns and the stitching yarns. The lower layer CMD yarns weave in a repeat pattern with respect to the lower layer MD yarns such that the lower layer CMD yarns weave in a repeat defined by floats under groups of at least three lower MD yarns and over pairs of adjacent lower MD yarns. Each lower layer MD yarn is in at least two different pairs relative to at least two different lower layer CMD yarns within the repeat which maintains a substantial degree of stacking of the lower layer MD yarns below respective upper layer MD yarns. 
     The weave repeat is also selected such that the stitching yarns are woven so as to be recessed within the fabric away from the surfaces defined by the interweaving of the upper MD and CMD yarns and the lower MD and CMID yarns, respectively. 
     In an alternative embodiment, the stitching yarns are incorporated into the upper layer of the fabric where selected upper layer MD yarns interweave with one or more layer CMD yarns in manner such that the interweavings are recessed from the bottom surface of the fabric. Preferably, less than half of the upper layer MD yarns are selected for the additional stitching function. 
     Other objects and advantages of the present invention will become apparent from the following description of a presently preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevated view of the machine side of a papermakers fabric made in accordance with the teachings of the present invention; 
     FIG. 2 is a schematic diagram of each of the upper layer CMD yarns within a repeat of the fabric shown in FIG. 1; 
     FIG. 3 is a schematic diagram of each of the lower layer CMD yarns within a repeat of the fabric shown in FIG. 1; 
     FIG. 4 is a schematic diagram of each of the stitching layer CMD yarns within a repeat of the fabric shown in FIG. 1; 
     FIG. 5 is a schematic diagram of each of the upper layer MD yarns within a repeat of the fabric shown in FIG. 1; 
     FIG. 6 is a schematic diagram of each of the lower layer MD yarns within a repeat of the fabric shown in FIG. 1; 
     FIG. 7 is a schematic diagram of the weave pattern for upper layer CMD yarns within the repeat of a second embodiment of a fabric made in accordance with the teachings of the present invention; 
     FIG. 8 is a schematic diagram of each of the lower layer CMD yarns within the repeat of the fabric shown in FIG. 7; 
     FIG. 9 is a schematic diagram of each of the stitching layer CMD yarns within the repeat of the fabric shown in FIG. 7; 
     FIG. 10 is a schematic diagram of each stacked set of upper and lower layer MD yarns within the repeat of the fabric illustrated in FIG. 7; 
     FIG. 11 is a weave chart for a repeat of a third embodiment of a fabric made in accordance with the teachings of the present invention; and 
     FIG. 12 is a weave chart for a repeat of a fourth embodiment of a fabric made in accordance with the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1-6, a papermakers fabric I made in accordance with the teachings of the present invention is illustrated. The fabric I is woven with a selected repeat pattern having sixteen upper layer CMD yarns  1 - 16 , eight lower layer CMD yarns  21 - 28 , eight stitching CMD yarns  31 - 38 , eight upper layer MD yarns  41 - 48  and eight lower layer MD yarns  51 - 58  within each repeat of the repeated pattern. All of the MD yarns  41 - 48 ,  51 - 58  have substantially the same cross sectional dimension. 
     As best seen in FIG. 2, an upper layer of CMD yarns  1 - 16  interweave in a selected pattern exclusively with respect to the upper layer MD yarns  41 - 48  and do not interweave with the lower MD yarns  51 - 58 . For example, the first CMD yarn  1  within the upper layer CMD repeat only weaves under upper MD yarns  41 ,  44  and  47  and weaves over all other MD yarns in the fabric I. The preferred weave pattern illustrated for the upper layer of the fabric I includes two, two yarn CMD floats in a broken twill. For CMD yarn  1 , the two floats are over upper MD yarns  42 ,  43  and yarns  45 ,  46 , respectively. Alternative weave patterns for the upper fabric layer weave can readily be employed such as a plain weave or twill patterns with or without MD or CMD floats. Preferably, the upper layer CMD yarns  1 - 16  are substantially the same size as the MD yarns. 
     As best seen in FIG. 3, the lower layer CMD yarns  21 - 28  of the fabric I interweave exclusively with the lower layer MD yarns  51 - 58 , but do not weave with the upper layer MD yarns  41 - 48 . The lower layer CMD yarns  21 - 28  are significantly larger in diameter than the MD yarns  41 - 48 ,  51 - 58 , the upper layer CMD yarns  1 - 16 , and the stitching yarns  31 - 38 . Additionally, there are preferably one half as many lower layer CMD yarns  21 - 28  as there are upper CMD yarns  1 - 16  within the fabric&#39;s repeat pattern. 
