Abstract:
A double layer forming fabric for a paper making machine is woven to an overall repeating pattern, which comprises in combination paper side layer weft yarns, machine side layer weft yarns, and warp yarns. The paper side layer weft yarns interweave with the warp yarns in a first repeating weave pattern, the machine side layer weft yarns interlace with the warp yarns in a second repeating weave pattern, and the fabric is woven in an overall repeating pattern requiring 3N sheds, in which N is an integer and is at least 2. For each warp yarn, the first repeating weave pattern includes interweaving locations comprising a first and a second interweaving point, each of the interweaving points being separated by an internal warp float of at least two paper side layer weft yarns. For each warp yarn, the second repeating weave pattern includes a series of interlacing points in which each interlaced machine side layer weft yarn also passes substantially below an interweaving location of paper side layer weft yarns and each adjacent warp yarn. In addition, for each warp yarn, each of the first and second interweaving points with a paper side layer weft yarn is separated from an adjacent machine side layer interlacing point by at least two machine side layer weft yarns. The machine side face of the machine side layer includes exposed machine side layer weft yarn floats having a float length L defined as L=3N−M, wherein M is an integer and is at least 1.

Description:
FIELD OF THE INVENTION 
     This invention relates to a double layer forming fabric, consisting of a single set of warp yarns, and two layers of weft yarns, for use in the forming section of a paper making machine. 
     BACKGROUND OF THE INVENTION 
     In the forming section of a paper making machine, an aqueous stock is deposited onto the paper side surface of a moving forming fabric. The machine side surface of the forming fabric is in contact with the static fabric support elements in the forming section of the paper making machine. The forming fabric allows water to drain from the stock, and retains a proportion of the paper making solids in the stock on its surface to form an incipient paper web. 
     It has been found that the desirable characteristics for such fabrics are to a degree mutually incompatible, both in achieving an acceptable balance between the drainage, formation and retention properties of the fabric, and in other factors affecting the selection of weave patterns to achieve optimal properties for the paper side and the machine side of the forming fabric. The forming fabric must be capable of withstanding the mechanical and abrasive stresses imposed on it, which, in modern paper making machines where the forming fabric moves at a speed in excess of 70 kph, are substantial. To produce acceptable quality paper, the forming fabric should not cause marking, known as wire mark, on the sheet, and the percentage of the paper solids in the stock retained in the incipient paper web, known as first pass retention, should be as high as possible. In order to achieve a high first pass retention, the forming fabric must have good drainage characteristics and low water carrying properties, so that the removed water is readily transported through the fabric, without excessive drainage and loss of the paper solids. The fabric should also provide a uniform, planar support surface onto which the stock is delivered so that the paper making fibres are evenly supported by the component yarns of the paper side surface and the resulting sheet does not exhibit substantial variation in its fibre distribution and is stated to be “well-formed”. In addition, as a significant proportion of the fibres in the stock delivered onto the moving forming fabric tend to be oriented in the machine direction of the forming fabric, the fabric should provide adequate fibre support in the cross machine direction. 
     The need for a high drainage rate calls for a fabric with an open weave, but such a weave tends to cause wire mark and the incipient paper web tends to be formed somewhat in, rather than mostly on, the forming fabric paper side surface. A closely woven fabric provides better paper support and results in good first pass retention, and the paper is formed on, rather than somewhat in, the fabric, and is thus easier to release from the forming fabric. However, a closely woven fabric drains relatively poorly. 
     It has been found that improved drainage and fibre support characteristics can be achieved by ensuring that the frame openings in the paper side layer are substantially regular, and if the openings are rectangular, it is preferable that the longer side be oriented in the cross-machine direction. However, a related factor is the undesirable effect of forces which tend to induce adjacent pairs of weft yarns to move closer together, creating an asymmetry, known as “twinning”. This reduces alignment and registration of the paper side and machine side yarns, and the resulting different sized drainage passages adversely affect paper quality. Various methods have been suggested to resolve this problem. 
     Wilson, in U.S. Pat. No. 6,112,774, suggests that twinning results from excessive tension in machine direction yarns where those yarns interlace in the machine side layer with the cross-machine direction yarns in an “under 1, over 1, under 1” configuration, for example in the zig-zag -machine side layer weave pattern disclosed by Wright in U.S. Pat. No. 5,025,839. Wilson discloses a weave pattern which maintains the zig-zag pattern of Wright for the machine side layer, but suggests an arrangement of alternating machine direction yarns, in which the machine side layer interlacing points on adjacent machine direction yarns are offset by at least two cross machine direction yarns, as a means of reducing tension in the machine direction yarns. 
