Low caliper mechanically stable multi-layer papermaker's fabrics with paired machine side cross machine direction yarns

Abstract

Multi-layer papermaker's fabrics are provided which include in the bottom fabric layer pairs of cross machine direction yarns that are woven in the same shed so as to provide paired bottom fabric layer cross machine direction yarns. Typically, these paired yarns comprise two smaller yarns that replace what otherwise would have been a larger single yarn, thereby reducing the thickness, void volume and water carrying propensity of the fabric. These fabrics may include a relatively large number of cross machine direction yarns on the papermaking surface and/or a papermaking surface having single float machine direction knuckles, so as to provide a high level of fiber support and good papermaking qualities.

Description

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 (i.e., 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 rollers 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. A number of auto-joiner machines are now commercially available, which for certain fabrics may be used to automate at least part of the joining process. 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 yarns 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 yarns 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 yarns of the fabric rather than penetrate the gaps between yarns. As used herein, "coplanar" means that the upper extremities of the yarns 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.

Typically, such finely woven fabrics include at least some relatively small diameter machine direction or cross machine direction yarns. Regrettably, however, such yarns tend to be delicate, leading to a short surface life for the fabric. Moreover, the use of smaller yarns can also adversely effect the mechanical stability of the fabric (especially in terms of skew resistance, narrowing propensity and stiffness), which may negatively impact both the service life and the performance of the fabric.

To combat these problems associated with fine weaves, multi-layer forming fabrics have been developed with fine-mesh yarns on the paper forming surface to facilitate paper formation and coarser-mesh yarns on the machine contact side to provide strength and durability. For example, fabrics have been constructed which employ one set of machine direction yarns which interweave with two sets of cross machine direction yarns to form a fabric having a fine paper forming surface and a more durable machine side surface. These fabrics form part of a class of fabrics which are generally referred to as "double layer" fabrics. Similarly, fabrics have been constructed which include two sets of machine direction yarns and two sets of cross machine direction yarns that form a fine mesh paperside fabric layer and a separate, coarser machine side fabric layer. In these fabrics, which are part of a class of fabrics generally referred to as "triple layer" fabrics, the two fabric layers are typically bound together by separate stitching yarns. As double and triple layer fabrics include additional sets of yarn as compared to single layer fabrics, these fabrics typically have a higher "caliper" (i.e., they are thicker than) comparable single layer fabrics. An illustrative double layer fabric is shown in U.S. Pat. No. 4,423,755 to Thompson, and illustrative triple layer fabrics are shown in U.S. Pat. No. 4,501,303 to Osterberg, U.S. Pat. No. 5,152,326 to Vohringer, and U.S. Pat. No. 5,437,315 to Ward.

Although these fabrics have performed successfully, they have some shortcomings. For instance, the separate stitching yarns that are included in typical triple layer fabrics can adversely affect the appearance of the paper that is formed on the fabric, since portions of the stitching yarns form part of the papermaking surface of the fabric. Additionally, these traditional triple layer fabrics are also susceptible to interlayer wear problems, which may occur as the result of the top and bottom layers of the fabric shifting relative to one another (in the machine direction and the cross machine direction) during operation. This shifting can cause the fabric to wear out prematurely, and may also cause the layers to become offset from one another, which can adversely affect the drainage, and hence the papermaking performance, of the fabric. Moreover, the use of coarser yarns on the machine side of the fabric can increase the tendency of the fabric to curl at the edges, which may negatively impact the performance and life of the fabric.

Additionally, many double layer, triple layer and other "multi-layer" forming fabrics have a large "void volume", which refers to the volume of the open space in the interior of the fabric. Large void volumes typically translate into high water carry, meaning that the fabric tends to carry a large amount of undrained water which may negatively impact the fabrics ability to drain water from the paper web which is being formed, thereby increasing the water removal requirements of the press and dryer sections of the papermaking machine. It is generally preferable, however, to remove as much water as possible in the forming section of the fabric, because the energy costs in the press and dryer sections of the papermaking machine typically exceed the energy costs of the forming section. Moreover, in situations where water carry is excessive, vacuum drainage may be impaired, and sheet consistency off the couch roll may be degraded which may negatively impact the quality of the resulting paper.

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 that reduces the amount of water carry.

It is another object of the present invention to provide a multi-layer forming fabric construction that has a fine papermaking surface that provides a high amount of fiber support.

It is a further object of the present invention to provide a multi-layer forming fabric that has a good life potential and low edge curl tendencies.

These and other objects are satisfied by the present invention, which relates to multi-layer papermaker's fabrics which includes pairs of cross machine direction yarns in the bottom (machine side) fabric layer that are woven in the same shed (i.e., the two yarns are woven in direct succession with an identical weave pattern relative to the other yarns in the fabric to provide paired bottom fabric layer cross machine direction yarns). Typically, these paired yarns comprise two smaller yarns that replace what otherwise would have been a larger single yarn, thereby reducing the "caliper" (thickness) and void volume of the fabric. Furthermore, the use of paired machine side CMD yarns can reduce the effective top to bottom pick ratio from 1:1 to perhaps 2:1. This, in turn, results in a fabric which on the machine side has fewer drainage holes that are larger in size, which have less hydraulic resistance and therefor lower water carrying propensity. Preferably, a set of "primary" top CMD yarns are stacked above the paired bottom fabric layer CMD yarns, and additional top layer CMD yarns (such as paired stitching yarns, "x-pick" yarns or paired auxiliary yarns) are included between adjacent primary top CMD yarns. Such a configuration creates good drainage between the paired bottom fabric layer CMD yarns, which otherwise may close-up the bottom of the fabric. It is also preferred that the fabrics of the present invention also include a relatively large number of cross machine direction yarns or "picks" on the papermaking surface and/or a papermaking surface having single float machine direction knuckles, so as to provide a high level of fiber support and good papermaking qualities.

In one aspect of the present invention, a triple layer papermaker's forming fabric is provided that includes a set of top machine direction yarns, a set of top cross machine direction yarns and a set of stitching cross machine direction yarns which are interwoven to form a top fabric layer having a papermaking surface, as well as a set of bottom machine direction yarns and a set of bottom cross machine direction yarns that are interwoven to form a bottom fabric layer having a machine side surface. Each yarn in the set of bottom cross machine directions yarns may be interwoven with the bottom machine direction yarns in a pattern identical to the weave pattern of an adjacent yarn in the set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine direction yarns. In a preferred configuration of this fabric, the yarns in the set of bottom machine direction yarns pass under no more than one of any two adjacent paired bottom fabric layer cross machine direction yarns.

In another configuration of the above-described triple layer forming fabric, the stitching cross machine direction yarns comprise pairs of adjacent stitching yarns that are woven into the fabric such that when the first yarn of a pair is weaving in the top fabric layer the other yarn of the pair is passing downwards from the top fabric layer to interweave with the bottom fabric layer. In this embodiment, the pairs of adjacent stitching yarns may be positioned between adjacent yarns in the set of top cross machine direction yarns. Preferably, the yarns in the set of top machine direction yarns pass over no more than one of any two adjacent yarns in the set of top cross machine direction yarns, and the pairs of adjacent stitching yarns each serve as locator yarns at the points where the yarns of the pair cross each other in entering or leaving the papermaking surface. Additionally, in one particular embodiment, the set of stitching yarns may be used to complete a weave pattern (such as a plain weave or a 1.times.2 twill) on the papermaking surface which is partially formed by the interweaving of the set of top machine direction yarns and the set of top cross machine direction yarns.

In another aspect of the present invention, auto-joinable triple layer papermaker's forming fabrics are provided which include a set of top machine direction yarns, a set of top cross machine direction yarns and a set of stitching cross machine direction yarns interwoven to form a top fabric layer having a papermaking surface, and a set of bottom machine direction yarns and a set of bottom cross machine direction yarns interwoven to form a bottom fabric layer. At least some of the bottom cross machine direction yarns are woven in parallel so as to provide paired bottom fabric layer cross machine direction yarns. In one embodiment of this fabric, the ratio between the number of top cross machine direction yarns and bottom cross machine direction yarns is approximately one-to-one and the fabric preferably has at least 80 picks per inch on its papermaking surface. Preferably, in this embodiment adjacent of the paired bottom fabric layer cross machine direction yarns are spaced apart by at least 0.1 mm to facilitate use of various auto-joining machines.

Additionally, multi-layer papermaker's fabric are also provided which have a base fabric structure that includes machine direction yarns, primary top layer cross machine direction yarns and bottom layer cross machine direction yarns interlaced to form a top fabric layer and a bottom fabric layer, as well as auxiliary top cross machine direction yarns that are positioned between each pair of adjacent primary top layer cross machine direction yarns. In these fabrics, each yarn in the set of bottom cross machine direction yarns is woven in a pattern identical to the weave pattern of an adjacent yarn in the set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine yarns. Moreover, the fabric is constructed such that the yarns in the set of bottom machine direction yarns pass under no more than one of any two adjacent paired bottom fabric layer cross machine direction yarns, and so that each bottom machine direction yarn passes under at least two paired bottom fabric layer cross machine direction yarn in a repeat of the fabric.

In other aspects of the present invention, methods of making and methods of using a multi-layer papermaker's fabric which has paired bottom fabric layer cross machine direction yarns are provided.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a top view of the top fabric layer of an embodiment of a 20 harness triple layer forming fabric of the present invention having a plain weave top surface.

FIG. 2 is a top view of the bottom fabric layer of the triple layer forming fabric of FIG. 1.

FIG. 3 is a cross-section view of the triple layer fabric depicted in FIGS. 1 and 2 taken along the line 3--3.

FIG. 4 is a top view of an embodiment of a 24 harness multi-layer forming fabric of the present invention having a plain weave top surface.

FIG. 5 is a top view of the bottom fabric layer of the fabric of FIG. 4.

FIG. 6 is a cross-section view of the triple layer fabric depicted in FIGS. 4 and 5 taken along the line 6--6.

FIG. 7 is a top view of a 24 harness triple layer forming fabric of the present invention having a top surface with a 1.times.2 weave pattern.

FIG. 8 is a top view of the bottom fabric layer of the fabric of FIG. 7.

FIG. 9 is a cross-section view of the triple layer fabric depicted in FIGS. 7 and 8 taken along the line 9--9.

FIG. 10A is a cross section of the bottom fabric layer of a multi-layer papermaker's fabric according to the present invention in which the pictured machine direction yarn passes under no more than one consecutive paired bottom fabric layer cross machine direction yarn.

FIG. 10B is a cross section of the bottom fabric layer of a multi-layer papermaker's fabric according to the present invention in which the pictured machine direction yarn passes under more than one consecutive paired bottom fabric layer cross machine direction yarn.

FIG. 11 is a top view of the top fabric layer of an embodiment of a triple layer forming fabric of the present invention having pairs of stitching yarns that serve as both fiber support and locator yarns.

