The differentiated selection of needles provides greater possibilities to create patterns with different levels of complexity to meet the needs of a fashion market which requires increasingly sophisticated and innovative garments.
Needle selection can be carried out on circular knitting machines with different degrees of complexity: from the differentiated control of cams acting on high and low butt needles, to needles with multilevel butts (where each butt level matches a cam track), to the selection techniques based on Jacquard motions.
The most common selection system for the creation of plain (or simple operated) patterns on single and double bed machines, are the needles provided with multilevel butts matching the corresponding cam tracks to carry out the knitting cycle.
The operating principle is quite simple: when the needle reaches a specific knitting level, it generates a knit stitch, a tuck stitch or a miss stitch according to the type of cam sliding in the track corresponding to the specific needle butt level.
Single bed machines can incorporate needles with up to 4 to 5 butt levels for as many cam tracks, while on double bed machines, cylinder needles have 4-level butts and dial needles 2-level butts. The reduced number of tracks on the dial is determined by the fact that the dial has a fixed diameter, therefore the grooves cannot exceed a certain length, with the result that dial needles are relatively shorter.
Jacquard selection systems allow the knitting of elaborated patterns. On Jacquard selection systems, large clusters of needles can work independently and each single needle in a cluster can work independently from the others (needle-by-needle selection).
In the past, a variety of mechanical Jacquard selection systems were available on the market, but today manufacturers privilege only two types of mechanical Jacquard systems, equipped respectively with
The slider selection system is the most commonly used system. Under each needle there is a slider with 3 butts: a lower butt for needle raising, an upper butt for loop knocking-over and an intermediate selection butt that can be positioned at different levels. A selection lever acts on the intermediate selection butt. When the lever is idle (i.e. in external position) the slider maintains a vertical position and is pushed upward by the raising cams so that the corresponding needle can assume a working position. When the selection lever is active (i.e. in internal position) the slider is pushed into the guide groove and the corresponding needle assumes a non-knitting position. The selection lever is controlled externally and each manufacturer has developed its own solution to control the selection lever (e.g. fixed turrets with plug-in cartridges for pattern selection, punched cards system, reeds etc.)
The most recent models of circular knitting machines incorporate 37 selection levels (mini-jack machines). On double-bed machines, the selection levels are limited to the cylinder. In addition to the pattern selection levels, these machines also feature two extra levels to facilitate the selection change-over procedures.
Picture 93 - Arrangement of the selection butts of sliders
Picture 93 - Arrangement of the selection butts of sliders
With the slider butts arranged diagonally, the pattern design includes in its width a number of wales equal to the number of selectors. In symmetrical arrangement, the width of the pattern design is twice the number of selection levels. The sliders arrangement can remain unchanged for each machine revolution (fixed arrangement), and in this case the height of the design pattern is given by the number of feed systems of the machine divided by the number of pattern colours.
If the operator wishes to modify the height of the pattern, the arrangement of the sliders must be changed after every machine revolution. This operation can be carried out by applying different methods, i.e.:
The mechanical selection system with pattern wheels is based upon the use of a wheel incorporating special grooves on its circumference. The distance between the grooves equals the distance between one needle and the next on the machine bed. Special pegs are positioned into the grooves of the pattern wheel according to the patterns to be carried out. The wheel is mounted in slant position so that the needle butts can enter the grooves of the wheel and make it rotate. When the needle butt finds an empty space, it is not raised and remains in non-knitting position. When the butt meets a peg, the needle assumes its working position. Pegs have different heights: higher pegs select knitting needles while shorter pegs select tucking needles.
Each feed system of the machine has to be equipped with a pattern wheel, therefore the number of pattern wheels corresponds to the number of feed systems on the machine.
Picture 94 -Mechanical selection with pattern wheels
The width of the pattern ratio is strictly related to the number of teeth on the pattern wheel and to the number of machine needles.
If the number of machine needles is exactly divisible by the number of the teeth on the pattern wheel, the design can be repeated serially lengthwise and crosswise.
However, this solution does not give particularly appreciable results in terms of originality and aesthetics. Therefore, it is advisable that the number of needles be not exactly divisible by the number of the teeth on the pattern wheel, as a result slightly shifting the design pattern position with every revolution of the machine. In this way, the pattern develops diagonally on the fabric giving the impression of a higher covering ratio.
Today, the application scope of pattern wheels has become rather limited compared to the past, yet some manufacturers still offer single and double bed machines equipped with pattern wheels selection above all for the manufacturing of continuous Jacquard fabrics.
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