     The repeat of the larger lower layer CMD yarns  21 - 28  is selected to predominate the bottom surface of the fabric by weaving floats under six adjacent lower MD yarns and over a pair of lower MD yarns. For example, lower CMD yarn  21  weaves over paired lower MD yarns  51 ,  52  and under lower MD yarns  53 - 58 . 
     As best shown in FIG. 1, this weave pattern results in relatively long floats F of bottom layer CMD yarns  21 - 28  predominating the bottom surface of the fabric with MD knuckle pairs P retaining the lower layer CMD yarns  21 - 28  within the fabric. When weaving in knuckle pairs P under lower layer yarns  21 - 28 , the lower layer MD yarns  51 - 58  tend to migrate together in a substantially contiguous relationship. This pairing both enhances fabric stability and wear resistance. 
     In order to maintain the separation between the lower layer MD yarns to facilitate drainage, each lower layer MD yarn  51 - 58  weaves a knuckle around at least two different lower layer CMD yarns  21 - 28  such that in one instance the lower layer MD yarn is paired with its preceding adjacent lower layer MD yarn and in another instance the lower layer MD yarn is paired with its succeeding lower layer MD yarn. For example, with respect to lower layer MD yarn  52 , it is paired with preceding lower layer MD yarn  51  when weaving a knuckle under lower layer CMD yarn  21  and lower layer MD yarn  52  is paired with succeeding lower layer MD yarn  53  when weaving a knuckle under lower layer CMD yarn  24 . Accordingly, the lower layer CMD yarn  52  maintains a substantially stacked relationship with upper layer MD yarn  42  to facilitate fabric drainage, even though the lower layer MD yarn  52  is not directly under upper layer MD yarn  42  when it weaves under the lower layer CMD yarns  21 ,  24 . 
     As best seen in FIGS. 4-6, a layer of stitching yarns  31 - 38  interconnects the systems of interwoven upper MD and CMD yarns and interwoven lower MD and CMD yarns. Preferably, the stitching yarns have a diameter no greater than the diameter of the MD yarns. Each stitching yarn weaves under one lower MD yarn and over one upper MD yarn in its repeat. The stitching CMD yarns  31 - 38  are interwoven such that their interweavings with the upper and lower MD yarns are recessed within the fabric I from the surfaces defined by the upper and lower systems of interwoven MD and CMD yarns. For example, stitching yarn  31  weaves under lower layer MD yarn  53  and over upper MD yarn  47 . As best seen in FIG. 5, when upper layer MD yarn  47  is weaving under stitching yarn  31 , it is also weaving under upper CMD yarn  2  which blocks stitching yarn  31  from rising to the upper surface of the fabric I. As best seen in FIG. 6, when lower layer MD yarn  53  is weaving over stitching yarn  31 , it is also weaving over lower larger CMD yarns  21  and  22  so that stitching yarn  31  is recessed above the bottom surface of the fabric I by at least the difference in diameters of the larger lower CMD yarns and the smaller stitching yarns. Accordingly, the stitching yarns  31 - 38  are protected from wear by not being exposed on the surface of either side of the fabric I. 
     Such protection from wear is important to prevent delamination of the fabric I. Since the stitching yarns are the only yarns which maintain the upper and lower interwoven systems of yarns together, the fabric will delaminate if the stitching yarns wear and break. 
     With reference to FIGS. 7-10, there is shown a second embodiment of a fabric II made in accordance with the teachings of the present invention. The fabric II is woven with a selected repeat pattern having ten upper layer CMD yarns  60 - 69 , five lower layer CMD yarns  70 - 74 , five stitching CMD yarns  80 - 84 , ten upper layer MD yarns  90 - 99 , and ten lower layer MD yarns  100 - 109  within each repeat of the repeated weave pattern. Preferably, all of the MD yarns  90 - 109  have the same diameter. 
     As best seen in FIG. 7, upper layer CMD yarns  60 - 69  interweave exclusively in a selected pattern with respect to the upper layer MD yarns  90 - 99  and do not interweave with the lower MD yarns  51 - 58 . Preferably, the upper CMD yarns  60 - 69  weave in a plain weave such that CMD yarns  60 ,  62 ,  64 ,  66 ,  68  each weave under MD yarns  90 ,  92 ,  94 ,  96 ,  98  and over alternate MD yarns  91 ,  93 ,  95 ,  97 ,  99  and alternate CMD yarns  61 ,  63 ,  65 ,  67 ,  69  each weave over MD yarns  90 ,  92 ,  94 ,  96 ,  98  and under alternate MD yarns  91 ,  93 ,  95 ,  97 ,  99  within the repeat. Alternate weave patterns for the upper MD and CMD yarns to define the upper paper carrying surface of the fabric II can be used. 
