The ends-down rate in ring spinning has a direct influence on the profitability of the spinning process. In view of the persistent trend of realizing spindle speeds up to 25,000 rpm in industrial practice, the probability of ends-down as a result of higher yarn tension and other factors of influence is getting very important.
1. Exposed section of the yarn path
In all probability, ends-down in ring spinning always occur between the front roller pair of the drafting system and the yarn guide, arranged centrically to the spindle. In this section, the yarn has a more or less reduced twist due to the twist-retaining effect of the yarn guide.1 (Fig. 1)
This connection is inherent in ring spinning and can be influenced to a certain extent by re-arranging the spinning elements (spinning geometry). Twist reduction affects the stability of the spinning process. This negative effect is even increased by the following factors:
- high yarn tension
- smaller deviation radius of the yarn at the yarn guide
- smaller diameter of the yarn guide material
- reduced twist multiplier
- smaller elasticity module of the fibre
Ends-down will always occur just at the weakest point of the yarn within the described section. This may be either in the spinning triangle itself at the front roller pair of the drafting system or in the subsequent yarn section between spinning triangle and yarn guide.
2. Conditions in the spinning triangle
In a spinning triangle, fibres are always subjected to uneven load due to the spinning tension, while maximum load is exerted on marginal fibres. The fan-shaped fibre band is transferred into the more or less round cross-section of the yarn. The wider the spinning triangle, the more different is the pre-tension of the marginal fibres at the moment of twist impartation. As a result of this pre-tension, especially the marginal fibres are prevented from migrating between the different layers of the yarn cross-section.
The size of the spinning triangle is determined by width B of the fibre band leaving the drafting system and length L, which is confined by twisting point Z. The position of point Z depends on width B, the amount of turns per meter and the yarn tension applied.
When the outer marginal fibre arrives at its elongation at rupture, it will break. Then the next fibre will break and finally the resistance of the spinning triangle is exhausted and the yarn will break. On the assumption that fibre distribution is uniform and fan-like in the spinning triangle and that all fibres are clamped on both sides, which is coming very close to practice, there exist in theory the following two fundamental load scenarios:
- The marginal fibre arrives at its elongation at rupture. The central fibre (middle of spinning triangle) is not yet under load.
This is the case in conventional ring spinning with higher twist multipliers, wide spinning triangle or low elongation at break of the fibres (cotton) (Fig. 3).
- The marginal fibre
arrives at its elongation at break, when the central fibre is already under
This is the case in conventional ring spinning with low twist multiplier, narrow spinning triangle and high elongation at break of the fibres (synthetic fibres) (Fig. 4).
3. Factors influencing the strength of the spinning triangle
If the twist multiplier is increased at a constant width B of the fibre band, the strength of the spinning triangle will decrease accordingly. The reason is the increasing slope of the marginal fibres and consequently their higher load. When the fibre band is wider, but yarn tension maintained, the strength of the spinning triangle will also decrease, as the load of the marginal fibres is increasing in this case, too. When spinning tension is raised, the other conditions being unchanged, the spinning triangle will get longer. As a result, the number of marginal fibres clamped on both sides will decrease. Simultaneously, however, the pre-tension of the fibres still being clamped on both sides is increased. A high spinning tension results in higher hairiness and more fibre loss.
Fibre length has an analogous influence, but is not so important in practice, because the fibre length is significantly longer than the length of the spinning triangle.
All the above-mentioned factors alone are not decisive for the ends-down rate, but only take effect in combination with the elongation at break of the fibres.
W. Krause and A. Soliman could prove both mathematically and by way of experiment, that above all the elongation at break of the fibres has a decisive influence on the strength of the spinning triangle.
When fibres have a higher elongation at break, the spinning triangle will extend, so that the marginal fibres have less slope and can contribute better to the strength. As a result, cotton yarns and synthetic yarns have a fundamentally different ends-down distribution between spinning triangle and yarn section.
4. Ends-down distribution
For industrial practice and technical development of the ring spinning technique it is of importance - apart from knowing the factors of influence in which relation the two critical areas, i.e. spinning triangle and yarn section between spinning triangle and yarn guide, contribute to the total ends down rate.
Trials have shown that with cotton yarns with an αe range from 3.5 to 5.5 almost 100% of all ends-down happen at the spinning triangle.2 Some yarns with a twist multiplier αe < 3.5 can have a lower strength than the spinning triangle. So, when spinning cotton, the spinning triangle is clearly the weak point.
Synthetic yarns on the other hand have an ends-down portion of about 50% at the spinning triangle, largely irrespective of yarn twist, what underlines again the vast influence of elongation at break of the fibres on the spinning triangle strength.
5. Influence of compacting
By adding a compacting zone to the actual drafting process, the spreading of fibres at the front roller pair of the drafting system is minimized until the spinning triangle is virtually eliminated. The re-arrangement of the fibres from two-dimensional to three dimensional structure is drastically reduced. Twist is imparted to a compacted bundle of largely parallel fibres and can extend almost up to the clamping line. Optimum results are achieved, if compacting takes place without fibre elongation and only with a slight tension of the fibre band. This is perfectly put into effect with the EliTeQCompact Spinning System. A fibre band compacted in such a way helps the fibres to migrate better between the different yarn layers when twist is imparted.
Inter-fibre friction of the fibre band is increased all through the yarn cross section, and this is the reason for higher yarn strength in compact spinning.
Contrary to load scenarios 1 and 2 (see item 2), we now have load scenario 3. All fibres have almost the same load. This permits to achieve strength values with cotton at the drafting system front roller pair, which correspond to the yarn strength or often are even better. The reduction of the ends-down rate by 40 to 50% in compact spinning is mainly due to the drastic decrease of ends-down at the front roller pair of the drafting system. In comparison with conventional ring yarn, the ends down distribution between front roller pair of the drafting system and subsequent yarn section is different (Fig.7).
The great influence of elongation at break of the fibres almost disappears as a result of the eliminated spinning triangle, so that compact yarns from cotton and man-made fibres now have a very similar ends-down distribution.
As a result of different elongation at break of cotton and man-made fibres, the ends-down rate at the spinning triangle of such yarns is different. A compacting zone installed subsequent to the drafting system largely eliminates the influence of this elongation at break, so that compacted cotton and synthetic yarns have a similar ends-down distribution between drafting system and subsequent yarn section. Ends-down of a compact yarn mainly occur at the yarn portion between front roller pair of the drafting system and yarn guide.
- G. Trommer, Faserforschung and Textiltechnik 3/1967
- W. Krause, A. Soliman, Melliand 04/ 1991
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