Get the Offline ArchiveFree lifetime updates

Shedding

5 min read

Parent
🧵
Weaving Mechanics
The three essential motions of weaving
Current
↕️
Shedding
Raising and lowering warp threads to create openings

Shedding splits the warp into an upper and a lower layer so the shuttle can cross between them, and the opening at the reed must exceed the shuttle’s height, 30 mm in the overview, by a clearance. On an assumed geometry that needs a 40 mm shed at the reed, which stretches every warp end by about 0.66% on every pick, 48,000 times in an 8 hour shift at 100 picks a minute, so the warp yarn must be strong and extensible enough to survive that repeated stretch on top of its tension.

Part of Power Loom Construction

Raising and lowering groups of warp threads to open the gap the shuttle passes through, and what that does to the yarn.

The Shed Opens Fully Only Where The Reed Is, Not Where The Heddles Are

The overview describes the shed as half the warp rising and half dropping to make a V-shaped opening. The warp is held at the cloth edge (the fell) and at the back of the loom, and the heddles lift it in between, so the opening is zero at the fell and grows toward the heddles. The shuttle passes where the reed is, between fell and heddles, so the lift at the heddles must be larger than the opening needed at the reed.

With similar triangles, if each layer moves ±h at the heddles, a distance a from the fell, the separation at distance x from the fell is 2h × x ÷ a. Take assumed distances (the overview gives none): a = 300 mm, the reed x = 150 mm from the fell when the shuttle passes, and the shuttle 30 mm tall (overview) plus 10 mm clearance, so a separation of 40 mm at the reed.

  • 2h × 150 ÷ 300 = 40, so h = 40 mm, a 80 mm separation at the heddles.

Opening The Shed Stretches Every End

Lifting a thread by h at a point turns two straight segments into two sloped ones, which are longer. With the front segment 300 mm and the back segment 400 mm (assumed) to the back rest:

  • front: √(300² + 40²) − 300 = 302.655 − 300 = 2.66 mm
  • back: √(400² + 40²) − 400 = 401.995 − 400 = 2.00 mm
  • total 4.65 mm over 700 mm, a strain of 0.66% on an end at the extreme of the shed

This is on top of the steady warp tension. It happens on every pick: 100 picks a minute × 60 × 8 = 48,000 times a shift, and 100 m of warp at 20 picks per cm is 100 × 100 × 20 = 200,000 picks over the life of the warp. Each end is also rubbed through its heddle eye and the reed on each pick. A warp end that stretches only a little before breaking, or loses strength with repeated stretching, will fail here; the twist, strength and extension tests in Twist Testing and Thread Quality Control exist for that reason. A warp that is soft, low-twist and fuzzy snags and breaks in the shed first.

What You Need

Method: Setting The Shed

Step 1: Measure the shuttle height and add clearance; that is the opening needed at the reed.

Step 2: Fix the heddle frame travel from the geometry of your loom with the similar-triangles rule.

Step 3: Set the timing so the shed is fully open while the shuttle crosses (Weaving Mechanics gives the minimum window of 40-60° of crank turn).

Step 4: Check the clearance by running the shuttle through slowly; it should not rub the upper or lower layer.

Step 5: Check the strain with the stretch calculation, and reduce lift if the warp breaks at the heddles.

Worked Example

If the shed is made half as high, 20 mm per side instead of 40, the strain on the extreme end becomes: front √(300² + 20²) − 300 = 0.67 mm, back √(400² + 20²) − 400 = 0.50 mm, total 1.17 mm over 700 mm, 0.17%, about a quarter of 0.66%. The saving is large because the stretch rises with the square of the lift. But the opening at the reed is then 20 mm, less than the 30 mm shuttle, so the shuttle would saw through the warp. The limit is set by the shuttle, so a smaller shuttle is a way to protect the warp.

Common Mistakes

  • A shed lifted from the wrong distance, which opens at the heddles but not at the reed.
  • Uneven heddle lengths, which make some ends lift less (Heddle System).
  • A shed too small, so the shuttle rubs and breaks ends.
  • Ignoring the cycle count when choosing the warp yarn.

Safety

Heddle frames, cams and levers move on every pick

Frames, cams and levers close on a hand reaching in to tie a broken end, and a broken warp end can whip. Join threads only on a stopped, locked loom; the stop for the loom is at the belt, and the plant’s stop is at the water gate (Power Drive System). This is adult work; children are kept away from running machinery (Task Assignment by Age).

What’s Next