US 3222246 A
Descripción (El texto procesado por OCR puede contener errores)
C. A. LEE
BACKUP WIRE FOR FOURDRINIER MACHINE Dec. 7, 1965 2 SheetsSheet 1 Filed Dec. 14, 1961 0m Nu Dec. 7, 1965 c. A. LEE 3,222,246
BACKUP WIRE FOR FOURDRINIER MACHINE Filed Dec. 14, 1961 2 Sheets-Sheet 2 United States Patent BACKUP WIRE FOR FOURDRINIER MACHINE Charles A. Lee, Knoxville, Tenn assignor to Huyck Cor poration, New York, N.Y., a corporation of New York Filed Dec. 14, 1961, Ser. No. 159,415
This present invention relates to the manufacture of felted fibrous products such as paper, board, pulp, asbestos-cement products and the like, in continuous web form on machines which for convenience will be referred to in this specification as papermaking machines. More particularly, the present invention relates to the operation of the Wet end of papermaking machines of the Fourdrinier type. At the wet end of such machines the fibrous web is formed from stock consisting primarily of a suspension of fibers in water which is continuously deposited from an orifice or similar device onto a travelling endless belt of screen-like, material. Such screen belts may be woven from metal wire or from synthetic yarns, including monofilaments, multifilaments, staple fibers, or any combination thereof, and will be referred to herein at Fourdrinier screens.
In papermaking machines of the type referred to above, the Fourdrinier screen is driven through an endless path around a breast roll the upper surface of which is located adjacent the point of deposit of the stock, a couch roll and one or more guide rolls and tensioning rolls. The portion of the path extending from the upper surface of the breast roll to the upper surface of the couch roll is generally horizontal and constitutes the web-forming and water extracting portions of the path. The guide rolls and tension rolls are located beneath the machine and lie in the return portion of the path which extends from the couch roll back to the breast roll.
In the web-forming portion of the path of the Fourdrinier screen the screen passes over various devices to In the Web-forming portion of the path of the Fourdrinier screen the screen passes over various devices to control or to promote the drainage of water through the screen. Ordinarily the screen passes over forming boards adjacent the breast roll and the screen then passes over a plurality of table rolls or stationary drainage units, or combinations thereof, which cause a controlled drainage through the screen ofa major portion of the water in the stock, leaving the major portion of the fibers in the form of a felted web on the upper surface of the screen. The screen then progresses through the portion of the path in which water is extracted from the freshly laid web. Thus the screen usually passes over a plurality of suction boxes having flat perforated tops across which the screen is dragged while suction is applied through the screen to extract water from the Web of fibers carried on the upper surface of the screen. The couch roll is frequently provided with interior chambers arranged in such a manner as to apply further suction to the web and then to release the suction so that the web may be continuously picked off the screen by a pick-up felt for further dewatering. The web is eventually transferred continuously to drying and reeling apparatus.
Formation, that is the pattern of distribution of fibers in the web, is primarily established in the early part of the web-forming path where about 90% or more of the water originally in the stock rapidly drains through the screen. Formation therefore is dependent upon a complex combination of machine speed and rate of drainage of the water through the screen, and the partciular type of formation desired varies quite widely between different grades of paper or other felted fibrous products being manufactured. For any particular product and formation it is always the desire to operate the machine at as high a rate of speed as is practicable, and in many instances the upper limit upon production speed is established by physical limitations imposed upon the screen as a result of the mechanical demands placed upon it. Higher speeds increase the physical demands and thus call for stronger constructions which must be either coarsely woven, with large yarns or wires and large openings, or tightly woven with small yarns or wires and small openings. The coarse structures may afford rapid drainage but they cause marking of the screen pattern in the paper. The tightly woven structuresmay not mark the paper but the drainage rate is much slower. Theslower drainage rate means that a proportionately greater length of wire must be provided and this compound the physical strength demands as well as increases the cost of the screens and the paperrnaking machine.
Accordingly the design of Fourdrinier screens has requiredcorn'promises between the physical demands and the desired drainage characteristics with resultant limitation upon the speed at which paper or similar products may be produced with the fiber formation, uniformityof caliper, freedom from screen marking and other characteristics that may be required.
The present invention provides supplemental screenlike belts made from yarns of synthetic materials and preferably woven in endless form, which are arranged to run in endless paths inside the endless paths of Fourdrinier screens. In all modifications of the present invention the supplemental belt lies under and in direct contact with the Fourdrinier screen in'one or more parts of the webforming and/ or the water extracting portions of the path of the Fourdrinier screen. In one embodiment a supplemental belt may lie beneath the Fourdrinier screen as it passes over thesuctionboxes. In otherembodiments the beltrnay remain in contact with the Fourdrinier screen as it passes around the couch roll or the breast roll or both, or substantially throughout the endless path thereof. These supplemental belts perform at least'one and usually several functions such as protecting the Fourdrinier screen from wear, relieving it, in varyingdegrees in different embodiments, from the stresses due to tensioning, power transmitting and supporting of the stock, as well as increasing the rate at which water may be uniformly drained from the stock in the critical formation zone.
In one sense it is a purpose of the present invention to reduce the mechanical demands placed upon Fourdrinier screens whereby the design'of the screens may be pointed much more sharply toward the drainage, formation or marking characteristics or combinations of these characteristics required for the production of any particular product, in many cases at higher rates of speed than heretofore practical. However, the' present invention affords additional advantages which are not directly related to the reduction of mechanical demands. For example, the multitudinous points of contact between the Fourdrinier screen and the woven synthetic fabric beneath it promotes rapid and uniform drainage of water through the Fourdrinier screen irrespective of its construction and this factor permits the use, where desired, of Fourdrinier screens of exceptionally closely-woven construction.
