1 As the guides swing through the needles to start their next overlap, the back guide bar is first to lay its underlap on the technical back (Fig. 25.2) and the front bar is the last, so its underlaps lie on top on the back of the fabric (Fig. 25.3).
2 The front bar thread is the first to strike the needle on the return swing after the overlap (Fig. 25.4) and as its bar swings furthest to the front of the machine, it tends to occupy a lower position on the needle. If this position is retained it will show prominently on the under surface, which is the technical face (Fig. 25.2).
3 A low setting of a guide, a longer run-in, an open instead of a closed lap, or a short underlap movement, will all tend to cause a warp thread to occupy a low position on the needle, either reinforcing or reversing the normal front guide bar/ back guide bar plating relationship.
Carefully arranged lapping movement can thus overcome the normal plating dominance of the front guide bar threads on both surfaces of the structure. A structure showing the underlaps of the front guide bar on the surface of the
technical back, but the overlaps of the back guide bar on the surface of the technical face, is produced if the front bar makes a 2 x 1 closed lap and the back bar makes a 1 x 1 open lap in opposition to it.
If the two bars overlap in opposition, the yarns tend to twist over each other in the overlap so that the back bar thread tends to partly show on top of one side limb.
If the two bars underlap in opposition they tend to balance the tension at the needle head, producing a more rigid upright overlap stitch. On the technical back, the underlaps will cross over each other in the middle between wales and this improves the strength of the structure.
6 A short movement will cause the underlap to lie at an angle and its laps will be under the greatest tension. If the front guide bar makes the shortest underlap, it will tie the longer underlaps of the back bar securely into the rigid structure. If the front bar makes the longer underlap, this floats freely across the back and allows more movement of the yarn within the structure giving it more elasticity and a greater tendency to curl towards the technical face at the top and bottom.
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