FIG. 12 is a top view of the bottom fabric layer of the triple layer forming fabric of FIG. 11.

FIG. 13 is a cross-section view of the triple layer fabric depicted in FIGS. 11 and 12 taken along the line 13--13.

FIG. 14 is a top view of the paper side of an eight harness double layer fabric of the present invention.

FIG. 15 is a section view taken along lines 15--15 of FIG. 14 showing the interrelationship between an exemplary MD yarn, primary and auxiliary CMD yarns of the fabric layer on the paper side of the fabric, and the CMD yarns of the fabric layer on the machine side of the fabric.

FIG. 16A is a cross sectional view, taken between adjacent machine direction yarns, of a conventional 16 harness triple layer fabric.

FIG. 16B is a cross sectional view, taken between adjacent machine direction yarns, of the conventional 16 harness triple layer fabric of FIG. 16A, wherein the bottom fabric layer cross machine direction yarns are replaced with paired bottom fabric layer cross machine direction yarns that are positioned beneath every other top layer cross machine direction yarn.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated or other embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. In the figures, the dimensions of some components may be exaggerated for clarity.

Pursuant to one aspect of the present invention, triple layer papermaker's forming fabrics are provided that comprise a top fabric layer, a bottom fabric layer, and stitching yarns which bind the layers together. Pursuant to the teachings of the present invention, such fabrics may be constructed to have a relatively low caliper, and hence a lower void volume and lower water carry, by weaving the fabric so that it has a large number of relatively small cross machine direction yarns instead of a smaller number of relatively large cross machine direction yarns in the bottom fabric layer. At least some of these relatively small cross machine direction yarns are woven into the fabric in pairs, with both of the yarns that comprise a pair woven in an identical pattern next to the other yarn of the pair to provide paired cross machine direction yarns in the bottom fabric layer. By using these smaller diameter paired yarns in the bottom fabric layer, it is possible to construct relatively low caliper forming fabrics which have good mechanical stability and low edge curl. Moreover, the use of paired machine side CMD yarns can reduce the effective top to bottom pick ratio from 1:1 to perhaps 2:1. Pursuant to the teachings of the present invention, it will be understood that this results in a fabric which on the machine side has fewer drainage holes that are larger in size, which have less hydraulic resistance and therefor lower water carrying propensity.

In one embodiment of the triple layer forming fabrics of the present invention, pairs of adjacent stitching yarns are provided that are woven into the fabric such that when the first yarn of the pair is weaving in the top fabric layer the second yarn of the pair is passing downwards from the top fabric layer to interweave with the bottom fabric layer. Preferably, such pairs of stitching yarns are provided between every, or every other, pair of adjacent top layer cross machine direction yarns

One such triple layer forming fabric 20 is illustrated in FIGS. 1 and 2. FIG. 1 depicts a top view of the top fabric layer 21 of fabric 20 (i.e., a view looking down onto the papermaking surface) while FIG. 2 depicts a top view of the bottom fabric layer 22 of fabric 20 which underlies the top fabric layer depicted in FIG. 1 (i.e., a view looking at the top of the bottom fabric layer 22 with the top fabric layer 21 removed). The triple layer fabric of FIGS. 1 and 2 is woven on 20 harnesses, and hence a single repeat of the fabric encompasses ten top layer machine direction yarns and ten bottom layer machine direction yarns. While FIGS. 1 and 2 only show a single repeat unit of the fabric, those of skill in the art will appreciate that in commercial applications the repeat unit shown in FIGS. 1 and 2 would be repeated many times, in both the machine and cross machine directions, to form a large fabric suitable for use on a papermaking machine.

As seen in FIG. 1, the repeat unit of the fabric 20 includes a set of top layer MD yarns 21-30 and a set of top layer CMD yarns 31-40. These yarns 21-30 and 31-40 are interwoven such that each yarn in the set of top layer CMD yarns 31-40 passes over and beneath the yarns in the set of top layer MD yarns 21-30 in an alternating fashion, with each yarn in the set of top layer CMD yarns 31-40 passing over and under the same top MD yarns. For example, top CMD yarn 31 passes under top MD yarn 21, over top MD yarn 22, under top MD yarn 23, over top MD yarn 24 and so on until it passes over top MD yarn 30. Similarly, top CMD yarn 32 passes under top MD yarn 21, over top MD yarn 22, under top MD yarn 23, over top MD yarn 24 and so on until it passes over top MD yarn 30.

Referring now to FIG. 2, a repeat unit of the bottom fabric layer 22 of the fabric 20 is shown. The repeat unit includes a set of bottom layer MD yarns 41-50 which are interwoven with a set of bottom layer CMD yarns 51-70. As shown in FIG. 2, the yarns comprising the set of bottom layer CMD yarns 51-70 are interwoven with the set of bottom layer MD yarns 41-50 in pairs, such as pair 51/52. Each of the yarns which comprise one of the pairs of yarn, such as yarns 51, 52 of paired yarn 51/52, are woven together in the same shed of the fabric, and thus the yarns forming each of these paired bottom fabric layer CMD yarns (such as pair 51/52) have an identical weave pattern in the fabric. By woven in the same shed it is meant that the yarns are woven adjacent to each other and have an identical weave pattern with respect to the machine direction yarns with which they weave. Note that herein, unless the context demands otherwise, references to a "paired bottom fabric layer cross machine direction yarn" are intended to refer to a single yarn witch is formed from two yarns that are woven in the same shed. Accordingly, a reference to a fabric having paired bottom fabric layer cross machine direction yarns that is woven, for example, in a 1.times.4 twill pattern, refers to a fabric woven in a 1.times.4 twill pattern if the paired bottom fabric layer cross machine direction yarns are treated as a single yarn. In FIG. 2, the set of bottom layer MD yarns 41-50 are interwoven with the pairs of yarns that comprise the set of bottom layer CMD yarns 51-70 in a 1.times.4 twill type pattern, meaning that each of the yarn pairs 51/52, 53/54, 55/56, 57/58, 59/60, 61/62, 63/64, 65/66, 67/68, 69/70 pass above one yarn of the set of bottom MD yarns 41-50, below the next four yarns of the set of bottom MD yarns 41-50, above the next yarn of the set of bottom MD yarns 41-50, and below the next four yarns of the set of bottom MD yarns 41-50. For example, bottom CMD yarn pair 51/52 passes above bottom MD yarn 41, below bottom MD yarns 42-45, above bottom MD yarn 46, and below bottom MD yarns 47 through 50. The other paired bottom fabric layer CMD yarns 51/52 through 69/70 follow a similar "over-one/under-four" weave pattern, although this pattern is offset by two bottom layer MD yarns for adjacent paired bottom layer CMD yarns 51/52 through 69/70. Thus, for example, paired bottom fabric layer CMD yarn 53/54 passes above bottom MD yarns 43 and 48, whereas adjacent paired bottom fabric layer CMD yarn 55/56 passes above bottom MD yarns 45 and 50.

The top fabric layer 21 (pictured in FIG. 1) and the bottom fabric layer 22 (pictured in FIG. 2) are stitched together with twenty stitching yarns, designated herein as pairs 71a, 71b through 80a, 80b. These stitching yarns are positioned in pairs between adjacent yarns of the set of top layer CMD yarns 31-40. For example, stitching yarn pair 71a, 71b are positioned between top CMD yarns 31 and 32 and between paired bottom fabric layer CMD yarns 51/52 and 53/54. The stitching yarns interweave with the top MD yarns and bottom MD yarns to bind the top fabric layer 21 and the bottom fabric layer 22 together.

FIG. 3 is a cross section taken along the line 3--3 of FIGS. 1 and 2 which shows a pair of stitching yarns interweaving with the top MD yarns and bottom MD yarns. Each of the stitching yarns of the repeat unit can be subdivided into two portions: a fiber support portion which interweaves with the top MD yarns, and a binding portion which interweaves with a bottom MD yarn. These fiber support and binding portions are separated at "transitional" top MD yarns (such as top MD yarn 24 in FIG. 3), below which one stitching yarn of a pair crosses the other stitching yarn of the pair. The stitching yarns of each pair are interwoven relative to one another such that the fiber support portion of one yarn of the pair is positioned above the binding portion of the other yarn of the pair. In the repeat pattern shown in FIGS. 1-3, one of the yarns (e.g., yarn 71a) of each pair of stitching yarns includes a fiber support portion which interweaves in an alternating fashion with five top MD yarns (yarns 25-29 in the case of yarn 71a), alternately passing over three top MD yarns (yarns 25, 27, 29) and under two top MD yarns (yarns 26, 28). The other stitching yarn of the pair (yarn 71b) likewise has a fiber support portion which passes over two top MD yarns (yarns 21, 23) while passing below a top MD yarn (yarn 22) positioned between those two MD yarns. As shown in FIG. 3, there are two transitional top MD yarns 24, 30 in the repeat, which, as noted above, refer to a top layer MD yarn beneath which the yarns of a pair of stitching yarns cross.

As best seen in FIG. 1, in its fiber support portion, each stitching yarn passes over the top layer MD yarns 21-30 that the top layer CMD yarns 31-40 which are adjacent to the pair of stitching yarns pass beneath, and passes below the top layer MD yarns that each adjacent top layer CMD yarn pass over. In this manner, the stitching yarns 71a-80a and 71b-80b (which as a pair weave as the equivalent of a single yarn on the papermaking surface) and top layer CMD yarns 31-40 form a plain weave pattern with the top layer MD yarns 21-30 (see FIG. 1).

As can also be seen in FIG. 3, in its binding portion, stitching yarn 71a passes below top layer MD yarns 21-23 while passing above bottom layer MD yarns 41 and 43 and below bottom layer MD yarn 42 to stitch the bottom layer 22 of the fabric 20. Stitching yarn 71a then passes beneath the transitional top layer MD yarn 24 and over bottom layer MD yarn 44. Similarly, stitching yarn 71b also has a binding portion which passes below top layer MD yarns 25-29 while passing above bottom layer MD yarns 45-46 and 48-49 and below bottom layer MD yarn 47 to stitch the bottom layer 22 of the fabric 20. As shown in FIG. 3, when the stitching yarn 71a is weaving in the top fabric layer 21 (i.e., in its fiber support portion), stitching yarn 71b is passing downwards from the top fabric layer 21 to interweave with the bottom fabric layer 22 (i.e., in its binding portion).

As shown in FIG. 1, top layer CMD yarns 31-40, stitching yarns 71a-80a and 71b-80b and top layer MD yarns 21-30 combine to form a top surface with the "over-one/under-one" pattern of a plain weave on the top layer 21. Additionally, as shown in FIG. 2, the paired bottom fabric layer CMD yarns 51/52 through 69/70, stitching yarns 71a-80a and 71b-80b and bottom layer MD yarns 41-50 combine to form an "over-four/under-one" pattern on the bottom surface of the fabric 20.