     Nevertheless, it has been found that twinning of paper side layer weft yarns continues to occur in weave patterns such as disclosed by Wilson in U.S. Pat. No. 6,112,774. Wilson further suggests, in WO 01/59208, that cross-machine direction yarns can be maintained in their original positions, i.e. that twinning can be reduced, by the use of suggested preferred materials for the manufacture of the machine direction yarns. These materials are said to encourage crimping, particularly where the machine direction yarns interweave with the cross machine direction yarns in an “over 1, under 1, over 1, under 1, over 1” configuration. 
     However, it has recently been found that twinning of paper side layer weft yarns adjacent to interweaving points in that layer can be avoided, or significantly reduced, in a double layer fabric, by using weave patterns which do not involve the close proximity of interweaving points on adjacent paper side layer warp yarns. This advantage is further enhanced where the weave pattern additionally does not involve the close proximity on a single warp yarn of the last one of a series of interweaving points in the paper side layer and an immediately adjacent interlacing point in the machine side layer. It has thus been found that the undesirable twinning effect can be significantly reduced by providing a weave pattern which maximizes the distance between interweaving points in the paper side layer on adjacent warp yarns, while increasing the internal float length of the warp yarns between the interweaving points on the paper side layer and the interlacing points on the machine side layer. 
     The degree of twinning of adjacent yams can be described in terms of the ratio of the difference of the distance (W) between one of a specific twinned pair of yarns and the adjacent non-twinned yarn, and the distance (T) between the twinned yam pair, to the distance W. This can be expressed as the ratio (W−T):W; or as a percentage (W−T)/W×100. 
     In a fabric with minimal twinning, this ratio would approach 0:1, or 0%; whereas in a highly twinned fabric, this ratio can be as high as 1:2, or 50%. It has been found for the fabrics of this invention that the ratio can be reduced to at least 0.1:1, or 10%, and more preferably can be reduced to between 0.05:1 and 0:1, or 5% to 0%. 
     The reduction of the twinning of the paper side weft yarns, together with the fact that all of the paper side layer weft yarns contribute to the support of the paper making fibres, leads to a greater regularity in the frame openings on the paper side surface of the paper side layer, and hence to a corresponding greater uniformity in the fibre support. It is well known that the overall frame size and the frame length in the machine direction are important parameters in the design of forming fabrics, and these topics are discussed by Helle, Torbjorn, “Fibre Web. Support of the Forming Wire”,  Tappi Journal , Vol. 71, No. 1 (January 1988), pp. 112–117; and Johnson, D. B., “Retention and Drainage of Forming Fabrics”,  Pulp  &amp;  Paper Canada,  Vol 85, pp. T167–172 (1984). The authors indicate that frame opening configurations have a significant influence on the drainage of the incipient paper web, and on the first pass retention characteristics of the forming fabric. It has been found that greater cross machine direction support is achieved by the use of designs having rectangular frame openings. 
     It has previously been considered that drainage problems in double layer forming fabrics result from the use of weave patterns requiring more than 8 sheds in the loom. For example, one aspect of such problems is noted in CPPA data sheet No. G18 (Rev. November 1994), at page 9. However, it has been found that suitable weave patterns can be created using designs requiring 9 sheds or more, with advantageous results, and without the expected disadvantages. 
     Consequently, it has been found that the lengths of the exposed floats of the machine side layer weft yarns on the machine side surface of the machine side layer in a double layer forming fabric can be increased. The resultant increased volume of weft material which is subjected to the abrasive forces of the machine can significantly extend the operational life of the forming fabric. 
     The present invention therefore seeks to provide a double layer forming fabric for a paper making machine, having increased resistance to machine side layer wear and abrasion. The invention provides for relatively long machine side layer weft yarn floats in the machine side surface, which are exposed to the abrasive wear experienced by the forming fabric as it is running in contact with the various stationary and moving elements in the forming section of the paper making machine. The invention also enables the use of larger diameter weft yarns than have previously been found feasible for use in double layer forming fabrics. 