The present invention also affords the possibility of valuable changes in papermaking machine construction. Drastically shorter Fourdrinier screens may be used to obtain equivalent or superior products and production speeds with resultant reduction in size and cost of the machine and building in which it is housed.
It also has been'discovered that the use of a durable synthetic fabric as a Fourdrinier screen coming into direct contact with the suction'boxes or the use of such a fabric as a supplemental belt, as disclosed herein, with a synthetic or metallic Fourdrinier screen makes possible a new combination in a paperrnaking machine of such a fabric with suction boxes having perforated tops made of a substantially permanent material such as stainless steel. Since Fourdrinier screens heretofore used almost always have been made of metallic wire the suction box tops have been made of wood or, more recently, of plastic or plastic-coated materials to reduce wear on the metal wire fabric. These soft materials have wasted power because of their relatively high frictional characteristics and have required frequent replacement or resurfacing because of irregularities, ridges and the like, worn into them by the metallic fabric. With synthetic fabrics the wood or plastic surfaces are not only not required but indeed are undesirable. The life of the synthetic fabrics may be increased several fold by using smooth metal-surfaced perforate plates for the tops of the suction boxes and stainless steel has been found to be particularly desirable for such use. The amount of power required to drag the fabric over the suction boxes is greatly reduced with the result that even greater freedom of fabric design is afforded.
Other and further objects of the present invention will be understood from the following description of preferred, illustrative forms of the present invention together with the drawings forming a part of this specification.
In the drawings:
FIGURE 1 is a diagrammatic side elevational view of the wet end of a typical papermaking machine embodying one form of the present invention;
FIGURE 2 is an enlarged fragmentary sectional view taken along the line 2-2 in FIGURE 1;
FIGURE 3 is a view similar to FIGURE 1 showing a different embodiment of the present invention;
FIGURE 4 is a view similar to FIGURE 1 showing still another embodiment of the present invention;
FIGURE 5 shows a further embodiment thereof; and
FIGURE 6 is an enlarged fragmentary plan view of a portion of any of the machines shown in the preceding figures, looking down upon one of the suction boxes with the supplemental belt, the Fourdrinier screen and the paper mat successively broken away.
Referring now to the drawings, in FIG. 1 there is shown a Fourdrinier screen 10 which is movable in an endless path around a breast roll 12, a couch roll 14, a return guide roll 16 of well-known construction having edge sensing paddles 17 for automatic operation, a return roll 18 and a movable screen-tensioning roll 20. The tensioning roll 20 may be mounted in a conventional manner, for example, on an arm 22 carried on a shaft 24 for angular adjustment about a horizontal axis 26. For convenience in illustration, the frame of the papermaking machine has not been shown and various supporting structures, wash rolls, showers and the like have been omitted.
The screen 10, in a typical machine, is driven only by the couch roll 14 which is rotated by suitable means, not shown, in a clockwise direction as indicated by the arrow in FIG. 1. The remaining rolls are idlers and are rotated by the screen 10.
Above the couch roll 12 there is indicated a head box 28 having an orifice or slice 30 from which the stock 32, in the form of an aqueous suspension of fibers flows continuously in a ribbon-like stream, frequently several inches deep, onto the screen 10 as the latter leaves the breast roll to enter the formation zone. Illustratively there is shown a forming board 34 positioned beneath the screen 10 and extending from a position closely adjacent the periphery of the breast roll 12 to a position downstream of the screen path. When a forming board or boards are used their function is to partially impede drainage of water through the screen 10 for a brief period in which formation may start with generally horizontally disposed fibers bridging the interstices of the screen 10.
The screen 10 progresses to a series of table rolls 36 which are rotatably supported on horizontal axes extending transversely of the path of the screen 10. The table rolls 36, as is well known, not only serve to support the screen 10 with the layer of stock thereon but also serve to extract Water through the screen 10, separating the water from the fibers which are deposited in a layer upon the upper surface of the screen 10 in desired random orientation and relatively uniform thickness. As the screen 10 passes over each table roll 36 the surface of the latter moves downwardly away from the screen creating a zone of subatmospheric pressure tending to draw water downwardly and also tending to draw the screen itself downwardly. The diameter of the table rolls and the speed of the screen must be carefully related to the stiffness and porosity of the screen and with the nature of the stock to minimize dipping and recovery of the screen as it passes over each table roll. Such action frequently causes stock jump and the present invention affords a highly effective combination for preventing stock jump while providing for rapid removal of water.
In addition to the table rolls 36 there may be provided any of the well-known forms of stationary dewatering devices. For example a transversely extending board 38 may be interposed between any or all pairs of adjacent table rolls 36 or such a board may be substituted for one or more of the table rolls. The particular form of the board 38 or equivalent stationary dewatering device or devices is not material to the present invention. Usually these devices are adjustably mounted at the sides of the machine so that the leading portion of the upper surface may be brought into contact with the lower surface of the Fourdrinier screen 10 and the trailing portion of the upper surface may be inclined downwardly away from the screen to promote drainage of water through the screen.
As shown in FIG. 1 the Fourdrinier screen 10 next passes over one or more suction boxes 40 having essentially fiat tops 42 which are perforated, usually with a staggered pattern of holes or slots. For example, in FIG. 2 there is shown a fragment of a suction box top 42 having staggered rows of holes 44 and 46 which extend transversely of the path of travel of the screen 10. Instead of being circular, as shown, the holes 44 and 46 may take the form of slots which may be elongated in any desired direction relative to said path of travel. The suction boxes 40 are connected to a suitable pump (not shown) which exhausts the air and water from the interior of the boxes which enters through the screen 10 and the slots 44 and 46.