Cross sections similar to the cross section of FIG. 3 may be drawn for each of the remaining pairs of stitching yarns. As is apparent from FIGS. 1 and 2, the stitching yarns in each of these cross sections would follow the same weave pattern as stitching yarns 71a and 71b in FIG. 3. However, as shown in FIGS. 1 and 2, pairs of stitching yarns that are positioned adjacent to and on opposite sides of a top CMD yarn are interwoven with the top or bottom MD yarns such that there is an offset of two MD yarns between such stitching yarn pairs. For example, stitching yarn 71a passes above top MD yarns 25, 27 and 29 and below bottom MD yarn 42. Stitching yarn 72a passes above top MD yarns 27, 29 and 21 (with top MD yarn 21 being a continuation of the pattern on the opposite side) and below bottom MD yarn 44. Thus, stitching yarn 71a is offset from stitching yarn 72a by two top and two paired bottom MD yarns. This same two MD yarn offset is followed for the interweaving of the other stitching yarns.

It can also be seen in FIGS. 1 and 2 that the stitching yarns are interwoven with the top and bottom MD yarns as "reversed picks." This term can be understood by examination of stitching yarn pairs 71a, 71b; 72a , 72b; and 73a, 73b. As shown in FIG. 1, stitching yarn 71a is positioned nearer to top CMD yarn 32 than is stitching yarn 71b. However, on the other side of top CMD yarn 32, stitching yarn 72a is positioned nearer to top CMD yarn 32 than is stitching yarn 72b. As a result, the fiber support portions of stitching yarns 71a and 72a are positioned nearer to top CMD yarn 32 than are the fiber support portions of stitching yarns 71b and 72b. This relative proximity to the top CMD yarn between adjacent pairs of stitching yarns is reversed with stitching yarn pairs 72a, 72b and 73a, 73b, with stitching yarns 72b and 73b being positioned nearer top CMD yarn 33 than stitching yarns 72a, 73a.

This "reversed picks" configuration is particularly effective in masking the presence of stitching yarns in the top surface of the fabric 20. When a top layer MD yarn passes over a pair of stitching yarns at a "transition point" (i.e., where the stitching yarns of a pair cross as they enter and leave the top fabric layer 21) to form a top surface knuckle, that knuckle tends to receive less upwardly-directed support from the stitching yarns at that location than at other locations where the top MD yarn passes over a stitching yarn or top CMD yarn. As a result, the knuckle at the transition point tends to be positioned slightly lower than the other top MD knuckles. As seen in FIG. 1, the top fabric layer MD knuckles which are formed above the transition points form a diagonal line. Thus, as the knuckles of this diagonal may all be positioned somewhat lower than the remaining top MD knuckles, paper formed on such a fabric may show this diagonal pattern, which can in turn affect images printed thereon. By including the stitching yarns as reversed picks, such as is illustrated in fabric 20, however, the diagonal formed by the transitional top MD knuckles is disturbed somewhat and is less distinctly defined. As such, paper formed on fabric 20 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 "reverse picks" configuration is created in the fabric by weaving the stitching yarns into the top and bottom MD yarns so that first an "a" stitching yarn immediately follows the weaving of a top layer CMD yarn and the two yarns comprising a paired bottom fabric layer CMD yarn (followed by a "b" stitching yarn), then a "b" stitching yarn immediately follows the next top layer CMD yarn and the two yarns comprising a paired bottom fabric layer CMD yarns (followed by an "a" stitching yarn). This pattern can be repeated throughout weaving. Although it is preferred that all of the stitching yarn pairs follow this pattern (i.e., that 50 percent of the stitching yarn pairs be "reversed"), some benefit can be obtained by reversing a smaller percentage (for example 25, 33, or 40 percent) of the stitching yarn pairs.

In one embodiment of the fabric depicted in FIGS. 1-3, both the top machine direction yarns and the stitching cross machine direction yarns are 0.13 mm in diameter, while the top cross machine direction yarns are 0.15 mm in diameter. The bottom machine direction yarns and the yarns which form the paired bottom fabric layer cross machine direction yarns are 0.20 mm in diameter. This fabric may be implemented with nylon or polyester yarns, or with a combination thereof.

Another embodiment of the present invention is illustrated in FIGS. 4, 5 and 6, wherein a repeat unit of a 24 harness multi-layer forming fabric designated broadly as 100 is shown. The fabric 100 comprises a set of top layer machine direction yarns 101-112, top layer CMD yarns 121-132, a set of bottom layer MD yarns 141-152, a set of paired bottom fabric layer CMD yarns 161/162-183/184, and stitching yarns 185a, 185b through 196a, 196b. One pair of stitching yarns is positioned between adjacent top layer CMD yarns and adjacent paired bottom fabric layer CMD yarns.

Like the fabric 20, the top MD and CMD yarns of the fabric 100 are interwoven such that each top CMD yarn passes over and under alternate MD yarns, and so that every top CMD yarn passes over and under the same MD yarns. These, in combination with the stitching yarn pairs, form a top papermaking surface that has a plain weave pattern (FIG. 4). The bottom MD yarns 141-152 are interwoven with the paired bottom CMD yarns 161/162 through 183/184 so that each paired bottom CMD yarn follows an "over-one/under-five" pattern relative to the bottom MD yarns. As also shown in FIG. 5, the knuckles formed by the bottom MD yarns take a "broken twill" pattern, in which the knuckles formed under adjacent CMD yarns are first offset by two MD yarns in one direction, then by three MD yarns in the opposite direction. Thus, the knuckles form a zig-zag diagonal pattern.

Each of the stitching yarns of the fabric 100 has a fiber support portion, which interweaves with the top MD yarns, and a binding portion, which stitches with the bottom layer of the fabric 100. As in the fabric 20 these portions of the stitching yarns are separated at transitional top MD yarns, under which both stitching yarns of a pair pass under and cross. The fiber support portion of each stitching yarn is positioned above the binding portion of the other stitching yarn of its pair. FIG. 6 illustrates the weave pattern for stitching yarn pair 185a, 185b.

As shown in FIG. 6, in a repeat each of the stitching yarns 185a, 185b have a fiber support portion in which they interweave with five top layer MD yarns in an over-one/under-one/over-one/under-one/over-one pattern. Each of the remaining stitching yarns of the fabric 100 follows this same weave pattern in its fiber support portion, such that each stitching yarn passes over three top MD yarns and under two top MD yarns in an alternating fashion in each repeat of the fabric. As best seen in FIG. 4, the stitching yarns pass over the top MD yarns 101-111 passed under by the top CMD yarns 121-132, then pass over the top MD yarns 101-111 passed under by the top CMD yarns 121-132, with the result that the top layer of the fabric 100 has a plain weave surface. Pairs of stitching yarns are interwoven with the top MD yarns such that each group of four adjacent stitching yarn pairs falls within a pattern in which the fiber support portions of three of the four pairs of stitching yarns are not offset from one another in the MD direction at all; i.e., the fiber support portions of each pass over the same top MD yarns. The fiber support portion of the fourth pair of stitching yarns of the group is offset from the others within the group by two top MD yarns. For the fiber support portions of the next group of four yarn pairs, the entire group is offset by two top MD yarns in the direction opposite of the offset of the individual stitching yarn pair.

As an example of this pattern, the stitching yarns 192a, 193a, 194a, and 195a form a group of four stitching yarns in adjacent stitching yarn pairs. Of these, stitching yarns 192a, 194a, and 195a pass over top MD yarns 105, 107 and 109. Stitching yarn 193a passes over top MD yarns 107, 109, and 111, which represents a two MD yarn offset. The next group of four stitching yarn pairs would then begin with stitching yarn 195a, which passes over top MD yarns 103, 105 and 107; this represents a two top MD yarn offset in the direction opposite that of the offset of stitching yarn 193a. This pattern continues for each group of four stitching yarn pairs.

As best seen in FIG. 6, in its binding portion, each stitching yarn (e.g., 185a, 185b) passes below five top MD yarns and above four bottom MD yarns while passing below one bottom MD yarn to stitch the top and bottom layers together. The bottom MD yarn stitched by the stitching yarn binding portion follows one of three different patterns; it is either the second, third or fourth bottom MD yarn reached by the stitching yarn after passing below a transitional top MD yarn. For example, stitching yarn 185a passes below bottom MD yarn 147, the third bottom MD yarn it approaches after passing below transitional top MD yarn 104. In contrast, as best seen in FIG. 5, stitching yarn 186a passes below bottom MD yarn 144, the second bottom MD yarn it approaches after passing below transitional top yarn 102, and stitching yarn 187a passes below bottom MD yarn 146, the fourth bottom MD yarn it approaches after passing below transitional top MD yarn 102.

As shown in FIG. 5, the stitching yarns of each pair follow the same weave pattern in their binding portions as the other stitching yarn of that pair (i e., like stitching yarn 187a, stitching yarn 187b also stitches the fourth bottom MD yarn it approaches after passing below a transitional top MD yarn). Also, it can be seen from FIG. 5 that the stitching yarn pairs follow a pattern in which the stitching yarns of the first pair stitch the third bottom MD yarn they approach, the stitching yarns of the second pair stitch the second bottom MD yarn they approach, the stitching yarns of the third pair stitch the fourth bottom MD yarn they approach, and the stitching yarns of the fourth pair stitch the third bottom MD yarn they approach. This "third/second/fourth/third" pattern is repeated three times within the repeat unit.

Those skilled in this art will also appreciate that other plain weave patterns in which the stitching yarns 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 yarn of a pair passes over two or four top MD yarns in its fiber support portion (instead of three top layer MD yarns as in the example of FIGS. 4 and 6). As illustrated in FIG. 1, the stitching yarns can pass over different numbers of top MD yarns, 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 yarn pattern on the top surface.

Those of skill in the art will also appreciate that fabrics similar to the illustrative fabrics of FIGS. 1-3 and FIGS. 4-6 may be constructed that have a papermaking surface other than a plain weave surface. For instance, as disclosed in FIGS. 7, 8 and 9, a multi-layer forming fabric 200 in which the papermaking surface is woven in a 1.times.2 broken twill pattern may be provided. As shown in FIG. 7, the repeat unit includes 12 top layer MD yarns 201-212, 6 top layer CMD yarns 221 through 226, 12 bottom layer MD yarns 241-252, 12 bottom layer CMD yarns 261-272, and 12 stitching yarns 281a, 281b through 286a, 286b.

