MULTI-LAYER FORMING FABRIC WITH STITCHING YARN PAIRS INTEGRATED INTO PAPERMAKING SURFACE

Abstract: A multi-layer papermaker's forming fabric of the present invention utilizes stitching yarns in the papermaking surface. The fabric of the present invention is formed of repeating units, each of which comprises: a set of top machine direction yarns; a set of bottom machine direction yarns; a set of bottom cross-machine direction yarns interwoven with the set of bottom machine direction yarns; and pairs of first and second stitching yarns extending in the cross-machine direction. The first and second stitching yarns of each pair are interwoven with the top and bottom machine direction yarns such that, as a fiber support portion of the first stitching yarn is interweaving with the top machine direction yarns, a binding portion of the second stitching yarn is positioned below the top machine direction yarns, and such that as a fiber support portion of the second stitching yarn is interweaving with the top machine direction yarns, a binding portion of the first binding yarn is positioned below the top machine direction yarns. The first and second stitching yarns cross each other as they pass below a transitional top machine direction yarn. Each of the binding portions of the first and second stitching yarns passes below at least one of the bottom machine direction yarns. The fiber support portions of the stitching yarns and the top machine direction yarns form the papermaking surface.

Description: MULTI-LAYER FORMING FABRIC WITH STITCHING YARN PAIRS INTEGRATED INTO PAPERMAKING SURFACE Field of the Invention The present invention relates generally to papermaking, and relates more specifically to fabrics employed in papermaking. Background of the Invention In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper "stock") is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a "forming fabric," provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (L , the "machine side") of the fabric. After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more pairs of pressure rolls covered with another fabric, typically referred to as a "press felt." Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a "batt" layer of the press felt. The paper is then transferred to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging. Typically, papermaker's fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin-seamable flap or a special foldback on each end, then reweaving these into pin-seamable loops. In a flat woven papermaker's fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In the second technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yams extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms "machine direction" (MD) and "cross machine direction" (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermakers' fabric on the papermaking machine, and a direction parallel to the fabric surface and traverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term "endless belt" as used herein refers to belts made by either method. Effective sheet and fiber support and an absence of wire marking are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Wire marking is particularly problematic in the formation of fine paper grades, as it affects a host of paper properties, such as sheet mark, porosity, see through, and pin holing. Wire marking is the result of individual cellulosic fibers being oriented within the paper web such that their ends reside within gaps between the individual threads or yams of the forming fabric. This problem is generally addressed by providing a permeable fabric structure with a coplanar surface that allows paper fibers to bridge adjacent yams of the fabric rather than penetrate the gaps between yams. As used herein, "coplanar" means that the upper extremities of the yams defining the paper- forming surface are at substantially the same elevation, such that at that level there is presented a substantially "planar" surface. Accordingly, fine paper grades intended for use in quality printing, carbonizing, cigarettes, electrical condensers, and like grades of fine paper have typically heretofore been formed on very finely woven or fine wire mesh forming fabrics. Regrettably, such finely woven forming fabrics often are delicate and lack dimensional stability in either or both of the machine and cross machine directions (particularly during operation), leading to a short service life for the fabric. In addition, a fine weave may adversely effect drainage properties of the fabric, thus rendering it less suitable as a forming fabric. To combat these problems associated with fine weaves, multi-layer forming fabrics have been developed with fine-mesh yams on the paper forming surface to facilitate paper formation and coarser-mesh yams on the machine contact side to provide strength and durability. For example, fabrics have been constructed to include one fabric layer having a fine mesh, another fabric layer having a coarser mesh, and stitching yarns that bind the layers together. These fabrics, known as "triple layer" fabrics, are illustrated in U.S. Patent No. 4,501 ,303 to Osterberg, U.S. Patent No. 5,152,326 to Vohringer, and U.S. Patent No. 5,437,315 to Ward. Although these fabrics have performed successfully, they have some shortcomings that relate to the inclusion of the stitching yams. In a typical triple layer forming fabric, one or more stitching yams are positioned between some of