The screen 10 next progresses to the couch roll 14 which, as shown in FIG. 1, may be provided with a suction chamber 48 fixed against rotation and communicating, through a perforated rotary shell 50 of the couch roll 14 with the lower surface of the screen 10. Water is extracted from the stock on the screen 10 into the chamber 48 by means of a suitable pump (not shown) which continuously exhausts water and air from the chamber 48.
The structure so far described is merely illustrative of a convention combination of papermaking machine elements and the combination as well as the elements may take many different forms as is well known in the art. It will be recognized that in any such combination the Fourdrinier screen 10 must have the mechanical strength to withstand longitudinal forces due to tension and the transmission of power and to withstand bending forces at the table rolls, and it also must withstand abrasion as it comes into contact with the various rolls and stationary surfaces in its path.
In FIG. 1 the present invention is illustrated in an embodiment which greatly reduces frictional drag with attendant reduction in abrasive wear and in the amount of power which the screen 10 is required to transmit. In this embodiment an endless belt 52 is positioned inside the endless path of the screen 10 and is conducted through a path such that the belt 52 lies between the screen 10 and the tops 42 of the suction boxes 40. The endless belt 52 is preferably wider than the screen 10 so that the screen 10 will be supported on the belt 52 throughout its width durlng travel over the suction boxes, and so that the ls)c1ieen and belt may be independently guided as described e ow.
The belt 52 may be conducted over rolls 54 and 56 located respectively ahead of and beyond the suction boxes 40 and return rolls 58 and 60 located beneath the suction boxes. One of the return rolls, for example the roll 60, may be a guide roll having edge sensing paddles 61 and well-known provisions for maintaining the belt 52 centered in its desired path. One of the rolls 54 or 58 may be adjustable transversely of its axis of rotation to apply such degree of tension upon the belt 52 as may be desired.
It is known in the art to provide a very heavy rubber belt with perforations extending through it to run in a path approximately the same as that just described for the belt 52. These rubber belts usually are associated with special suction boxes so that the belts, in effect, constitute a movable suction box top and the perforations in them are quite comparable with those normally formed in fixed suction box tops. The rubber belts of the prior art usually are dragged along by the Fourdrinier screen, and while they may be effective to reduce, to a limited extent, the abrasive wear upon the screen they are necessarily very heavy and stiff, and they place an additional power demand upon the screen. Also, they require special suction box tops having such a perforation pattern as to be usable only in combination with rubber belts of a particular design.
In contrast with the prior art the present invention makes use of a belt 52 woven from synthetic yarns and preferably woven in endless form to avoid the need for transverse joints. The belt 52 is thus a fabric com parable to the non-metallic fabric from which Fourdrinier screens have been made, but in most cases the belt fabric may be made of heavier or coarser yarns and the weave may be more open. Such a coarse open fabric would normally not be useful as a Fourdrinier screen because it would tend to pass too many fibers, it would mark the paper or other fibrous product being produced, and would also cause the formation to be so uneven as to be unacceptable for almost any sort of fibrous product. However, such a coarse, relatively open fabric has been found to be an ideal support for a Fourdrinier screen which may may be of such construction as not to be practical for use alone in the conventional manner.
In the form of the invention shown in FIGURE 1, the supplemental screen-like belt 52 is an idler belt whereby the power required to drag it across the suction boxes 40 must be derived from the Fourdrinier screen 10. However, because of the light weight and flexibility of the belt 52, the power demand is very much less than it has been for the heavy perforated rubber belts heretofore used. Beyond this, however, the surface textures of the belt 52 and screen are such that they tend to mesh with one another whereby the belt 52 readily moves with the screen 10. As diagrammatically shown in FIGURE 2, the Fourdrinier screen 10 is made up of transversely extending yarns or wires 58 and longitudinally extending yarns or Wires 60, and the fabric belt 52 is made up of transversely extending yarns 62 and longitudinally extending yarns 64. While a plain weave is shown in each instance it will be understood that any of the various weaves heretofore used for Fourdrinier screens, for example, may be used for the screen 10 and belt 52 disclosed herein. Also, while it is customary in the manufacture of Fourdrinier screens from metallic wire to have the warps run lengthwise of the screen, with welded or brazed joints to form an endless belt, it is preferred, when Fourdrinier screens are made from synthetic yarns to weave them endless so as to avoid the need for a joint or seam. In the present invention the screen 10 may be metallic or non-metallic as desired, but for purposes of illustration the screen 10 shown in FIGURE 2 is of the configuration assumed in an endless-woven structure. Thus, the transversely extending wires or yarns 58 have the undulating shape characteristic of warps, while the longitudinally extending wires or yarns 60 have the straight shape characteristic of wefts.
Irrespective of the construction of or material used for the screen 10 is it a particular feature of the present in-- vention that the fabric belt 52 be non-metallic and it is preferred that it be woven in tubular form on the loom whereby the longitudinally extending yarns 64 are wefts and the transversely extending yarns 62 are warps. It IS believed that maximum benefits of the present invention are achieved when both the screen 10 and belt 52 are non metallic endless-woven fabrics, and FIGURE 2 is intended to illustrate the combination of these preferred fabrics. Thus, the transversely extending yarns 58 and 62 of both fabrics shown in FIGURE 2 have been illustrated as undulating above and below the wefts 60 and 64 in the manner which is characteristic of warp yarns in a woven fabric.