     The present invention also seeks to provide a double layer forming fabric having an improved balance between water drainage and paper solids retention. The invention provides substantially rectangular paper side layer frame openings, having substantially the same width in the machine direction. The regular spacing of the yarns forming the perimeters of the frame openings provides a high degree of uniformity of support for the paper making fibres, so that the resulting sheet has a substantially uniform appearance and structure. 
     The present invention still further seeks to provide a double layer forming fabric having a weave pattern which produces a substantial reduction in the twinning of the paper side layer weft yarns. 
     SUMMARY OF THE INVENTION 
     The present invention provides a double layer forming fabric for a paper making machine, woven to an overall repeating pattern, which comprises in combination
         (a) paper side layer weft yarns;   (b) machine side layer weft yarns; and   (c) warp yarns,   wherein
           (i) the paper side layer weft yarns interweave with the warp yarns in a first repeating weave pattern;   (ii) the machine side layer weft yarns interlace with the warp yarns in a second repeating weave pattern;   (iii) the fabric is woven in an overall repeating pattern requiring 3N-sheds, in which N is an integer and is at least 2;   (iv) for each warp yarn, the first repeating weave pattern includes interweaving locations comprising a first and a second interweaving point, each of said first and second interweaving points being separated by at least two paper side layer weft yarns;   (v) for each warp yarn, the second repeating weave pattern includes a series of interlacing points in which each interlaced machine side layer weft yarn also passes substantially below an interweaving location of paper side layer weft yarns and each adjacent warp yarn;   (vi) for each warp yarn, each of the first and second interweaving points with a paper side layer weft yarn is separated from an adjacent machine side layer interlacing point by at least two machine side layer weft yarns; and   (vii) a machine side surface of the machine side layer includes exposed machine side layer weft yarn floats having a float length L defined as L=3N−M, wherein M is an integer and is at least 1.   
               

     In the double layer forming fabrics of this invention, each warp yarn is intrinsic to the weave pattern in both the paper side layer and the machine side layer of the fabric, so that each warp yarn contributes to the structural integrity and properties of both layers, particularly in relation to consistency in the paper support, and allows for long weft yarn floats in the machine side layer, thus increasing the operational life of the fabric. 
     Furthermore, in the double layer forming fabrics of this invention, each warp yarn follows an identical path, the weave pattern for each warp yarn being displaced from the weave pattern of adjacent warp yarns by an identical predetermined number of paper side layer and machine side layer weft yarns. Within each pattern repeat, the warp yarn path includes interweaving locations comprising pairs of interweaving points with the paper side layer weft yarns, and interlacing points with the machine side layer weft yarns, such that the interlacing points are approximately centralized between the second interweaving point of a preceding interweaving location and the first interweaving point of the next succeeding location. The displacement distance of one pair of interweaving points of one warp yarn from the preceding pair of interweaving points of the immediately preceding warp yarn, measured in terms of the predetermined number of paper side layer and machine side layer weft yarns, is selected so that the machine side interlacing points on one warp yarn are located approximately beneath a portion of an interweaving location on each adjacent warp yarn. 
     In addition, the warp yarns are arranged so that each warp yarn interlaces with the same machine side layer weft yarn as also interlaces with either the second preceding or second subsequent warp yarn. This enables the use of designs including relatively long external weft floats in the machine side surface of the machine side layer, by providing stability to the long weft floats which, by the increased wear volume in the machine side layer, contribute to the desired increased operational life of the fabric. Such designs include substantially regular frame openings on the paper side surface, which provides greater uniformity of the paper support. 
     Preferably, in all embodiments, each warp yarn has an internal float, between the machine side layer and the paper side layer, passing over at least two machine side layer weft yarns between each interlacing point with a machine side layer weft yarn and the immediately preceding and subsequent interweaving point in the paper side layer. 
     Preferably, the ratio of the number of paper side layer weft yarns to the number of machine side layer weft yarns is chosen from between 2:1 and 1:1, and more preferably the ratio is 2:1. 
     Preferably, the paper side layer weave pattern is selected from a satin weave design, or a twill or a broken twill weave design. More preferably the paper side layer weave pattern is a 3N×6N design, where 3N is the number of sheds. Where the ratio of the number of paper side layer weft yarns to the number of machine side layer weft yarns is 2:1, the machine side layer weave pattern would thus preferably be a 3N×3N design. 
