Repeated Weft Breaks at the Same Pick Position on a Rapier Loom
A 190 cm rapier loom with a 20-shaft dobby runs a 30 Ne × 20 Ne, 68 × 60 plain quality at 620 rpm. Over two weeks, weft stops cluster at the same crank position, and the weaver finds the pick broken 4–6 cm from the left-hand selvedge. Start marks appear every 40–60 cm, and a faint crack line is visible at each restart. Maintenance records show that the pick-finding wheel was replaced 10 days ago, the weft brake spring tension was increased to stop pick run-back, the cutter blade was adjusted, and the feeler timing was left unchanged. Warp let-off tension is 3.0 kN against a standard 2.6 kN. Efficiency has fallen from 89% to 78% and seconds have risen from 4.5% to 7.2%. The supervisor wants to reduce loom speed to 520 rpm to stop the breaks and asks for your opinion before the change is made.
1. Diagnose the likely cause of the breaks occurring repeatedly 4–6 cm from the left selvedge, and explain what the evidence says about yarn quality versus mechanism.
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Why this is a mechanism fault, not a yarn fault
The break position is the strongest clue. It is always 4–6 cm from the same selvedge, which is the zone where the rapier head releases the pick and where the weft brake and cutter act. Random yarn faults would be distributed along the width and vary in position. A fixed position points to a fixed setting or component.
The timeline makes the mechanism case stronger: the problem began after four changes were made together - pick-finding wheel replacement, increased brake tension, cutter adjustment, and a let-off tension change. Any one of these could produce the symptom, and a combined change is the classic way to lose the reference setting.
Most probable causes
- Pick-finding wheel fitted one or two teeth out of position. The loom then stops and restarts at a crank angle that does not match the last pick, so the pick is left partly in the shed and is pulled against the release point on restart. The break appears near the gripper release zone and the restart leaves a crack.
- Weft brake over-tensioned to stop run-back. Excess brake force holds the pick back during insertion, stretching and thinning it at the point where the rapier releases. The yarn then breaks at that thin place, 4–6 cm from the selvedge.
- Cutter blade gap or timing wrong. A blunt or badly gapped cutter tears rather than cuts, leaving a tail that catches in the shed on the next pick.
- Let-off at 3.0 kN instead of 2.6 kN. This tightens the shed and raises warp tension, but it mainly makes the restart mark more visible; it is a contributing factor rather than the primary cause.
Before touching anything, confirm the crank angle at which the loom stops and the pick-finding position against the machine manual.
2. Evaluate the proposal to reduce speed to 520 rpm and set out the checks and settings you would make, with reasoning for the order of work.
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On the speed reduction
Dropping from 620 to 520 rpm cuts output by 16% before any break is fixed. It may lower peak weft tension slightly and mask the symptom, but it does not correct the pick-finding error or the over-tight brake, so the crack and start mark will remain. It also hides the true cause, making later diagnosis harder. Speed should be reduced only as a temporary containment measure, and only after the timing checks, because the loom must be re-timed at the final speed anyway.
Order of work
- Establish the reference. Confirm 0° at beat-up and record the stop angle and pick-finding angle. Everything else depends on this datum; if it is wrong, every adjustment is guesswork.
- Pick-finding and pick wheel. Check that the replaced wheel is the correct part, correctly keyed, and that the pick-finding position matches the manual. An 8° error here is enough to leave a pick half inserted.
- Weft cutter. Check blade sharpness, gap and the angle at which it operates, normally shortly after beat-up while the pick is still under control. A clean cut is needed before the brake acts.
- Weft brake. Release the tension back towards the previous setting and set the brake to apply after the cutter has operated. Test with a length of yarn pulled by hand and check for stretch marks.
- Rapier head and tape. Inspect the gripper jaws and tape guides for wear or misalignment at the release point; the break position is exactly in this zone.
- Let-off tension. Return to 2.6 kN unless there is a documented reason to change it.
- Trial run. Run at 620 rpm for at least 10⁵ picks and record weft stops by position. Only then decide whether speed needs changing.
Restart on slow speed for the first two or three picks after any stop until the crack disappears.
3. Explain how a crack line and a start mark are formed at restart, and how pick-finding practice should be controlled to prevent them.
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How the marks form
When a loom stops with the pick part-way through the shed, the pick is left in one of three states: fully inserted but uncut, partly inserted, or pulled back. On restart, the pick spacing at that point differs from the normal beat-up by a fraction of a pick. The warp ends also relax or stay stretched unevenly while the loom is stopped, so the first few picks after restart are beaten up at a slightly different tension and pick density.
The result is a visible band: a start mark where pick spacing is irregular, and a crack where the weft is missing or doubled for one or two picks. If the pick-finding position is wrong by one pick, the loom may weave a missing pick and the crack becomes a running fault, not just a mark.
Control of pick-finding
- Set pick-finding to the crank angle specified for the loom, so the shed is opened to release the trapped pick and the reed is clear of the fell.
- Ensure the weft is cut and the loose end removed from the shed before restart; a tail left in the shed is pulled through and creates the crack.
- Keep the brake and cutter timing consistent, because pick-finding assumes the pick is under control at the stop.
- Restart on slow speed for the first picks, and use a soft-start or automatic pick removal if the loom has it.
- Record every stop angle and inspect the first metre after restart; a repeating stop angle is the early warning of a pick-finding fault.
Prevention is cheaper than repair: one crack line can downgrade a full roll, while the correct pick-finding angle costs nothing but discipline.