As shown in FIG. 7, the top surface of the fabric 200 has a 1.times.2 twill pattern formed by the set of top layer MD yarns, the set of top layer CMD yarns and the fiber support portions of the stitching yarns. More specifically, each top layer CMD yarn interweaves with the top MD yarns in an "over-two/under-one" pattern; this is demonstrated by top CMD yarn 221, which passes over top MD yarns 201 and 202, under top MD yarn 203, over top MD yarns 204, 205, under top MD yarn 206, over top MD yarns 207, 208, under top MD yarn 209, over top MD yarns 210, 211, and under top MD yarn 212. The remaining top CMD yarns follow the same "over-two/under-one" pattern, but are laterally offset from their adjacent CMD yarns by two MD yarns. For example, top CMD yarn 222 passes over top MD yarn 201, under top MD yarn 202, over top MD yarn 203 and 204, and under top MD yarn 205 before continuing in an over-two-under-one pattern. Thus, the "over-two" portion of top CMD yarn 222 is first seen as it passes over top MD yarns 203 and 204, which are offset from the top MD yarns 201, 202 passed over by top CMD yarn 221 by two MD yarns.

Referring now to FIG. 8, the upper surface of the bottom layer of fabric 200 is depicted. As shown in FIG. 8, the bottom layer CMD yarns 261-272 are woven in pairs, with each yarn of the pair having an identical weave pattern with respect to the bottom layer MD yarns. Thus, the twelve bottom layer CMD yarns 261-272 depicted in FIG. 8 form a total of six paired bottom fabric layer CMD yarns 261/262 through 271/272. As is also depicted in FIG. 8, the paired bottom fabric layer CMD yarns and the bottom layer MD yarns are woven in the pattern of a "broken twill." Each bottom layer CMD yarn has an "under-five/over-one" repeat pattern with the bottom layer MD yarns. For example, paired bottom fabric layer CMD yarn 261/262 passes over bottom layer MD yarn 241, under bottom layer MD yarns 242 through 246, over bottom layer MD yarn 247, and under bottom layer MD yarns 248 through 252. This "under-five/over-one" pattern is repeated by the remaining paired bottom fabric layer CMD yarns. However, the machine side knuckles formed by the bottom layer MD yarns as they pass below the paired bottom fabric layer CMD yarns are arranged in a broken twill pattern which fail to form a clear diagonal as is characteristic of twill fabrics.

The top and bottom layers of the fabric 200 are bound together by the stitching yarns listed above, each of which has both a fiber support portion and a binding portion. As with the fabrics 20 and 100 described earlier, the fiber support portion and binding portion of each stitching yarn are divided by transitional top layer MD yarns below which stitching yarns of a pair cross each other. The fiber support portion of each stitching yarn follows an "over-two/under-one/over-two" pattern. In its binding portion, each stitching yarn passes between the top and bottom layer MD yarns with the exception of passing below one bottom layer MD yarn to stitch the top and bottom layers together. The bottom layer MD yarn that is stitched is located either two or three MD yarns away from the transitional MD yarns that separate the fiber support and binding portions of each stitching yarn.

This pattern is exemplified by stitching yarn 281b, the stitching pattern of which is illustrated in FIG. 9. Stitching yarn 281b passes over top layer MD yarns 205 and 206, under top layer MD yarn 207, and over top layer MD yarns 208, 209 before passing below transitional top layer MD yarn 210. In its binding portion, stitching yarn 281b passes above bottom layer MD yarns 251, 252 and 241, below bottom layer MD yarn 242 and above bottom layer MD yarn 243 before passing below transitional top layer MD yarn 204 and above bottom layer MD yarn 244. The pairs of stitching yarns are interwoven with the top layer MD yarns relative to one another such that their fiber support portions, the top layer MD yarns, and the top layer CMD yarns form a 1.times.2 twill pattern.

FIG. 7 demonstrates that the stitching yarns are interwoven with the top and bottom layer MD yarns relative to top layer CMD yarns such that an "over-two" segment of each fiber support portion is offset by one MD yarn from an "over-two" segment of the top layer CMD yarns that flank that stitching yarn. For example, the stitching yarn 281b passes over top layer MD yarns 202 and 203. The nearest top CMD yarns, which are 221 and 222, pass over top MD yarns 201, 202 and 203, 204, respectively. Thus, the distinctive diagonal of a twill is formed by the top layer CMD yarns and the fiber support portions of the stitching yarns.

FIG. 8 also illustrates how the stitching yarns are stitched into the bottom layer MD yarns. It can be seen in FIG. 8 that the knuckle formed by each stitching yarn as it passes below a bottom layer MD yarn is positioned such that, in one direction, two paired bottom fabric layer CMD yarns reside between the stitching yarn knuckle and the knuckle formed by that bottom layer MD yarn under a paired bottom fabric layer CMD yarn, and in the opposite direction, three paired bottom fabric layer CMD yarns reside between the stitching yarn knuckle and the next knuckle formed by that bottom layer MD yarn under over a paired bottom fabric layer CMD yarn. For example, stitching yarn 284b forms a knuckle as it passes under bottom layer MD yarn 241. The bottom layer MD yarn 241 forms a knuckle as it passes under paired bottom fabric layer CMD yarn 261/262, which is separated from the knuckle formed by stitching yarn 284b by three paired bottom fabric layer CMD yarns (263/264, 265/266, 267/268). Continuing with the pattern in the other direction, paired bottom fabric layer CMD yarns 269/270 and 271/272 are positioned between the knuckle formed by stitching yarn 284b and the knuckle that would be formed by bottom MD yarn 241 under the next paired bottom fabric layer CMD yarn after paired bottom fabric layer CMD yarn 271/272 (which would have the same weave pattern as paired bottom fabric layer CMD yarn 261/262). Thus, the stitching yarn knuckle of stitching yarn 284b is separated from bottom layer MD yarn knuckles by three paired bottom fabric layer CMD yarns in one direction and by two paired bottom fabric layer CMD yarns in the other direction.

Those skilled in this art will 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 1.times.3 or 1.times.4 twill top layer. Any of these twill patterns can be a conventional twill, or can take a broken twill pattern, such as those embodied in 4 or 5 harness satin single layer fabrics. Fabrics can also be constructed in which fiber support portions of stitching yarn pairs pass over different numbers of top MD yarns. In each instance, the skilled artisan should understand the appropriate modifications to the binding portions of the stitching yarns to accommodate differences in the fiber support portions.

Note that in the fabrics 20, 100 and 200 illustrated in FIGS. 1, 4 and 7, respectively, the combination of the set of top layer MD yarns, the set of top layer CMD yarns, and the set of stitching CMD yarns, forms a papermaking surface having single float machine direction knuckles. By "single float machine direction knuckles" it is meant that on the papermaking surface no machine direction yarn passes over more than one consecutive cross machine direction yarn before passing back down below the top surface of the fabric. In a preferred embodiment of the triple layer forming fabrics of the present invention, the top MD yarns, top CMD yarns, and stitching CMD yarns form a papermaking surface having such single float machine direction knuckles. However, as will be apparent from the discussion below, a papermaking surface having such single float machine direction knuckles is not required, and in fact, with respect to some weave patterns, it may instead be preferable to provide a top fabric layer wherein the combination of just the top MD yarns and top CMD yarns results in a fabric having single float machine direction knuckles, but if the stitching yarns are also considered, the top fabric layer includes some double float machine direction knuckles (see, e.g., FIG. 11 herein).

In another embodiment of the present invention, triple layer papermaker's forming fabrics are provided which have a bottom fabric layer woven in any of a variety of weave patterns, so long as the yarns comprising the bottom set of machine direction yarns pass under no more than one of any two adjacent paired bottom fabric layer cross machine direction yarns. FIG. 2 illustrates one such papermaker's fabric having this characteristic of the fabrics of the present invention. As seen in FIG. 2, the bottom machine direction yarns 41-50 are woven such that any given machine direction yarn weaves with the paired bottom fabric layer cross machine direction yarns 51/52 through 69/70 in an "under-one/over-four" pattern. By way of example, bottom MD yarn 41 weaves under paired bottom fabric cross machine direction yarn 51/52, over paired bottom fabric layer cross machine direction yarns 53/54, 55/56, 57/58, 59/60, under paired bottom fabric layer cross machine direction yarns 61/62, and over paired bottom fabric layer cross machine direction yarns 63/64, 65/66, 67/68,69/70. FIGS. 10A and 10B illustrate one reason why such a configuration may be advantageous.

FIG. 10A depicts a cross section of the bottom layer of fabric 20 taken along the line 10--10 in FIG. 2. As is clear from FIG. 10A, bottom layer MD yarn 46 never passes under more than one adjacent paired bottom fabric layer CMD yarn (i.e., it only passes under paired yarns 51/52 and 61/62). As a result, bottom layer MD yarn 46 slopes upwardly on either side of each of the paired bottom layer CMD yarn 51/52, 61/62 which it passes under, thereby urging yarns 51 and 52 together and yarns 61 and 62 together. This can be seen by the arrows in FIG. 10A which indicate the direction of the force applied by yarn 46 on yarns 61 and 62 in the machine direction. Thus, FIG. 10A illustrates that by weaving the bottom MD yarns such that they pass under no more than one paired bottom fabric cross machine direction yarn 51/52, it is possible to exert a relatively high lateral force on each of the bottom cross machine direction yarns which form the paired bottom fabric cross machine direction yarns. As a result, these yarns can become completely paired (i.e., they touch each other over substantially their entire length) within the fabric, and thus tend to act in the fabric like a single yarn, which may significantly increase the stability of the fabric.

If, instead, bottom layer MD yarn 46 is woven so that it passes under two or more adjacent paired bottom fabric layer CMD yarns, the paired bottom fabric layer CMD yarns will not be located in a trough as in FIG. 10A, since the bottom layer MD yarn 46 will only slope upwardly on one side of each paired bottom fabric layer CMD yarns. This is illustrated in FIG. 10B with respect to paired bottom fabric layer CMB yarns 61/62 and 63/64, and shows that a lateral force (depicted by the arrows in FIG. 10B) is only applied on one side of each of paired yarns 61/62 and 63/64. Such an arrangement may not fully pair the yarns which comprise a paired bottom fabric layer CMD yarn. If this occurs, the mechanical stability of the fabric may be significantly reduced. Thus, pursuant to the teachings of the present invention, it will be realized that mechanical stability may be increased in many fabrics which include paired bottom layer CMD yarns if the machine direction yarns pass under no more than one such paired yarn before passing over another paired bottom fabric layer CMD yarn.

Those of skill in the art will appreciate that a variety of different weave patterns may be used to provide a bottom fabric layer in which no MD yarn passes under more than one adjacent paired bottom fabric layer CMD yarn in a row. Such weave patterns include, for example, 1.times.2 twills, 1.times.3 twills, 1.times.4 twills, 1.times.5 twills, 1.times.6 twills, plain weave patterns. One of skill in the art will also appreciate that broken twill patterns may also be used.