     In the following description and claims, certain terms have the following meanings: 
     The term “paper side layer” refers to the layer of weft and warp yarns in the double layer forming fabric onto which the stock is deposited, and the associated term “paper side surface of the paper side layer” refers to the exposed surface of the paper side layer which directly supports the incipient paper web. 
     The term “machine side layer” refers to the layer of weft and warp yarns in the double layer forming fabric which is in contact with the support means of the paper making machine, and the associated term “machine side surface of the machine side layer” refers to the exposed surface of the machine side layer which is in direct contact with the stationary and rotating elements of the machine. 
     The term “machine direction” or “MD” refers to a line parallel to the direction of travel of the forming fabric when in use on the paper making machine, and the associated term “cross machine direction” or “CD” refers to a direction transverse to the machine direction. 
     The term “frame” refers to the substantially rectangular area defined by the longitudinal axis of four interwoven yarns in the paper side surface of the paper side layer of a forming fabric. The associated term “frame size” refers to the size determined by measurement from four selected yarns which define in plan view a distinct frame. This term is synonymous with the term “top surface open area” as used in CPPA Data Sheet No. G18 (Rev. Nov. 1994), at page 3. The associated term “frame opening” refers to the actual open area in between the yarns within a given frame in the paper side surface of the paper side layer of the fabric. 
     The term “fibre support index” refers to a calculation made according to the method described by Beran and summarized in CPPA data sheet No. G-18 (Rev. November 1994) at page 4; it provides an indication of the level of support given to the incipient paper web by the forming fabric. The method is further detailed in Helle, Torbjorn, “Fibre Web Support of the Forming Wire”,  Tappi Journal , supra at p. 115. 
     The term “interlace” refers to a locus at which a specific warp yarn wraps about a machine side layer weft yarn; the associated term “interweave” refers to a locus at which a specific warp yarn wraps about a paper side layer weft yarn. 
     The term “float” refers to a yarn which passes over a group of other yarns without interweaving or interlacing with them; the associated term “float length” refers to the length of a float, which can be expressed as a number indicating the number of yarns passed over. 
     The term “internal float” refers to a float which passes between the adjacent surfaces of the machine side layer and the paper side layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of reference to the drawings, in which: 
         FIGS. 1 to 18  inclusive are sequential schematic cross-sectional views of a first embodiment of the invention, showing the paths of each successive warp yarn in one repeat of the forming fabric weave pattern; 
         FIG. 19  depicts the paper side layer of the first embodiment of the invention; 
         FIG. 20  depicts the machine side layer of the first embodiment of the invention; 
         FIG. 21  is a weave diagram of the first embodiment of the invention; 
         FIG. 22  is a weave diagram of a second embodiment of the invention, also showing the path of one warp yarn in one repeat of the forming fabric weave pattern; and 
         FIG. 23  is a weave diagram of a third embodiment of the invention, also showing the path of one warp yarn in one repeat of the forming fabric weave pattern. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIGS. 1 to 18 , these figures taken together show the path of each of eighteen single successive warp yarns  140  of the overall fabric repeat pattern of a first embodiment of the forming fabric  100  of the invention. The warp yarns  140  in consecutive figures are identified consecutively as warp yarn A, warp yarn B, warp yarn C up to and including warp yarn R. In each of  FIGS. 1 to 18 , the weft yarns  120  in the paper side layer  102  are shown in cross-section as the upper layer, and the weft yarns  130  in the machine side layer  104  are shown in cross-section as the lower layer. The two sets of weft yarns  120  and  130  are numbered from 1 to 54. 
     It can be seen that each warp yarn  140  follows an identical path, forming in one repeat of the paper side layer  102  weave pattern two interweaving locations  105  and  107 , each comprising two interweaving points  106  and  108 , and  110  and  112  ( FIG. 1 ), and in one repeat of the machine side layer  104  weave pattern two interlacing points  114  and  116 , shown for warp yarn A as  114   a ,  116   a , for warp yarn B as  114   b ,  11 , 6   b  etc. In each repeat, reading from the left of the figures, interlacing point  114  follows interweaving point  108  and precedes interweaving point  110 . 
     Referring to the path of warp yarn A shown in  FIG. 1 , at each interweaving location  105 , each pair of interweaving points  106 ,  108  and  110 ,  112  respectively is separated by, and thus forms an internal warp float of, two paper side layer weft yarns  120 , seen in  FIG. 1  as paper side layer weft yarns  2  and  4 , and  29  and  31 . 
