EP1092797B1 - Knitwear - Google Patents

Description

The present invention relates to a single or multi-layer Knitted fabric with high stability, variable pattern and design, defined elasticity and a method for its Manufacturing, as well as the use of the knitted fabric for a self-supporting Seat cover and a seat with the knitted fabric according to the invention.

Such knitted fabrics are especially designed for applications where on the one hand a high mechanical resilience and on the other hand specifically requested a desired elasticity is. Seating is a preferred application. The technical Construction of seating furniture with textiles as a seat was carried out So far, using a framework in which a - usually woven - textile was fixed laterally by springs. The woven, largely rigid textile showed a high mechanical Resilience on and the elasticity was due to the connection the textiles are realized by springs on the frame.

In connection with the conception of new covers one would like to Replace the use of springs as this will change the visual appearance of the seating furniture have a significant say. Farther there is always a risk when using springs that lighter garments in the feathers or in the spring suspensions get stuck. The rigid fabric, however, fits only partially different body shapes, supports on the other hand not at essential points, e.g. the lordosis area.

It is therefore an object of the present invention to provide a method for Creating a mechanically stable knitted fabric, not only high mechanical strength, but also elasticity variable in different areas and Designation with possible use of different, in particular has elastic materials. A relevant procedure e.g. in EP-A-0 503 836.

Such methods are the subject of claims 1 and 2. Advantageous Further training is the subject of the corresponding Dependent claims.

In a first process for producing a - in particular multilayer - knitted fabrics with selectively adjustable cross stability and transverse elasticity, one works with a basic bond in at least one knitted layer, which basic weave stitch rows contains, for the most part fleets with a length of Have a distance from at least two active needles, which offset in successive courses are. The lateral stability can be partially changed here the fleet length can be set and additionally if necessary about the use of more or less elastic materials in different areas of the knitted fabric. According to the invention, the basic weave is in the seat and / or back area of a piece of seating furniture is at least partially replaced by a net tie.

In a second production method according to the invention one - especially multi-layer - knitted fabric with specifically adjustable Cross stability and elasticity become a basic bond used in at least one knitted layer, which Basic weave a sequence of first and second courses has, with in the first courses with the predominant Part of the active needles forms a stitch, and in the second row of stitches mainly with fleets a length of the distance of at least two active needles. By choosing the successive sequence of the first and second rows according to the requirements of the Elasticity and transverse stability of the knitted fabric can be combined in one Knitted fabric partially elastic and stable Set ranges. In cross-elastic knitted areas the proportion of first rows of stitches is higher than in cross-stable ones Knitted areas. Called cross-elastic or cross-stable the direction of the needle bed. According to the invention, the basic weave is in the seat and / or back area of a piece of seating furniture is at least partially replaced by a net tie. The stability can be not just through the sequence of the first and second rows of stitches regulate, but also by the length of the fleets, being a longer fleet length a higher lateral stability with reduced Longitudinal stability. However, this can be increase with a warp thread entry, for example.

The knitted fabric preferably contains at least one basic weave two layers of knitted fabric, preferably separately on at least a front and rear needle bed of a flat knitting machine getting produced. Each layer contains an alternating one Sequence of first courses in which at least with the majority of the active needles formed a stitch and second courses where fleets are trained are. The fleets are generated by at most everyone second active needle is knitted so that the generated Fleet at least the length of the distance between two active needles to have. The first courses are preferably with knitted at least approximately all active needles.

During the first row of stitches, a typical knit Elasticity is inherent in the second row of stitches a higher transverse stability, i.e. stability in the direction of the needle bed or in the direction of the courses. This stability is higher the longer the fleets and the closer the sequence of the second rows of stitches in the Are basic. On the other hand, the fleet length increases also the longitudinal stability, i.e. the stability in the direction of the rod from. Here the strengths in longitudinal as well in the transverse direction, however, by selected warp and / or weft insertion increase.

Due to the sequence and number of the first and second courses, e.g. a first course followed by two second courses or two rows of stitches followed by one second course or alternating sequence of one first and second rows of stitches, can be the desired property of the knitted fabric, either a higher elasticity or a higher transverse stability, individually in the desired way, in particular also partially and with the selection of different ones Materials can be controlled. This can - like already described - also by the length of the fleets in the second rows of stitches.

