Compared with the latch needle, the compound needle is more intricate and expensive to manufacture. Each of its two parts must be separately and precisely controlled during knitting. In circular knitting, yarn feeding is very critical because, if the yarn lands on the tongue, it will not enter the open hook, whereas in latch needle knitting the closing latch will flick the yarn into the hook. It is particularly a problem when knitting multiple tucks. Adjustment of a machine setting is therefore a very skilled operation. Lifting of the tongue out of its guide groove at high speeds or as the result of dirt or fly can also be a problem, particularly if it splits filament yarns. In addition, differential heat expansion between the hook and its closer can cause problems.
On the other hand, the vertical clearing height for the compound needle is not so high because only the open hook and not an open latch spoon has to be cleared. The shorter vertical stroke can be achieved with a smaller cam system in V-bed flat knitting.
Also, when clearing, the compact head of the compound needle does not cause stretching of needle loops and robbing of yarn from adjacent sinker loops as the needle rises to clear or descends to knock-over, as is the case with the latch needle. The needle can knit tight, uniform stitches that tend to be rounder than the long, narrow loops produced by latch needles.
The compound needle has a short, smooth, simple harmonic movement without latch and beard inertia problems, so there is less vibration. Also, there is no stress on needle loops to open and close latches. The hook of the compound needle does not have to withstand the shock of a latch spoon hitting it. It can therefore be tapered to a slimmer diameter, producing a larger area inside the hook that can accommodate thicker yarns. This is particularly useful in the case of fine gauge, V-bed flat machinery.
Its slim construction and short hook make it particularly suitable for knitting fine warp knitted structures at high speed. It can knit chain stitches without the loops rising up the needles, and its sturdy construction resists the deflection generated by elastic yarns or thick places in yarns. Accumulations of lint are pushed out of the hook by the action of the closing element.
It is now employed in all types of warp knitting machines apart from double needle bar raschels and raschel simplex machines. Horizontal yarn tension between front and back needle bars can cause the two sets of needle hooks to be drawn towards each other and away from contact with their hook-closing sliders.
The compound needle has not lived up to its earlier promise in circular weft knitting. It has failed to gain a foothold in hosiery and even in simple plain knit single jersey. Vignoni are now the only circular machine builder to continue to include it as an option.
In V-bed flat knitting, Shima Seiki are successfully employing an open-slot compound needle in their coarse gauge (3 to 5 gauge) V-bed flat machines, resulting in a more compact cam box and reduced width of machine. The needle has conventional knit, tuck, miss and rib loop transfer facilities. The closing element passes through a slot in the hook element to the back, so that the two elements are held in contact with each other. Stop ledges on the two elements engage so that, after a certain distance, the individual movement of the element is converted into a collective movement of the two elements together.
Shima Seiki also used compound needles in their prototype four needle bed model SWG-X WholeGarment machine because the four needle beds are so close to each other that there is no space for latches to turn-over. Shima, again employ compound needles in their model FIRST machine. These ascend during knitting to only half the height of latch needles. They have a uniquely designed hook closer whose leading-end shoulder can project across to receive or transfer a loop from a needle in the opposite bed. This closing element also has a small cut-away section on its outward surface that can be used for retaining loops separately from those inside the hook. On the Shima machine, the slide needles are centre-mounted, minimising yarn stress and damage.
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