In the fabrics depicted in FIGS. 1-9 above, pairs of stitching yarns were generally used to "complete" a particular weave pattern on the papermaking surface. Thus, for example, in FIG. 1, stitching yarns 61a-70a and 61b-70b completed an over-one/under-one or "plain weave" pattern on the papermaking surface of the fabric, and in FIG. 7 the stitching yarns 281a-286a and 281b-286b completed a 1.times.2 twill pattern. However, in light of the discussion below, those of skill in the art will appreciate that paired bottom fabric layer cross machine direction yarns may advantageously be used to provide a variety of other low caliper multi-layered fabrics in which the stitching yarns do not complete a particular weave pattern on the papermaking surface.

FIGS. 11-13 depict a portion of a triple layer embodiment of one such fabric 300. As seen in FIG. 11, which is a top view of the top fabric layer 301 of fabric 300 (i.e., a view of the papermaking surface), the top fabric layer 301 includes a set of top layer MD yarns 310-319 and a set of top layer CMD yarns 320-324. These yarns 310-319 and 320-324 are interwoven in an over-one/under-one pattern to form a plain weave base top fabric layer. Thus, for example, top CMD yarn 321 passes under top MD yarn 310, over top MD yarn 311, under top MD yarn 312, over top MD yarn 313 and so on until it passes over top MD yarn 319. Those of skill in the art will appreciate that FIG. 11 only depicts one repeat unit of the fabric.

Referring now to FIG. 12, the portion of the bottom layer 302 of the fabric 300 corresponding to the portion of the fabric shown in FIG. 11 is shown. As illustrated in FIG. 12, the bottom layer 302 of fabric 300 includes a set of bottom layer MD yarns 330-339 which are interwoven with a set of bottom layer CMD yarns 340-349. As shown in FIG. 12, the yarns comprising the set of bottom layer CMD yarns 340-349 are interwoven with the bottom layer set of MD yarns 330-339 in pairs, such as pair 340/341. Each of the yarns in these pairs of yarn are woven together in the same shed of the fabric, thereby forming paired bottom fabric layer CMD yarns such as yarn 340/341 which have an identical weave pattern in the fabric. The bottom layer set of MD yarns 330-339 are interwoven with the paired bottom fabric layer set of CMD yarns 340-341 through 346/349 in a 1.times.4 twill type pattern, meaning that each of the yarn pairs 340/341, 342/343, 344/345, 346/347 and 348/349 pass above one yarn of the set of bottom MD yarns 330-339, below the next four yarns of the set of bottom MD yarns 330-339, above the next yarn of the set of bottom MD yarns 330-339, and below the next four yarns of the set of bottom MD yarns 330-339. For example, paired bottom fabric layer CMD yarn 340/341 passes below bottom MD yarns 330-333, above bottom MD yarn 334, and below bottom MD yarns 335-338 and above bottom MD yarn 339. The other paired bottom fabric layer CMD yarns 342/343, 344/345, 346/347 and 348/349 follow a similar "under-four/over-one" weave pattern, although this pattern is offset to the left by two bottom layers MD yarns for adjacent paired bottom fabric layer CMD yarns.

The top fabric layer 301 (pictured in FIG. 11) and the bottom fabric layer 302 (pictured in FIG. 12) are stitched together with pairs of stitching yarns, designated herein as pairs 350a, 350b; 351a, 351b; 352a, 352b; 353a, 353b; and 354a, 354b. These stitching yarn pairs are positioned between adjacent yarns of the set of top layer CMD yarns and adjacent paired bottom fabric layer CMD yarns. For example, stitching yarns 350a and 350b are positioned between top CMD yarns 320 and 321 and between paired bottom fabric layer CMD yarns 340/341 and 342/343. The stitching yarns interweave with the top MD yarns and bottom MD yarns to bind the top fabric layer 301 and bottom fabric layer 302 together.

FIG. 13, which is a cross section taken along the line 13--13 of FIGS. 11 and 12 illustrates one repeat of the weave pattern followed by stitching yarns 352a, 352b. Note that each of stitching yarns 352a and 352b has a fiber support portion where it interweaves with the top MD yarns, and a binding portion where it interweaves with a bottom MD yarn. However, in this particular embodiment of the present invention, the "transitional" points where the stitching yarns stop weaving with the top MD yarns and pass down into the fabric occurs between two top MD yarns (e.g., MD yarns 314, 315 in FIG. 13) as opposed to beneath one of the top MD yarns as was the case with the fabrics illustrated, for example, in FIGS. 1-9. As is also shown in FIG. 13, the stitching yarn pairs are interwoven such that the fiber support portion of one yarn of the pair is positioned above the binding portion of the other yarn of the pair. As can be seen in FIGS. 11 and 13, one of the yarns (e.g., yarn 352b) of each pair of stitching yarns includes a fiber support portion where it interweaves in an alternating fashion with five top MD yarns (yarns 310-314) by passing over top MD yarns 310, 311, under top MD yarn 312, and over top MD yarns 313, 314. The other stitching yarn of the pair, yarn 352a, likewise has a fiber support portion in which it passes over top MD yarns 315, 316, under top MD yarn 317 and over top MD yarns 318, 319. In its binding portion, stitching yarn 352b passes below top layer MD yarns 315-319 while passing above bottom layer MD yarns 335, 336, 338 and 339 and below bottom layer MD yarn 337 to stitch the bottom layer 302 of the fabric 300 (see FIG. 13). Similarly, stitching yarn 352a also has a binding portion in which it passes below top layer MD yarns 310-314 while passing above bottom layer MD yarns 330, 331 and 333, 334 and below bottom layer MD yarn 332 to stitch the bottom layer 302 of the fabric 300. As shown in FIG. 13, the stitching yarns are woven such that when one yarn of the pair is in its binding portion the other yarn of the pair is in its fiber support portion.

The other pairs of stitching yarns illustrated in the plan views follow the same weave pattern as shown for stitching yarns 352a and 352b in the cross section view of FIG. 13. However, as shown in FIGS. 11 and 12, the pairs of stitching yarns that are positioned adjacent to and on opposite sides of a top CMD yarn (or paired bottom fabric layer CMD yarn) preferably are interwoven with the top or bottom MD yarns such that there is an offset of one or more MD yarns between such stitching yarn pairs. In the illustrated embodiment, this offset is three MD yarns to the right. Thus, for example, the fiber support portion of stitching yarn 350a occurs in the vicinity of top MD yarns 310-313, and 319 while the fiber support portion of stitching yarn 351a occurs in the vicinity of top MD yarns 312-316. However, one of skill on the art will appreciate that a variety of different offsets may be used depending upon the number of harnesses on which the fabric is constructed and the desired papermaking qualities.

As is best illustrated in FIG. 11, each of the stitching yarns 350a-354a and 350b-354b serves both a fiber support function and a locator function. By "fiber support function" it is meant that the yarn weaves with the top fabric layer 301 of the fabric 300 so as to provide support to the paper slurry fibers during the papermaking process. By "locator function" it is meant that the yarn exerts a force on the other stitching yarn at the transition point so as to urge the other stitching yarn towards its proper position, which typically is midway between adjacent top cross machine direction yarns. In a preferred embodiment, the stitching yarns are either approximately the same diameter as the top fabric layer cross machine direction yarns, or are slightly (e.g., 35% smaller) than the top fabric layer CMD yarns.

One method of implementing the locator function can be seen in FIG. 11 at the transition points where each of the pairs of stitching CMD yarns cross each other entering and leaving the top fabric surface. Focusing on stitching yarns 351a, 351b, one such transition point occurs between top layer MD yarns 311, 312. At that point, top MD yarn 312 has just crossed over top CMD yarn 321 and passes under the top CMD yarn 322; consequently, top MD yarn has a downward slope as it travels between top CMD yarns 321 and 322. Similarly, top MD yarn 311 has just passed under top CMD yarn 321 and over top CMD yarn 322. Thus, top MD yarn 311 has an "uphill" slope as it travels between top CMD yarns 321 and 322. Because top MD yarn 312 is on a "downhill" slope at the aforementioned transition point, it applies a force to stitching yarn 351a towards top CMD yarn 322. Consequently, at the transition point, stitching yarn 351a would tend to "pair" with top CMD yarn 322 in the absence of a countervailing force. Similarly, since top MD yarn 311 is on an uphill slope at this transition point, it applies a force to stitching yarn 351b which (in the absence of a countervailing force) urges yarn 351b to pair with top CMD yarn 321. However, since at the transition point the top MD yarns 311, 312 are urging stitching yarns 351a, 351b in opposite directions with substantially equal forces, stitching yarns 351a, 351b apply generally opposite forces on each other, which tends to maintain the yarns in a central position between top CMD yarns 321, 322 at the transition point.

As is best illustrated in FIG. 13, in a preferred embodiment of the present invention the stitching yarns are woven into the fabric in pairs. Moreover, these stitching yarns may advantageously be woven in the same weave pattern, except that they are offset by some number of top MD yarns, so that when one of the stitching yarns is performing a fiber support function the other of stitching yarn of the pair is performing a binding function. Preferably, the combined weave of the top layer MD yarns, the top layer CMD yarns and the stitching yarns is such that each yarn of a pair of stitching yarns exerts a force on the other yarn at the crossover or "transition" points as described above so as to help locate the yarn in its proper position in the fabric. Moreover, while those of skill in the art will appreciate that it generally is preferable to have such a locating function occur at every transition point, the fabric may also be woven such that the locating function only occurs at some of the transition points.

In fabric 300 (see FIG. 11), the set of top layer MD yarns and the set of top layer CMD yarns form a fabric having single float machine direction knuckles (i.e., no machine direction yarn passes over more than one of the top layer cross machine direction yarns 320-324 before passing back down below the surface of the fabric). This results in a papermaking surface which largely, but not exclusively, has single float machine direction knuckles (since in some instances a top MD yarn passes over a top layer CMD yarn and a stitching yarn before passing back down below the surface of the fabric). This results in a fabric which has a high level of cross machine direction support and provides excellent papermaking qualities.

Pursuant to another aspect of the present invention, multi-layer papermaker's fabrics having a relatively large number of picks on its papermaking surface and a relatively low void volume are provided. The void volume (V.sub.o) of a forming fabric can be determined experimentally by the following equation:

V.sub.o =V.sub.s -V.sub.f where:

V.sub.s =the volume of a forming fabric of unit area A.sub.s ; and

V.sub.f =the volume of the constituent monofilaments in the forming fabric of unit area A.sub.s.

V.sub.s and V.sub.f may be determined as: V.sub.s =A.sub.s S where:

A.sub.s : is the forming fabric unit area (mm.sup.2 /cm.sup.2); and

S: is the forming fabric caliper (mm) and

V.sub.f =W/.sub..rho.f where

W=the weight of a forming fabric sample of size A.sub.s ; and

.sub..rho.f =the average weighted density of the constituent monofilament yarns.