     Referring to  FIGS. 1 ,  2  and  3 , showing the paths of warp yarns A, B and C respectively, it will be seen that machine side layer weft yarn  3 , which interlaces with warp yarn B at interlacing point  116   b , also passes directly under the first interweaving location  105  of warp yarn A, which occurs at paper side layer weft yarns  1  and  5 , and passes substantially under the first interweaving location  107  of warp yarn C, which occurs at paper side layer weft yarns  4  and  8 . 
     Similarly, machine side layer weft yarn  33 , which interlaces with warp yarn B at interlacing point.  114   b , passes directly under the second interweaving location  105  of warp yarn C, which occurs at paper side layer weft yarns  31  and  35 , and also passes substantially under the second interweaving location  107  of warp yarn A, which occurs at paper side layer weft yarns  28  and  32 . 
     It can further be seen, referring to  FIGS. 1 to 18 , that for each of warp yarns A, B, C, and each succeeding warp yarn D to R, each first interlacing point  114   a ,  114   b ,  114   c  etc. is separated from both the immediately preceding interweaving point  108  and each succeeding interweaving point  110  on the same warp yarn by at least three machine side layer weft yarns  130 . For example, in  FIG. 1 , first interlacing point  114   a  at machine side layer weft yarn  18  is separated from first interweaving point  108  at paper side layer weft yarn  5  by machine side layer weft yarns  6 ,  9 ,  12  and  15 , and from second interweaving point  110  at paper side layer weft yarn  28  by machine side layer weft yarns  21 ,  24  and  27 . Similarly, each second interlacing point  116   a ,  116   b ,  116   c  etc. is separated from the immediately preceding interweaving point  112  and each succeeding interweaving point  106  by at least three machine side layer weft yarns  130 . 
     Still referring to  FIGS. 1 to 18 , it will further be noted that each machine side layer weft yarn  130  has an external float length in the machine side surface of the machine side layer of 15 warp yarns  140 . For example, the machine side layer weft yarn  18  has an interlacing point  114   a  with warp yarn A, and a second interlacing point  116   c  with warp yarn C, but has no further interlacing points in the machine side layer weave pattern repeat, thus passing below and on the machine side of each of the fifteen warp yarns D to R. Similarly, machine side layer weft yarn  42  has an interlacing point  116   a  with warp yarn A, and a second interlacing point  114 q with warp yarn Q, but no further interlacing points in the machine side layer weave pattern repeat, thus passing below and on the machine side of each of the fifteen warp yarns B to P. 
     It can further be seen that for any group of three adjacent warp yarns  140 , in one repeat of the overall weave pattern, the first and third warp yarns  140  each interlace once, i.e., at either interlacing point  114  or interlacing point  116 , but not both, with a common machine side layer weft yarn  130 . Thus warp yarns A and C at their respective interlacing points  114   a  and  116   c  are separated by warp yarn B. Similarly, warp yarns Q and A at their respective interlacing points  114   q  and  116   a  are separated by warp yarn R. The effect of this aspect of the second repeating weave pattern can be seen in  FIG. 20 , where interlacing point  114  is indicated. 
     One result of this pattern of pairs of interlacing points  114  or  116  is an increase in the crimp differential of the machine side layer weft yarns  130  at these points, which causes them to bow outwards away from the machine side surface of the machine side layer  104 , thus increasing their prominence. This results in an increase in the available wear volume of the machine side layer weft yarns  130  exposed to abrasion, thus increasing the operational life of the fabric. 
     Still referring to  FIGS. 1 to 18 , it will be seen that each successive warp yarn  140  follows an identical path, the pattern of which is displaced from the pattern of the immediately preceding warp yarn  140  by the same number of paper side layer weft yarns  120 , and the same number of machine side layer weft yarns  130 . For example, referring to  FIGS. 1 to 4 , the first interweaving point  106  of warp yarn A is with paper side layer weft yarn  1 , and the first subsequent interweaving point  106  of warp yarn B is with paper side layer weft yarn  16 . The first subsequent interweaving point  106  of warp yarn C is with paper side layer weft yarn  31 , and the first subsequent interweaving point  106  of warp yarn D is with paper side layer weft yarn  46 . Thus in this first embodiment, the displacement can be seen to comprise  10  paper side layer weft yarns  120 , the subsequent interweaving point  106  being on the tenth paper side layer weft yarn  120  from the interweaving point  106  on the preceding warp yarn  140 . Similarly, the displacement also comprises five machine side layer weft yarns  130 , each interlacing point  114  or  116  being on the sixth machine side layer weft yarn  130  from the respective interlacing point  114  or  116  on the preceding warp yarn  140 . 
