The double needle bar raschel

The double needle bar raschel, as designed by Redgate, later developed into a general-purpose machine, mainly knitting shawls and scarves. At first, the needle bars were arranged back-to-back alternately, as on rib weft knitting machines, but they were soon placed exactly behind each other for convenience of guide bar swinging. Between six and eight guide bars were employed, together with various attachments such as a fall-plate, a crepeing motion (which could disengage one needle bar for a pre-selected number of courses), a switching device for moving the guide bar push-rod from one track of the pattern chain to another, and simple weft inlay or insertion. The front needle bar could be replaced with a point bar for making plush and pile structures or removed altogether so that the back needle bar could knit single-faced fabrics driven by a new set of cams, which doubled its knitting speed.

Improvements in weft knitting and single-bed warp knitting machinery left the double needle bed raschel isolated as a slow, coarse-gauge and very cumbersome type of machine until comparatively recently. However, the arrangement of the elements and knitting action of the raschel is less complex than that of the simplex machine, thus offering greater possibilities for adaptation and modification in order to knit special structures at economical speeds, so it is in this direction that developments have occurred.

29.5.1 The conventional knitting action

On the conventional double needle bar machine, each needle bar in turn is active only for half of the 360 degrees of the knitting cycle. Holding-down sinkers are therefore unnecessary as the other needle bar is in the low inactive position and will restrain the fabric loops.

Figure 29.4 shows the knitting action on the front needle bar; a similar action occurs on the back needle bar (for simplicity, only one guide bar is illustrated).

  • a) The front needle bar rise. The front needle bar is raised to clear the previous course of overlaps from the latches whilst the back needle bar holds the fabric loops.
  • b) The overlap. The guide bar swings through between the needles to the front of the machine. It is shogged for the overlap and then swings back.
  • c) The knock-over and underlap. As the needle bar descends to knock-over, the guide bar performs the underlap shog.
  • d) The third swing of the guide bar. The guide bar now swings over the front needle bar in order to allow the back needle bar to rise and begin its knitting cycle.
Waffle Knit Diagrammatic Notation

Fig. 29.4 Knitting action of a double needle bar raschel.

Fig. 29.4 Knitting action of a double needle bar raschel.

In order to increase knitting speeds on modern machines, the 180-degree dwell period of each needle bar has either been reduced or eliminated. In the latter case, one needle bar is rising as the other is falling so that the two needle bars are almost continuously moving in opposition, thus effectively doubling the knitting speed. Some machines also have a counter needle bar motion so that the needle bars and trick-plates move towards the guide bars and thus reduce the guide bar swing. As the needle bar in these cases has only a short dwell period and sometimes separate fabric sections are being knitted on each needle bar, holding-down sinkers are necessary.

29.5.2 Double needle bar raschel products

In the past, double needle bar raschels of 24-gauge and coarser were used to knit fancy fabrics in woollen yarn for baby-wear, nightwear and knitwear. Two such structures were rib and crepe. In the former, certain needles were never overlapped, whereas the latter is actually a knop fabric produced by taking the back needle bar out of action for between two and four courses to hold its loops whilst the front bar continues to knit. Fabrics of this type have faced increasing competition from the improved design possibilities now offered by flat weft knitting machines.

Two other structures that occasionally achieve a limited success in underwear or outerwear are waffle fabric and Brynje string vest, both of which were originally developed in the early 1950s as thermal underwear fabrics for US forces serving in cold climates. Both are produced with two half-threaded guide bars although two other guide bars are often also used to produce the selvedge edges for making up. In 24 gauge, 22/1 NeB combed cotton would be a suitable yarn count.

String vest is a double-faced net structure with the underlaps hidden inside. Because it is a double needle bed fabric, the net openings are only half as large as the lapping movement representation.

Waffle fabric is a solid fabric composed of a series of open pockets alternately placed on both sides of the fabric. Each guide bar makes overlaps over two needles, which draws their two adjacent wales together thus leaving a gap between every two wales. Gaps on one side are opposite the two connected wales on the other. This arrangement would give the fabric the appearance of a 2 x 2 rib but after five courses, the lapping movement is changed causing the gaps and connected wales to change positions.

29.5.3 Length-sequenced articles

Some raschel double needle bed products are in the form of articles, a number of which can be simultaneously knitted side-by-side across the needle bed. These articles have a length repeat composed of sections of fabric where the lapping cycle of one or more of the guide bars has been altered. The sequence involves a pattern-change device for counting the number of repeat lapping cycles in each section and for initiating a changeover of guide bar push-rod control from scanning links in one chain track on to those in another track, in order to alter the lapping repeat for a particular guide bar. By this method, a guide bar may be controlled from a choice of two or more chain tracks, each having a short, simple repeat of chain links that may be used any number of times, instead of being controlled from one track of an excessively long and expensive chain containing links for every repeat cycle throughout the length of article.