Generally, the fabrics illustrated herein can be woven to have relatively large numbers of picks on the papermaking surface and low void volumes. Preferably, the papermaking surface has at least 80 picks (i.e., cross machine direction yarns) per inch, although pick counts of at least 85 and higher are even more preferred. It will be understood that in determining the pick count on the papermaking surface, pairs of stitching yarns having both fiber support and binding portions which weave substantially equivalent to a single yarn on the papermaking surface are counted as a single "pick." It is also preferred for some papermaking applications that the void volume of the fabric is no greater than 50 mm.sup.3 /cm.sup.2, and void volumes of less than 45 mm.sup.3 /cm.sup.2 are even more preferred for such fabrics.

In yet another aspect of the present invention, triple layer forming fabrics which have a high level of top layer cross machine direction support are provided which are particularly suitable for auto-joining. As will be understood by those of skill in the art, the free space between bottom layer CMD yarns in any weave may be calculated as:

S.sub.f =25.4/PPI-D where

S.sub.f =the free space between bottom CMD yarns in mm;

PPI=the bottom CMD yarn count in picks per inch; and

D=the diameter of the bottom CMD yarns in mm.

Accordingly, the maximum picks/inch for a minimum free space and yarn diameter is: PPI.sub.max =25.4/(S.sub.f +D) One potential problem of designing fabrics to have a large number of picks per inch is that many auto-joining machines which are currently in use have reed wires that are no smaller than 0.1 mm for some machines and 0.15 mm for others. The size is limited by the materials available to provide sufficient reed strength and rigidity. As these auto-joining machines may only be used on fabrics in which the free space between yarns is at least the size of the reed wire, a fabric shall have a free space of at least 0.1 (or 0.15) mm to be auto-joined on these machines. However, if paired bottom fabric layer CMD yarns are employed, both yarns of the pair are woven in the same fashion in the auto-joining process, and as such, no free space is required between the two yarns which form each of the paired bottom fabric layer CMD yarns. Accordingly, by weaving the fabric with paired bottom fabric layer CMD yarns it is possible to increase the number of bottom CMD yarns per inch while maintaining the necessary free space between adjacent paired bottom fabric layer CMD yarns. This, in turn, allows an overall increase in the number of picks per inch.

The potential increase in top pick count for any weave which is converted from having single bottom layer CMD yarns to paired bottom layer CMD yarns may be calculated as follows:

PI=[2(S.sub.f +D.sub.s)/(S.sub.f +2D.sub.D)-1]100 where

PI is the percent increase in the top pick count;

D.sub.s =the diameter of the bottom CMD yarns in mm if single bottom CMD yarns are used;

D.sub.D =the diameter of the bottom CMD yarns in mm if paired bottom fabric layer CMD yarns are used.

Thus, for example, if the required free space is 0.1 mm, and the diameter of the bottom CMD yarns (whether single yarns or paired yarns are used) is 0.2 mm, the percent increase is 20%. In this example, if the required free space is increased to 0.1 5 mm, the percent increase further increases to 27%.

In a preferred embodiment of such an auto-joinable triple layer fabric, the ratio between the number of top layer CMD yarns and bottom layer CMD yarns is approximately one-to-one. One of the advantages provided by this embodiment is illustrated in FIG. 16. FIG. 16A is a cross section (taken between adjacent MD yarns) of a repeat of a conventional 16 harness triple layer fabric having eight top layer CMD yarns 501-508, eight bottom layer CMD yarns 511-518 and one stitching yarn 519. The top and bottom layer CMD yarns are arranged in a stacked configuration. As the bottom layer CMD yarns are of larger diameter than the top layer CMD yarns (to provide good wear characteristics and mechanical stability), it is the bottom fabric layer, as opposed to the top fabric layer, which limits the pick count (i.e., the number of CMD yarns per inch). Thus, for example, if the bottom layer CMD yarns 511-518 in FIG. 16A are 0.33 mm in diameter, and the top layer CMD yarns are 0.2 mm in diameter, and a free space of 0.1 mm is required between all CMD yarns (to allow use of an auto-joining machine), then the maximum pick count for the fabric may be calculated as:

PPI.sub.max =25.4/(0.1+0.33)=59 If the fabric of FIG. 16A is modified to include paired bottom fabric layer CMD yarns, then a fabric such as that shown in FIG. 16B results. In this fabric, the bottom layer CMD yarns 531-538 are woven in pairs of two yarns (531/532, 533/534, 535/536 and 537/538), where each yarn in the pair has an identical weave pattern with respect to the fabrics machine direction yarns. Assuming that the bottom layer CMD yarns 531-538 are once again 0.33 mm in diameter, and that the top layer CMD yarns 521-528 are 0.20 mm in diameter, the maximum pick count for the fabric of FIG. 16B may be calculated as: PPI.sub.max =2.times.25.4/(S.sub.f +2D)=2.times.25.4/(0.1+0.66)=67 Thus, by modifying the fabric of FIG. 16A is can be seen in the above example that the maximum pick count may be increased from 59 to 67 picks per inch. Consequently, the fabric of FIG. 16B may be designed to be auto joinable while have a greater number of top layer CMD yarns, and hence superior cross machine direction support on the papermaking surface, which may provide improved papermaking qualities.

Pursuant to another aspect of the present invention, methods of making paper are provided. Pursuant to these methods, one of the exemplary papermaker's forming fabrics described herein is provided, and paper is then made by applying paper stock to the forming fabric and by then removing moisture from the paper stock. As the details of how the paper stock is applied to the forming fabric and how moisture is removed from the paperstock is well understood by those of skill in the art, additional details regarding this aspect of the present invention will not be provided herein.

Pursuant to another aspect of the present invention, methods of making a multi-layer papermaker's fabric having paired bottom fabric layer CMD yarns are provided. Pursuant to these methods, both the yarns which comprise each paired bottom fabric layer CMD yarn are woven in direct succession. As techniques for weaving forming fabrics are generally well known to those of skill in the art, the details of the weaving process will not be described herein. However, pursuant to the teachings of the present invention it will be realized that in weaving fabrics having paired bottom fabric layer CMD yarns for use in certain applications, it may be preferable to weave the yarns which comprise each paired yarn in direct succession. Such a weaving technique may advantageously pair the yarns more tightly, which may be advantageous in terms of both mechanical stability and wear characteristics and which may facilitate drainage through the openings between adjacent paired bottom fabric layer CMD yarns. However, it will also be appreciated that in other applications weaving considerations or the desired characteristics of the fabric may make it more preferable to weave the yarns which comprise the paired bottom CMD yarn in non-successive picks.

FIGS. 14 and 15 illustrate an exemplary fabric embodying another aspect of the present invention, wherein paired bottom fabric layer CMD yarns are employed in multi-layer fabrics which include auxiliary CMD yarns. As will be apparent to those of skill in the art, FIGS. 14 and 15 depict an eight harness double layered fabric broadly designated as 400. For ease of understanding this aspect of the present invention, fabric 400 will be described as if a base fabric layer were initially woven and then additional yarns added. The hypothetical base fabric layer includes MD yarns, top layer CMD yarns and bottom layer CMD yarns. With respect to fabric 400, the top layer CMD yarns which are used to form the base fabric layer are referred to as "primary" top layer CMD yarns, so as to distinguish them from the additional or "auxiliary" top layer CMD yarns (described below) which are added to the base fabric layer. Of course, the papermaker's fabric 400 will typically be woven in a one step weaving process.

As shown in FIG. 14 (which encompasses its 8.times.8 repeating unit), the fabric 400 includes MD yarns 401-408 and primary top layer CMD yarns are interwoven in a twill pattern such that each primary top layer CMD yarn passes over seven MD yarns, passes beneath an MD yarn, and then repeats this pattern. The 8 harness fabric 400 is constructed so that the knuckles adjacent primary CMD yarns are offset in the cross-machine direction by three MD yarns.

FIG. 15 illustrates the bottom (machine) side layer of the base fabric 400. As (partially) shown in FIG. 15, the bottom fabric layer CMD yarns 421-436 are woven in groups of two yarns which have identical weave patterns so as to form eight paired bottom fabric layer CMD yarns 421/422 through 435/436. These paired yarns are positioned below the primary top layer CMD yarns 411-418 described above. A typical MD yarn 401 is shown passing from the papermaking surface of the fabric 400 to interweave the paired bottom fabric layer CMD yarns; specifically in this instance, the MD yarn 401 passes over the paired bottom fabric layer CMD yarns 421/422, 423/424 and 425/426, under paired bottom fabric layer CMD yarn 427/428, over paired bottom fabric layer CMD yarn 429/430, under paired bottom fabric layer CMD yarn 431/432 and over paired bottom fabric layer CMD yarns 433/434 and 435/436.

As shown in FIGS. 14 and 15, in addition to the primary top layer CMD yarns 411-418, the fabric 400 also includes auxiliary top layer CMD yarns. As discussed in more detail below, each of the auxiliary top layer CMD yarns follows a weave pattern through the MD yarns that is identical to the weave pattern of a nearby primary top layer CMD yarn. For clarity, the auxiliary top layer CMD yarns are designated with the same component numeral as the primary top layer CMD yarn which has the identical weave pattern, but the auxiliary yarn includes either an "a" or a "b" after the component numeral.

As shown in both FIGS. 14 and 15, a pair of auxiliary top layer CMD yarns is positioned between each pair of adjacent primary top layer CMD yarns. These auxiliary yarns are positioned such that each primary top layer CMD yarn has on either side of it an auxiliary top layer CMD yarn which has an identical weave pattern. Thus, for example, primary top layer CMD yarn 413 has an auxiliary top layer CMD yarn on both its left (yarn 413a) and its right (yarn 413b). As is also seen in both FIGS. 14 and 15, the primary top layer CMD yarns are separated from the auxiliary yarns which have the same weave pattern by one auxiliary yarn (i.e., auxiliary yarn 412b falls between primary yarn 413 and auxiliary yarn 413a, and auxiliary yarn 414a falls between primary yarn 413 and auxiliary yarn 413b). Thus, each of the primary top layer CMD yarns has a pair of auxiliary top layer CMD yarns which have its identical weave pattern with respect to the MD yarns, where those auxiliary yarns are separated from their corresponding primary top layer CMD yarn by one auxiliary top layer CMD yarn.

As will be appreciated by those of skill in the art in light of the present disclosure, positioning an auxiliary top layer CMD yarn between two primary top layer CMD yarns, one of which has the identical weave pattern, the auxiliary top layer CMD yarn is urged toward the primary top layer CMD yarn having the identical weave pattern. However, because there is a second auxiliary CMD yarn positioned between each auxiliary top layer CMD yarn and its corresponding primary top layer CMD yarn, and because the second auxiliary top layer CMD yarn is biased in the opposite direction from its counterpart auxiliary top layer CMD yarn (because it is biased toward its own corresponding primary top layer CMD yarn due to its identical weave pattern), the pairs of auxiliary top layer CMD yarns tend to center one another within the gap between the primary top layer CMD yarns.