     It can further be seen from  FIGS. 1 to 18  that each interlacing point  114  in the machine side layer  104  is located respectively substantially below a central location  115  in the paper side layer  102  between the second interweaving point  108  and the next following interweaving point  110 . Similarly, each interlacing point  116  in the machine side layer is located substantially below a central location between the second interweaving point  112  and the next following interweaving point  106 . In the embodiment shown in  FIGS. 1 to 18 , the first central location  115  is separated from interweaving point  108  by eight paper side layer weft yarns  120 , and from interweaving point  110  by six paper side layer weft yarns  120 . The second central location  117  is separated from interweaving point  112  by six paper side layer weft yarns  120 , and from the next following interweaving point  106  by eight paper side layer weft yarns  120 . This arrangement of interlacing and interweaving points is constant for each of the warp yarns A to R in  FIGS. 1 to 18 . 
     Still referring to  FIGS. 1 to 18 , it can further be seen that in the repeating weave pattern of the paper side layer  102 , the interweaving points are aligned so that for each interweaving location  105  or  107  on a selected warp yarn  140 , comprising a pair of interweaving points  106 ,  108  or  110 ,  112 , one interweaving point on each of the second preceding and second subsequent warp yarns  140  is located on a paper side layer weft yarn  120  between the paper side layer weft yarns with which the selected warp yarn  140  interweaves. For example, considering warp yarn C in  FIG. 3  as being the selected warp yarn  140 , the second interweaving location  105  comprises interweaving points  106  and  108  at paper side layer weft yams  31  and  35  respectively. The second preceding warp yarn  140  would be warp yarn A ( FIG. 1 ), which has an interweaving point  108  with paper side layer weft yarn  5 . The second subsequent warp yarn  140  is warp yarn E ( FIG. 5 ), which has an interweaving point  106  with paper side layer weft yarn  7 . Similarly for the second interweaving location  107  on warp yam C ( FIG. 3 ), the interweaving points  110 ,  112  are with paper side layer weft yarns  31  and  35 . The corresponding interweaving point  112  on warp yarn A ( FIG. 1 ) is with paper side layer weft yarn  32 , and the corresponding interweaving point  110  with warp yam E ( FIG. 5 ) is with paper side layer weft yarn  34 . A similar pattern can be identified in considering the interweaving points  106 ,  108  and  110 ,  112  on each warp yarn  140 . 
     It can further be seen from  FIG. 21  that this spatial relationship of interweaving points  106 ,  108 , and  110 ,  112  on successive alternate warp yarns  140  comprises a series of substantially rhomboid bracing zones  142 , of identical configuration. Two examples are shown in  FIG. 21 , in which warp yarns  1 ,  3  and  5  correspond with warp yarns A, C and E in  FIGS. 1 ,  3  and  5 . 
     The effect of these bracing zones  142  is to provide a bracing effect on the paper side layer weft yarns  120  at each interweaving location  105  and  107 , which has been found to have the advantage of further reducing any tendency to twinning of pairs of paper side layer weft yarns  120 . 
     As discussed above, the degree of twinning of pairs of yarns in the fabrics of the present invention can be reduced so that the ratio of the distance between twinned yarns and adjacent non-twinned yarns is less than 0.1:1, or 10% and is preferably between 0.05:1 and 0:1, or 5% to zero. 
     Referring to  FIGS. 1 to 18 , and  FIG. 21 , and as already noted above, the first embodiment thus comprises a forming fabric  100  having an overall repeating pattern requiring eighteen sequential warp yarn paths, and having a first repeating weave pattern, in the paper side layer  102 , comprising  36  paper side layer weft yarns  120 . The second repeating weave pattern, in the machine side layer  104 , over the same distance comprises  18  machine side layer weft yarns  130 . Thus the forming fabric of this embodiment can be seen as having a first repeating weave pattern of 3N by 6N, and a second repeating weave pattern of 3N by 3N. For the fabric of this embodiment, it can thus be seen that 3N is 18, and N=6. 
