Understanding Weaving Loom Shed
The shed is the space between raised and lowered warp threads in which the weft passes through. Floor looms have a range of mechanisms for creating the shed; each has its benefits and disadvantages. The shed shown in the image right demonstrates how the warp is lifted and separated. This small loom creates a shed using a simple rigid heddle system.
The system for creating the shed is more complex on larger floor looms. The key to a good floor loom is a good wide parallel shed.
Counter Balance Looms
These looms have the simplest mechanism, and are therefore cheapest to buy. The shafts are suspended above the warp, and as one shaft is pulled down by the treadle it lifts the opposite shaft. The tie-up of shafts and treadles is simple and works well for a balanced weave on an even number of shafts. The shafts both rise and sink to give an even warp tension in balanced weaves. For larger numbers of shafts and unbalanced weaves, the shed does not open equally for each shaft, the distribution of tension in the warp is therefore uneven and the shuttle can readily skip warps. Adaptations, including a roller fitment in place of the 'horses' or pulleys shown, can help to ensure the shed is parallel across the width of the warp. This loom is therefore best for simple weaving, where the fabric uses the full width of the loom.
Jack Looms are a relatively recent development, with the potential for a good shed. Inelastic warps can however limit the depth of shed produced. The shafts are raised independently by the treadles. For visibility only one Jack is shown in the diagram alongside. Since the size of the shed is largely dependant on the raised warps, these are often at a much higher tension than in a loom with both rising and sinking shafts. Their mechanical simplicity is used in the Schacht folding looms and for adaptation to a computer dobby. They are best used for shorter lengths or lighter fabrics.
In weaving, the unraised warps are held down well below the centre line by the weight of the shafts and heddles, remaining under tension against the shuttle race. Metal heddles give the weight to hold the lower shafts in place and allow the raised shafts to fall back after release. Springs or weights can also be attached to the shafts where the warp is under higher tension. This shedding arrangement reduces the possibility of skipping warps in weaving. It gives a positive action, is easy to thread and additional 'repair' heddles can be inserted to overcome threading errors. However it is noisy and can wear the warp.
These looms give a positive and equal separation of the rising and sinking warps.
In Glimåkra looms the shed comes from either a horizontal or vertical jack lever action at the top of the loom. The preference is essentially national and makes little difference. Each treadle is linked to all of the rising and sinking shafts for that weft shot. The separating force is applied near to the centre of the shaft to avoid tilting the shafts.
With the Louët looms the entire shaft is linked by a parallel tie-up action, attached to the side of the shaft, for raising and sinking, ensuring that the shed opens fully and remains truly horizontal. Since this raises the possibility of high levels of warp tension, or the requirement for heavy footwork on the treadles, the Louët looms have a sprung breast beam. The movement of the beam allows the shed to open without any great increase in warp tension.
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