Although some selection devices, including pattern wheels, operate onto element butts of one height position, many patterning arrangements involve the use of a single selection butt for each element, placed at one of a choice of height positions. The total number of different heights often directly influences the width repeat in wales. It is generally most convenient to arrange and retain a butt set-out that is a factor of the needle bed, so that the pattern widths exactly repeat into it.
The two most common geometrical butt set-outs are straight and mirror repeats, although combinations of the two are possible.
A straight (diagonal, echelon, or up-and-up) butt set-out is arranged in an ascending order in the direction of knitting (Fig. 11.2). Each butt position is used once only in the set-out repeat, so the pattern width is equal to the number of available pattern butt positions.
A mirror repeat (reflex chevron, up-and-down, or geometric) butt set-out is a mirrored continuation of the straight set-out, with the butts descending in sequence after the highest position (see Fig. 11.3). The top and bottom butts are not used in the descending sequence as the former would produce two identical adjacent wales in the same repeat and the latter would produce two identical adjacent wales with the first wale of the next repeat. This set-out thus produces a symmetrical design width about a common centre wale, with the right side identically mirroring the left side.
With geometric selection, the top butt position is used only in mirror repeats so that these are exactly twice the width of straight set-outs and both mirror repeats and straight set-outs are a factor of the number of cylinder needles.
For example, an E 18,30-inch diameter machine with 1728 cylinder needles, using a small-area fixed selection, might have 24 butt positions (and pattern comb teeth) for a straight set-out repeating 72 times around the cylinder, and an extra top butt and tooth used only for mirror repeat set-outs, making 25 up and 23 down, giving a width of 48 butts that repeats 36 times around the cylinder.
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