PAPERMAKER'S WET PRESS FELT HAVING MULTI-LAYERED BASE FABRIC

Abstract: A papermaker's multilayer fabric having at least top and bottom layers of machine direction yarns which are inter­ woven with cross machine direction yarns systems to estab­ lish a first interwoven fabric layer which is bound to a bottom layer fabric with the two being united by interweaving cross machine direction yarns from the bottom fabric layer in the upper fabric layer. The effective pressing surface area of the first fabric layer being equal to or greater than (x + 1) + (0.5y). Wherein (x + 1) defines the contribution of the upper fabric layer to the effective pressing surface area and the contribu­ tion of the binding yarn is defined by the y factor of the equa­ tion. Accordingly, the total effective pressing area is defined by the contributions of the upper ply and the binding yarns.

Description: Background of the Invention This invention relates to papermaker's fabrics and in particular to fabrics, generally known as wet press felts or wet felts which are used in the wet press section of a papermaking machine. Papermaker's wet felts are desired to transport an aqueous-partially formed web of paper through the press rollers in the wet press section of a papermaking machine and to assist in the further dewatering thereof. In its most common form, a papermaker's wet felt is constructed from a woven base fabric having fibrous batts needled to one or both sides. The amount of void volume within the base fabric of a wet felt and its dewatering ability is directly related to the amount of water which can be internally handled by the felt construction while in the press nip. In other words, felts which can run with a "dry nip" (no water puddling behind the nip) are less likely to result in crushing or other hydraulic phenomena which are known to those skilled in the art. It has been recognized in the art that it is possible to maintain a desired controlled void volume within the fabric construction by employing multi-layered base fabrics. One example of such a felt is disclosed in U.S. Patent No. 4,356,225 which is assigned the assignee of the present invention. The fabric of the '225 patent is disclosed as having increased stability while maintaining the machine direction layers in vertical alignment. Other examples are also cited in that patent. The surfaces of the base fabric of conventional fabrics are predominantly defined by the top and bottom layers of machine direction yarns. The cross machine direction yarns which interweave the multiple layers of machine direction yarns of such fabrics protrude beyond the surfaces of the base fabric with sharp infrequent knuckles. It has been discovered that under the intense pressure of the nip, water removal can be impaired by the extremes of high and low pressure caused by the cross machine direction yarn knuckles on the paper bearing side of the fabric. Also, on the other or machine side of the fabric, the knuckles represent high pressure points which result in accelerated wear of the fabric. It has been recognized in the art that it is possible to achieve improved pressing service and machine surface contact while maintaining controlled void volume within the fabric construction of multi-layered base fabrics. One example of such a felt is disclosed in U.S. Patent 4,461,803 which is assigned to the assignee of the present invention. The fabric of the '803 patent comprises a multi-layered base fabric having both a smooth pressing surface and a roller surface resistant to wear in which the cross machine direction yarns define the predominate surfaces of the base fabric without creating sharp knuckles on either surface of the base fabric. However, in the '803 construction the binder yarns extend between the top layer and the bottom layer of the fabric as machine direction yarns extend under or above the respective layer, and do not contribute to the pressing pressure points of the water removal surface, the top layer, or the machine roller contact surface, the bottom layer. It has been recognized in the art that it is possible to utilize two separate fabric layers and to join the two fabric layers by means of an independent binder or a binder system comprised of threads from one of the fabric layers. One example of such a felt is disclosed in U.S. Patent 3,214,326. The fabric of the '326 which utilizes binder threads from one of the fabric layers results in the binder threads reducing the sheet contact surface area rather than in increasing the sheet surface contact area. The '326 construction using individual binder threads need not result in such a reduction of the sheet contact area, however, the binder threads do not contribute to the sheet surface contact area. The present invention provides a papermaker's wet felt for use in the wet press section of a papermaker's machine. The disclosed papermaker's felt comprises a multi-layered base fabric having cross machine yarns which interweave with the multiple layers of machine direction yarns such that two fabrics layers are formed and united with predominantly the cross machine direction yarns defining the top and bottom surfaces of the base fabric. The cross machine direction yarns are woven in a repeat pattern having floats which extend above the top layer of machine direction yarns so as to define the surfaces of the base fabric. Additionally, the bottom cross machine direction system is used to bind the two fabric layers together. Such a weave is illustrated in Figure 3 of EP-A-0 239 207 which is relevant under Article 54(3) EPC. In the present invention, as defined in claim 1, the binding yarn complements the weave pattern to increase sheet contact area as will be later explained in greater detail. It is an object of the present invention to provide a papermaker's wet felt comprising a multi-layered base fabric having