CH629268A5 - DRYING CANVAS FOR A PAPERMAKING MACHINE. - Google Patents

Description

The present invention aims to provide a drying fabric in which permeability and the coefficient of elasticity are reduced.

The drying fabric according to the invention comprises warp and weft strands of monofilaments of plastic polymer, woven at a fill rate by the warp of about 100%, at least the warp strands, which are intended to extend. in the direction of operation of the machine,

having a flattened cross section whose major axis is parallel to the plane of the fabric. Preferably, the flattened chain has an almost rectangular section whose resistance to bending on its major axis is less than the resistance to bending of a circular section of the same area and therefore, at equal force from the blow of the loom during the weaving, the spacing of the weft strands can be reduced considerably compared to the spacing obtained with a circular warp. Also, because of the lower profile of the flattened chain, the diagonal openings in the mesh which allow the passage of air, are reduced.

The major axis of the rectangular section being parallel to the weft threads, the fabric is made more resistant to distortion on its own plane at the same time as it allows easy bending of the fabric on the axis parallel to the weft strands, making it easier to fold or bend the fabric around the drying cylinders and rollers with smaller diameters in the drying system.

Although the reduction in permeability is achieved primarily by using a flattened warp, a further reduction in permeability can be achieved by using weft strands of monofilaments shaped transversely to accommodate the horizontal interstices of the naturally formed mesh. by the woven warp strands to thereby reduce the space between the adjacent weft strands.

Preferably, the weft is round or relatively malleable in comparison with the warp, so that during weaving, and therefore under stress conditions, it will tend to adapt to the shape of the interstices of the mesh to reduce them and thus further reducing the permeability.

Advantageously, the weft is shaped or tubular, so that it can more easily match the shape of the interstices of the mesh and further reduce the permeability.

An important advantage of the flattened monofilament warp, with a round or shaped monofilament weft, is that it gives low permeability in an all-monofilament drying fabric without having to add coarse threads, as described in the patent. Canadian No. 861.275, which absorb dirt and moisture, or puffy weft threads comprising fine cut fibers of low flexural strength and help reduce the resistance of the fabric to distortion in its own plane.

Yet another advantage of the flattened warp is that the fabric from which it is woven is relatively thin and it happens to have a coefficient of elasticity which is only half that of a similar fabric woven with a classic round chain. As explained above, a small thickness and a low coefficient of elasticity are particularly advantageous factors if the fabric is used in a single fabric drying system.

A preferred embodiment of the present invention is described below with reference to the examples illustrated by the accompanying drawings in which:

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Figure 1 is a schematic view of a typical drying section used in a paper making machine.

Figure 2 is a schematic view of a single fabric drying section.

Figure 3 is an enlarged cross-section of a portion of a drying fabric showing circular strands of weft and warp monofilaments as they are currently used.

Figures 3A and 3B are cross sections along A-A and B-B in Figure 3.

Figure 4 is an enlarged cross section of a fabric structure similar to that shown in Figure 3 but whose transverse chain strands are flattened and form the drying fabric of the present invention.

Figures 4A and 4B are cross sections along the section lines A-A and B-B in Figure 4.

Figure 5 is a cross section of a drying fabric all 4-harness monofilament, compared to 8, double layer, of the prior art.

Figures 5 A and 5B are cross sections along lines A-A and B-B of Figure 5.

Figure 6 is an enlarged cross-section of a dryer fabric as shown in Figure 5 but using the flattened warp strands to obtain the dryer fabric of the present invention.

Figures 6A and 6B are cross sections along lines A-A and B-B in Figure 6.

Figure 7 is an enlarged cross-section of the flattened monofilament warp strand as used in the drying fabric of the present invention.