Another advantage offered by the fabric 400 is the presence of three separate twill lines. One twill line is formed by the floats of adjacent primary top layer CMD yarns. Another twill line is formed by the set of auxiliary top layer CMD yarns positioned above their corresponding primary top layer CMD yarns. The third twill line is formed by the set of auxiliary top layer CMD yarns positioned below their respective corresponding primary top layer CMD yarns in FIG. 14. Thus, every float of every CMD yarn, whether primary or auxiliary, resides within a twill line. This multiplicity of twill lines usually helps to obscure markings of the fabric 400 on the paper formed thereon.

As shown in FIG. 15, the double layered fabric includes paired bottom fabric layer CMD yarns. By using such paired yarns as opposed to single bottom layer CMD yarns, it is possible to reduce the diameter of the bottom layer CMD yarns without necessarily reducing the wear characteristics or the mechanical stability of the bottom layer. Consequently, the caliper of the fabric 400 may be reduced, which reduces both the void volume and water carrying propensity of the fabric 400.

Those of skill in the art will also appreciate that the concepts of the present invention will also advantageously work in certain fabrics of a class of fabrics commonly referred to as "single x-pick double layer" fabrics, which comprise a set of machine direction yarns, sets of primary and auxiliary top cross machine direction yarns, and a set of bottom cross machine direction yarns. These single x-pick double layer fabrics typically are similar to the fabric 400, but only have a single auxiliary top cross machine direction yarn positioned between each pair of adjacent primary top cross machine direction yarns. Pursuant to the teachings of the present invention, the single x-pick double layer fabric is modified to include paired bottom fabric layer cross machine direction yarns, where such a paired bottom fabric layer cross machine direction yarn is stacked below each primary top cross machine direction yarn. As will be understood by those of skill in the art in light of the present disclosure, in implementing a single x-pick double layer fabric to have paired bottom fabric layer cross machine direction yarns, it is preferred that the yarns in the set of bottom machine direction yarns pass under no more than one of any two adjacent paired bottom fabric layer cross machine direction yarns. By implementing the fabric in this manner, the paired bottom fabric layer cross machine direction yarns rest in a trough formed by the machine direction yarn which, as discussed above with respect to FIGS. 10A and 10B, advantageously exerts forces on the individual yarns which comprise each paired bottom fabric layer cross machine direction yarn that urge the yarns together so that they act as a single yarn within the fabric. It is further preferred that each bottom machine direction yarn pass under at least two paired bottom fabric layer cross machine direction yarns in each repeat of the fabric. In this manner, the number of "troughs" where the bottom machine direction yarns work to pair the paired bottom fabric layer cross machine direction yarns is increased, which further assists in ensuring that the paired bottom fabric layer cross machine direction yarns remain fully paired throughout the fabric.

Those skilled in this art will appreciate that this concept of auxiliary yarns oppositely biased toward adjacent primary CMD yarns can be applied to virtually any fabric, including plain weaves, twills, satins, and the like. It can be employed as the paper side of both double and triple layer fabrics, whether interlaced by common MD yarns (such as the fabric 400) or formed as separate fabric layers, such as those described in U.S. Pat. No. 5,277,967 to Zehle. It is preferred that the fabrics of the present invention have a harness repeat of greater than 2.

Preferably, the auxiliary CMD yarns are of a smaller diameter than the MD and CMD yarns making up the base structure fabric. The size of the smaller diameter auxiliary CM yarns is typically governed by the size and spacing of the papermaking surface CMD yarns of the base fabric. Generally, the diameter of the auxiliary CMD yarns is about one half the diameter of the primary cross machine direction yarns.

As will be appreciated by those of skill in the art in light of the present disclosure, the use of paired bottom fabric layer CMD yarns may be particularly advantageous in multi-layer fabrics, such as fabric 400 and the single x-pick double layer fabrics discussed above, which include auxiliary top layer CMD yarns which are not stacked above a bottom layer CMD yarn. The presence of these auxiliary top layer CMD yarns may facilitate drainage when paired bottom fabric layer CMD yarns are used, since the fabric is relatively open beneath these yarns, thereby providing a good drainage path. Consequently, even if relatively large yarns are used to form the paired bottom fabric layer CMD yarns, sufficient drainage should exist due to the openings under the auxiliary top layer CMD yarns.

As will also be understood by those of skill in the art in light of the present disclosure, the use of paired bottom fabric layer cross machine direction yarns may be particularly advantageous in triple layer forming fabrics which include pairs of stitching yarns between every pair of top layer cross machine direction yarns. If the bottom CMD yarns of a particular triple layer fabric are replaced with paired bottom fabric layer CMD yarns, the amount of open area in the bottom fabric layer is decreased to the extent that the diameter of each of the paired yarns exceeds one-half the diameter of the single yarn which they replaced. However, in triple layer fabrics which include pairs of stitching yarns, the bottom fabric layer tends to be quite open, as each of the pairs of stitching yarns typically only stitches with the bottom layer MD yarns at one place within a repeat (which is typically 10 or 12 bottom layer MD yarns), thus leaving a significant amount of open area. Accordingly, even if each of the paired bottom fabric layer CMD yarns are 75%, or even more, of the diameter of the single bottom layer CMD yarn typically employed in top layer fabrics, typically there will still be sufficient drainage through the openings provided along the paths followed by the pairs of stitching yarns. Accordingly, the concepts of the present invention are particularly suitable for low caliper triple layer forming fabrics which include pairs of stitching yarns between each top layer CMD yarn.

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 may be constructed, that employ paired bottom fabric layer CMD yarns.

The configurations of the individual yarns utilized in the fabrics of the present invention can vary, depending upon the desired properties of the final papermakers' fabric. For example, the yarns may be multifilament yarns, monofilament yarns, twisted multifilament or monofilament yarns, spun yarns, or any combination thereof. Also, the materials comprising yarns employed in the fabric of the present invention may be those commonly used in papermakers' fabric. For example, the yarns may be formed of cotton, wool, polypropylene, polyester, aramid, nylon, or the like. The skilled artisan should select a yarn material according to the particular application of the final fabric.

Regarding yarn dimensions, the particular size of the yarns is typically governed by the size and spacing of the papermaking surface. In a typical embodiment of the triple layer fabrics disclosed herein, preferably the diameter of the top CMD yarns is between about 0.11 and 0.17 mm and the diameter of the top MD yarns is between about 0.11 and 0.15 mm. For these triple layer embodiments preferably the diameter of the bottom MD yarns is between about 0.17 and 0.33 mm, and the diameter of each of the individual yarns used to form paired bottom fabric layer CMD yarns is between about 0.14 and 0.30 mm. The diameter of the stitching yarns is typically between about 0.11 and 0.17 mm. Those of skill in the art will appreciate that yarns having diameters outside the above ranges may be used in certain applications. For example, top CMD yarns and MD yarns of up to 0.25 mm in diameter and bottom CMD yarns of up to 0.40 mm in diameter are often used for brown paper applications. In any event, it is preferable that the yarns which form the paired bottom fabric layer CMD yarns have a diameter in the range of 50% to 125% the diameter of the bottom MD yarns. In a more preferred embodiment, the yarns which form the paired bottom fabric layer CMD yarns have a diameter in the range of 60% to 85% the diameter of the bottom MD yarns.

It should also be noted that in many applications which include a relatively high number of floats on the papermaking surface that are formed by stitching yarns, the stitching yarns are of the same diameter as the top layer CMD yarns (to provide a coplanar papermaking surface). However, in situations where paired bottom fabric layer CMD yarns are employed in this type of fabric, it may be advantageous to use slightly smaller stitching yarns (e.g., with a diameter 10-20% less than the diameter of the top layer CMD yarns), as this may provide for a better drainage path between the paired bottom fabric layer CMD yarns.

As will be appreciated by those of skill in the art in light of the present disclosure, by using paired bottom fabric layer CMD yarns as taught herein, it is possible to significantly reduce the caliper of multi-layered papermaker's fabric without significantly reducing or otherwise affecting the mechanical stability of the fabric. Consequently, the fabrics of the present invention can have reduced void volume, and hence water carry, as compared to similar fabrics which have single bottom layer CMD yarns, yet should have similar performance from a mechanical perspective. Moreover, as will also be appreciated by those of skill in the art, the use of large CMD yarns may, in many situations, result in edge curl problems. The fabrics of the present invention provide a means for overcoming this problem, as smaller individual (MD) yarns may typically be used when they are woven as paired bottom fabric layer yarns, and the use of such smaller yarns in many cases may reduce, or even eliminate, the edge curl problem. In a preferred embodiment of the present invention, the fabrics further include a high density of stitching CMD yarn pairs (or other auxiliary top layer CMD yarns) which provide a high degree of cross direction support on the papermaking surface and which firmly bind the top and bottom fabric layers together, thus reducing or even eliminating interlayer wear. In these embodiments, the paired bottom fabric layer CMD yarns are typically stacked underneath the non-stitching top layer CMD yarns, so as to provide good drainage paths through the fabric adjacent and underneath the pairs of stitching yarns.

While the present invention has primarily been described with respect to forming fabrics, the use of paired bottom fabric layer CMD yarns may also advantageously be used in press felt applications in which low caliper is desired without compromising mechanical stability. Accordingly, the concepts of the present invention are not limited to forming fabrics.

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

1. A triple layer papermakers fabric comprising:

a set of top machine direction yarns, a set of top cross machine direction yarns and a set of stitching cross machine direction yarns which are interwoven to form a top fabric layer having a papermaking surface; and

a set of bottom machine direction yarns and a set of bottom cross machine direction yarns interwoven to form a bottom fabric layer having a machine side surface;

wherein each yarn in said set of bottom cross machine directions yarns is interwoven with said bottom machine direction yarns in a pattern identical to the weave pattern of an adjacent yarn in said set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine direction yarns; and

wherein said set of stitching cross machine direction yarns comprises pairs of adjacent stitching yarns that are woven in said fabric such that when the first yarn of the pair is weaving in the top fabric layer the second yarn of the pair is passing downwardly from said top fabric layer to interweave with said bottom fabric layer.

2. The papermaker's fabric of claim 1, wherein said set of stitching yarns completes a weave pattern on the papermaking surface which is partially formed by the interweaving of the set of top machine direction yarns and the set of top cross machine direction yarns.

3. The papermaker's fabric of claim 1, wherein at least one of said pairs of adjacent stitching yarns is positioned next to each yarn in said set of top cross machine direction yarns.

4. The papermaker's fabric of claim 2, wherein said set of top machine direction yarns, said set of top cross machine direction yarns and said set of stitching cross machine direction yarns form a papermaking surface having single float machine direction knuckles.