     In the first embodiment, shown in  FIGS. 1 to 18  and  21 , the ratio of the paper side layer weft yarns  120  to the machine side layer weft yarns  130  is 2:1. 
     The machine side layer weft yarns  130  are not necessarily of the same diameter as, and are preferably of a larger diameter than, the paper side layer weft yarns  120 . Wilson, in U.S. Pat. No. 6,112,774, suggests that each CD yarn in the machine side layer may require to be substantially aligned with a CD yarn in the paper side layer. However, it has been found that although the 18 machine side layer weft yarns  130  occupy the same distance in the machine direction as the 36 paper side layer weft yarns  120 , none of the machine side layer weft yarns is required to be aligned specifically with any of the paper side layer weft yarns  120 . 
     Referring to  FIG. 19 , showing the paper side surface of the paper side layer  102 , it can be seen that the first repeating weave pattern results in regular frame openings  150 . As discussed above, this feature has been found to contribute to improved drainage properties of the paper side layer of a double layer forming fabric. It can further be seen that the substantially rectangular openings  150  are to some extent longer in the CD than in the MD. As discussed above, this feature contributes to CD support of the paper making fibres, which are predominantly MD oriented in the incipient paper web. The Beran&#39;s “b” figure used in the calculation of the fibre support index, as determined by the method described in the CPPA Data Sheet, noted above, for the fabrics of this invention is at least 0.8, and is more preferably between 0.8 and 1.0, and most preferably is 1.0, indicating that all of the paper side layer weft yarns  120  contribute to supporting the papermaking fibres. 
     Further referring to  FIG. 19 , a typical interweaving location  105 , of paper side layer weft yarns  120  and warp yarns  140 , comprises interweaving points  106  and  108 . A bracing zone  142  is also shown. 
     Referring to  FIG. 20 , showing the machine side surface of the machine side layer  104 , the interlacing points  114  and  116  of machine side layer weft yarns  130  and warp yarns  140  can be seen. By following the path of each warp yarn  140  on either side of an interlacing point  114  or  116 , it can further be seen that each two warp yarns  140  which appear to be adjacent at their respective interlacing points  114  and  116  are in fact separated by a third warp yarn  140 . 
     A second embodiment of the double layer forming fabric of the invention is shown in  FIG. 22 . In this embodiment, the paper side layer  102  and the machine side layer  104  are each woven to a 9-shed satin weave pattern, for which N=3. The weave diagram of  FIG. 22  shows one repeat in the MD and two repeats in the CD of both the paper side layer and machine side layer weave patterns. As can be more clearly seen from the diagram showing the path of one warp yarn  140 , in each repeat of the repeating weave pattern in the paper side layer  102 , there is a single interweaving location  105 , at which each pair of interweaving points  106 ,  108  is separated by two paper side layer weft yarns  120 . For example warp yarn  1  interweaves with paper side layer weft yarns  1  and  5 , which are separated by paper side layer weft yarns  2  and  4 . However, in this embodiment, at each interlacing point  114 , each warp yarn  140  interlaces with an adjacent pair of machine side layer weft yarns, shown in the warp yarn path diagram of  FIG. 22  as machine side layer weft yarns  15  and  18 . 
     It can further be seen from  FIG. 22  that for any three warp yarns  140 , the first and third warp yarn  140  interlace with a common machine side layer weft yarn  130 . Thus, for example, warp yarn  1  interlaces with machine side layer weft yarns  15  and  18 , and warp yarn  3  interlaces with machine side layer weft yarns  18  and  21 . Similarly, warp yarn  2  interlaces with machine side layer weft yarns  3  and  6 , and warp yarn  4  interlaces with machine side layer weft yarns  6  and  9 . As has already been noted in relation to the first embodiment, this pattern of double interlacing points  114  has been found to increase the crimp differential of the machine side layer weft yarns  130 , causing them to become more prominent on the machine side surface of the machine side layer and, together with the effects of the longer float lengths of the machine side layer weft yarns  130 , results in a corresponding increase in the operational life of the fabric. 
     It has been found that the interlacing of each warp yarn  140  with two adjacent machine side layer weft yarns  130  in this embodiment provides the additional advantage that a larger diameter yarn can be used for the machine side layer weft yarns, which can further increase the operational life of the fabric. 