In all preferred forms of the present invention the supplemental belt 52, at least, is Wovenin such a manner that the transversely extending yarns are undulating and therefore form knuckles which make up the surface of the fabric. These knuckles give the belt 52 a surface texture which tends to mesh with the Fourdrinier screen 10 whereby the belt 52 will readily move in a longitudinal direction with the screen 10 when the papermaking machine is in operation. In the specifically preferred form shown in FIGURE 2 wherein the Fourdrinier screen 10 also has the undulating yarns extending transversely, the resistance to slipping in the longitudinal direction in which the screen 10 and belt 52 move in use is unusually good.
Beyond the fact that driving of the supplemental screenlike idler belt 52 as shown in FIGURE 1 requires very little power from. the Fourdrinier screen 10, the interpositioning of an idler belt 52 constructed in accordance with the present invention between the Fourdrinier screen 10 and the suction box tops brings about a very great reduction in the total amount of power which must be exerted through the Fourdrinier screen 10. The relatively coarse woven structure of the belt 52 presents many fewer points of frictional contact with the suction box tops than is presented by a Fourdrinier screen of ordinary construction. These points of contact are between the knuckles of the transversely extending warps 62 and the suction box tops. Consequently abrasive wear is on the knuckles and not on the longitudinally extending yarns 64 whereby the longitudinal tensile strength of the belt 52 is not progressively reduced by such wear as would be the case if the knuckles were in the longitudinal yarns. Full advantage of this characteristic of the belts of the present invention may be realized by using suction box tops which are highly resistant to abrasion and preferably are smoothly polished. For example, the suction box tops 42 may be made of stainless steel and the upper surfaces thereof may be polished prior to installation in the machine. After the machine is placed in operation the movement of the belt 52 across the polished surfaces of the suction box tops 42 may further polish and smooth the surfaces, but grooving or galling of the surfaces has not been observed, even after extremely long periods of operation. The coeificient of friction between the belt 52, which is woven from synthetic yarns, and the smooth hard suction box tops 42 is extremely low with the result that the force required to pull the belt 52, the Fourdrinier screen 10 and the fibrous stock 68 (see FIGURE 2) carried by the screen 10 across the suction boxes 40, with the latter in normal operation, is very much less than has been required in any combination of which applicant is aware.
The customary practice in the past has been to use metallic Fourdrinier screens which are dragged across the suction box tops in direct contact therewith. Since the bronze or other alloys normally used in metallic Fourdrinier screens have relatively low abrasion resistance and because of the almost inevitable presence of gritty or metallic particles in the pulp, the suction box tops have been made of wood or relatively soft plastic materials which wear much more rapidly than the metal in the screen. Steel suction box tops would be impractical for use with metallic Fourdrinier screens not only because of the expected rapid wear upon the screens but also because the gritty particles in the pulp cause grooving of the metal box tops and the metallic particles in the pulp cause galling between the metal Wire and metal tops with consequent destruction of the screen.
The wood or plastic suction box tops heretofore necessarily used with metallic Fourdrinier screens require frequent resurfacing to remove grooves and other imperfections caused by wear and such resurfacing causes frequent shut-down of the papermaking machine. They also offer very substantial frictional resistance to the dragging of the screen across the suction boxes when the machine is in operation, and the power required to overcome this resistance must be exerted upon that portion of the screen which extends forwardly from the suction boxes to the couch, or driving, roll. As discussed above, the demand for longitudinal tensile strength to overcome this resistance alone has placed severe limitations upon the design of metallic Fourdrinier screens.
It is a feature of the present invention that hard metal suction box tops may be used when a non-metallic Fourdrinier screen woven from synthetic materials, such as nylon and other materials discussed hereinbelow, is used, with or Without the non-metallic supplemental belt 52, and it is a further feature of this invention that such metal suction box tops may be used with a metallic Fourdrinier screen when the non-metallic belt 52 is interposed between the screen and the suction box tops as shown in FIGURE 1. In all such cases the material in contact with the suction box tops is the non-metallic material described herein and the advantages of long service life for the last mentioned material, reduced power demand and substantially complete elimination of resurfacing of suction box tops, are achieved. When the non-metallic belt 52 is used as Shown in FIGURE 1 with either a metallic or non-metallic screen 10 the further advantage of vastly reduced wear upon the screen 10 and even greater reduction in power demand are achieved.
In the form of the invention shown in FIGURE 3, all of the major elements of the papermaking machine already described in connection with FIGURE 1 are shown and are identified by the same reference characters as used in FIGURE 1, whereby description of said elements need not be repeated at this point. In FIGURE 3, however, the supplemental screen-like belt is differently arranged. Thus, the supplemental belt 152 not only extends from a roll 54 over the suction boxes 40 but it continues beneath the Fourdrinier screen 10 around the couch roll 14 to the return guide roll 16. At the return guide roll 16 the belt 152 is separated from the Fourdrinier screen 10 and the edge sensing paddles 17 may be arranged to engage the edges of belt 152. In such event the return guide roll 16 will guide both the belt 152 and screen 10 in response to the sensing of the belt 152. The belt 152 may be tensioned by movement of the roll 54 in some cases, or by a belt tensioning roll 160 carried on an arm 162 secured to a shaft 164 for rotatable adjustment about the axis 166 of the shaft 164 as may be desired. The belt 152 then continues to the belt guide roll 54 to complete its endless path. The screen 10 should be separately guided and to this end the roll 18 may be a return guide roll with edge sensing paddles 19.
The modification of the invention illustrated in FIG- URE 3 affords all of the advantages obtained by use of the modification shown in FIGURE 1, and in addition affords the very great advantage of relieving the Fourdrinier screen 10 from a substantial portion of the longitudinal stresses incident to the driving of the screen through its endless path. In this form of the invention the supplemental screen-like belt 152 is not an idler, as in FIGURE 1, but rather serves to drive the Fourdrinier screen 10 because it lies beneath the screen at the couch roll 14. Thus, the power required to drag the belt 152 and the Fourdrinier screen 10 over the suction boxes 40 is exerted through the belt 152 instead of through the screen 10.