Referring to Figure 1, a subsection of a typical drying section in a paper making machine (not shown) is shown schematically. The upper row of the drying cylinders is indicated generally at 10 and the lower row at 11. The continuous sheet 13 passes in a serpentine shape on the upper and lower drying cylinders as indicated. An endless upper fabric 14 holds the continuous sheet 13 strongly against the upper cylinders 10 passing partially around the first upper cylinder, around a felt roll 15, partially around the other upper cylinders 10 and around the other roll felt 15 then around the return roller 16; the upper fabric then passes over the guide and tension rollers 24 and 23 respectively and over a steam-heated drying roller 17 to remove the residual moisture in the fabric, finally on another return roller 16 before passing again on the first drying cylinder by completing the cycle. Likewise, an endless lower fabric 18 holds the continuous sheet of paper 13 tightly against the lower dryer cylinders 11 passing around them, lower felt rollers 19, return rollers 21, the adjustment roller tension 25, of the guide roller 26, of the lower drying roller 22 of the fabric and of the other return rollers 21, as shown. The regions enclosed by the continuous sheet of paper when approaching and leaving a drying cylinder, and by the drying fabric when leaving the previous cylinder, wrapping around the felt roll and approaching the next drying cylinder, are called pockets 12. It is in these pockets that a large amount of moisture is removed by evaporation from the continuous sheet of paper. Adequate ventilation of the pockets 12 serves to remove moisture from the system and to maintain the balance of the evaporation process.

FIG. 2 schematically represents a typical drying section in which all the cylinders have the same diameter and are driven with the same number of revolutions per minute by conjugate gears. As in the figure

1, the drying cylinders of the upper row are indicated in 10 and those of the lower row in 11. A single endless cloth 14, coils around the first upper cylinder, descends and winds around the first lower cylinder, rises and wraps around the second upper cylinder, descends and wraps around the second lower cylinder, and so on, then the fabric passes around a return roller 16, a guide roller 24, a tension adjustment roller 23, a steam heated drying roller 17 and another return roller 16, as shown in the figure. The continuous sheet of paper 13 is introduced under the fabric to the first upper cylinder and follows the fabric, passing between the fabric and the upper cylinders and outside the fabric for the lower cylinders. It will be seen that in relation to the canvas, because of the thickness of the continuous sheet of paper, the effective diameter of the upper cylinders is now greater than the diameter of the lower cylinders by a value equal to twice the thickness. of the continuous sheet of paper.

Figure 3 shows at 30 a prior art synthetic fabric structure where the number 31 indicates consecutive warp strands and the number 32 indicates consecutive weft strands. In this structure, each warp strand 31 passes over a first weft strand 32,

under the second weft strand 32, above the third,

and so on. Likewise, the adjacent warp strand passes under the first weft strand, over the second, under the third, and so on. SI represents the wheelbase of adjacent weft strands 32. In Figure 3B, x indicates the shortest distance between adjacent warp strands 31 vertically in the section taken at the point of tangency between warp and weft, thus representing the diagonal opening the largest which allows the passage of air through the fabric 30.

With reference to FIGS. 4,4A and 4B, the same fabric structure is indicated in 30 ′ made of monofilament warp strands 31 ′ which have been flattened until the small axis b (see FIG. 7) is only the half the diameter of the round chain strand 31 of the corresponding cross section.

By comparing the fabrics of FIGS. 3 and 4, it is clear that, due to the smaller flexural strength of the rectangular section, the flattened chain 31 'undulates more easily so that the wheelbase between the weft strands, S2 in Figure 4, is smaller than SI in Figure 3. Also, because of the flat profile of the flattened strand, the distance y in Figure 4B is much smaller than the corresponding distance x in Figure 3 B. From the similarly, due to the reduced spacing of the weft strands 32 ', the distance S2, the roughly triangular gap based on y in Figure 4B is much smaller than that based on x in Figure 3B.

Figures 5.5A and 5B show a drying fabric 40 all 4-harness monofilament, compared to 8, double layer, a type commonly used in the papermaking industry. In Figure 5, the numbers 41,42,43, and 44 indicate consecutive chain strands. The frame is in two layers, in pairs and is numbered from 48 to 57, as shown in the figure. In this structure, a warp strand 41 passes in order over a first pair of weft strands 50-51, between the second pair 52-53, under the third pair 54-55, between the fourth pair 56-57 , and so on. The next consecutive warp strand passes between the first pair of weft strands, over the second pair, between the third pair and under the fourth pair. Similarly, the third and fourth consecutive warp strands are woven starting below and between the first pair of weft strands respectively.

S3 represents the wheelbase between the pairs of weft strands, 52-53 and 54-55 and x (see Figure 5B) is again the

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smallest distance between adjacent warp strands in the vertical section taken at the point of tangency between warp and weft. With reference to FIG. 5 A, P represents the smallest distance between the pairs of warp strand crossing taken in the vertical plane halfway between the pairs of weft strands.