     In this embodiment, in a similar manner to the first embodiment, the repeating weave pattern in the paper side layer  120  also includes bracing zones  142 . For example, again referring to  FIG. 22 , warp yarn  3  interweaves with paper side layer weft yarns  4  and  8 , warp yarn  1  interweaves with paper side layer weft yarn  5 , and warp yarn  5  interweaves with paper side layer weft yarn  7 . 
     A third embodiment of the double layer forming fabric of the invention is shown in  FIG. 23 . In this embodiment, the paper side layer  102  and the machine side layer  104  are each woven to a 9-shed satin weave pattern, for which N=3. The weave diagram of  FIG. 23  shows one repeat in the MD and two repeats in the CD of both the paper side layer and machine side layer repeating weave patterns. In this embodiment, in each repeating weave pattern in the paper side layer  102 , there is a single interweaving location  105 , at which each pair of interweaving points  106 ,  108  is separated by two paper side layer weft yarns  120 . For example, in  FIG. 23 , warp yarn  1  interweaves with paper side layer weft yarns  2  and  6 , which are separated by paper side layer weft yarns  3  and  5 . 
     In this embodiment, in a similar manner to the first and second embodiments, the repeating weave pattern in the paper side layer  120  also includes bracing zones  142 . For example, again referring to  FIG. 23 , warp yarn  3  interweaves with paper side layer weft yarns  5  and  9 , warp yarn  1  interweaves with paper side layer weft yarn  6 , and warp yarn  5  interweaves with paper side layer weft yarn  8 . It can be seen that the paper side layer  102  presents a uniform support surface for the incipient web, and has a fibre support index of approximately 1. 
     In this embodiment, the pattern of interlacing points  114  differs from that of the first two embodiments in that it does not include the interlacing of each of a first and third warp yarns  140  with a common machine side layer weft yarn  130 . The pattern of this embodiment may require a somewhat reduced maximum diameter which can be used for the machine side layer weft yarns  130  than can be used for the first or second embodiments. However, any restriction on the extended operational life of the fabric can be offset by the increased wear potential which is derived from the float lengths of 8 for the machine side layer weft yarns  130 . 
     The warp yarns  140  can be made of any suitable polymer material, and preferably have a substantially circular cross-section, although oval, elliptical and other geometric shaped cross-sections may be used. The dimensions of the warp yarns  140 , the paper side layer weft yarns  120  and the machine side layer weft yarns  130  can be selected depending on factors including the intended end use, particularly the intended paper grade. 
     Experimental fabrics woven according to the various embodiments of the invention utilized machine side layer weft yarns  130  having a circular cross-section, and a diameter of 0.45 mm. These were either polyethylene terephthalate (PET), or alternating polyester and nylon-6 or nylon-6/6. Wear resistant yarns comprised of polymer blends of PET and thermoplastic polyurethane such as are disclosed by Bhatt et al, in U.S. Pat. No. 5,502,120, were also found effective in increasing the wear potential of the forming fabric of the invention. Yarn diameters ranging from 0.40 mm to 0.50 mm have been found to provide satisfactory results. 
     For the paper side layer weft yarns  120 , a PET polyester was used having a circular cross-section and a diameter of 0.26 mm, but the results suggest that a range of 0.17 mm to at least 0.26 mm would give satisfactory results. 
     For the warp yarns  140 , high modulus yarns were found to be particularly suitable, such as those comprised of polyethylene naphthalate (PEN). These yarns have a circular cross-section and a diameter ranging from 0.20 mm to 0.25 mm. Yarns made from these materials tend to retain their crimp particularly well following weaving and heatsetting, and the resulting fabrics exhibit a reduced propensity to stretch. Due to their high modulus, it is possible to use smaller yarns than comparable yarns of PET, while retaining comparable physical properties. This provides the possibility of using warp yarns  140  of PEN to reduce the warp fill and thus allow for more rapid drainage of water from the incipient web, if this is desired in a particular situation. 
     Those of skill in the art may vary the yarn sizes and materials used in the fabrics of the invention so as to accommodate the prevailing conditions and parameters of use in the particular paper making machine. 
     The fabrics of the invention will generally be woven flat, and subsequently cut and seamed in order to provide the required endless loop of fabric.