Because the modification shown in FIGURE 3 relieves the Fourdrinier screen 10 from the longitudinal stresses due to movement across the suction boxes it will be apparent that the Fourdrinier screen 10 may be even lighter in construction, if so desired, than it might need to be for use in the modification shown in FIGURE 1. Accordingly, this particular form of the invention affords the designer of the Fourdrinier screen an even greater latitude than is afforded by the form shown in FIGURE 1.
FIGURE 4 illustrates a still further modification of the present invention. The principal elements of the papermaking machine illustrated in FIGURE 1 are also shown in FIGURE 4 and are indicated by the same reference numerals as are used in FIGURE 1 to avoid the necessity for repetition of description of such elements. However,
in the modification shown in FIGURE 4 a supplemental belt 252 is shown which is positioned inside of and extends with the Fourdrinier screen 10 through almost all of the path of the latter. Thus, the belt 252 is interposed between the Fourdrinier screen 10 and substantially all of the elements of the papermaking machine over which the Fourdrinier screen 10 would normally pass to protect the screen 10 from wear and to relieve it from almost all stresses.
In FIGURE 4 the supplemental belt 252 extends in face-to-face engagement with and beneath the Fourdrinier screen 10 around the breast roll 12 and over the forming board 34, over the table rolls 36 and other dewatering devices such as 38. The belt 252 continues beneath the screen 10 over the suction boxes 40 and around the couch roll 14, to the return guide roll 16. At the return guide roll 16 the belt 252 may be guided away from the Fourdrinier screen 10 to pass over a belt tensioning roll 260 carried by an arm 262 secured to a shaft 264 for rotatable adjustment about the axis 266 of the shaft 264. From the belt tensioning roll the belt 252 extends to the return roll 18 Where it rejoins the Fourdrinier screen 10 for travel with the same through the endless path just described. Separate guiding of the screen 10 must be provided. For example, an additional return guide roll (not shown) may be positioned in the independent portion of the path of the screen 10. Alternatively, separate guiding may be provided by making the tension roll 20 also serve as a guide roll, with edge sensing paddles 21 engaging the edges of screen 10.
By proper adjustment of the respective tensioning rolls 258 and 20 the belt 252 and screen 10 may be made to ride together smoothly with the belt 252 assuming almost all of the stresses and Wear to which the Fourdrinier screen 10 ordinarily would be subjected. From the standpoint of tension alone, the designer of a Fourdrinier screen 10 for use in an embodiment such as that shown in FIGURE 4, is almost unlimited in his ability to make use of light and delicate fabrics including many which could never be used alone for Fourdrinier screens.
In FIG. 5 a supplemental belt 352 is arranged and guided in the same manner as the belt 252 in FIG. 4, except that the belt 352 does not extend around the breast roll 12. As shown in FIG. 5 the belt 352 is separated from the screen 10 at the return roll 18 and extends upwardly to a special roll 336 which is positioned in place of one of the table rolls 36. The belt 352 may be sop arately tensioned in the manner shown in FIG. 4 or, alternatively, the special roll 336 may be mounted for horizontal adjustment. In this form of the invention the supplemental belt 352 is not beneath the Fourdrinier screen 10 as the latter passes beneath the slice 30, and over the forming board 34 and, if so desired, a selected number of the table rolls 36. Thus, the critical early formation of the sheet 68 occurs on the unbacked screen 10 and this may be preferred in certain instances as will be discussed below.
While FIG. 2 has been identified above as an enlarged fragmentary section taken along the line 22 of FIG. 1, it is equally characteristic of a similar section in FIGS.
3, 4 or 5. Also, it will be recognized that even at the enlarged scale employed for FIG. 2 the representation of the supplemental belt 52, screen 10, and layer of deposited paper fibers 58 is necessarily diagrammatic. This is also true of the fragmentary plan view presented in FIG. 6 which is enlarged to a lesser degree than FIG. 2. FIGS. 2 and 6 are intended to convey, within the limits of practicality, typical relative proportions of the suction box holes 44, 46, the layer of fibers 68 and the fabrics which comprise the belt 52 and Fourdrinier screen 10. The proportions are based on one illustrative installation particularly adapted for the manufacture of a corrugatingboard. In this particular installation the holes 44 and 46 in the suction box top 42 are circular and they are approximately one-half inch in diameter. The supplemental belt 52 is woven from synthetic yarns and there are twenty-four warp yarns 62 per inch and there are twenty weft yarns 64 per inch. The Fourdrinier sceen is woven from synthetic yarns with sixty warp yarns 58 per inch and forty weft yarns 60 per inch. Thus, since FIG. 2 is a section taken across the wefts and parallel with the warps it will be observed that there are approximately ten weft yarns 64 within the portion of the belt 52 which spans the diameter of a single suction box hole 44 or 46. Similarly, there are approximately twenty weft yarns 60 within the portion of the Fourdrinier screen 10 which spans the diameter of a single suction box hole 44 or 46. A typical supplemental belt 52 woven from synthetic yarns to the count illustratively set forth above may have a tensile strength in the machine direction, that is in the direction of the wefts, of about. 815 pounds per lineal inch. In contrast with this, a typical Fourdrinier screen 10 woven from synthetic yarns to the count illustratively set forth above may have a tensile strength in the machine or weft direction of about 380 pounds per lineal inch. In another typical installation suitable for the manufacture of fine paper or paper made from stock containing a substantial quantity of short'fibers or filling material where high fines retention is desirable, the Fourdrinier screen 10 may be woven from yarns of synthetic material to much higher counts. For example,a count of ninety warps per inch and ninety wefts per inch may be used. A typical fabric woven to such a count from carefully selected synthetic materials and stabilized as will be discussed more fully below, may have a tensile strength in the machine or weft direction of about ninety pounds per lineal inch. With such a screen 10 the supplemental belt 52 provided in accordance with the present invention may be the same strong, relatively coarse fabric described above in connection with the manufacture of a corrugating board, or it may be somewhat finer if so desired. In any event, the supplemental belt 52 can be substantially coarser in weave and, where desirable, it may have up to several times greater machine-direction tensile strength than the screen 10.