The principle of 'pattern changing' is used in the production of a scarf with knitted-in fringes on each end. Lapping for the scarf section is taken from one set of chain tracks and lapping for the fringe section from another. Each guide bar shog-ging lever may be controlled from either of two pattern chain drums; the upper drum chain tracks may produce the simple lapping repeat for the scarf section whilst the lapping for the fringe section is achieved by switching the shogging control to the chain tracks of the lower drum.

The scarf fabric is knitted as a continuous strip of double-faced fabric with the fringe sections composed of two-wale wide strips, each unconnected by underlaps to its neighbour. Each scarf piece is separated from the next by cutting through the centre of the fringe section and seaming the cut ends to secure them. The simple tricot lapping movement produces the width-wise elasticity required for scarves.

29.5.4 Tubular articles

A seamless tube of fabric may be knitted on a rectilinear double needle bed raschel in a similar manner to on a V-bed flat weft knitting machine. Each bed knits separate single-faced fabrics that are joined together only by underlaps of other partly-threaded guide bars between the beds at the two opposing selvedge needles at each edge. The underlaps may be arranged to be the same as for the needle beds, thus producing a seamless join to the fabric tube.

Figure 29.5 illustrates the basic principles using a base structure of single tricot lapping and four guide bars. The front bar laps only the front bed, the back bar laps only the back bed, and the two middle bars are threaded with only one thread to each complete one selvedge join.

In the first underlap movement towards the right, the warp threads will rotate anticlockwise by one needle space in producing the tube on the machine beds. Underlapping on the front bed will be towards the right. The right-hand selvedge

Slip Stitch Yarn Notation Bed
  1. 29.5 Lapping diagram and notation of a seamless tube knitted on a double needle bar raschel.
  2. 29.5 Lapping diagram and notation of a seamless tube knitted on a double needle bar raschel.

bar will underlap across from the front to the back bed. The back bar will underlap towards the left and the left-hand selvedge bar will underlap across from the back to the front bed. In the next underlap movement, the direction of lapping will be reversed for each of the guide bars, by a clockwise movement.

As one selvedge bar is always overlapping one needle in each bed, the threading of the front and back bars must be one less than the number of needles knitting the fabric in that bed. Two selvedge guide bars are required because when one is overlapping the front bed in a particular cycle, the other is overlapping the back bed.

Whilst knitting the tube, no guide bar must overlap on both the front and back beds during the same cycle, otherwise a single-thickness double-faced stitch is produced. If the base movement is a two-needle underlap, two selvedge threads will cross over the beds at each selvedge and each will require a separate guide bar. If the base movement was full tricot, a minimum of eight guide bars would be required, two for each bed and two for each selvedge. Inlay net or part-sett threaded net lapping movements may be used to produce tubes in a similar manner.

Some of the first tubular fabrics produced were for vests or for fishnet stockings, knitting eight to twelve tubes side by side. In 1967, the American Kidde Cocker company introduced the Fashion Master machine for knitting panty-hose and body stockings. By changing the lapping movement of an extra four bars that are lapping in the centre of the fabric, the large tube for the body portion can be divided into two smaller tubes with two of the bars joining two opposing needles across the needle beds for the inner selvedge of one leg, and the other two joining the adjacent needles for the other leg, thus knitting a bifurcated article. Graduating stiffening is achieved by infinitely-variable control of the fabric take-down and warp let-off, a shifting control moves the guide bar push-rods onto other chain tracks when required, and reinforcement is achieved by double-needle overlapping. For approximately two years, hosiery produced on these machines was highly popular.

The Karl Mayer HDR 16 EEW machine was introduced in 1970 for producing a range of simple garments such as seamless panties, brassieres and pocketings. The technique used, which has undergone continuous development, is to form the tube across the knitting width rather than down the wales. Although this causes the article in use to have its courses in a vertical direction, this is no major disadvantage and the possibilities for achieving simple shaping are considerably improved.

Figure 29.6 illustrates the production of a strip of briefs fabric; it is only necessary to cut through the centre of the connecting joins to separate each article from the next. These joins of short length are, in effect, knitted side seams, so the briefs are turned inside out after knitting to hide this seam. The first side seam is produced by guides lapping across between the two beds to form a solid double-faced fabric section. Guide bars inlaying on the left selvedge form the knitted-in waist band which is produced on each bed because the guide bars lap on the two needle beds separately in order to produce the waist opening on the left and the first leg opening on the right. Half-way through the courses for the sequence, the right selvedge needles are joined together for a number of courses to complete the first leg opening and close the crotch section of the brief. Single-bed fabric knitting then continues for the second leg after which the bars knit between the beds to form the second side seam and then commence the sequence for the next brief.