     As best seen in FIG. 8, lower layer CMD yarns  70 - 74  interweave exclusively with the lower layer MD yarns  100 - 109 , but do not weave with the upper layer MD yarns  90 - 99 . The lower layer CMD yarns  70 - 74  are significantly larger in diameter than the MD yarns  90 - 109 , the upper layer CMD yarns  60 - 69 , and the stitching yarns  80 - 84 . Additionally, there are preferably one half as many lower layer CMD yarns  70 - 74  as there are upper CMD yarns  60 - 69  within the fabric&#39;s repeat pattern. The repeat of the larger lower layer CMD yarns  70 - 74  is selected to predominate the bottom surface of the fabric by weaving two floats under three adjacent lower MD yarns and over two separate pairs of lower MD yarns within each repeat. For example, lower CMD yarn  70  weaves one float under lower MD yarns  100 ,  101 ,  102 , a second float under lower MD yarns  105 ,  106 ,  107 , and over pairs of lower MD yarns  103 ,  104  and  108 ,  109 . This weave pattern results in the floats of bottom layer CMD yarns  70 - 74  predominating the bottom surface of the fabric II with MD knuckle pairs retaining the lower layer CMD yarns  70 - 74  within the fabric II. When weaving in knuckle pairs under lower layer yarns  70 - 74 , the lower layer MD yarns  100 - 109  tend to migrate together in a substantially contiguous relationship. This pairing both enhances fabric stability and wear resistance. 
     In order to maintain the separation between the lower layer MD yarns to facilitate drainage, each lower layer MD yarn  100 - 109  weaves a knuckle around at least two different lower layer CMD yarns  70 - 74  such that in one instance the lower layer MD yarn is paired with its preceding adjacent lower layer MD yarn and in another instance the lower layer MD yarn is paired with its succeeding lower layer MD yarn. For example, with respect to lower layer MD yarn  104 , it is paired with preceding lower layer MD yarn  103  when weaving a knuckle under lower layer CMD yarn  70  and lower layer MD yarn  104  is paired with succeeding lower layer MD yarn  105  when weaving a knuckle under lower layer CMD yarn  73 . Accordingly, the lower layer CMD yarn  104  maintains a substantially stacked relationship with upper layer MD yarn  94  to facilitate fabric drainage, even though the lower layer MD yarn  104  is not directly under upper layer MD yarn  94  when it weaves under the lower layer CMD yarns  70 ,  73 . 
     As best seen in FIGS. 9-10, a layer of stitching yarns  80 - 84  interconnects the systems of interwoven upper MD and CMD yarns and interwoven lower MD and CMD yarns. Each stitching yarn weaves under one lower MD yarn and over one upper MD yarn in its repeat. 
     The stitching CMD yarns  80 - 84  are interwoven such that they are recessed within the fabric II from the surfaces defined by the upper and lower systems of interwoven MD and CMD yarns. For example, stitching yarn  80  weaves under lower layer MD yarn  101  and over upper MD yarn  95 . As best seen in FIG. 10, when lower layer MD yarn  101  is weaving over stitching yarn  80 , it is also weaving over lower larger CMD yarn  70  so that stitching yarn  80  is recessed above the bottom surface of the fabric due in part to the difference in diameters of the larger lower CMD yarns and the smaller stitching yarns. When upper layer MD yarn  95  is weaving under stitching yarn  80 , it is also weaving under upper CMD yarn  60  which tends to block stitching yarn  80  from rising to the upper surface of the fabric II. Accordingly, the stitching yarns are protected from wear by not being exposed on either side of the fabric. 