In the form of the invention shown in FIG. 1, wherein the supplemental belt 52 is an idler, the superior tensile strength of the belt 52 is not fully utilized, but the fact that the belt 52 may be relatively coarse in weave provides the great advantage, as pointed out above, of low friction movement across the suction box tops with resultant reduction in power demand upon the screen 10. However, the form of the invention shown in FIG. 1 may be further modified in a well-known manner by providing suitable means for driving the roll 56 at a peripheral speed matching that of the screen 10 whereby to relieve the screen 10 of additional longitudinal stresses.
In the form of the invention shown in FIG. 3 wherein the supplemental belt 152 extends across the suction boxes and around the couch roll 14, the great tensile strength of the belt 152 may be utilized inasmuch as it reinforces the screen 10 throughout the zone wherein the greatest longitudinal stress ordinarily is exerted upon the screen.
In the form of the invention shown in FIG. 4 wherein the supplemental belt 252 runs beneath the screen 10 around the breast roll 12, across all of the dewatering devices and table rolls, suction boxes and around the couch roll, the maximum advantage of the great tensile strength of the supple-mental belt 252 and the independent tensioning of the belt 252 and the screen 10 is realized. In this form of the invention the supplemental belt 252 may be placed under sufficient tension to prevent stock jump at the table rolls irrespective of the fineness and relatively low tensile strength of the screen 10. In this embodiment of the invention the screen 10 needs only to be placed under sufficient tension by means of its individual tensioning roll 20 as to insure that it will move with the supplemental belt 252 in a smooth and uniform manner.
In the form of the invention shown in FIG. 5, almost all of the advantages flowing from the superior tensile strength of the supplemental belt 352 may be realized. As shown in FIG. 5, the screen 10 travels independently of the belt 352 through a relatively short portion of its endless path, and this portion is in the zone wherein very little machine direction stress is placed upon the screen 10. By guiding the supplemental belt 352 into engagement with the screen 10 at a position within the table roll and dewatering section just ahead of the zone in which stock jump may present a problem, the screen 10 need not be placed under any great amount of tension because the tension upon the belt 352 may be relied uponto prevent stock jump in the critical areas. It will be understood that while FIG. 5 shows the belt 352 brought into engagement with the screen 10 after it has passed several table rolls 36, the belt 352 may be brought beneath the screen at any point ahead of or behind the particular point chosen for illustration. For example, the belt 352 might be brought into engagement with the screen 10 at the position occupied by the first table roll 36.
The problem of stock jump has been discussed above in so far as its relation to screen tension is concerned, but it will understood that, from a theoretical standpoint at least, the application of any increasing amount of longitudinal tension short of an infinite tension will not be completely effective to overcome the tendency of the screen 10 to wrap each of the table rolls over which it passes. However, a major cause of the tendency of the screen 10 to wrap a table roll is that each table roll acts as a pump carrying water and air away from the zone beneath the Fourdrinier screen. The screen is therefore caused to follow the table roll surface by the differential between the atmospheric pressure above the aqueous fibrous suspension carried by the screen, and the subatmospheric pressure on the downwardly moving side of the table roll. Since this differential sharply increases with increases in machine speed, it has been necessary to increase the screen tension very sharply or to make expensive structural alterations in a paper machine to accommodate it to higher speeds. In the forms of the present invention illustrated in FIGS. 4 and 5, the supplemental belts 252 and 352 may be utilized to control the differential and thus permit operation of a papermaking machine at greatly increased speeds without the need for increased tension to prevent stock jump. Reference to FIGS. 2 and 6 will reveal that the coarse open structure of the supplemental belt 52 affords openings of large volume between the yarns. The drainage rate of a screen 10 relative to a belt 252 or 352 in FIGS. 4 or 5, may be adjusted by relating the volume and the relative sizes of the openings between the yarns of the two fabrics so that the water that flows through the screen 10 as the fabrics pass over the table rolls 36 will not fill the opening in the belt 252 or 352. Under these conditions air may circulate freely in generally horizontal directions in and between the openings in the belt 252 or 352. This circulating air will maintain the gaseous pressure on the downwardly running sides of the table rolls at a considerably higher value than would be the case if the fabric in engagement with the table roll were tobe filled with water, as in the case in an ordinary installation involving a single Fourdinier screen. Thus, for a particular papermaking machine with table rolls of a given diameter, the supplemental belts 252 or 352 and the screen may be so designed as to establish the efficiency of the table rolls as pumps at a level where stock jump is avoided and the drainage rate is optimum for formation and water removal at almost any particular machine speed which may be desired.
In utilizing the present invention it must be taken into consideration that the rate at which water will drain through a screen 10 of any particular construction is substantially greater when the screen 10 is in contact with any of the supplemental belts herein disclosed than it would be through the screen 10 alone.
