Robbing back

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Knapton and Munden suggested the phenomenon of 'robbing back' to be the reason why the measured loop length in a knitted structure is smaller than the theoretical loop length when calculated from the depth of the stitch cam setting, as well as the reason for fluctuations in input tension producing large variations in loop length.

As the needles descend the stitch cam, the tension required to pull yarn from the package increases rapidly and it becomes easier to rob back yarn in the opposite direction from the already-formed loops of needles further back that are then beginning to rise from their lowest (knock-over) position.

With reference to Fig. 22.3, it was suggested that, under the dynamic conditions of loop formation, yarn tension increases (according to Amontons' Law of Friction) as it passes over the knitting elements from point A. Robbing back occurs from needles on the other side of the stitch cam. The lowest point of tension is reached at B. The tension on the yarn is determined by the yarn/metal friction and the number of angles of yarn wrap. Thus, a two-fold increase in yarn/metal friction can cause a six-fold increase in maximum knitting tension.

As robbing back reduces tension, flat-bottom cams would obviously be undesirable and a cam angle shape of 60 degrees was preferable to one of 45 degrees because the number of yarn/metal contacts was reduced. It was further proposed that smoothly designed, non-linear camming with a pressure angle of greater than 50 degrees could provide smooth acceleration of needles for much higher knitting speeds. Camming of this type has been incorporated into some simple high-speed single-jersey machines but it requires adaptation for more complex and alterable cam arrangements.

The TF in Imperial units is:

Robbing Back
Fig. 22.3 Model of weft-knitted loop formation indicating the mechanism of 'robbing-back' and the build-up in yarn tensions acting on the needles.

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