     Preferably polyester yarns having a diameter in the range of 0.0035-0.0200 in. (0.09 mm-0.51 mm) are used for the MD yarns. Either polyester or nylon yarns are preferably used for the CMD yarns with preferred diameters in the ranges of 0.0035-0.0200 in. (0.09 mm-0.51 mm) for the upper layer CMD yarns, 0.0050-0.0240 in. (0.012 mm-0.61 mm) for the larger lower layer CMD yarns and 0.0035-0.0150 in. (0.09 mm-0.35 mm) for the CMD stitching yarns. The use of nylon yarns for some of the CMD yarns and polyester yarns for CMD yarns within the same class or different classes of CMD yarns may be used depending upon the specific fabric characteristics desired. Preferred combination of yarn sizes include the weaving of fabric I or II with MD and CMD yarns all having the minimum diameters specified in the above-identified ranges or all having the maximum diameters specified in the above-identified ranges. For example, a preferred fabric is woven in accordance with the repeat pattern of fabric I having all polyester yarns with all MD yarns, upper layer CMD yarns and stitching yarns having a 0.0035 in. diameter and larger lower CMD yarns having a 0.0050 diameter. Another preferred fabric is a fabric woven with the repeat pattern of fabric II having polyester, 0.0200 in. diameter MD yarns and nylon CMD yarns, the upper layer CMD yarns having a 0.0200 in. diameter, the lower CMD yarns having a 0.0240 in. diameter and the stitching yarns having a 0.0150 in. diameter. 
     With reference to FIG. 11, a weave chart for a single complete repeat of a third embodiment of a fabric III is shown. The fabric III includes twenty MD yarns numbered  1 - 20  across the top of the weave chart; the odd number yarns representing lower layer MD yarns and the even number yarns representing upper layer MD yarns. The fabric III has forty CMD yarns within its repeat, the upper or top layer CMD yarns identified with a T, the lower or bottom layer CMD yarns identified with a B and the stitching CMD yarns identified with an S. The fabric III, accordingly, has ten upper MD yarns, ten lower MD yarns, twenty upper CMD yarns, ten lower CMD yarns and ten stitching CMD yarns within its repeat. 
     Within the weave charts illustrated in FIGS. 11 and 12, a blackened box represents that the respective CMD yarn weaves under the respective MD yarn. For example, in the first row of boxes of FIG. 11, the fourth and fourteenth boxes are blackened. This represents that, for the fabric III, the first CMD yarn which is an upper layer CMD yarn, weaves over all of the MD yarns except the fourth and fourteenth MD yarns, which are upper layer MD yarns since they are even numbered. Also, as with the fabric II, each lower CMD yarn in the fabric III weaves over only pairs of adjacent lower MD yarns and weaves two floats under three lower MD yarns in its repeat. 
     With reference to FIG. 12, a weave chart for a fabric IV is shown. The repeat of fabric IV has sixteen MD yarns and forty-eight CMD yarns. Within the weave chart of FIG. 12, the MD yarns are represented by numbers  1 - 16  with the upper layer MD yarns being even numbers and the lower layer MD yarns being odd numbers. Unlike the other embodiments references above, the fabric IV incorporates the function of its stitching yarns into selected upper layer CMD yarns. In the fabric IV of FIG. 12, the upper or top layer CMD yarns are indicated with a T and the lower or bottom layer CMD yarns are indicated with the letter B. The letters T/S are used to identify the top layer CMD yarns which also serve as stitching yarns. Preferably, no more than half of the top layer CMD yarns function as stitching yarns. 
     Within the repeat pattern of the fabric of IV, there are four top layer/stitching yarns identified by chart numbers  12 ,  30 ,  42  and  48 . Unlike the other top layer CMD yarns T, the four top layer/stitching CMD yarns T/S each weave under one of the bottom layer MD yarns within the repeat. Top layer/stitching CMD yarn  12  weaves under bottom MD yarn  7 ; top layer/stitching CMD yarn  30  weaves under bottom layer MD yarn  11 , top layer/stitching CMD yarn  42  weaves under bottom layer MD yarn  3 , and top layer/stitching CMD yarn  48  weaves under bottom layer MD yarn  15 . The interweaving of the top layer/stitching CMD yarns T/S with the bottom layer MD yarns are adjacent to floats of bottom layer CMD yarns B so that the top layer/stitching CMD yarns interweaving with the bottom layer MD yarn is recessed from the bottom surface of the fabric which is defined by the interweaving of the lower MD yarns and lower CMD yarns. 
     As with the other embodiments, the lower layer CMD yarns B of the fabric IV weave over only pairs of adjacent lower MD yarns and each of the lower MD yarns weaves under at least two different lower CMD yarns B in the repeat pattern such that each lower MD yarn is paired with a preceding adjacent lower MD yarn when it weaves under one of the lower CMD yarns B and is paired with a succeeding adjacent lower MD yarn when it weaves under another of the lower CMD yarns B. For the example represented in FIG. 12, lower MD yarn  3  weaves under lower CMD yarn  2  when it is paired with preceding lower MD yarn  1 , and lower MD yarn  3  weaves under lower CMD yarn  11  when it is paired with succeeding lower MD yarn  5 . 
     Other variations within the scope and spirit of the invention will be apparent to those of ordinary skill in the art.