The Mellor-Bromley rib jacquard (RJ) system uses revolving stacks of discs at each feed selection position. The replaceable disc stacks are rotated in unison with the machine drive. On 72-feeder machines, the stacks are accommodated at two alternately staged heights. When a disc tooth contacts the bottom half-butt of a presser (X in Fig. 11.7), it causes the jack tail (Y) which supports it to be retracted into the cylinder so that its tail butt misses the raising cam (Z) and the needle which is supported by the jack is not lifted to knit.
Presser half-butts are of two types: those with an upper half-butt (X in Fig. 11.7) are placed in odd cylinder tricks, and those with a lower half-butt are placed in even tricks.
A selection disc is actually composed of a pair of discs, the teeth of the upper one selecting odd needles by means of the upper half-butt and the teeth of the lower one selecting even needles by means of the lower half-butt (O in Fig. 11.8). As each only selects alternate needles, their teeth are cut twice as coarse as the machine gauge and are centred for these needles. The total number of teeth in a selection disc determines the pattern width, which may be 144 wales in 28 gauge.
At any cylinder revolution, a disc at the same height at each stack will be selecting. After each revolution, the pressers may be raised or lowered to a different height so that their half-butts are aligned with a different disc selection. In this way, as many as 18 discs, each for a selection at a different cylinder revolution, may be accommodated at each stack.
The height control of the pressers is achieved through their identically arranged and carefully-spaced guide butts, of which each may have as many as 10, depending upon the height of the disc stacks. During each cylinder revolution, two of these butts are in contact with a guide channel that surrounds the cylinder so that the pressers are held at a constant height. Three bolt cams, situated at a short break in the channel, provide the choice of serially lifting, lowering or retaining (bluffing or dwelling) the pressers at the same height for the next cylinder revolution. Introduction or withdrawal of each cam is controlled by separate tracks on a punched-hole film that racks once per cylinder revolution and thus has a major effect on the pattern depth.
Fig. 11.8 illustrates the change of presser height (S) at each of eighteen cylinder revolutions so that its half-butt obtains the selection from every disc (D) in the stack. Notice that, during the revolutions whilst the presser is being lifted, its guide butts occupy position (A) in the guide.
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