5. The papermaker's fabric of claim 4, wherein the papermaking surface has a plain weave pattern.

6. The papermaker's fabric of claim 4, wherein the papermaking surface has a 1.times.2 twill pattern.

7. The papermaker's fabric of claim 1, wherein the first yarn of each pair of stitching yarns passes over a first number of the set of top machine direction yarns and the second yarn of each pair of stitching yarns passes over a second number of the set of top machine direction yarns, and said first number differs from said second number.

8. The papermaker's fabric of claim 7, wherein said first number is larger than said second number, and wherein the second yarn of each pair of stitching yarns has a higher modulus of elasticity than the first yarn of each pair of stitching yarns.

9. The papermaker's fabric of claim 1, wherein the diameter of the yarns in the set of bottom cross machine direction yarns is less than the diameter of the yarns in the set of bottom machine direction yarns.

10. The papermaker's fabric of claim 1, wherein the diameter of the yarns in the set of bottom cross machine direction yarns is less than twice the diameter of the yarns in the set of top cross machine direction yarns.

11. The papermaker's fabric of claim 1, wherein the yarns in the set of top machine direction yarns pass over no more than one of any two adjacent yarns in the set of top cross machine direction yarns.

12. The papermaker's fabric of claim 1, wherein said pairs of adjacent stitching yarns are positioned between adjacent yarns in said set of top cross machine direction yarns.

13. The papermaker's fabric of claim 1, wherein said pairs of adjacent stitching yarns each serve as locator yarns at the points where they cross each other in entering or leaving the papermaking surface.

14. The papermaker's fabric of claim 1, wherein said pairs of adjacent stitching yarns are interwoven with said top fabric layer such that they pass over no more than two adjacent yarns in the set of top layer machine direction yarns before passing under a yarn in the set of top layer machine direction yarns.

15. The papermaker's fabric of claim 11, wherein said fabric has at least 80 picks per inch on its papermaking surface; and

wherein said fabric has a void volume of less than 55 mm.sup.3 /cm.sup.2.

16. A triple layer papermaker's fabric comprising:

a set of top machine direction yarns, a set of top, cross machine direction yarns and a set of stitching cross machine direction yarns which are interwoven to form a top fabric layer having a papermaking surface; and

a set of bottom machine direction yarns and a set of bottom cross machine direction yarns interwoven to form a bottom fabric layer having a machine side surface;

wherein each yarn in said set of bottom cross machine directions yarns is interwoven with said bottom machine direction yarns in a pattern identical to the weave pattern of an adjacent yarn in said set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine direction yarns;

wherein the yarns in the set of bottom machine direction yarns pass under no more than one of any three consecutive paired bottom fabric layer cross machine direction yarns, and

wherein the yarns in the set of bottom machine direction yarns and the paired bottom fabric layer cross machine direction yarns are interwoven in a twill pattern.

17. The papermaker's fabric of claim 16, wherein the yarns in the set of bottom machine direction yarns and the paired bottom fabric layer cross machine direction yarns are interwoven in a 1.times.4 twill pattern.

18. The papermaker's fabric of claim 16, wherein the yarns in the set of bottom machine direction yarns and the paired bottom fabric layer cross machine direction yarns are interwoven in a 1.times.3 twill pattern.

19. The papermaker's fabric of claim 16, wherein the yarns in the set of bottom machine direction yarns and the paired bottom fabric layer cross machine direction yarns are interwoven in a 1.times.2 twill pattern.

20. The papermaker's fabric of claim 16, wherein the yarns in the set of bottom machine direction yarns and the paired bottom fabric layer cross machine direction yarns are interwoven in a 1.times.5 twill pattern.

21. The papermaker's fabric of claim 16, wherein the diameter of the yarns in the set of bottom cross machine direction yarns is less than the diameter of the yarns in the set of bottom machine direction yarns.

22. The papermaker's fabric of claim 16, wherein the diameter of the yarns in the bottom set of cross machine direction yarns is less than twice the diameter of the yarns in the top set of cross machine direction yarns.

23. The papermaker's fabric of claim 16, wherein said set of stitching cross machine direction yarns includes yarns which serve as both fiber support yarns and as binder yarns.

24. The papermaker's fabric of claim 16, wherein said set of top machine direction yarns, said set of top cross machine direction yarns and said set of stitching cross machine direction yarns form a papermaking surface having single float machine direction knuckles.

25. An auto-joinable triple layer papermaker's forming fabric comprising:

a set of top machine direction yarns, a set of top cross machine direction yarns and a set of stitching cross machine direction yarns interwoven to form a top fabric layer having a papermaking surface;

a set of bottom machine direction yarns and a set of bottom cross machine direction yarns interwoven to form a bottom fabric layer having a machine side surface;

wherein at least selected yarns of said set of bottom cross machine direction yarns are woven parallel to and in an identical pattern with an adjacent yarn in the set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine direction yarns;

wherein the ratio between the number of yarns in the set of top cross machine direction yarns and the set of bottom cross machine direction yarns is approximately one-to-one; and

wherein said fabric has at least 80 picks per inch on its papermaking surface.

26. The papermaker's fabric of claim 25, wherein said set of stitching cross machine direction yarns are woven in pairs between each adjacent yarn in said set of top layer cross machine direction yarns; and

wherein said pairs of adjacent stitching yarns are woven in said fabric such that when the first yarn of the pair is weaving in the top fabric layer the second yarn of the pair is passing downwardly from said top fabric layer to interweave with said bottom fabric layer.

27. The papermaker's fabric of claim 26, wherein adjacent of said paired bottom fabric layer cross machine direction yarns are spaced apart by at least 0.1 mm.

28. A double layer papermaker's fabric, comprising:

a base fabric structure including machine direction yarns, primary top layer cross machine direction yarns and bottom layer cross machine direction yarns interlaced to form a top fabric layer and a bottom fabric layer;

a set of auxiliary top cross machine direction yarns that are interwoven with said machine direction yarns, wherein at least one auxiliary top cross machine direction yarn is positioned between each pair of adjacent primary top layer cross machine direction yarns;

wherein each yarn in said set of bottom cross machine direction yarns is woven in a pattern identical to the weave pattern of an adjacent yarn in said set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine yarns;

wherein the yarns in the set of bottom machine direction yarns pass under no more than one of any two adjacent paired bottom fabric layer cross machine direction yarns; and

wherein each bottom machine direction yarn passes under at least two paired bottom fabric layer cross machine direction yarns in a repeat of the bottom layer fabric.

29. The papermaker's fabric of claim 28, wherein said set of auxiliary cross machine direction yarns comprises first and second auxiliary top layer cross machine direction yarns positioned between each pair of adjacent primary top layer cross machine direction yarns;

wherein each first and second auxiliary top layer cross machine direction yarn passes over at least two adjacent machine direction yarns in each repeat of the fabric; and

wherein each first auxiliary top layer cross machine direction yarn has an interlacing pattern relative to said machine direction yarns that is identical to a first of said pair of adjacent primary top layer cross machine direction yarns, and wherein each second auxiliary top layer cross machine direction yarn has an interlacing pattern relative to said machine direction yarns that is identical to a second of said pair of adjacent top layer primary cross machine direction yarns, and wherein said first auxiliary top layer cross machine direction yarn is positioned between said second primary and auxiliary top layer cross machine direction yarns.

30. The papermaker's fabric of claim 29, wherein said second auxiliary top layer cross machine direction yarn is positioned between said first primary and auxiliary top layer cross machine direction yarns.

31. The papermaker's fabric of claim 28, wherein said set of auxiliary cross machine direction yarns comprises first and second auxiliary top layer cross machine direction yarns positioned between each pair of adjacent primary top layer cross machine direction yarns, and wherein said primary top layer cross machine direction yarns have a first diameter, and said auxiliary top layer cross machine direction yarns have a second diameter that is smaller than said first diameter.

32. The papermaker's fabric of claim 28, wherein each of said auxiliary top layer cross machine direction yarns passes over at least six adjacent yarns in said set of machine direction yarns before passing under a yarn in said of machine direction yarns.

33. The papermaker's fabric of claim 32, wherein each of said machine direction yarns passes over no more than two primary top layer cross machine direction yarns in a repeat of the fabric.

34. A method of making a multi-layer papermaker's fabric comprising the steps of:

interweaving at least one set of machine direction yarns, a set of top cross machine direction yarns and a set of bottom cross machine direction yarns to form a multi-layer papermaker's fabric having a papermaking surface and a machine side surface;

wherein each yarn in said set of bottom cross machine directions yarns is woven in a pattern identical to the weave pattern of an adjacent yarn in said set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine direction yarns; and

wherein the yarns comprising each of said paired bottom fabric cross machine direction yarns are woven into the fabric in successive picks.

35. The method of making a multi-layer papermaker's fabric of claim 34, wherein said multi-layer fabric is a triple layer forming fabric having a top fabric layer and a bottom fabric layer; and

wherein said set of top cross machine direction yarns includes pairs of adjacent stitching yarns that are woven in said fabric such that when the first yarn of the pair of stitching yarns is weaving in the top fabric layer the second yarn of the pair of stitching yarns is passing downwards from said top fabric layer to interweave with said bottom fabric layer.

36. The method of making a multi-layer papermaker's fabric of claim 35, wherein the diameter of the yarns in the set of bottom cross machine direction yarns is less than twice the diameter of the yarns in the set of top cross machine direction yarns.

37. A method of making paper, said method comprising the steps of:

(a) providing a triple layer papermaker's fabric comprising:

a set of top machine directions a set of top cross machine direction yarns and a set of stitching cross machine direction yarns which are interwoven to form a top fabric layer having a papermaking surface;

a set of bottom machine direction and a set of bottom cross machine direction yarns interwoven to form a bottom fabric layer having a machine side surface;

wherein each yarn in said set of bottom cross machine directions yarns is woven in a pattern identical to the weave pattern of an adjacent yarn in said set of bottom cross machine direction yarns to provide paired bottom fabric layer cross machine direction yarns; and

wherein said set of bottom machine direction yarns pass under no more than one of any two adjacent paired bottom fabric layer cross machine direction yarns,

wherein said set of stitching cross machine direction yarns comprises pairs of adjacent stitching yarns that are woven in said fabric such that when the first yarn of the pair is weaving in the top fabric layer the second yarn of the pair is passing downwardly from said top fabric layer to interweave with said bottom fabric layer;

(b) applying paper stock to said papermaker's fabric; and

(c) removing moisture from said paper stock.

38. The method of claim 37, wherein the diameter of the yarns in the bottom set of cross machine direction yarns is less than the diameter of the bottom set of machine direction yarns.

39. The method of claim 37, wherein the diameter of the yarns in the bottom set of cross machine direction yarns is less than twice the diameter of the yarns in the first set of cross machine direction yarns.