An alternating arrangement is preferably used in the basic weave used by four courses. The first two rows of stitches have already been described in advance. To the second row of stitches is followed by a third row of stitches, which is formed in the same way as the first course and the third row of stitches is followed by a fourth row of stitches, which in turn are formed in the same way as the second course is, however, the fleets of the fourth course are offset from the fleets of the second course. In this way, the overall stability of the knitted fabric is increased.

Mostly the two layers of knitted fabric are separately on a front and knitted back needle bed. So you knit ultimately two separate layers, in their edge area can be entangled or otherwise connected. In this way, a kind of large hose or even each creates any other contour that only on one Frame must be stretched. In this way there is no need the arrangement of separate fastening elements on the knitted fabric, which, however, are additionally or alternatively provided can.

The fleets of the second are also in a very stable structure and fourth courses in the two superimposed Layers were staggered, which in turn is analogous to offset clinker brick construction on house walls, positive for the overall stability of the knitted fabric.

The two layers of the basic weave can also be made with a pile thread or connected by other connection techniques are, which also changes the knitting thickness and others Parameters such as let the strength be influenced.

According to the invention in areas in which a higher elasticity one or more further knitted ties are to be provided provided in the form of a network tie, within the the two layers of the basic structure are connected to one another in this way be that a net-like high-strength, but also highly elastic strongly broken structure is created. Even in these areas can the elasticity and strength by a corresponding Choice of knitted weave and the materials used direct control. This structure is for seating To be provided in the back area and / or seating area, in which there is good ventilation in addition to the required elastic properties have to be. This network tie is characterized by a connected single or multi-layer structure, in the first Stitches form a stitch on each active needle however, the mesh thread at defined needle intervals on the needles of at least one second needle bed Catch is put. In one or more subsequent second Rows of stitches are separately floated on the first needle bed formed, these fleets in several successive second rows of stitches preferably offset from each other are. In the third row of stitches that follows, repeat knitted a structure of the first course. In the fourth row of stitches will finally catch the needles of the second bed hung back on the first needle bed, whereby the holes in the network structure arise. These four courses are successively used to manufacture the network structure knitted. About the distance of the catches on the second needle bed and possibly repeating the second course the mesh size of the network structure can be set as desired. The network structure can of course also be done via set other parameters, e.g. the selected needles, the distance between the catches and the selection of the straps Needles. Larger holes are usually made by pure Covering / transferring of stitches. For stabilization can be a moving one, especially in the hole edge area Weft inserted. The optional or integrated fleets serve as pure stabilization elements and have no significant influence on the optics. additionally can increase the stability between the two layers Pole thread may be provided.

By combining the basic structure with the network structure can also achieve a desired structuring of the knitted fabric become. Furthermore, by using the Network structure in the basic structure at targeted points e.g. at the Seat or back area of a piece of furniture is more elastic and Provide openwork areas. By a defined - in particular also partial / sequential - connection of basic structure and additional knitted fabrics, preferably different ones Network structures, with different ones (Mesh) strength and rigid or elastic materials can be self-supporting and especially patterned Manufacture seat covers.

The knitted structures mentioned above are excellent suitable for producing a self-supporting structured Seat cover, with the self-supporting knitted fabric both the mechanical properties are taken into account as well as the optical properties due to the shapely and structure of any structure. That way you can the stability properties as well as the seating comfort realized what was previously not considered possible. On such a seat element made from the above knitted fabric can be as two-dimensional, two-and-a-half or three-dimensional Structure are made. Of course you can the knitted fabric, especially in the edge area, by additional Fasteners are attached to the frame. Preferably However, the tubular structure is to be selected, in which no separate fasteners between the knitted fabric and the supporting tubular frame must be provided.

Fig. 1

das Nadelschema einer Grundbindung des erfindungs gemäßen Gestricks;

Fig. 2

das Nadelschema einer Netzbindung des erfindungsge mäßen Gestricks;

Fig. 3

die Anwendung von Grundstruktur und Netzstruktur in der Rückenlehne eines Sitzmöbels,

Fig. 4

einen Querschnitt IV-IV durch die Rückenlehne aus Fig. 3,

Fig. 5

Fäden, die in Randbereichen des Bezugs verwendet werden können,

Fig. 6

die Herstellung eines Loches 40 in einer Netzstruktur,

Fig. 7

ein Verfahren zum sauberen Stricken von bogenförmigen Rändern, und

Fig. 8

einen Ausschnitt einer Grundbindung.