the CMD yams of the top and bottom layers and interwoven with the top and bottom MD yams. In such a construction, portions of the stitching yams form part of the papermaking surface of the fabric. As a result, the appearance of paper formed with the fabric can be affected (sometimes adversely) by the presence of the stitching yams. In addition, triple layer fabrics have proven to have problems with interlayer wear. As the fabric is used on a paper machine, the top and bottom layers tend to shift relative to one another, both in the machine direction and the cross machine direction, due to the tension imparted to the fabric by the rolls. This effect is exacerbated on paper machines, such as the so-called "high-wrap" machines, that include multiple rolls, including some which contact the top layer of the fabric. This shifting can cause the fabric to wear and decrease in thickness, which can adversely affect the drainage of the fabric and, accordingly, its performance in papermaking. In many instances, it is this interlayer wear, rather than the wear of the machine side surface of the fabric machine against the paper machine, that determines the longevity of the fabric. Further, because the stitching yams of a triple layer fabric have a different weave pattern than the top CMD yams (i.e., they interweave with the bottom CMD yams also, whereas the top CMD yams do not), there can be differences in tension between the stitching yams and the top CMD yams. These differences can induce the fabric to distort out-of-plane, which can in turn reduce the quality of paper produced with the fabric. Also, the stitching yarns of a triple layer fabric should be sufficiently strong and durable to bind the top and bottom layers and to resist the wear and abrasion conditions that the bottom layer experiences while in contact with the paper machine, yet should be delicate enough to produce high quality paper. This balance can be quite difficult to strike. Summary of the Invention In view of the foregoing, it is an object of the present invention to provide a multi-layer forming fabric construction with little distortion in the top fabric layer. It is also an object of the present invention to provide a multi-layer forming fabric construction that produces a high quality paper. It is another object of the present invention to provide a multi-layer forming fabric construction that maintains the top and bottom layers in a tightly bound condition. It is a further object of the present invention to provide a multi-layer forming fabric that addresses the problem of interlayer wear. These and other objects are satisfied by the present invention, which relates to a multi-layer papermaker's forming fabric that utilizes stitching yarns in the papermaking surface. The fabric of the present invention is formed of repeating units, each of which comprises: a set of top machine direction yams; a set of bottom machine direction yams; a set of bottom cross-machine direction yams interwoven with the set of bottom machine direction yams; and pairs of first and second stitching yams extending in the cross machine direction. The first and second stitching yams of each pair are interwoven with the top and bottom machine direction yams such that, as a fiber support portion of the first stitching yam is interweaving with the top machine direction yams, a binding portion of the second stitching yam is positioned below the top machine direction yams, and such that as a fiber support portion of the second stitching yam is interweaving with the top machine direction yams, a binding portion of the first binding yarn is positioned below the top machine direction yams. The first and second stitching yarns cross each other as they pass below a transitional top machine direction yam. Each of the binding portions of the first and second stitching yams passes below at least one of the bottom machine direction yams. The fiber support portions of the stitching yams and the top machine direction ya s form the papermaking surface, which includes no more than 33 percent, and preferably is free of, cross machine direction yams that fail to pass below at least one bottom machine direction yam. In this configuration the stitching yams both stitch the bottom layer to the top machine direction yams and form a significant portion of the papermaking surface. . In a preferred embodiment, the fiber support portions of the stitching yams form a plain weave papermaking surface with the top machine direction yams. In another preferred embodiment, they form a 1x2 twill papermaking surface. In each instance, the density of stitching yams tightly and reliably binds the layers of the fabric. Brief Description of the Figures Figure 1A is a top view of an embodiment of a 20 harness multi-layer forming fabric of the present invention having a plain weave top surface. Figure IB is a top section view of the bottom surface of the fabric of Figure 1. Figures 2 A through 2J are section views of the stitching yams of the fabric of Figures 1A and IB. Figure 3 A is a top view of a 24 harness multi-layer forming fabric of the present invention having a 1x2 twill top surface. Figure 3B is a top section view of the bottom surface of the fabric of Figure 3A. Figures 4A through 4F are section views of the stitching yams of the fabric of Figures 3A and 3B. Detailed Description of the Preferred Embodiments The present invention will be described more particularly hereinafter with reference to the