The conventional fabric of FIG. 5, in a commonly used mesh range, gives air permeabilities ranging from 11.326 to 25.385 m Vmin / m2. In order to reduce the permeability in this type of construction as indicated above, it is usual to add puffy threads between certain monofilament weft strands as shown in the figure at 58. The puffy threads are normally made of woolen strands, linen, or cotton which become puffy and fill the space between the wefts.

Figures 6,6 A and 6B show the same fabric 40 'as in Figure 5 but with the flat strands 41' -44 'as in Figure 4. It is clear that the distances S4 and y of Figures 6 and 6B are smaller than the corresponding distances S3 and x in FIGS. 5 and 5B. The distance g in Figure 6A is not very different from the corresponding distance p in Figure 5A, but again, due to the reduced spacing S4, the gap based on g is much smaller than the gap based on p.

As also shown in FIG. 6, provision is made,

as an alternative to the bulky fibers of wool, linen or hemp, additional 59 monofilament strands woven into the fabric. As also shown in FIG. 6, the additional strands can have a diamond-shaped or rectangular section, indicated at 60, to fill the passages 61 of the fabric without making it liable to pick up more foreign substances or to retain more of water. Although not shown, when three or more weft strand layers 50,51 are used, two or more passages 61 will form in the area between the adjacent pairs of weft strands, i.e. in the region bounded by S4 through the canvas, and some or all of the passages can be filled with the shaped weft of the invention.

In addition, all weft strands can be made of a plastic polymer which is more malleable and which, under stress conditions during weaving or other processing of the fabric, will allow the weft strands to deform to fill even better the interstices of the mesh and further reduce the permeability of the fabric.

In the case of each of these types of canvas, the reductions s in the dimensions in S2 and S4 and the reduction of x in y are the cause of the reduction in the size of the interstices of the canvas and therefore the cause of the reduction permeability. By using suitable flattened monofilament warp strands and with suitably shaped and more malleable weft strands, the permeability of the fabric can be reduced to a value between 15 and 76 m3 / min / m2 without being necessary to have recourse to bulky "stuffing" threads with the drawbacks that follow.

îs A monofilament drying fabric like that shown in FIG. 5 has a thickness generally greater than 1.778 mm and a coefficient of elasticity greater than 89.280 kilograms per meter. The experimental fabric woven according to the invention and represented in FIG. 6, having 2 "strands of warp flattened in a ratio of 2: 1 and heated normally, had an average thickness of 1.473 mm and an average coefficient of elasticity of 48,032 kilograms per meter. In general, the woven fabric according to the invention will have a thickness between 0.889 and 1.778 mm and a coefficient of elasticity between 26.784 and 53.568 kilograms per meter.

Warp threads and shaped weft threads can be manufactured by a mechanical rolling device to roll monofilament strands of roundness with a diameter ranging from 0.2 30 mm to 1 mm, between pairs of rollers so as to flattening, or similarly flat or shaped strands can be extruded from a die using split wire to produce tape of a material similar to monofilament. The section of a flattened monofilament strand is shown in FIG. 7, where a is the width and b is the thickness. The cross-sectional area of a flattened monofilament warp strand could be in the range of 0.07 mm2 to 0.5 mm2 and the ratio range of a: b would be 1.1: 1 to 3: 1.

The fabric of the present invention would have a com-40 warp, preferably between 12 and 40 strands per centimeter and a weft, preferably between 4 and 40 strands per centimeter.

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Claims (4)

