Ring-frame end breakage after a roving count change and humidity drop
A ring frame spins 40s Ne carded yarn at 15,500 rpm with 25 TPI, 42 mm rings and 5/0 travellers; it has 1,000 spindles and runs 8-hour shifts. After a roving lot change from 1.2 Ne to 0.9 Ne and a fall in spinning-room RH from 55% to 42%, end breakage rose from 18 to 52 per 1,000 spindle-hours. CSP fell from 2,400 to 2,150, U% rose from 12.2 to 13.5, and thin places increased. The top roller cots are 18 months old and show flat spots; the traveller number has not been changed. The humidification plant is running but several nozzles are choked. The production manager proposes raising spindle speed to 16,500 rpm to recover output. The yarn is for hosiery and the customer has complained of hairiness and slubs.
1. Calculate delivery speed, production per 1,000 spindles per 8-hour shift at 96% efficiency, and twist multiplier. Is 25 TPI suitable for hosiery yarn?
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Delivery speed = spindle speed / TPI = 15,500 / 25 = 620 in/min = 15.75 m/min.
40s Ne linear density = 590.5 / 40 = 14.76 tex = 14.76 g/km.
At 100% efficiency, length per spindle per shift = 15.75 × 480 = 7,560 m. Mass = 7.56 km × 14.76 g/km = 111.6 g. At 96% efficiency = 111.6 × 0.96 = 107.1 g per spindle per shift. For 1,000 spindles this is 107.1 kg per shift.
Twist multiplier = TPI / √Ne = 25 / √40 = 25 / 6.32 = 3.95.
A TM near 3.95 is at the warp-yarn end. Hosiery normally uses about 3.2–3.6 TM for a softer, more extensible yarn. The 25 TPI is therefore not ideal for hosiery: it will give a harder yarn, lower loop extensibility and a harsher hand. It is not, however, the main explanation for a jump in breaks from 18 to 52; the humidity, cots and draft change must be examined first.
2. Rank the likely causes of the breakage increase and justify the ranking with the data. Include the roving draft change, humidity, cots and traveller. What immediate actions would you take before increasing speed?
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Ranking: (1) humidity, (2) top roller cots, (3) roving draft change, (4) traveller choice.
Humidity: at 42% RH, cotton regain falls and the fibre is stiffer and less extensible; static and hairiness rise, and thin places become weak spots. The simultaneous rise in U% and thin places is consistent with this. Restoring 55% RH is the first action.
Cots: flat spots cause periodic slip and drafting waves, producing thick and thin places and repeated breaks at the same spindle positions. An 18-month-old cot with visible flats is due for replacement regardless of the breakage data.
Roving change: draft = 40/0.9 = 44.4, compared with 40/1.2 = 33.3 before, a 33% increase. A finer roving with the same roller settings gives poorer floating-fibre control and more irregularity. This is a secondary but real contributor.
Traveller: the yarn count is still 40s, so the traveller is not obviously wrong, but a lighter traveller may be needed if tension is high. Before raising speed, restore humidity, replace the cots, check roller settings and try one traveller number lighter in a controlled position. Raise speed only after breakage is back below about 25 per 1,000 spindle-hours.
3. If the manager raises speed to 16,500 rpm at the same TPI and 96% efficiency, calculate the new production and explain why this is unlikely to solve the problem. Recommend a controlled trial with acceptance criteria.
Read the worked answer
New delivery speed = 16,500 / 25 = 660 in/min = 16.76 m/min. Length per spindle per shift = 16.76 × 480 = 8,045 m. Mass at 100% = 8.045 km × 14.76 g/km = 118.8 g. At 96% = 114.0 g per spindle, or 114.0 kg per 1,000 spindles per shift.
The gain is only 114.0 − 107.1 = 6.9 kg, about 6.4%. At the same time, higher spindle speed raises balloon tension, traveller friction and yarn hairiness. With weak thin places, breaks will rise, so efficiency will fall and the net gain may be negative. The speed increase treats output as if efficiency were fixed, which the breakage data contradict.
Run a trial with 200 spindles for 24 hours: restore RH to 55%, fit new cots, correct roller settings, and test one lighter traveller. Do not change speed during the first trial. Acceptance criteria: end breakage at or below 25 per 1,000 spindle-hours, CSP at least 2,300, U% not more than 12.5, and no increase in customer-visible hairiness or slubs. Only then consider a small speed increase.