accompanying drawings. The invention is not intended to be limited to the illustrated embodiments; rather, these embodiments are intended to fully and completely disclose the invention to those skilled in this art. A 20 harness multi-layer forming fabric, generally designated at 10, is illustrated in Figures 1A and IB, in which a single repeat unit of the fabric is shown. The repeat unit of the fabric 10 includes ten top MD yams 11-20, ten bottom MD yams 21-30, ten bottom CMD yams 31-40, and stitching yarn pairs 41a, 41b through 50a, 50b. Referring first to Figure IB, a repeat unit of the bottom layer of the fabric 10 is shown. The bottom MD yams 21-30 are interwoven with the bottom CMD yams 31-40 in a harness satin type pattern, with each bottom CMD yam passing above one bottom MD yam, below four bottom MD yams, then repeating this "over 1/under 4" pattern. For example, bottom CMD yam 31 passes above bottom MD yarn 21, below bottom MD yams 22-25, above bottom MD yam 26, and below bottom MD yams 27 through 30. The other bottom CMD yams follow the "over 1/under 4" weave pattern, but each is offset from its nearest bottom CMD yam neighbors by two bottom MD yams. Consequently, bottom CMD yam 32 passes below bottom MD yams 21 and 22, above bottom MD yam 23, below bottom MD yam 24 through 27, above bottom MD yam 28, and below bottom MD yams 29 and 30. Thus the "knuckle" formed by bottom MD yam 23 as it passes below bottom CMD yam 32 is offset from the "knuckle" formed by bottom MD yam 21 as it passes over bottom CMD yam 31 by two bottom MD yams. Referring now to Figure 1A, the top layer of the fabric 10 is formed by the top MD yams and by portions of the stitching yam pairs. As can be seen in Figure 1A, the stitching yams and the top MD yams combine to form a plain weave top surface. The interweaving of the stitching yams and the top and bottom MD yams can be understood by examination of Figures 1A and 2A through 2J. Each of the stitching yams of the repeat unit can be subdivided into two portions: a fiber support portion that interweaves with the top MD yams, and a binding portion that interweaves with a bottom MD yarn. These are separated at "transitional" top MD yams, below which one stitching yam of a pair crosses the other stitching yam of the pair. The stitching yams of each pair are interwoven relative to one another such that the fiber support portion of one yam of the pair is positioned above the binding portion of the other yam of the pair. The fiber support portion of stitching yams of each pair designated with an "a" (e.g., 41a, 42a, 43a) interweaves in an alternating fashion with five top MD yams (alternately passing over three top MD yams and under two top MD yams), and the other stitching yam of the pair (those designated with a "b") passes over two top MD yams while passing below a top MD yam positioned between those two MD yams. In its fiber support portion, each stitching yam passes over top MD yams that fiber support portions of stitching yams of adjacent pairs pass beneath, and passes below top MD yams that fiber support portions of stitching yams of adjacent pairs pass over. In this manner, the stitching yarns form a plain weave pattern with the top MD yams (see Figure 1 A). In its binding portion, each stitching yam passes below one bottom MD yam in the repeat unit. Each stitching yarn passes below the bottom MD yarn that is located between two knuckles formed by adjacent bottom MD yams over the bottom CMD yams that sandwich the stitching yam. The combined binding portions of the stitching yam pairs establish an "over 4/under 1" pattern on the bottom surface of the fabric 10 (see Figure IB). The weaving pattern of the stitching yams is exemplified in Figure 21, which illustrates stitching yams 49a, 49b interweaving with top and bottom MD yarns. In its fiber support portion, stitching yam 49a passes over top MD yams 11, 13 and 15, and below top MD yams 12 and 14. It then passes below transitional top MD yam 16 and above bottom MD yam 26. In its binding portion, stitching yam 49a passes below top MD yams 17 through 19 while passing above bottom MD yams 27 and 29 and below bottom MD yam 28 to stitch the bottom layer of the fabric 10. Stitching yarn 49a then passes between top transitional MD yam 20 and bottom MD yam 30. Figure 21 also illustrates that stitching yam 49b is interwoven such that its binding portion is below that of stitching yam 49a; stitching yam 49b passes below top MD yams 11 through 15 while passing above bottom MD yams 21, 22, 24, 25 and below bottom MD yam 23. In its fiber support portion, stitching yam 49b passes above top MD yam 17, below top MD yam 18 and above top MD yam 19, and below transitional top MD yam 20 to continue the alternating weave established by stitching yam 49 a. As can be seen in Figures 2A through 2H and 2J (which depict the interweaving patterns of the other stitching yam pairs with the top and bottom MD yams), the same pattern described hereinabove for the stitching yarns 49a, 49b relative to each other is followed by the other stitching yam pairs. Referring back to Figures 1A and IB, adjacent pairs of stitching yarns are interwoven with the top and bottom MD yams such that there is an offset of three MD yams between such stitching yam pairs. For example, stitching yam 41a passes above top MD yams 15, 17 and 19 and below bottom MD yam 32. Stitching yarn 42a passes above top MD yams 12, 14 and 16 and below bottom MD yam 39. Thus, stitching yam 42a is