A dobby replaces the cams with a chain of bars, one per pick, each carrying pegs or holes that choose which heddle frames lift for that pick, so it can weave any pattern that fits the number of frames and the length of the chain. A repeat of 16 picks on 8 frames is 16 x 8 = 128 peg positions, and the chain must advance exactly one bar per pick or the whole pattern shifts.
Part of Power Loom Construction
A peg-chain pattern mechanism for looms with more heddle frames than a cam shaft can sensibly drive.
The Chain Is The Program
The overview describes a chain or cylinder with pegs or holes, where each row of pegs selects which heddle frames lift for that pick, one row per weft insertion, and the pattern repeats when the chain completes a cycle. It calls this one of the earliest forms of automation.
For a pattern of P picks on F frames, the chain has P bars and each bar has F positions, so P × F positions in all, each peg either present or absent. A 16-pick repeat on 8 frames is 16 × 8 = 128 positions, 16 bytes of information if each position is one bit (a derived comparison). The chain is read at the loom’s picking rate: at 100 picks a minute the dobby must present 100 bars a minute and place each one in the right position before the shed opens, which is the timing window of Weaving Mechanics.
Why Not Just More Cams?
The cam shaft of Cam Shedding Mechanism gives each frame a fixed lift pattern repeating every shaft turn. A pattern that repeats over 16 picks on 8 frames would need cams cut to that pattern and a shaft turning once per 16 picks, and a new pattern would need new cams. A chain is changed by moving pegs. The overview uses cams for up to four frames and a dobby beyond that.
What You Need
- Bars (lags) of hardwood with holes bored at each frame position, and pegs of wood or iron.
- A chain or cylinder that carries them, indexed one step per pick.
- Feelers or hooks that sense each peg and pass the lift to the frame levers.
- A pattern draft on paper: a grid of picks by frames.
Method: Building A Pattern Chain
Step 1: Draw the pattern on a grid, picks down and frames across, marking which frames are up on each pick.
Step 2: Make as many lags as the repeat has picks, with F holes each.
Step 3: Peg each lag from the grid, one row at a time. Check every row by a second person.
Step 4: Link the lags in order and fit them to the cylinder.
Step 5: Time the chain so each lag arrives before the shed is wanted (Shedding).
Step 6: Run slowly, weaving a few repeats while comparing the cloth to the drawing.
Worked Example
A herringbone of 8 picks on 4 frames: 8 × 4 = 32 positions. A 3/1 twill needs 4 picks on 4 frames: 16 positions. In the first, every pick lifts one or two frames at a time; if the person pegging puts a peg on the wrong frame on one lag, the cloth carries a visible flaw every 8 picks. At 20 picks per cm that is a flaw every 8 ÷ 20 = 0.4 cm, 4 mm, for the whole length of the cloth until someone notices. The fault is cheap to find if the first few centimetres are checked against the drawing.
Common Mistakes
- A lag mispegged, which weaves a repeating fault.
- An indexing mechanism that slips a step, shifting the pattern by one pick.
- Pegs that loosen under vibration and drop out.
- Using a dobby where cams would do, adding wear parts for a plain weave.
Safety
The chain is heavy and indexed by force
A loaded lag chain, its sprocket and the feelers move with the loom and pinch fingers, and pegs can fly out. Peg and change chains only with the loom stopped and locked off; the stop is at the belt and the plant’s is at the water gate (Power Drive System). Children keep away from running looms (Task Assignment by Age).
What’s Next
- Learn next: Shuttle Mechanism — the weft side of the loom
- Combine with: Weave Patterns — the patterns a dobby can make