24.3.1 The history and development of the raschel machine
In 1855, German warp knitters in Apolda used warp rib machines made by Redgate of Leicester to knit lace stoles which they sold under the name of Raschel Felix, the famous French actress , so that when Wilhelm Barfuss began to build his latch needle rib machines, he named them raschel machines . Originally, two vertical needle bars arranged back-to-back, mid-way between each other, were employed for producing simulated rib fabrics. In 1914, when the needle bars were placed directly back-to-back, only even-numbered chain links were required.
Until the mid-fifties, the raschel industry tended to be small, employing slow, cumbersome but versatile coarse-gauge universal raschels. These had two needle bars, one of which could be removed or replaced with plush points, changeable cams and patterning mechanisms that might include fall plate, crepe and fringing motions, chain switching, and possibly weft insertion or jacquard.
The development of modern specific-purpose raschels dates from 1956, when a twelve guide bar raschel machine led to the rise of the raschel lace industry . There are now single needle bar raschels for simple and multi-guide bar dress and household fabrics, elastic laces, trimmings and curtain nets; high-speed standard raschels for simple structures such as suitings; versatile multi-purpose raschels for fancy fabrics, weft insertion; and jacquard raschels and double needle bar raschels for plush, tubular articles, scarves and string vests.
There is an increasing demand for finer, lighter fabrics with minimum elongation and transparency. Warp knitting is able to meet this requirement by producing fabrics weighing much less than 100g/m2 with very little edge-curling.
The finest gauge single bed raschel is E 40. It can knit lightweight foundation and swimwear at speeds between 1900 and 2200 rpm in a yarn count of approximately 80dtex.
Raschel machines (Figures 24.4 and 24.5) originally had a gauge expressed in needles per 2 inches (5 cm), so that, for example, a 36-gauge raschel would have eighteen needles per inch. Now, the standard E gauge (needles per inch) is generally used. There is a wide gauge range, from E 1 to E 32.
Their chain links are usually numbered in even numbers, 0, 2, 4, 6 etc., generally with two links per course. Raschel sinkers perform only the function of holding down the loops whilst the needles rise. They are not joined together by a lead across their ends nearest to the needle bar so they can move away clear of the needles, towards the back of the machine, for the rest of the knitting cycle. The needle trickplate verge acts as a fabric support ledge and knock-over surface.
The fabric is drawn downwards from the needles, almost parallel to the needle bar, at an angle of 120-160 degrees, by a series of take-down rollers. This creates a high take-up tension, particularly suitable for open fabric structures such as laces and nets.
The warp beams are arranged above the needle bar, centred over the rockershaft, so that warp sheets pass down to the guide bars on either side of it. The beams are placed above the machine so that it is accessible at the front for fabric inspection and at the back for mechanical attention to the knitting elements. The guide bars are threaded, commencing with the middle bars and working outwards from either side of the rocker-shaft. They are numbered from the front of the machine.
With the raschel arrangement, there is accommodation for at least four 32-inch diameter beams or large numbers of small diameter pattern bars. The accessibility of the raschel machine, its simple knitting action, and its strong and efficient
take-down tension make it particularly suitable for the production of coarse gauge open-work structures employing pillar stitch, inlay lapping variations and partly-threaded guide bars. These are difficult to knit and hold down with the tricot arrangement of sinkers. Additional warp threads may be supplied at the selvedges to ensure that these needles knit fabric overlaps, otherwise a progressive press-off of loops may occur.
24.3.3 The knitting action of the single needle bar raschel machine
Raschel needles tend to have longer latches than weft knitting machine needles, to ensure that the wrapped yarns of the overlap goes onto and not below the open latch (Fig. 24.6).There is a trick-plate extending the full width of the machine, whose
walls preserve the needle spacing and whose verge provides an edge for a clean knock-over. Holding-down sinkers that are thin blades, unleaded at their forward edges, move in a horizontal plain over the top of the trick-plate.
1 Holding down. The guide bars are at the front of the machine, completing their underlap shog. The sinker bar moves forward to hold the fabric down whilst the needle bar starts to rise from knock-over.
2 Clearing. As the needle bar rises to its full height, the old overlaps slip down onto the stems after opening the latches, which are prevented from flicking closed by latch wires. The sinker bar then starts to withdraw to allow the guide bars to overlap.
3 Overlap. The guide bars swing to the back of the machine and then shog for the overlap.
4 Return swing. As the guide bars swing to the front, the warp threads wrap into the needle hooks.
5 Latch closing. The needle bar descends so that the old overlaps contact and close the latches, trapping the new overlaps inside. The sinker bar now starts to move forward.
6 Knocking-over and underlap. As the needle bar continues to descend, its head passes below the surface of the trick-plate, drawing the new overlap through the old overlap which is cast-off and as the sinkers advance over the trick-plate, the underlap shog of the guide bar is commenced.
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