629268 1. Drying cloth for a paper-making machine, comprising warp and weft of monofilaments of plastic polymer, woven at a filling rate by the warp of approximately 100% at least the warp strands (31 ′, 41 '), which are intended to extend in the direction of operation of the machine, having a flattened cross section whose major axis is parallel to the plane of the fabric. 2. The drying fabric of claim 1, wherein the flattened warp strands have a width to thickness ratio of 1.1: 1 to 3: 1. 2 3 629268 to improve the extraction of moisture from the paper sheet. To achieve these results, the threads are woven together tightly and sometimes with several folds to form an even more waterproof canvas. To further reduce permeability, sometimes threads of bulky cut fibers, some containing asbestos, are woven there. These fabrics have an annoying tendency to keep enough water to rewet the sheet. They also become more and more difficult to clean from various foreign bodies such as priming agents, incrustations of bleaching earth and resins, waxes, pitch, and the canvas becomes blocked so that it is necessary clean it frequently or change it. The drying fabrics are usually woven with a filling rate by the warp of about 100%, as shown in the attached drawing and as is well known. “Chain fill rate” is understood to mean the quantity of warp threads in a given space relative to the total space considered. The filling rate can exceed 100% when there are more warp threads piled up in the available space than it can receive in a single plane. Fabrics with a nominal chain fill rate of around 100% generally have an actual calculated rate of 80 to 125%. The rates exceeding 100% are obtained by piling up the warp threads and causing them to undulate laterally. Permeability is an important characteristic of the drying fabric and it is a measure of its air passage capacity. A fabric with low permeability will resist the passage of air and will tend to absorb steam while a fabric with high permeability will allow the free passage of air and vapor. As previously mentioned, the drying fabrics were made of cotton or wool and sometimes contained asbestos fibers. With the development of synthetic materials, traditional fabrics are gradually being replaced by fabrics containing synthetic threads. These can be woven in a simple or complex weaving, in two or three plies or more with either monofilaments with a relatively large diameter, or multifilaments spun from numerous filaments of small diameter. New synthetic yarns, monofilaments, are preferred because the fabric obtained has an increased longevity, is easy to clean, does not lose its fiber and does not retain too much moisture. During the part of the cycle where the fabric is in contact with the sheet on a drying cylinder, a low moisture content and a high permeability improve the heat transfer of the sheet. Also, a high permeability of the fabric can have a beneficial effect on the ventilation of the drying bags, thus producing a more homogeneous moisture profile in the sheet. However, the high permeability of fabrics made of all-monofilament yarns is sometimes a disadvantage because it is the cause of an excessive movement of air in the drying pockets which gives vibrations to the sheet. This problem gets worse with the speed of the machine and you reach a point where the vibrations, especially in the first and second drying sections where the sheet is wet and brittle, are violent enough to cause the sheet to break . The effect of the permeability of the fabric on the ventilation of the drying bags and on the vibration of the sheet has been described by Race, Wheeldon et al. (Tappi, July 1968, vol. 51 No. 7) and they showed that the movement of air in the drying bags is influenced by the permeability rather than by the roughness of the surface of the fabric as previously assumed. . The movement of air is due to the fact that a moving canvas carries layers of air with it. At the surface of the canvas the speed of the air layer is the same as that of the canvas and as the distance from the surface of the canvas increases, the speed of the air decreases. When the fabric is wrapped around a roller, the air layer inside is trapped in the constriction between the roller and the fabric and, if the fabric is sufficiently permeable, the air in the interior is pumped through, joins the air stream on the outer part of the fabric and the combined speed of the two streams is greater than the speed of the fabric. When the fabric is wrapped around the roller, the layers of air on the outside tend to be rejected towards the outside by the centrifugal force generating a tangential air movement. This results in a large mass of air moving laterally towards the outside of the pockets when very permeable fabrics are used on machines at high speeds. The experiences of Race, Wheeldon et al. show that as the speed of the fabric increases, the air which is pumped through the fabric by the felt rollers of the dryer increases in speed, particularly at speeds greater than 1500 rpm. They also show that when the permeability of the fabric is reduced, the amount of air blown into the drying bags is reduced accordingly. Thus, at low speeds a drying fabric of high permeability can be tolerated and, in fact, it is useful for obtaining high drying percentages, but at high speeds, in particular in the first drying section or in the second, fabrics with low permeability-2s of the order of 14 to 56 m3 / min / m2 are necessary. Thus, on high speed machines, it is often not possible to take advantage of the characteristics of ease of cleaning of monofilament fabrics because of their inherent high permeability. 