Reference to FIG. 2 will reveal that the knuckles in the warp yarns 62 on the upper surface of the fabric belt 52 provide an enormous number of separate contact points with the lower surface of the screen. Each such contact point will serve to break the surface tension of the water in the screen 10 with the result that the flow of water through the screen is constantly being initiated and promoted in the vicinity of each contact point. This phenomenon is similar to that observed in a canvas camping tent, the roof of which may be substantially impervious to water except at any point where it may be touched on the inside. As a result of this phenomenon it is possible to use a screen 10 woven very closely of water repellent yarns or wires having openings between yarns or wires so small that water would not flow therethrough without inducement. With such fine-mesh screen 10 resting upon the supplemental belt provided in accordance with the present invention, the flow of water through the screen is induced throughout the area of contact with the belt rather than only at the table rolls, suction boxes or other dewatering devices. The advantages of using such finemesh screens 10 are manifold and include exceedingly high fines retention, early and rapid build-up of a lowermost layer of fibers and other papermaking constituents upon the screen, and extremely smooth and uniform surface on the screen side of the paper.
Because of the presence of the relatively coarse mesh supplemental belts provided by the present invention directly beneath the screen 10, shadow marking might be expected. However, this is not the case even with exceedingly finely woven Fourdrinier screens so long as water is not forced by the table rolls, for example, to fiow through the screen at such a high rate as would be objectionable anyhow from the standpoint of poor formation throughout the paper sheet. It it believed that one of the factors which contribute to the absence of shadow marking in the practice of the present invention, is the fact that the screen 10 and the supplemental belts 52, 152, 252 or 352 are separate fabrics which more or less continuously move or shift relative to one another as they are driven through their respective paths. For example, it has been found that the screen and the supplemental belt drift back and forth relative to one another in directions transverse the path of travel for distances sometimes as great as about two inches. For this reason it is preferable, as shown in FIG. 6, to make the supplemental belt 52 somewhat wider than the screen 10 so that the longitudinal edges of the screen 10 will not overlap the edges of the supplemental belt. While the drifting referred to is kept to a small amount by the automatic individually acting return guide rolls described above, there inevitably will be sufiicient drifting in transverse direction to cause the patterns of the two fabrics to shift relative to one another. It also has been observed that the outer fabric, that is the screen 10, will creep ahead of the lower fabric because of its tendency to run at a slightly higher average longitudinal speed. When it is considered that the diameter of the yarns and the spacing between the yarns in even the coarsest fabric might be used as a supplemental belt in practicing this invention, are only a small fraction of an inch, it will be recognized that even the small amount of drifting or creep between the fabrics which occurs in each circuit of the machine is sufficient to continuously change the flow patterns through the interstices of the two fabrics.
It has been pointed out above that any of the supplemental belts employed in accordance with the present invention are preferably made of a synthetic woven material and several reasons for such preference have already been stated. An additional reason for the use of such materials rather than a woven metal screen, for example, is that by proper choice of materials or combinations of materials utilized in the yarns from which the belts of the present invention are woven, the fatigue resistance of the synthetic fabrics can far exceed the fatigue resistance of a metal screen of equivalent coarseness under the conditions encountered in a papermaking machine. It will be recognized that endless belts are repeatedly bent back and forth as they pass over the driving rolls, guide rolls and the like, as well as when they pass over the table rolls and suction boxes. Such repeated flexure of metal wire screens causes workhardening thereof which eventually leads to fracture. Furthermore, in a papermaking machine the Fourdrinier screen inherently is subjected to varying amounts of tension in the direction of movement thereof, the tension normally being the greatest in that portion of the screen which extends from the last suction box to the couch roll. As the screen passes around the couch roll the tension is progressively reduced from this maximum to substantially the minimum longitudinal tension which exists after the screen leaves the lower surface of the couch roll and enters its return path. At high machine speeds the cycling between high and low longitudinal tension is very rapid and leads to fatigue failure in metal screens. Fabrics in accordance with the present invention can be so constructed as to withstand a substantially greater number of flexural or tension cyclings than can be withstood by metal screens of similar coarseness under the tensile loading conditions which are encountered in a papermaking machine.
All of the supplemental belts of the present invention substantially reduce, and some almost entirely eliminate, the ordinarily high longitudinally stresses to which the Fourdrinier screen 10 is subjected. They also reduce at least some of the flexural stresses thereon, for example, at the table rolls and at the suction boxes. Accordingly, the screens 10 for use in the present invention may be designed with much less than usual regard for the danger of fatigue failure. It is for this reason that Fourdrinier screens woven from metal wire or combinations of metal and synthetic or glass fiber yarns, may be used with the supplemental fabric belts of the present invention although, as stated above, the full advantages of this invention are believed to be best achieved by the use of synthetic fabrics for both the screen and belt. When synthetic screens 10 are used they may be woven from yarns selected from the same groups of materials and yarn constructions discussed below in connection with the supplemental belts.
The supplemental screen-like fabric belts 52, 152, 252 and 352 are preferably woven from a synthetic material having high tensile strength and great abrasion resistance whether wet or dry, and nylon has been mentioned above as a suitable material having these characteristics in such degree as to be particularly useful for the purposes of this invention. Nylon yarns in the form of monofilament, multifilament or staple fiber yarns may be used as warp or weft, or both, and different forms may be used as warp and weft respectively. There are other synthetic fiber forming materials available at the present time which have characteristics such as to be adequate for use in this invention, and it is reasonable to expect I that new materials of similar or superior characteristics will be developed in the future. Some of these other materials may be used alone to produce yarns of multifibers or yarns to form heat-settable compositions.