On a 75-inch (190 cm) wide machine, three brief fabric strips can be knitted side by side giving a production of 360 briefs per hour. It is possible to achieve a cotton terry effect on the inside if desired. Upper and lower pattern chain drums are employed to control the guide bar shogging levers and these drums may have a split drive and chain stop facilities to further economise on links and provide greater versatility in lapping movements. The double needle bar raschel in 12-16 gauge has proved particularly useful for the production of packing sacks for fruit and vegetables made from poly olefin in fibrillated tape or mono-filament form [4,5]. The base structure is usually a pillar stitch inlay that provides a secure non-slip construction (Fig. 29.7). The polyolefin sheets may, if necessary, be fed directly into the back of the machine where they are split into separate ends without the need for warping.

The sacks are knitted sideways at a rate of 250 courses per minute on each bed in a similar manner to the briefs. Their depth can thus be varied according to the number of needles knitting in each section. The two fabrics are joined together at the top and bottom to form the side seams and at one selvedge to form the bottom of the sack. At the open end, a draw-thread may be knitted into each side of the fabric and separation of the sacks from the continuous warp knitted strip is achieved afterwards with a hot wire.

Double Needle Fabric
Fig. 29.6 The principle of knitting tights on a double needle bar raschel.

29.5.5 Pile fabrics

There are two main groups of pile fabrics produced on double-bar raschels: cut pile and point pile. Cut pile is achieved by knitting a separate base fabric on each needle bed but joining the two together by the lapping movement of the pile, which is later slit to produce the two cut pile fabrics. Point or looped pile is produced by replacing the front bar needles by a point or pin bar around which the pile yarns are overlapped. For security, the pile yarn may be overlapped in the base fabric on the needle bar or it may be inlaid to economise on yarn and produce a lighter-weight fabric.

Cut pile fabrics are employed for a wide range of high pile end-uses such as simulated fur and skin fabrics, upholstery and coat linings. The Karl Mayer HDR 5PLM is designed specifically for this type of fabric. Its raschel gauges range from 18 to

Fig. 29.7 Fruit and vegetable sacks knitted on double needle bar raschel machines [Karl

Mayer].

Fig. 29.7 Fruit and vegetable sacks knitted on double needle bar raschel machines [Karl

Mayer].

36, with 32 being most common, in widths of 75-180 inches (190-457 cm) and speeds of approximately 250-300cpm per needle bar (five-times faster than weaving). The fabric made from polyester yarns weighs between 300 and 600g/m2 and is particularly used for automotive upholstery.

Each bed knits alternately and has a cam-shaft, needle bar, trick-plate, sinker bar and two guide bars with no swinging action. The needle bar and trick-plate swing through the two guide bars to produce the base structure on that particular needle bed. The middle (pile) guide bar has normal swinging facilities for lapping the pile alternately on each needle bed. As the pile is severed in the centre, its height is half the distance between the two trick-plates; this distance may be altered to give a range of pile heights between 2.5 and 30 mm.

Figure 29.8 shows a simple three guide bar construction and Fig. 29.9, a more popular construction using five guide bars. By lapping the pile yarn into two wales, any irregularity in the yarn is disguised.

The effect produced is determined by a combination of type of fibre, denier, lapping movement and finishing process sequences whose operations may include one or more of the following: raising, cropping, setting, dyeing or printing, and electro-polishing.

Front Middle Back bar bar bar

pile pile

Front Middle Back bar bar bar

pile pile

Fig. 29.8 Notation for a three guide bar cut plush.

Fig. 29.9 Notation for a five guide bar cut plush.

In point pile, the loops lie at right angles to the base fabric and on some machines the points are sharpened or contain rotating cutting blades for cutting the pile loops. The structures are particularly suitable for floor coverings and carpeting. On a five guide bar machine in 12 gauge, the front two bars might knit pillar stitches in opposition, threaded with spun polyester; the inlay might be supplied by 2/10 NeK spun polyester from the back bar; whilst the two middle bars might supply 5/400 denier textured polyester for the pile, overlapping the points and laying-in on the needle bar.

Using an eight-link-per-course cycle, the overlap for the points occurs between the first two links, the overlap for the pillar stitches on the needle bar occurs between the second two links, whilst the last four links allow the points and needles to descend for knock-over and for the underlap inlay on the back bar.

An unusual use is three guide bar structure for the artificial turf, Astro-turf [6], whose pile is composed of four, six or eight ends of 500 denier dope-dyed nylon ribbon on a nylon polyester knitted and inlaid base fabric.

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Responses

  • ferdinanda
    How heavy is a karl mayer hdr16?
    7 years ago
  • Paula
    What is the difference between double guide bar and double needle bed knitting structure?
    1 year ago

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