both a smooth pressing surface with increased paper contact and a machine roller side surface resistant to wear. In particular, it is an object of the invention to provide a system of cross machine direction yarns which interweave multiple layers of machine direction yarns without creating sharp knuckles on either surface of the base fabric of the wet felt. The present invention will become further apparent from the following description, with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram of a papermaker's wet press felt embodying the present invention; Figure 2 is a top plan view schematic diagram of the fabric of Figure 1; Figure 3 is a perspective schematic view of the weave of the upper layer of the fabric according to the present invention; and Figure 4 is a perspective schematic view of the bottom layer fabric according to the present invention. With reference to Figure 1, there is shown an illustrative section cut in the cross machine direction of a fabric embodying the invention which illustrates a weave which may be improved by the present invention as will be described later with reference to Figures 3 to 6. Machine direction yarns are numbered as 1 to 8 and cross machine direction yarns are numbered as 9 to 16. All yarns which weave in the upper ply are odd numbered yarns and they weave so as to independently form a complete fabric layer. All yarns which weave primarily in the lower ply are even numbered yarns. As can be seen from Figure 1, the machine directions yarns 1 to 8 are substantially vertically aligned and each of the yarns is surrounded and retained in position by a number of cross machine direction yarns. Such a weave pattern contributes to the stability of the fabric and improved void volume control of the final fabric. CMD yarn 9 weaves under MD yarn 1 and over MD yarns 3, 5 and 7. CMD yarn 10 weaves over MD yarns 1 and 2 and under MD yarns 4, 6 and 8. CMD yarn 11 weaves over MD yarns 7, 1 and 3 and under MD yarn 5, CMD yarn 14 weaves over MD yarns 7 and 8 and under MD yarns 2, 4 and 6. CMD yarn 15 weaves over MD yarns 7, 1 and 5 and under MD yarn 3. CMD yarn 16 weaves under MD yarns 2, 4 and 6 and weaves over MD yarns 3 and 4. It will be observed that the weave repeats of the CMD yarns in the upper and lower ply are substantially mirror images. As can be seen from the above described weave, the CMD yarns will always have a weave repeat which produces a float on the upper and lower surface planes of the fabric of at least two yarns and preferably at least three MD yarns in length. The preferred weave pattern is generally referred to as a 3/1 weave pattern. Likewise, it will be seen that the upper ply will weave in the reversed direction of the bottom ply. It will also be recognized that float lengths of more than three may be used in view of the design application. With reference to Figure 2, there is shown an exploded schematic top plan view of a fabric as woven in accordance with the description of Figure 1. It will be recognized by those skilled in the art that Figure 2 is illustrative of the weave pattern which has been previously described. By reference to Figure 2, it can be seen that the binding effect accomplished by CMD yarns 10, 12, 14 and 16 are such that each of the yarns will produce a knuckle on the surface plane on the upper layer of the fabric. Through the utilization of such a binding construction, it is therefore possible to supplement the cross machine direction yarns which appear on the upper ply surface by approximately 25%. As will be recognized by those skilled in the art, a 3/1 construction must by definition eliminate approximately 20% to 25% of the cross machine direction yarns from effecting the surface pressing area of the top ply. However, by placing the binder pick from the CMD lower ply yarns in the upper ply surface, it is possible to substantially replace the missing contact surface so as to achieve a top surface plane approximating 100% contact from the CMD yarns. The above relationship of the effective pressing area of the upper ply surface may be expressed generally by the equation, EPA = (x + 1) + (at least .5y) , where both x and y are always at least one. In general, the contribution of the y yarn can never be more than 90% and will generally be greater than 50%. The contribution of the y yarn will not exceed 90% due to the normal loss of surface area which results from the interweaving. Likewise, the minimum contribution will preferably always be at least 50% of the y yarn. Those skilled in the art will recognize that the percentage contribution will obviously be related to the beat up during the weaving process and the density of the yarns. As the weave is made loser, the curvature of the knuckle will increase and the percentage contribution will decrease. Likewise, as the fabric is more tightly woven, the percentage contribution will increase. As will be appreciated by those skilled in the art, it can be seen that the construction of Figure 1 provides a construction where each of the binder yarns 10 to 16 provides a knuckle or float on the surface of the upper ply layer which is positioned adjacent to the void in the float surface resulting from the interweaving of CMD yarns 9, 11, 13 and 15. With reference to Figures 3 and 4, there is presented a graphic illustration of this phenomena. As can be seen from Figure 3, each of the lower ply CMD yarns 10 to 16 will substantially replace the sunken CMD yarn 9, 11, 13 and 15 so as to accomplish a virtually continuous float length for the CMD yarns on the upper ply. Since the binding yarn knuckle complements the weave on the upper ply, it will not produce the objectionable knuckle marking problems noted with the prior art constructions. As noted previously, the binding knuckle complements the top float weave construction so as to improve the surface contact and to avoid