offset from stitching yam 43a by three top and bottom MD yarns. This same three MD yarn offset is followed for the interweaving of the other stitching yams. It can also be seen in Figures 1A, IB and 2A through 2J that the stitching yam pairs are interwoven with the top and bottom MD yams such that each "a" yam (the stitching yam that passes over three top MD yams) is positioned between two "b" yams (stitching yams that pass over two top MD yams), and each "b" yam is positioned between two "a" yams. This arrangement is demonstrated by examination of stitching yam pairs 41a, 41b, 42a, 42b. As shown in Figures 1A and IB, stitching yarn 41b is positioned between stitching yams 41a and 42a. As seen in Figures 1A, IB and 2B, stitching yam 42a is positioned between stitching yams 41b and 42b. It has been discovered that this configuration is effective in improving the papermaking properties of the top surface of the fabric, particularly in regard to the transitional top MD yams. When a transitional yam passes over the stitching yams of a pair to form a top surface knuckle, that knuckle tends to receive less upwardly- directed support from the stitching yams at that location than other locations on the top MD yam where it passes over a stitching yam or top CMD yam. As a result, that knuckle tends to be positioned slightly lower than the other top MD knuckles. As seen in Figure 1 A, the knuckles formed by the transitional top MD yams define a series of broken diagonal lines; one example of this diagonal is formed by the knuckles formed by top MD yarn 14 over stitching yam yams 41a, 41b and 43a, 43b, top MD yam 15 over stitching yams 44a, 44b and 46a, 46b, top MD yam 16 over stitching yarns 47a, 47b and 49a, 49b, and top MD yam 17 over stitching ya s 50a, 50b. The broken diagonal line defined by these top MD yarn knuckles may have a slight depression because of the lesser upward support described above. Because the knuckles of this diagonal may all be positioned somewhat lower than the remaining top MD knuckles, paper formed on such a fabric can show this pattern, which can in turn affect images printed thereon. However, by including the stitching yams in the alternating fashion of an "a" stitching yam (which passes above three top MD yams) followed by a "b" stitching yam (which passes above two top MD yams), and by offsetting the "a" and "b" ya s as indicated, the fiber support portions of stitching yams of adjacent pairs that are positioned on one side of closely positioned top MD knuckles (e.g.. the fiber support ' portions of stitching yams 46b, 47a, which pass below the knuckles formed thereon by top MD yams 15, 16) are adjacent to each other, while the fiber support portions of their paired yams ( g., stitching yams 46a, 47b) that are positioned on the other side of these knuckles are separated from each other by the other stitching yams of the pair. As a result, the diagonal formed by the transitional top MD knuckles is disturbed somewhat and is less distinctly defined. As such, paper formed on fabric 10 has a less distinct diagonal pattern due to these knuckles, and printing on the paper is improved. Those skilled in this art will appreciate that the afore-described configuration is created in the fabric by weaving the stitching yams into the top and bottom MD yams so that the weaving of an "a" stitching yarn of a pair precedes the weaving of the "b" stitching yam of the pair, then maintaining this sequence throughout the repeat unit. Although it is prefeπed that all of the stitching yam pairs follow this pattern, some benefit can be obtained by varying the sequence so that only a smaller percentage (for example 25, 33, 40, or 50 percent) of the stitching yam pairs follow this alternating sequence. Those skilled in this art will also appreciate that other plain weave patterns in which the stitching yams are divided differently into fiber support portions and binding portions can be constructed. For example, the fabric can include a top layer in which each stitching yam of a pair passes over two, three, four or even more top MD yams in its fiber support portion. The stitching yams can pass over different numbers of top MD yams, or can pass over the same number. Of course, appropriate adjustment of the positioning of the bottom knuckles in the binding portions of such stitching yarns should be made with changes to the stitching yam pattem on the top surface. Another embodiment of a multi-layer forming fabric of the present invention is illustrated in Figures 3A, 3B and 4A through 4F, in which a repeat unit of a multi-layer forming fabric, designated broadly at 100, is illustrated. The repeat unit includes twelve top MD yams 101 through 112, twelve bottom MD yams 141 through 152, six bottom CMD yams 161 through 166, and twelve stitching yams 181a, 181b through 186a, 186b. Referring first to Figure 3B, the machine side surface of the fabric 100 formed by the bottom MD and CMD yams takes the pattem of a "broken twill." Each bottom CMD yam has an "over 1/under 5" repeat pattem with the bottom MD yams. For example, bottom CMD yam 161 passes over bottom MD yam 141, under bottom MD yams 142 through 146, over bottom MD yam 147, and under bottom MD yams 148 through 152. This "over 1/under 5" pattern is repeated by the remaining CMD yams. However, the bottom side knuckles formed by the bottom MD yams as they pass below the bottom CMD yams are arranged in a broken twill pattem, with the bottom side knuckles being formed