The invention is described below, for example, using the schematic drawing. In this show:

Fig. 1

the needle diagram of a basic weave of the inventive knitted fabric;

Fig. 2

the needle diagram of a net binding of the knitted fabric according to the invention;

Fig. 3

the application of the basic structure and network structure in the backrest of a piece of seating furniture,

Fig. 4

3 shows a cross section IV-IV through the backrest from FIG. 3,

Fig. 5

Threads that can be used in the edge areas of the cover

Fig. 6

the production of a hole 40 in a network structure,

Fig. 7

a method for cleanly knitting arcuate edges, and

Fig. 8

a section of a basic tie.

Fig. 1 shows the structure of the basic weave of a knitted fabric present invention.

According to Fig. 1 a) are on the needles of the front needle bed Stitches formed, one stitch being formed on each active needle becomes. In the same row of stitches is also according to Fig. 1 b) on the rear needle bed in each active Needle formed a stitch. 1 c) and d) now show the Formation of the second row of stitches. According to Fig. 1 c) are on the front needle bed fleets formed by the mesh thread only on every second needle. He can alternatively, it is also used on every third, fourth, etc. needle become. It is still possible not to run through the fleet But to form the thread at the connection points by laying catch. In the same way in Fig. 1 d) the fleets formed on the rear needle bed, the Fleets on the back needle bed opposite the fleets are offset from the front needle bed. These fleets according to Fig. 1 c) and 1 d) give according to the chosen length and the Sequence in the knitted fabric of the basic weave increased Stability in the longitudinal direction of the needle bed. In the procedural steps e) and f) is again first on the front and then a stitch on the back needle bed on each active needle educated. In Figs. G) and h) are again in one fourth row of stitches creates fleets by turning the mesh thread only with every second, fourth, sixth, eighth, tenth etc. active needle is connected. It should be emphasized that the liquors according to process steps g) and h) in the fourth stitch row compared to the fleets in the process steps c) and d) in the second row of stitches by one each Needle position are offset. Of course, the sequence of meshes and fleets chosen arbitrarily become. For example twice in a row the first or third rows of stitches according to process steps a), b) or e) and f) are formed and / or several times in succession, i.e. in several rows of stitches in a row, fleets formed according to process steps c) and d) or g) and h) become. By choosing this sequence and by choosing the fleet lengths in process steps c), d) and g) and h) is the elasticity or stability of the knitted fabric set in the desired manner. If the fleets according to Process steps c), d) and g) and h) not cumulated, but can only be placed on catch, a further increase can be made of stability. The basic structure can also be only by offset fleets according to steps c + d, g + h produce.

Fig. 2 shows the needle diagram for the net binding, which e.g. in insertable into the basic binding in the manner shown in FIG. 3 is. Due to the net tie a heavily broken and provided more elastic structure than the basic weave, which primarily has a high stability and as a support structure serves. The network binding is a connected double-surface structure.

In a first row of stitches, as shown in FIG. 2a, on the back Needle bed cumulated on each active needle during the stitch thread on needles of the front needle bed in defined Distances, in the present case at a distance of two needles, is caught. Then in a second and third row of stitches as a succession of two second rows of stitches can be viewed on the back needle bed Fleets knitted that are each six meters long Extend needles, the fleets of the second and third Row of stitches are staggered about 1: 1. In a The fourth stitch row according to FIG. 2d is the method step 2a repeated and the catch is in the following fifth row of stitches as shown in FIG. 2e hung back again. In process steps f) to k) the education of these five courses repeated, with the fleets of the seventh and eighth course according to the procedural steps g) and h) compared to the fleets in the second and third course are again offset according to b) and c). The net-like Structure is created when the catches hang back from the front needle bed on the back needle bed according to the procedural steps e) and k) reached. The applied catches, selected needles, Length and integration of the fleets and transfer of the needles or However, catches can also be chosen individually.

