A Loom Dobby Mechanism is a shedding device fitted to a weaving loom that lifts and lowers individual heald shafts in a programmed sequence to form the warp shed for each pick. It works by reading a pattern — historically a lag-and-peg chain, today an electronic file — and using hooks, jacks and a knife or rotating cam to select which shafts rise. This lets a single loom weave geometric patterns up to 40 shafts wide without the cost of a full Jacquard. Modern rotary dobbies like the Stäubli 2670 run at 1000+ picks per minute on commercial weaving lines.
Loom Dobby Mechanism Interactive Calculator
Vary loom speed, dwell angle range, selection time, and hook gap to see whether the dobby has enough time to engage cleanly.
Equation Used
The calculator uses the dobby selection-window equation: the time available equals the pick period multiplied by the crankshaft dwell-angle fraction. At 600 PPM, a 108 to 180 degree dwell range gives the article's 30 to 50 ms selection window. The margin compares the lower window against the hook engagement time.
- One dobby selection is required for each loom pick.
- Dwell angle is the crank angle available for hook selection.
- Selection must finish inside the lower dwell-window time for reliable operation.
- The hook-to-knife gap is compared with the stated 0.4 mm critical setting.
How the Loom Dobby Mechanism Actually Works
A dobby sits on top of the loom over the heald frames and decides, every single pick, which shafts go up and which stay down. The pattern source — a lag chain on an old Hattersley, a paper card on a 1950s Crompton & Knowles, or a memory file on a modern Stäubli or Picanol — tells the dobby which hooks to engage. Engaged hooks catch a reciprocating knife (in a negative dobby) or lock onto a rotating eccentric (in a rotary dobby), and that engagement pulls a jack lever that lifts the corresponding heald shaft. Disengaged hooks sit idle and their shafts stay at the bottom shed. The whole selection happens in the dwell between picks — typically 30 to 50 milliseconds at 600 PPM.
The physics is brutal in its repetition. At 800 picks per minute the dobby has to make a clean shaft-by-shaft selection 13.3 times every second. If a hook lifts late by even 5 milliseconds you get a mis-pick — the weft lays in the wrong shed and the fabric is scrapped. If the timing of the knife or rotary disc drifts out by more than about 2° of crankshaft rotation you start seeing broken warp ends because the shaft is still moving when the reed beats up. The hook-to-knife clearance on a Stäubli 2660 is specified at 0.4 mm — not 0.3, not 0.5 — and that gap is what separates clean selection from missed lifts.
Failure modes are predictable. Worn hook noses round off and start slipping past the knife under load, which shows up as random missing lifts on a specific shaft. Broken jack springs cause shafts to float instead of returning to the bottom shed, producing weft floats on the face of the cloth. On lag-driven dobbies the pegs themselves wear, and a peg sitting 0.5 mm too low simply will not push its hook into the knife path. Modern electronic dobbies eliminate the peg wear problem but add solenoid coil failures — a single dead solenoid takes one shaft permanently out of action until you swap the module.
Key Components
- Heald Shaft (Heddle Frame): The wooden or aluminium frame that carries the heddles through which warp ends pass. A dobby loom typically controls between 8 and 40 shafts. Each shaft weighs 2-6 kg depending on width and must be lifted and dropped clean every pick without overshoot.
- Jack Lever: The rocker arm that converts hook engagement into shaft lift. One jack per shaft, pivoted on a common shaft running across the dobby head. Lever ratios are typically 1:1.5 to 1:2.5 to convert hook stroke into the 70-90 mm shed opening.
- Hook (Top and Bottom): The selection element. Each shaft has two hooks in a double-lift dobby — one for the up-pick, one for the down-pick — which doubles weaving speed by halving idle dwell. Hook nose hardness is specified at 58-62 HRC to resist the cyclic impact against the knife.
- Knife or Rotary Eccentric: On a negative dobby a pair of reciprocating knives sweeps past the hooks once per pick, lifting any engaged hook. On a rotary dobby (Stäubli 2670, Fimtextile) a continuously rotating eccentric replaces the knife, which removes the impact load and lets the head run at 1000+ PPM.
- Pattern Lag, Card, or Solenoid Block: The selection memory. Mechanical lags carry steel pegs in a 1:1 mapping to shafts. Electronic dobbies use a solenoid per hook, switched in 3-8 ms by the loom controller from a stored weave file. Solenoid coil resistance must stay within ±5% of nominal or selection becomes unreliable.
- Return Spring: Pulls each jack back down when its hook is not engaged, returning the shaft to the bottom shed. Spring rate matters — too stiff and the dobby fights itself, too soft and the shaft floats. Typical preload is 15-25 N per jack.
Who Uses the Loom Dobby Mechanism
Dobbies live in any weaving operation that needs more pattern complexity than a tappet (cam) loom can deliver but does not need the thousands of independent ends a Jacquard provides. The split is roughly: tappets for plain and twill up to 8 shafts, dobbies for geometric patterns up to 40 shafts, Jacquard for figured and pictorial fabrics. That middle band — shirting, denim, fancy suiting, terry towelling, technical fabrics — is where dobby weaving dominates worldwide, and it is why every major loom builder still ships dobby heads as standard fitment.