by bottom MD yams 141, 143, 145, 142, 146, 144 on bottom CMD yams 161 through 166, respectively, and by bottom MD yams 147, 149, 151, 148, 152, 150 on bottom CMD yams 161 through 166, respectively. As can be seen in Figure 3B, these knuckles fail to form a clear diagonal as is characteristic of twill fabrics, but instead form a "broken twill" pattern. As shown in Figure 3A, the top surface of the fabric 100 has a 1x2 twill pattern formed by the top MD yams and portions of the stitching yams. As with the fabric 10 described earlier, each stitching yam has a fiber support portion and a binding portion; these are divided by transitional top machine direction yams, below which stitching yams of a pair cross each other. The fiber support portion of each stitching yarn follows an "over 2/under 1/over 2" pattern as it interweaves with the top MD yams. In its binding portion, each stitching yam passes between top and bottom MD yams with the exception of passing below one bottom MD yam. The bottom MD yam that is stitched is located either 1 or 2 MD yams away from the transitional MD ya s that separate the fiber support and binding portions of each stitching yarn. This pattern is exemplified by stitching yam 185a, the stitching pattern of which is illustrated in Figure 4E. Stitching yam 185a passes over top MD yams 101 and 102, under top MD yam 103, and over top MD yams 104, 105 before passing below transitional top MD yam 106. In its binding portion, stitching yarn 185a passes above bottom MD yarns 147 and 148, below bottom MD yam 149 and above bottom MD yams 148, 149 before passing below transitional top MD yam 112. Pairs of stitching yams are interwoven with the top MD yarns relative to one another such that their fiber support portions and the top MD yams form a 1x2 twill pattern. Referring again to Figure 4E, and as described above, stitching yam 185a passes above top MD yams 101, 102 under top MD yam 103, and over top MD yams 104, 105. Both stitching yams 185a, 185b pass below transitional top MD yarn 106, after which the fiber support portion of stitching yam 185b continues the over 2/under 1 twill pattern first established by stitching yam 185a. In doing so, stitching yam 185b passes above top MD yams 107, 108, below top MD yam 109 and above top MD yams 110, 111 before passing below transitional top MD yarn 112. Figure 5A demonstrates that the stitching yams are interwoven with the top MD yarns such that a "over 2" segment of each fiber support portion is offset by one top MD yam from the "over 2" segments of stitching yams of adjacent stitching yarn pairs that flank that stitching yam. For example, stitching yam 181a passes over top MD yams 102 and 103. The nearest top CMD yams, which are 121 and 122, pass over top MD yarns 101, 102 and 103, 104 respectively. Thus, the distinctive diagonal of a 1x2 twill is formed by the fiber support portions of the stitching yarns. Figure 3B also illustrates how the stitching yams are stitched into the bottom MD yams. It can be seen in Figure 3B that the knuckle formed by each stitching yam as it passes below a bottom MD yam is positioned such that, in one direction, two bottom CMD yams reside between the stitching yam knuckle and the knuckle formed by that bottom MD yam over a bottom CMD yam, and in the opposite direction, three bottom CMD yams reside between the stitching yam knuckle and the next knuckle formed by that bottom MD yarn over a CMD yam. For example, stitching yam 184a forms a knuckle as it passes under bottom MD yarn 141. The bottom MD yam 141 forms a knuckle as it passes under bottom CMD yam 161, which is separated from the knuckle formed by stitching yam 184a by three bottom CMD yams (162, 163, 164). Continuing with the pattern in the other direction, bottom CMD yams 165 and 166 are positioned between the knuckle formed by stitching yarn 184a and the knuckle that would be formed by bottom MD yam 141 under the next bottom CMD yam after bottom CMD yam 166 (which would have the same weave pattern as bottom CMD yarn 161). Thus, the stitching yam knuckle of stitching ya 184a is separated from bottom MD yam knuckles by three bottom CMD yams in one direction and by two CMD yams in the other direction. Those skilled in this art will further appreciate that fabrics of the present invention can be constructed with other twill patterns in the top layer. For example, a fabric can have a 1x3 or 1x4 twill top layer. Any of these twill patterns can be a conventional twill, such as that of the fabric 100, or can take a broken twill pattern, such as that of the surface of a conventional 4 or 5 harness satin single layer fabric. Fabrics can also be constructed in which fiber support portions of stitching yam pairs pass over different numbers of top MD yams. In each instance, the skilled artisan should understand the appropriate modifications to the binding portions of the stitching yams to accommodate differences in the fiber support portions. Those skilled in this art will recognize that, although the plain weave and twill fabrics illustrated and described in detail herein are preferred, other fabric weaves, such as other twill weaves and satins, that employ pairs of stitching yams integrated into the papermaking surface of a fabric with the top MD yams can also be made. It is also contemplated that, although the fabrics 10 and 100 illustratively and preferably are free of CMD yams that fail to pass below at ' least one bottom MD yam, fabrics of the present invention can