Fig. 3 shows the backrest of a seat cover 10, the one Tubular frame 12 is stretched. The double-layer cover can be single or double-bed knitted. The tubular frame 12 is preferably made of painted or chromed Steel pipe or another precious metal, plastic or Light metal or composite materials from the above materials. The Seat cover 10 is in the entire base area 14, especially in the area the suspension formed by the basic bond according to FIG. 1. There are several mesh areas 16 in the backrest area provided that are knitted with the net tie. In these Mesh areas 16 is the elasticity due to the binding character the net tie increases. In addition, the knitted fabric is in this network areas 16 heavily broken, so that the corresponding Passage of the back is well ventilated. Through commitment these areas 16 can be made using other binding techniques Create inlaid patterns of variable design. The seat cover 10 thus looks visually appealing and at the same time has one high functionality. The edge area 13 of the cover is with Rubber thread worked. This is the case with arcs or semicircular shapes usual staircase structure - in the picture at the top Edge area - neatly laminated. The one with a rubber thread Knitted edge area also serves as a shock absorber that occurs quickly high mechanical stress on the cover. hereby edge areas can also be made insensitive to impact. in the lower end of the cover can be reinforced Perforated strip or perforated edge strip 17 for fixing the cover on a crossbar can be integrated. An improved connection the reference to the perforated edge strip 17 can be used of fuses. When pinching the cover between two heated parts of the perforated edge, the Knitted structure in the perforated edge area are fused, so that in the area of the holes no overuse of individual Stitches or stitch areas occurs. The area 15 between Perforated edge strip 17 and frame 12 is preferably linked or also knitted with melting thread, so that the subsequent Heating this area 15 by fusing the knitted structure a clean and mechanically insensitive Conclusion is achieved.

At the lower edge of the cover, a fixing strip is worked into the cover,

FIG. 4 shows the cross section IV-IV from FIG. 3. Here it is clear to see that the basic bond consists of the two Layers 18 and 20 in the base region 14 between the tubes 12 of the Chair frame is stretched. It is also easy to see that the two layers 18, 20 of the basic bond in the area of Net tie 16 are interconnected, this network area 16 for the appropriate ventilation of the backrest surface provides.

Fig. 5 shows threads used in edge areas 13, 15 of the cover can be. The thread 28 shown in Fig. 5a) has a PES / PA base 30 and is defined with a thermal thread 32 Shrink wrapped. When heating with such Thread 28 knitted area 15 pulls the knitted fabric together in a defined manner and possibly merges something so that a good attachment to the frame 12 or one defined voltage in the corresponding area 15 and a smooth finish are generated. Fig. 5b) shows an elastic Thread 33 with a PES / PA base thread 34, which with a Rubber thread 36 is twisted. Even with such a thread can be a good fit of the cover on the frame and a lamination reach from arched staircase areas (upper backrest edge). By using this thread in the frame area 13 an area knitted with this thread 33 also shock absorbing or -damping properties.

6 shows the production of a hole 40 in a network structure 42. The hole is created by transferring stitches 44 Knit outwards according to the arrows and then knit together of stitches. There are two in the edge of the hole Warp threads 46, 48 guided, which give the hole good stability. In this way, stable network structures can also be created with large holes that are visually appealing and ensure good ventilation for use in the seat cover area.

Fig. 7 shows the method for clean knitting of arcuate Edges. To become one or more rows of stitches Knitted rubber thread. Then the rubber coating produced in this way Bar 50 gradually connected to areas with are knitted with normal thread. First, one goes to the bar 50 central section 52 knitted, the height of one or has several rows of stitches. Then the neighboring ones Areas 54 additionally knitted, again over a or several rows of stitches. The knitted area expanded over the areas 56 and 58 until finally over the entire area The width of the last 50 is continued with normal thread becomes. You get a smooth clean curved edge that by using the rubber thread, e.g. 33 from FIG. 5, resiliently applied to the frame.

Fig. 8 shows a section with a length of four courses A to D and a width of four meshes 1 to 4 a basic bond. The courses A to D of this basic weave contain staggered fleets at the points 1 to 4 are meshed. You can also click on these points Be caught. By choosing different thread materials and different fleet lengths, the elasticity or stability properties of the basic weave in each stitch row A to D and individually at all meshing points 1 to 4 be managed.

3 and 4 illustrate that not only by the execution the basic weave and net weave per se the elasticity or Stability of the knitted fabric defined at different points is adjustable, but this choice can continue through targeted Arrangement of network areas 16 within the basic bond areas 14 are supported. Here too, the high Functionality of the arrangement of these areas with a corresponding Decor effect can be combined.