13 filament, monofilament or staple fiber types, or filaments or fibers made of such materials may be blended with others or with nylon filaments or fibers to produce multifilament or staple fiber yarns suitable for use as warp or weft, or both, in the manufacture of fabrics for this invention. No effort will be made herein to make a tie tailed classification of specific characteristics of fabrics woven from such materials or blends, or combinations thereof. However, nylon6, nylon 6-6, nylon 6-10, nylon 11, Dacron, polyethylene and glass are presently available and have been found to be particularly suitable. Dacron or glass may be formed into monofilaments or independent fine filaments for use in multifilament yarns, and individually encased in a suitable type of nylon, if
so desired, whereby the composite filament will exhibit desirable surface characteristics of nylon and yet have such desirable additional characteristics as the encased, or core, material may afford. Any of the materials named above, or composites or blends thereof, may be utilized as multifilament yarns or staple fiber yarns of cabled or plied construction, as may be desired, and any particular fabric may have yarns made of the same or different material or of the same or different construction in the warp or weft.
Fabrics for use as the supplemental belts 52, 152, 252 or 352 disclosed herein, are preferably woven endless. Among the advantages thus achieved is avoidance of the need for splicing or otherwise joining the ends to form endless belts. However, in many papermaking machines it may be inconvenient or time-consuming to install an endless belt in any one or more of the positions illustrated herein, and particularly when such machines are in production of some product in which marking of the product is not objectionable it may be economically prefer-able to provide the belts with some form of fastening means by which the ends may be joined after installation. For such instances, the belts may be Woven endless so that the wefts run longitudinally and they may be cut transversely. Also, they may be Woven as flat fabrics with the warps running longitudinally. In either event, the ends may be joined, after installation, by sewing, or lacing, or by adhesives, or any combination thereof, or the ends of the belts may be provided with loops or metal clips adapted to receive a pintle to form a hinged joint as is well-known in the art.
The fabrics for use as supplemental belts 52, 152, 252 or 352 are preferably stabilized to prevent shoving of the yarns and to minimize dimensional changes under the varying tensile stresses and moisture conditions to which they are subjected in use and during start-ups and shutdowns of the machine. The fabrics may be stabilized by heat-setting of some or all of the fibers therein with or Without the application of chemicals which react with or coat, .or both, some or all of the fibers or yarns to stiffen or otherwise to change the characteristics of the fibers or yarns, or the surfaces thereof. The applied chemicals may themselves be heat-settable or curable, or they may be of such a nature as to react with some or all of the It is preferred to stabilize the fabrics while in endless form whereby they may be brought to or maintained in a specific yarn count, both warp and weft, which is best calculated to promote stability, and stabilized at that count. For example, a fabric having at least some yarns which consist at least in part of nylon may be stabilized by heatsetting of the nylon, and'stability may be enhanced by applying an aldehyde resin to the fabric and heating the fabric, while held to desired yarn count, to cure and heatset the resin-coated nylon. The endless fabric which is thus stabilized may have been woven endless or may have been woven flat, and joined to form an endless belt. After stabilization they may be installed and used in endless form or they may be cut in the cross machine direction for ease of installation and joined, as described above, after they have been threaded upon the machine.
The service life of any of the fabrics used as supplemental belts as disclosed herein, will be enhanced if the fabric is stabilized in such a manner that the transversely extending yarns undulate above and below the longitudinally extending yarns to protect the latter from abrasive wear. Thus, when the fabric is manipulated to bring it to desired count or to hold it in desired count care may be taken to assure that the lengthwise-extending yarns are substantially straight and the cross-wise extending yarns undulate above and below the crossing points. The fabric is then set with the yarns so disposed. As indicated above, this result is most readily achieved when an endless woven fabric is used.
What I claim is:
1. For use in a paper-making machine and the like having an endless forming screen traveling through an endless path to receive an aqueous suspension of fibers from which water drains through said screen to leave a layer of fibers on the outer surface of said screen, and having one or more dewaterin-g devices positioned along a portion of said endless path to promote the flow of water through said screen, an endless supplemental belt made of screen-like fabric substantially the entire area of which is woven exclusively from synthetic fiber-forming material characterized by having whether wet or dry great tensile strength great resistance to abrasion and great resistance to damage due to flexing, said belt being arranged on said papermaking machine to move in an endless path inside the endless path of said forming screen, the endless paths of said forming screen and said belt substantially coinciding in a portion of the endless path of said forming screen in which said forming screen is advanced over at least one of said dewatering devices whereby in said portion of said path said belt is interposed between said forming screen and said dewatering devices with the outer surface of said belt in face-to-face contact with the inner surface of said forming screen and the inner surface of said belt in contact with said dewatering devices, the fabric from which said belt is formed having the weft yarns extending longitudinally and the warp yarns extending transversely of the path of movement of said belt, and the fabric of said belt being stabilized with the transversely extending warp yarns set in undulating form to provide knuckles extending above and below the longitudinally extending Weft yarns to protect said Weft yarns from abrasive wear and to mesh with the inner surface of said forming screen to provide improved longitudinal driving engagement between those portions of said belt and said forming screen which are in face-to-face contact.
2. An endless supplemental belt in accordance with claim 1 for use in a papermaking machine and the like having a woven forming screen of relatively fine mesh, the fabric of said belt being substantially coarser in mesh than said forming screen.
References Cited by the Examiner UNITED STATES PATENTS 635,266 10/-1899 Marshall 162-349 1,514,011 11/1'924 Pope 162-349 1,880,686 10/4932 Berry 162349 2,903,021 9/1959 Holden 162-348 FOREIGN PATENTS 762,894 112/ 1956 Great Britain.
Pye Pulp and Paper Magazine of Canada: pages 124-- 136.
DONALL H. SYLVEST-ER, Primary Examiner. RICHARD D. NEVIUS, Examiner.
Citas de patentes