30 "Permeability" is usually expressed by the number of cubic meters of air per minute passing through a square meter of fabric when the pressure drop across its section is 12.7 millimeters of water. One instrument used to measure air permeability is a 35 Frazier air perméometer. In this instrument, air is drawn in by a variable speed suction fan through a one inch square section of fabric taken as a test sample, then through an upper chamber and a lower chamber. 40 which communicate by one of the replaceable calibrated orifices used to measure the volume by differential pressure. The fan speed is increased until the vacuum in the upper chamber reaches 0.5 inch of water as indicated on a pressure gauge. The depression, in inches 45 of water, in the lower chamber is read on another interconnected pressure gauge and this value is plotted on a graph so as to convert the reading into cubic feet of air per minute per square foot of fabric. While with the known drying system the problem of so vibrating the sheet can be overcome by using a drying fabric having low permeability, another method of alleviating this problem is known as a single drying system. canvas. In this method, a simple drying cloth is used to guide the serpentine sheet of paper through the drying sections of the paper machine. The paper, for example, is introduced under the canvas to the first upper cylinder and passes more or less in contact with the canvas through a whole drying section so that it rests between the canvas and the cylinders in the upper row. outside and outside of the fabric around the cylinders in the bottom row. The main advantage of the single cloth drying system is that the sheet of paper and partially supported by the cloth passing between the rows of drying cylinders and the vibration of the sheet is thereby reduced or can be entirely eliminated. Other important advantages of the single fabric drying system are the reduced cost of the 629268 The drying fabric of claim 2, wherein the ratio is about 2: 1. 4. Drying cloth according to one of claims 1 to 3, in which at least some (60) of the weft strands are shaped to effectively adapt to the interstitial passages of the mesh directed along the weft and horizontally, formed naturally by woven warp strands. 5. Drying fabric according to one of claims 1 to 3, in which at least two layers of weft strands are provided, the warp strands forming between them interstices in the zone between the adjacent weft strands, the interstices forming, on at least one horizontal plane of the fabric, horizontal passages, at least some of which are crossed by strands (60) of monofilament-weft of plastic polymer, these strands being shaped to adapt to these passages. 6. The drying fabric of claim 4, wherein at least some of the weft strands (60) have a section which is roughly diamond-shaped. 7. The drying fabric of claim 4, wherein at least some of the weft strands are hollow. 8. A drying fabric according to claim, wherein at least some of the weft strands are more malleable than the warp strands. 9. Drying fabric according to claim 1, having a single layer of weft strands at least some of which are shaped to adapt to the horizontal passages formed by the warp strands. 10. Drying fabric according to one of claims 1 to 3, wherein the flattened chain strands have a substantially rectangular cross section. 11. Drying fabric according to claim 1, the permeability of which is between 15 and 76 m3 / min / m2 depending on the cross-sectional area of the flattened chain strands. 12. Drying fabric according to claim 11, the elastic modulus of which is between 26,784 and 53,568 kg / m. 13. Use of the drying cloth according to claim 1 in a single cloth drying installation in which the cloth follows a serpentine path between upper and lower rows of drying cylinders by supporting a continuous sheet of paper along its path around cylinders. 14. Use according to claim 13, wherein all the drying cylinders are coupled to each other by a set of gears. In papermaking with a Fourdri-nier papermaking machine, for example, an aqueous suspension of cellulose fibers, comprising one part or less of fibers in 99 parts or more of water by weight, flows onto a screen of endless rotary woven formation of metallic or synthetic filaments. This forming strip or fabric or "wire", as it is called, through water extraction devices such as roller tables, drip sheets and vacuum boxes * the water content of the suspension supported by the fabric is reduced to about 80-85 percent. The fine fiber fabric, which now supports itself, is removed from the forming fabric and goes through a series of pressing sections where it is deposited on other endless strips of relatively thick fabric, one or more both surfaces are made of a sewn bat made of synthetic or natural fibers. These bands, called coating felts transport the fabric or sheet through the throttles of the pressure rollers where another part of the water remaining in the sheet is expressed and forcefully absorbed by the felts until the water content is lowered. about 65%; at this stage, it is generally not practical to try again to extract the water by direct extraction such as by pressure or suction. The sheet of paper then passes into the drying section of the machine where the rest of the water is extracted by an accelerated heat evaporation process. The drying section consists of a number of large hollow cast iron or steel cylinders through which the sheet of paper passes in a serpentine shape. The rotation of the cylinders is synchronized to make it easier to pass the sheet. Heat is supplied by vapor condensation inside each cylinder and the sheet is kept in close contact with the heated surface portions of the drying cylinders by the drying fabric. To have sufficient drying capacity, the drying section of a newspaper, for example, can consist of 50 drying cylinders of approximately 