- Denim Weaving: Picanol OmniPlus-i air-jet looms running 16-shaft Stäubli dobbies for selvedge denim at mills like Cone Mills' White Oak (historically) and Kaihara in Japan.
- Shirting Fabrics: Itema R9500 rapier looms with 20-shaft electronic dobbies producing dobby-pattern cotton shirting for brands sourced through Albini Group in Italy.
- Terry Towel Production: Dornier P2 rapier looms with dedicated terry dobby heads at Welspun India running Egyptian cotton bath towel programs at 600 PPM.
- Heritage and Handloom Weaving: Hattersley domestic dobby looms still in service in Harris Tweed weaver croft houses on the Isle of Lewis, running 24-shaft lag-and-peg pattern chains for tweed twill structures.
- Technical and Industrial Fabrics: Sulzer projectile looms fitted with rotary dobbies weaving multi-layer filtration fabrics and seatbelt webbing at producers like GKD in Düren, Germany.
- Wool Suiting: Vamatex Leonardo rapier looms with 24-shaft Bonas dobbies at Italian mills such as Reda and Loro Piana for fancy weave wool suitings.
The Formula Behind the Loom Dobby Mechanism
The core question on any dobby installation is whether the head can keep up with the loom speed. The selection cycle has to complete inside the crankshaft dwell window, otherwise hooks engage late and you mis-pick. At low loom speeds the dobby has loads of margin — at 300 PPM the dwell is huge and even a sloppy mechanical dobby selects cleanly. At nominal weaving speeds of 600-800 PPM you are inside the design envelope of most modern rotary dobbies. Push past 1000 PPM and you are at the limit of what hook-and-knife geometry can do — beyond that, only true rotary heads with no reciprocating mass survive. The formula below tells you the available selection time per pick.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| tsel | Available selection time per pick during crankshaft dwell | seconds (s) | seconds (s) |
| NPPM | Loom speed in picks per minute | picks/min | picks/min |
| θdwell | Crankshaft dwell angle during which the shed is fully open and selection can occur | degrees (°) | degrees (°) |
Worked Example: Loom Dobby Mechanism in a 16-shaft electronic dobby retrofit
A bespoke wool blanket mill in Otago New Zealand is retrofitting an electronic 16-shaft dobby head onto a Dornier HTVS rapier loom that previously ran a 12-shaft mechanical dobby. The plant engineer needs to confirm the new solenoid-driven head can complete shaft selection inside the crankshaft dwell window at the loom's nominal 720 PPM running speed, with margin at the low end (overhaul mode at 360 PPM) and at the planned production push to 900 PPM. The crankshaft dwell on this loom is 110° of the 360° cycle. Solenoid switching time on the new dobby is quoted at 6 ms.
Given
- NPPM,nom = 720 picks/min
- NPPM,low = 360 picks/min
- NPPM,high = 900 picks/min
- θdwell = 110 degrees
- tsolenoid = 6 milliseconds
Solution
Step 1 — at nominal 720 PPM, calculate cycle time per pick:
Step 2 — multiply by the dwell-angle fraction to get available selection time at nominal speed:
That is comfortable. The 6 ms solenoid switch fits inside 25.5 ms with over 4× margin — plenty of room for hook travel, jack lift and crankshaft tolerance stack-up.
Step 3 — at the low end of the operating range, 360 PPM (overhaul / start-up mode):
At 360 PPM the dobby has 50.9 ms to do a 6 ms job. The head is loafing — useful for fault-finding because every selection is visible and clean.
Step 4 — at the high-end production push, 900 PPM:
20.4 ms versus a 6 ms solenoid is still safe on paper, but real-world margin is tighter than it looks. The 14 ms buffer has to swallow hook lift travel, jack acceleration, mechanical lash and crankshaft phase drift. Above 900 PPM on this geometry you will start seeing intermittent mis-picks on the heaviest shafts because the jack hasn't fully completed its lift before reed beat-up begins.
Result
Available selection time at the nominal 720 PPM is 25. 5 ms — well inside the 6 ms solenoid switching budget, so the retrofit is safe at design speed. At the low overhaul speed of 360 PPM the head has 50.9 ms of dwell and runs visibly clean; at the planned 900 PPM push the window collapses to 20.4 ms, which is still inside spec but with much less margin for mechanical drift. If the mill measures mis-picks above predicted at 900 PPM, check first for crankshaft phasing drift between the loom and the dobby drive shaft (more than 2° of phase error eats most of the remaining margin), second for sluggish solenoid response on individual shafts where coil resistance has drifted above the ±5% band, and third for jack-spring fatigue allowing shafts to float and miss bottom-shed reset. Any one of those three will produce random shaft-specific weft floats that look like an electronic fault but are actually a timing or mechanical problem.