interruption generally associated with the sinking of the yarn beneath the machine direction thread. In addition to replacing the sunken CMD yarn of the upper ply, the lower ply CMD binder yarn also provides a counter tension on the respective MD yarn so as to avoid knuckle marking which results from the MD yarn pushing into the surface ply of the fabric. With reference to Figure 4, there is illustrated the construction of the lower ply CMD yarns. As can be seen from Figure 4, each of the binder loops of the lower ply CMD yarns will have a height at least equal to the diameter of 2 MD yarns. In addition, it can be seen that the lower ply does not form a complete fabric construction as does the weave pattern of the upper ply. In addition, it will be seen that through the disclosed system of binder yarns, each pair of vertically arranged MD yarns will be under the influence of a number of binder yarns which will tend to maintain the MD yarns in vertical alignment and stabilize the fabric during operation. With respect to weaving of the present fabric it will be appreciated by those skilled in the art that the elimination of a third machine direction yarn system about which to bind the yarns will reduce the time and cost of weaving the fabric. Use of on independent binder yarn which does not contribute to the overall weaving of the fabric results in a condition where an additional shuttle or an additional warp is required. Thus, fabric according to the present invention can be woven in an endless system without the need for a dedicated shuttle to insert the binder yarn. Likewise, flat weaving of the fabric is simplified since there is no need to account for an additional warp system for the binder yarn and no need to account for the separate binding system in the idle weave repeat pattern. Accordingly, utilization of cross machine direction binder allows for the elimination of a separate system devoted to binding yarns and increases the weaving speed. Likewise, cross machine direction binding yarns are preferable, as water removal is greatly enhanced by uniform pressure in the nip area. Long floats running parallel to the nip provide optimum pressure points or surface contact as the fabric and paper sheet travel through the nip. In addition, the potential of the cross machine direction binder yarn to recapture a substantial portion of the CMD surface area, which is lost due to the sinking of the upper ply CMD yarns, adds significantly to the dewatering capabilities of the construction. When the increased dewatering capacity is coupled with the stability of the present construction, dewatering and void volume retention are greatly enhanced. Constructions according to the present invention may find use in certain dryer fabric and forming fabric applications, however, the primary benefit is obtained in using the fabric in the press felt position of the papermaking equipment. In press felt applications, the preferred fabric base will be provided with a felt batt. Generally, the felt batt will comprise upper and lower batt layers which are needled to the fabric base by techniques known to those skilled in the art. Likewise, it will be recognised by those skilled in the art that a single upper batt layer may be sufficient for certain design applications. It will be further understood by those skilled in the art that the terms "machine direction yarns" and "cross machine direction yarns" refer to the direction of the yarns as positioned to and operated on the papermaking equipment.

Claims: A papermaker's wet press felt comprising: a top layer of machine direction yarns (1,3,5,7); a bottom layer of machine direction yarns (2,4,6,8); and a system of cross-machine direction yarns (9-16) selectively interwoven with said machine direction yarns for providing the dominant surface yarns of both the top and the bottom surfaces of the fabric, the system of cross-machine direction yarns comprising: a first sub-system of cross-machine direction yarns (9,11,13,15) woven with said top layer machine direction yarns in a repeat pattern having floats extending over at least two top layer machine direction yarns and under one top layer machine direction yarn to form a first fabric layer; and a second sub-system of cross-machine direction yarns (10,12,14,16) each woven with said bottom and top layers of machine direction yarn in a repeat pattern having floats extending under at least two bottom layer machine direction yarns and over at least one vertically aligned pair of top and bottom layer machine direction yarns (1,2), each second sub-system yarn (10) where it extends over an upper layer machine direction yarn (1) being adjacent a first sub-system cross-machine direction yarn (9) which extends beneath the same upper layer machine direction yarn (1). A fabric according to claim 1, wherein the first sub-system of cross-machine direction yarns has a repeat pattern extending over at least three of the upper layer machine-direction yarns. A fabric according to claim 1 or 2, wherein the second sub-system of cross-machine direction yarns has a repeat pattern extending under at least three of the lower layer machine-direction yarns. A fabric according to any preceding claim, wherein said first sub-system and said second sub-system are woven to be substantially mirror images of each other. A fabric according to any preceding claim, further comprising a batt needled adjacent to the top surface of the fabric. A fabric according to any preceding claim, further comprising a batt needled adjacent to the bottom surface of the fabric. A fabric according to any preceding claim, wherein the repeat pattern of the first sub-system is a 3/1 repeat.

Inventors: SUMMER, JR., WILLIAM S., SUTHERLAND, PAUL H.
Applicants: ASTEN GROUP INC. (A DELAWARE CORPORATION)
Application year: 1987
Main classification: D21F1
IPC codes: D21F1, D03D11, D21F7