also include some CMD yams that interweave only with top MD yams. These should comprise no more than 33 percent of the top surface of the fabric (i.e.. these should be no more than one of these CMD yams for every two pairs of stitching yams), and preferably should comprise no more than 25 or 20 percent of the top surface, with a top surface being free of such CMD yams being most preferred. Any number of configurations of the bottom layer in which stitching yam pairs stitch the bottom MD yams can be used. In addition, although the illustrated fabrics have equal numbers of top and bottom CMD yarns and stitching yam pairs, this need not be the case for the present invention; other ratios, such as two stitching yarn pairs for each bottom CMD yam, can also be employed. The configurations of the individual yams utilized in the fabrics of the present invention can vary, depending upon the desired properties of the final papermakers' fabric. For example, the yams may be multifilament yams, monofilament yams, twisted multifilament or monofilament yams, spun yams, or any combination thereof. Also, the materials comprising yams employed in the fabric of the present invention may be those commonly used in papermakers' fabric. For example, the yams may be formed of cotton, wool, polypropylene, polyester, aramid, nylon, or the like. The skilled artisan should select a yam material according to the particular application of the final fabric. Regarding yam dimensions, the particular size of the yams is typically governed by the size and spacing of the papermaking surface. Generally, the diameter of the top MD yams is about equal to or smaller than the diameter of the bottom MD yams, and the diameter of the bottom CMD yams is somewhat larger than that of the bottom MD yams. In a typical fabric, the diameter of the top MD yams is between about 0.11 and 0.15mm, the diameter of the bottom CMD yams is between about 0.20 and 0.40mm, and the diameter of the bottom MD yams is between about 0.17 and 0.25 mm. The diameter of the stitching yams is typically between about 0.11 and 0.17mm. Yams may also vary advantageously in modulus of elasticity. For example, stitching yarns that interweave with a fewer number of top MD yams than its paired stitching yam (such as the "b" yams of fabric 10) may have a higher modulus of elasticity (typically between about 10 and 50 percent higher) than its paired stitching yam. As the foregoing discussion demonstrates, the fabrics of the present invention address problems encountered with prior art triple layer forming fabrics. The fabrics of the present invention utilize the stitching yams as the top surface of the fabric, whether it be a plain weave, a twill, a satin, or other pattern, and therefore avoid the marring of the papermaking surface that can accompany the inclusion of stitching yams that comprise less of the papermaking surface. The integration of the fabric attributable to the stitching yams also greatly reduces (if not eliminating entirely) interlayer wear. In addition, because the stitching yams comprise the papermaking surface, the differences in tension between top CMD yams and stitching yams of prior art fabrics that can distort the papermaking surface are not present in the fabrics of the present invention. The density of the stitching yams also provides a more complete binding of the top and bottom layers of the fabric, which can provide the designer with a wider variety of yam choices as the balance of paper forming properties and durability and wear. The foregoing embodiments are illustrative of the present invention, and are not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims: THAT WHICH IS CLAIMED: 1. A papermaker's fabric, comprising top machine direction yams, bottom machine direction yams, bottom cross machine direction yarns, and sets of first and second stitching cross-machine direction yams, said fabric being formed in a plurality of repeating units, each of said repeating units comprising: a set of top machine direction yams; a set of bottom machine direction yams; a set of bottom cross-machine direction yams interwoven with said set of bottom machine direction yarns; and pairs of first and second stitching yam extending in the cross machine direction, said first and second stitching yams of each pair being interwoven with said top and bottom machine direction yams such that, as a fiber support portion of said first stitching yam is interweaving with said top machine direction yams, a binding portion of said second stitching yarn is positioned below said top machine direction yarns, and such that as a fiber support portion of said second stitching yarn is interweaving with said top machine direction yams, a binding portion of said first binding yam is positioned below said top machine direction yams, and such that said first and second yams cross each other as they pass below a transitional top machine direction yam, and such that each of said binding portions of said first and second stitching yams passes below at least one of said bottom machine direction yams; said fiber support portions of said stitching yams and said top machine direction yams defining a papermaking surface, said papermaking surface including no more than 33 percent of cross-machine direction yams that fail to pass under at least one bottom machine direction yam. 2. The papermaker's fabric defined in Claim 1, wherein fiber support portions of first alternate pairs of stitching yams pass over a first group of alternate top machine direction yams and under a second group of alternate top machine direction yams, and fiber support portions of second alternate pairs of stitching yams pass over said second group of top machine direction yams and under said first group of alternate top machine direction yams, said fiber support portions of said first and second alternate pairs of stitching yams, said first group of alternate top machine direction yarns, and said second group of alternate machine direction yams form a plain weave top surface of said fabric. 