Whereas butts of normal length extend into the track formed between cams and guide their elements by contact with the profiled edges, a butt of shorter length may not reach into the track and will thus pass across the face of the cam and be unaffected by its profile (Fig. 11.1).
The same principle is employed when cams are withdrawn into their cam-plate
or the elements are depressed into their tricks, thus reducing the effective length of their butts.
The principle of butt lengths is that the element with the longest butt is always contacted first as a cam is brought into operation and the shortest butt is affected only when the cam is fully in action.
For example, a tuck cam might be partly in action, raising long and medium butt needles but allowing short butt needles to pass across at miss height, whilst the succeeding clearing cam is set to raise only long butt needles, leaving medium butt needles at tuck height. If short, instead of long, butt needles are required to be lifted, it is necessary to contact and lower the long butt needles before they reach the raising cam that is placed fully in action to lift the short butt needles remaining in line with it.
Separately butted and cam-controlled elements known as push-jacks may be placed below the needles in their tricks. As their butt set-out need not correspond to that of the needles, a greater selection potential is available than through the set-out of the needle butts alone. Long butt jacks can thus be used to positively lift short butt needles. Jack butt set-outs are particularly suitable for obtaining predetermined rib set-outs in garment length sequences.
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