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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 key to a good floor loom is a good
wide parallel shed, and the points set out below will help
in deciding what you require for your loom.
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
 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.
Countermarch Looms
 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 and 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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