3. The papermaker's fabric defined in Claim 2, wherein said fiber support portions of said first stitching yarns pass over a first number of said machine direction yams, said fiber support portions of second stitching yams passes over a second number of said machine direction yams, and said first number differs from said second number. 4. The papermaker's fabric defined in Claim 3, wherein said first number is larger than said second number, and wherein said second stitching yarns have a higher modulus of elasticity than said first stitching yarn. 5. The papermaker's fabric defined in Claim 3, wherein said first number is three, and said second number is two. 6. The papermaker's fabric defined in Claim 1, wherein said first and second stitching yarns are of a smaller diameter than said top machine direction yarns. 7. The papermaker's fabric defined in Claim 1 , wherein each of said binding portions of said first and second stitching yams passes beneath only one of said bottom machine direction yams. 8. The papermaker's fabric defined in Claim 1, wherein said repeat unit includes 10 top machine direction yams and 10 bottom machine direction yams. 9. The papermaker's fabric defined in Claim 1, wherein said top machine direction yams and said fiber support portions of said first and second stitching yams are interwoven in a repeating pattern such that each of said fiber support portions passes over a first pair of adjacent top machine direction yams, under a third top machine direction yam adjacent to said first pair, and over a second pair of top machine direction yams positioned adjacent said third top machine direction yam, and wherein fiber support portions of adjacent stitching yams pass over top machine direction yarn pairs that are offset by one top machine direction yarns such that said top machine direction yams and said fiber support portions of said first and second stitching yams form a 1x2 twill pattern. 10. The papermaker's fabric defined in Claim 1, wherein said papermaking surface is free of cross machine direction yams that fail to pass below at least one bottom machine direction yam. 11. The papermaker's fabric defined in Claim 1, wherein said stitching yarns are interwoven with said top and bottom machine direction yams such that adjacent pairs of stitching yams cross beneath transitional top machine direction yarns that are offset by three top machine direction yams. 12. The papermaker's fabric defined in Claim 1, wherein between 25 and 50 percent of said first and second stitching yams are alternately interwoven with said top machine direction yams. 13. A papermaker's fabric comprising top machine direction yams, bottom machine direction yams, bottom cross machine direction yarns, and sets of first and second stitching cross-machine direction yams, said fabric being formed in a plurality of repeating units, each of said repeating units comprising: a set of top machine direction yams; a set of bottom machine direction yams; a set of bottom cross-machine direction yams interwoven with said set of bottom machine direction yams; and pairs of first and second stitching yarn extending in the cross machine direction, said first and second stitching yams of each pair being interwoven with said top and bottom machine direction yams such that, as a fiber support portion of said first stitching yam is interweaving with said top machine direction yams, a binding portion of said second stitching yarn is positioned below said top machine direction yams, and such that as a fiber support portion of said second stitching yam is interweaving with said top machine direction yams, a binding portion of said first binding yam is positioned below said top machine direction yarns, and such that said first and second yams cross each other as they pass below a transitional top machine direction yarn, and such that each of said binding portions of said first and second stitching yams passes below at least one of said bottom machine direction yams, and such that between 25 and 50 percent of said stitching yams are alternately interwoven with said top machine direction yams; said fiber support portions of said stitching yams and said top machine direction yams defining a papermaking surface, said papermaking surface including no more than 33 percent of cross machine direction yams that fail to pass under at least one bottom machine direction yam. 14. The papermaker's fabric defined in Claim 13, wherein fiber support portions of first alternate pairs of stitching yams pass over a first group of alternate top machine direction yams and under a second group of alternate top machine direction yams, and fiber support portions of second alternate pairs of stitching yams pass over said second group of top machine direction yams and under said first group of alternate top machine direction yams, said fiber support portions of said first and second alternate pairs of stitching yams, said first group of alternate top machine direction yams, and said second group of alternate machine direction yarns form a plain weave top surface of said fabric. 