When to Use a Loom Dobby Mechanism and When Not To
Dobby is one of three shedding strategies a weaving mill chooses between. The decision comes down to how many independent shafts you need, how fast you want to run, and what the fabric value supports. Here is how dobby compares to the simpler tappet (cam) shedding and the more complex Jacquard.
| Property | Loom Dobby Mechanism | Tappet (Cam) Shedding | Jacquard Mechanism |
|---|---|---|---|
| Maximum shafts / independent ends | Up to 40 shafts | 2-8 shafts typical | Up to 12,000+ independent ends |
| Maximum loom speed | 1100 PPM (rotary dobby) | 1500+ PPM | 600-800 PPM |
| Pattern repeat length | Up to several hundred picks | Limited to cam profile (8-16 picks) | Effectively unlimited |
| Capital cost per loom | Mid — $15-40k for the head | Low — $2-8k for cam set | High — $60-200k+ for the head |
| Best fabric fit | Shirting, denim, terry, suiting | Plain weave, basic twill, sheeting | Damask, brocade, figured upholstery |
| Pattern change time | Minutes (electronic) to hours (lag chain) | Hours — physical cam swap | Minutes — electronic file load |
| Mechanical complexity | Moderate — 2 hooks per shaft | Low — one cam per shaft | Very high — one harness cord per end |
Frequently Asked Questions About Loom Dobby Mechanism
Bench-testing a solenoid only checks coil continuity and pull-in current at room temperature. Inside a running dobby head the coil heats up to 60-80°C, and a coil with a marginal turn-to-turn short will pull cleanly cold and start missing as it heats. The dropped pick interval — once every 50-100 picks — corresponds to the thermal cycle of that specific coil hitting its drop-out threshold.
Quick check: pull the suspect solenoid module after a 30-minute production run and measure coil resistance immediately. If it reads more than 8% above the nominal value, the coil is failing. Swap the module rather than trying to repair it.
Yes, three of them. First, a 24-shaft head has 50% more reciprocating mass than a 16-shaft, which lowers your top usable PPM by roughly 8-12% on a negative dobby — less on a rotary. Second, the head is longer fore-to-aft, which on some loom frames forces the back rest further back and changes warp tension dynamics. Third, capital cost scales close to linearly with shaft count.
Rule of thumb: pick the shaft count that covers 95% of your planned pattern library plus 4 spare shafts. Buying capacity you will never weave costs you speed every single pick.
Single-shaft floats almost always trace to the jack-and-hook assembly for that shaft, not to anything systemic. The two most common causes that survive the obvious checks are: a bent jack lever (look for a jack that sits 1-2 mm below its neighbours at rest), and a hook nose that has rounded off to the point that it slips past the knife under heavy lift load but engages on light loads.
Diagnostic: swap the hook-jack module for that shaft with a known-good one from a low-use shaft like shaft 1 or 2. If the fault moves to the new shaft position, you've confirmed the module. If it stays, the problem is in the heald frame or its connecting strap.
A negative dobby uses a reciprocating knife — it accelerates, decelerates and reverses every pick, which generates impact noise and limits top speed. A rotary dobby replaces the knife with a continuously rotating eccentric disc that hooks engage onto. There is no reversal, so reciprocating mass is essentially zero in the selection path.
The practical result on a Stäubli 2670 versus an older 2660: about 6-8 dB(A) quieter at the same PPM and roughly 25% higher top speed before hook bounce becomes a problem. The cost is more complex hook geometry and tighter manufacturing tolerances on the rotating eccentric.
Three questions decide it. How often do you change patterns? If pattern changes are weekly or more often, electronic pays for itself in setup time alone — minutes versus hours of re-pegging. What's the fabric value? Low-margin tweed or hessian doesn't justify the retrofit; high-value bespoke shirting does. And is the loom restored as a museum piece or as a production tool? A working Harris Tweed loom under the Orb mark must use a treadle or non-electric dobby to qualify, so the retrofit isn't even legal for that fabric.
For genuine heritage production work, keep the lag chain. For commercial sample weaving, retrofit.
15 mm of lost motion in the lift path is too much to attribute to a single cause — you almost certainly have stack-up across three or four joints. The usual suspects in order of likelihood: heald-frame connecting strap stretch (synthetic straps can creep 3-5 mm under tension over months of use), worn jack pivot bushings adding 1-2 mm of slop per joint, and shed-stick or backing-shaft flex on wider looms over 220 cm.
Measure each link in turn with the loom on slow turn. Lift the jack by hand and watch where motion is lost — the joint that flexes most is the one to fix first. Don't just crank up the dobby lift to compensate, because that overloads every other shaft that wasn't losing motion in the first place.
Pattern file alone is enough only if the two fabrics use the same shaft count, similar shaft weights, and similar warp tensions. Change any of those significantly and you need to re-tune. Heavier shafts (more heddles per shaft, denser warp) need stiffer return springs and possibly increased lift to clear the shed cleanly. Different warp tensions change the force the jack has to overcome on the lift stroke.
A specific case: switching from 60 ends/cm cotton shirting to 24 ends/cm wool suiting on the same loom roughly halves the lift load per shaft. The dobby will work, but the shafts will overshoot and bounce at the top of stroke unless you back off the lift speed or add damping.
References & Further Reading
- Wikipedia contributors. Dobby loom. Wikipedia
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