The invention is not limited to the described embodiments limited. For example, the first (front) layer with the second (Rear) layer connected by inserted / bound pile threads become. To stabilize the knitted fabric in the longitudinal direction (Stick direction) or cross direction can be partial / sequential also warp / weft threads especially made of high tenacity Materials are inserted / integrated.

The first layer can be different from the second layer Strengths and materials are worked. Of course, the layers can also be partially different Strengths, different materials or material hybrids - in particular elastic or shrinking - or variable binding structures are worked. Using from shrinking textile threads, the knitting is only on the Frame stretched and then shrunk so that you have an optical attractive tight fit of the knitted fabric on the frame, if necessary, avoiding gradations or stairs receives.

You can also use elastic materials if there is a decrease / increase in the 'stairs / peaks' in the edge area be laminated in the horizontal / vertical border area the 'rubber thread mesh' serves as a shock absorber.

In addition to different network structures, other bonds can also be created can be integrated with similar properties.

A stable perforated edge as a finish serves for fixation and tensioning the cover in the direction of the rod. Of course you can other fasteners can also be integrated directly.

On a four-bed machine, a stable 'hose cover' are made with two layers of knitted fabric the advantage that the desired stability / elasticity through different binding structures easier, more economical and is more durable to achieve.

Knitting with a twin assembly offers in particular that easy way, about stable passages with a + b needles stretchy passages to work only with a needles.

Large network holes can also be caused by moving Stitches can be achieved. To stabilize the edge area an additional thread is integrated.

In addition to the different network structures or ties Similar properties can also be used effectively in patterns known intarsia technique, these elements are incorporated then preferably consist of elastic materials.

The boundaries that are necessarily open are linked or sealed by a fuse.

A tube cover can also be made using of elastic materials. This tube cover will then pulled over a foam core.

The basic binding can be done by different combinations of all or only selected needles with / without elastic threads as well like the network structure or bond structure with / without fleets in Elasticity and stability can be defined.

In elastic areas, loops or warp or weft threads can be used specifically made of elastic threads with a defined stretching behavior be manufactured or incorporated. In this way can be a specific tension in a knitted fabric adjust or marginal parts can be worked so that they are close to the frame.

The knitted fabric of a seat cover preferably consists of a Multi-layer structure. To achieve curved, cylindrical convex or concave deformations of the knitted fabric can be the different Layers with different mesh and / or Number of double crochet.

Preferably, a seat is made with a cover made of there is a multilayer structure. The layers are then advantageous different functions assigned. For example, a lower layer can be designed as a carrier layer, while a A seating position oriented towards the seat Function, e.g. perforated areas or moisture absorbing Properties. These layers can be together entangled or connected by a pile thread his.

In one embodiment of the invention is a seat provided with a cover made of an elastic knitted tube is made. This elastic knitted tube will stretched around the frame and thus fixes itself on Frame. Elastane or material hybrids can be used as the material elastic components are used.

Claims (22)

1. A method for producing a knitted fabric (10) featuring engineered crosswise stability and crosswise elasticity, for use on seating furniture, including a ground knit (14) in at least one ply (18, 20) of said knitted fabric, said ground knit (14) featuring courses comprising predominantly floats having a length of the spacing of at least two active needles staggered in said courses in sequence, said crosswise stability being engineered partially by varying the length of said floats, wherein said ground knit (14) is replaced at least in part by a netting knit (16) in the seat cushion and/or backrest region.
2. A method for producing a knitted fabric (10) featuring engineered crosswise stability and crosswise elasticity, for use on seating furniture, including a ground knit (14) in at least one ply (18, 20), said ground knit comprising a sequence of

   first courses in which a loop is formed by said predominant portion of said active needles, and

   second courses comprising floats predominantly having a length of the spacing of at least two active needles,

   and suitably selecting the successive sequence of said first and second courses in accordance with the requirements for said crosswise elasticity and crosswise stability of said knitted fabric (10), the proportion of first courses being higher in said more elastic portions of said knitted fabric than in said crosswise stable portions of said knitted fabric, wherein said ground knit (14) is replaced at least in part by a netting knit (16) in the seat cushion and/or backrest region.
3. The method as set forth in claim 1 or 2 wherein structuring said netting knit (16) comprises the following steps:

   a) sinking said interlooping thread in at least a first needle bed at each selected needle in first courses, said interlooping thread being tucked in defined needle spacings on said needles of at least one second needle bed,

   b) producing long floats on said first needle bed in second courses,

   c) repeating step b) as often as is needed, the floats of adjacent second courses being staggered relative to each other,

   d) repeating step a) in third courses, and

   e) returning said tuck from said needles of said second needle bed to said first needle bed in said fourth courses, said first, second, third and fourth courses following each other in succession in the above scheme.
4. The method as set forth in any of the preceding claims wherein knitting in elastic portions of said knitted fabric is done with an elastic thread such as e.g. of rubber or elasthane.
5. The method as set forth in claim 3 wherein knitting in said crosswise stable portions is done only with second courses, the floats of which are preferably staggered relative to each other.
6. The method as set forth in claim 3 wherein said crosswise stability is engineered via the float length of said second courses.
7. The method as set forth in claim 1 or 6 wherein in said crosswise stable portions the diminished lengthwise stability of said knitted fabric (10) due to the increased float length is compensated by inserting warp threads (46, 48).
8. The method as set forth in any of the preceding claims wherein disposed between said plies (18, 20) of said netting knit (16) is a pile thread joined in defined spacings alternating with both plies by sinking or tucking.
9. The method as set forth in claim 7 or 8 wherein said ground knit (14) and netting knit (16) are arranged in accordance with a predefined decorative pattern.
10. The method as set forth in claim 7 to 9 wherein said netting knit (16) is provided in more elastic portions of said knitted fabric and/or as a breathability zone.
11. The method as set forth in any of the preceding claims wherein disposed between said plies (18, 20) of a multi-ply ground knit (14) is a pile thread alternating joined in defined spacings to said two plies (18, 20) by sinking or tucking.
12. The method as set forth in any of the preceding claims wherein said floats in several courses in said netting knit (16) and/or in said ground knit (14) are interlooped staggered - more particularly 1x1 - or tucked.
13. The method as set forth in any of the preceding claims wherein said floats in successive courses in said netting knit (16) and/or in said ground knit (14) are interlooped staggered - more particularly 1x1 - or tucked.
14. A knitted fabric (10) featuring engineered crosswise stability and crosswise elasticity for use in seating furniture, said knitted fabric (10) including a ground knit (14) comprising

    a) courses comprising predominantly floats having a length of the spacing of at least two active needles staggered in said courses in sequence, said crosswise stability being engineered partially by varying the length of said floats
    and/or

    b) a sequence of first courses in which a loop is formed by said predominant portion of said active needles, and second courses comprising floats predominantly having a length of the spacing of at least two active needles, and suitably selecting the successive sequence of said first and second courses in accordance with the requirements for said crosswise elasticity and crosswise stability of said knitted fabric (10), the proportion of first courses being higher in said more elastic portions of said knitted fabric than in said crosswise stable portions of said knitted fabric, wherein said ground knit (14) is partially replaced in said seating cushion and/or backrest portion by a netting knit (16).
15. The knitted fabric as set forth in claim 14 comprising a multi-ply structure in which the various plies (18, 20) are knitted differingly in loop and/or wale count.
16. The knitted fabric as set forth in claim 14 or 15 wherein a visual ply of said knitted fabric (10) comprises an intarsia knit for incorporating logos or design elements.
17. Use of said knitted fabric (10) as set forth in any of the claims 14 to 16 for a self-supporting seat cover affixed to a frame (12).
18. A seat comprising a frame (12) between the rods of which a self-supporting knitted fabric (10) as set forth in any of the claims 14 to 16 is tautened as a seating cushion and/or backrest.
19. The seat as set forth in claim 18 wherein the seating cushion and outer portion of the backrest are formed by said ground structure (14) and a more central portion of said backrest and/or seating cushion is formed by said netting structure (16).
20. The seat as set forth in claim 18 or 19 wherein said ground knits (14) and/or netting knits (16) are fabricated partially of shrink-fit and/or elastic materials or hybrids.
21. The seat as set forth in claim 18 to 20 wherein said cover thereof comprises a multi-ply structure including a backing ply, and a ply facing said seat cushion is engineered for breathability, said plies (18, 20) being interknitted more particularly by a pile thread.
22. The seat as set forth in claim 18 to 21 wherein said cover is made of an elastic tubular knitted fabric tensioned on said frame (12).

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