15. The papermaker's fabric defined in Claim 14, wherein said fiber support portions of said first stitching yam pass over a first number of said machine direction yams, said fiber support portions of second stitching yarns passes over a second number of said machine direction yams, and said first number differs from said second number. 16. The papermaker's fabric defined in Claim 15, wherein said first number is larger than said second number, and wherein said second stitching yams have a higher modulus of elasticity than said first stitching yam. 17. The papermaker's fabric defined in Claim 15, wherein said first number is three, and said second number is two. 18. The papermaker's fabric defined in Claim 13, wherein said first and second stitching yams are of a smaller diameter than said top machine direction yams. 19. The papermaker's fabric defined in Claim 13, wherein each of said binding portions of said first and second stitching yams passes beneath only one of said bottom machine direction yams. 20. The papermaker's fabric defined in Claim 13, wherein said repeat unit includes 10 top machine direction yams and 10 bottom machine direction yams. 21. The papermaker's fabric defined in Claim 13, wherein said stitching yams are interwoven with said top and bottom machine direction yams such that adjacent pairs of stitching yams cross beneath transitional top machine direction yams that are offset by three top machine direction yams. 22. The papermaker's fabric defined in Claim 13, wherein said papermaking surface is free of cross machine direction yams that fail to pass below at least one bottom machine direction yam. 23. A method of making paper, comprising the steps of: (a) providing a papermaker's fabric, said fabric comprising top machine direction yams, bottom machine direction yams, bottom cross machine direction yams, and sets of first and second stitching cross-machine direction yams, said fabric being formed in a plurality of repeating units, each of said repeating units comprising: a set of top machine direction yarns; a set of bottom machine direction yams; a set of bottom cross-machine direction yams interwoven with said set of bottom machine direction yams; and pairs of first and second stitching yam extending in the cross machine direction, said first and second stitching yams of each pair being interwoven with said top and bottom machine direction yams such that, as a fiber support portion of said first stitching yarn is interweaving with said top machine direction yams, a binding portion of said second stitching yarn is positioned below said top machine direction yams, and such that as a fiber support portion of said second stitching yam is interweaving with said top machine direction yams, a binding portion of said first binding yam is positioned below said top machine direction yams, and such that said first and second yams cross each other as they pass below a transitional top machine direction yam, and such that each of said binding portions of said first and second stitching yams passes below at least one of said bottom machine direction yams; said fiber support portions of said stitching yams and said top machine direction yams defining a papermaking surface, said papermaking surface including no more than 33 percent of cross machine direction yams that fail to pass under at least one bottom machine direction yam; (b) applying paper stock to said fabric; and (c) removing moisture from said paper stock. 24. The method defined in Claim 23, wherein fiber support portions of first alternate pairs of stitching yams pass over a first group of altemate top machine direction yarns and under a second group of alternate top machine direction yams, and fiber support portions of second alternate pairs of stitching yams pass over said second group of top machine direction yams and under said first group of alternate top machine direction yams, said fiber support portions of said first and second alternate pairs of stitching yams, said first group of alternate top machine direction yarns, and said second group of alternate machine direction yams form a plain weave top surface of said fabric. 25. The method defined in Claim 23, wherein said top machine direction yams and said fiber support portions of said first and second stitching yams are interwoven in a repeating pattern such that each of said fiber support portions passes over a first pair of adjacent top machine direction yams, under a third top machine direction yarn adjacent to said first pair, and over a second pair of top machine direction yarns positioned adjacent said third top machine direction yam, and wherein fiber support portions of adjacent stitching yams pass over top machine direction yam pairs that are offset by one top machine direction yams such that said top machine direction yarns and said fiber support portions of said first and second stitching yams form a 1x2 twill pattem. 26. The papermaker's fabric defined in Claim 23, wherein said papermaking surface is free of cross machine direction yams that fail to pass below at least one bottom machine direction yam.

Inventors: WARD KEVIN J, WARD, KEVIN, J.
Applicants: WARD KEVIN J, WEAVEXX CORP, WARD, KEVIN, J., WEAVEXX CORPORATION
Application year: 1997
Main classification: D21F1
IPC codes: D21F1