1.5 m diameter each and placed in an upper row and one and lower in four or five individual subdivisions. In order to appreciate the importance of a drying section of a modern paper manufacturing machine, the overall dimensions are of the order of 60 m long up to 12 m wide and up to 12 m from above. The sheet of paper can pass through the drying section at speeds of up to 900 m per minute so that one point on the sheet remains in the drying section for only 15 seconds, during which time the sheet will be reduced to a normally dry sheet of paper. The drying fabric is used to hold the sheet of paper against the heated surfaces of the rotary drying cylinders to allow more efficient heat transfer to the sheet by partially removing an insulating air layer that adheres to the surface of the cylinders. The drying fabric is also used to prevent wrinkling of the paper sheet. In the classic drying section there is an upper drying cloth and a lower drying cloth. The upper fabric rolls up and holds the paper sheet against the upper peripheral parts of the upper drying cylinders while the lower canvas rolls up and holds the paper sheet against the lower peripheral parts of the lower drying cylinders. The fabrics are guided by rolls of intermediate fabric placed between the cylinders. The drying fabrics operate in a particularly unfavorable ambient environment in which they are alternately exposed to heat, humidity, heat and dry. They should be flexible in the machine so that they can fold easily around the felt rollers. They must have good dimensional stability and durability characteristics under tension, temperature and humidity conditions prevailing in the drying section of a paper machine. Generally, the drying fabrics are woven from natural or synthetic yarns to form a relatively bulky fabric which will have good absorption characteristics and a high porosity. s 10 15 20 25 30 35 40 45 50 55 60 65 4 drying, elimination "of the felt rollers and a set of extension and guide rollers which are no longer necessary. Also, since the lower row of the cylinders is not encumbered by a separate lower drying felt, waste paper caused by broken paper, which is called "breakage", can be removed more easily. A disadvantage of the single cloth system is that when it is applied to existing drying sections in which all the cylinders are the same size and are driven at the same rotational speed by gears, the traditional monofilament cloth having a coefficient of high elasticity, is not very extensible and will try to force the upper cylinders, which have a larger effective diameter due to the paper layer, to have a lower rotation speed. This braking action of the cylinders by a force tending to stretch the fabric, produces a considerable stress on the transmission system and even when the paper is rather thin, the stress forces are sufficient to cause abnormal wear of the teeth and bearings of the gear wheels, in some cases damaging them. The extension of the canvas, called elongation of the canvas due to the difference in the lengths of the path of the canvas on the cylinders, lies in the elastic range of the canvas and is proportional to the thickness of the sheet of paper. . The stress, expressed by the value of a torque, on the gears of the drying cylinders, is proportional to the product of the thickness of the paper and the elasticity coefficient of the canvas. For example, in a single cloth drying section where the thickness of the paper sheet is only 0.304 mm, the calculated torque developed on the drive wheel of an upper cylinder will total 2000 kg m. That said, it is clear that the problem of wear and damage to the gears will be substantially reduced by using a fabric having a lower coefficient of elasticity so that it elongates more easily and can absorb stress. developed by the differences in drying cylinder diameter due to the thickness of the paper. While the example above shows what may be the magnitude of the stress developed by a relatively thin sheet of paper, it should be noted that differences in cylinder diameters due to wear or thermal expansion due to temperature differences, can also have destructive effects which can be mitigated by the use of a drying cloth having a lower coefficient of elasticity. The stress problem can be overcome in all cases where it is possible to disconnect the upper transmission from the lower transmission so that only the upper cylinders or the lower cylinders are driven. In some cases the disconnected cylinders rotate in free rotation by the drying cloth and it does not matter if they rotate at a different speed. However, in some installations, it is not possible to disconnect drive wheels, and this is where it is advantageous to use a fabric having a small coefficient of elasticity. Another disadvantage of the single fabric drying system comes from the relative thickness of a conventional fabric. For example, when the wet sheet of paper goes from an upper drying cylinder, where it is under the canvas, to a lower drying cylinder where it is on the canvas, it is lengthened because of the difference in diameters. This elongation, or stretching of the paper, is proportional to the thickness of the canvas. Since it is easily stretched, the wet sheet of paper will accommodate the stretch. However, by continuing its course from a lower drying cylinder to an upper drying cylinder, a negative elongation is created and, because the sheet of wet paper is not elastic, it separates from the canvas and lifts up. forming waves so that it folds or overlaps on itself before passing under the canvas to the upper drying cylinder, thus canceling the support effect of the canvas. It is therefore clear that it is advantageous to use the thinnest drying cloth possible in the single cloth system.