GATE Textile Engineering Yarn Manufacture, Yarn Structure and Properties Flashcards

24 sample cards from the Yarn Manufacture, Yarn Structure and Properties deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What are the principal modern developments in the roving (speed) frame?

   Modern roving frames feature: individual servo drives for spindle, flyer, bobbin and bobbin-rail (eliminating cone-drive belts), automatic doffing and bobbin transport, microprocessor control of twist and winding tension, larger packages, and improved false-twisting top flyers/closed flyers for better roving control at higher speeds.

2. Why does the roving frame require a precisely coordinated differential/cone or servo drive for the bobbin?

   Because the roving must be wound at constant linear speed while the bobbin diameter grows with each layer, the bobbin rotational speed (relative to the flyer) and the lift traverse speed must continuously decrease as the package builds. Traditionally a differential gear plus cone drum belt managed this; modern frames use independent servo drives controlled electronically to maintain constant winding tension.

3. Why is twist inserted in the roving, and why must it be kept low?

   A small amount of twist gives the roving enough cohesion/strength to withstand winding and unwinding without false drafts, but it must be low enough to allow easy drafting at the ring frame. Excessive roving twist resists drafting and causes irregular attenuation; roving twist is typically expressed by twist multiplier and kept much lower than yarn twist.

4. What is the function of the flyer's false-twist (top) device on a roving frame?

   A false-twister (groove/closed top of the flyer) imparts temporary additional twist to the roving as it passes over the flyer top, increasing the roving's tensile strength at the weakest point (just below the front roller) to prevent breakage, while the twist is cancelled before winding so the wound roving retains only the intended real twist.

5. List the main categories of causes of end breakages in ring spinning.

   End breakages arise from: (1) spinning tension exceeding yarn strength (high spindle speed, large balloon, traveller faults), (2) weak/thin places and faults in the yarn (irregular drafting, poor fibre quality), (3) mechanical defects (bad traveller, rough ring, eccentric rollers, defective separators), and (4) environmental/material factors (low humidity, poor fibre cohesion). End breakage rate rises sharply with spindle speed.

6. How does spinning tension relate to balloon size and spindle speed, and why does this cause end breaks?

   Spinning (yarn) tension is generated mainly by the traveller drag and balloon centrifugal effects and increases roughly with the square of spindle (traveller) speed: $T \propto N^{2}$. As spindle speed rises, tension increases and the balloon enlarges; when peak tension exceeds the yarn's weakest-place strength, an end break occurs, which is why breakage rate climbs steeply at high speeds.

7. Why is the ratio of yarn strength to spinning tension important, and what is the typical safe relationship?

   For stable spinning the yarn strength at its weakest point must exceed the maximum spinning tension with a safety margin; if mean tension approaches the weak-place strength, breaks proliferate. Spinners aim to keep maximum spinning tension well below mean yarn strength (a large strength-to-tension ratio), since the weakest thin places, not the mean, determine breakage.

8. Describe key modern developments in the ring spinning machine aimed at higher productivity and quality.

   Developments include: compact (condensed) spinning to reduce the spinning triangle and hairiness, individual spindle monitoring with automatic single-spindle stop, higher spindle speeds with improved spindle/bolster design, automatic doffing and link to winding, longer machines with more spindles, electronic drives, and improved low-mass travellers/rings and ring lubrication.

9. What is compact (condensed) ring spinning and what is its main advantage?

   Compact spinning condenses the drafted fibre strand by aerodynamic (suction through a perforated drum/apron) or mechanical means just before twist insertion, drastically narrowing or eliminating the spinning triangle. The main advantages are markedly reduced hairiness, higher yarn strength, fewer end breaks, and better fibre utilization, allowing lower twist or finer counts.

10. Why does reducing the spinning triangle in compact spinning improve yarn strength?

    The spinning triangle is the flat fibre bundle leaving the front roller before twist consolidates it; edge fibres in a wide triangle are poorly bound and contribute little to strength while increasing hairiness. Condensing the strand narrows the triangle so nearly all fibres are integrated into the twisted structure, increasing the fraction of load-bearing fibres and thus tenacity.

11. State the relationship between the twist of a folded (plied) yarn and the twist of its single (component) yarns for a balanced two-ply yarn.

    For a balanced two-ply (folded) yarn, the folding twist is inserted in the opposite direction to the single-yarn twist, and the folding twist magnitude is typically about $0.6\text{-}0.7$ times the single-yarn twist (commonly quoted as $\approx \tfrac{2}{3}$). In terms of twist multipliers, the folding twist multiplier is usually lower than the single-yarn twist multiplier for balance.

12. For a balanced folded yarn, what is the relationship between single-yarn twist direction and folding-twist direction?

    They are opposite: if the single yarns are spun with Z twist, the folding (ply) twist is inserted as S twist (and vice versa). Folding in the reverse direction reduces the residual torque of the single yarns toward zero, giving a balanced (torque-free, snarl-free) folded yarn.

13. Define 'twist multiplier' (twist factor) and give its relation to twist per unit length and yarn count in the tex system.

    Twist multiplier (twist factor) characterizes the level of twist independent of count. In the tex system: $$\alpha_{tex} = T_{m}\sqrt{\text{tex}},$$ where $T_{m}$ is turns per metre. In the English (cotton) system $\alpha_{e} = \dfrac{TPI}{\sqrt{N_{e}}}$, where $TPI$ is turns per inch and $N_{e}$ is English count.

14. What is fibre packing density in a yarn, and how is it defined?

    Fibre packing density (packing fraction) $\phi$ is the fraction of the yarn's cross-sectional area actually occupied by fibre material, $$\phi = \frac{\text{area of fibres}}{\text{total yarn cross-sectional area}} = \frac{A_{fibre}}{A_{yarn}}.$$ It indicates how tightly fibres are packed; the remainder is air/voids. Typical staple yarns have $\phi \approx 0.5\text{-}0.6$.

15. How does fibre packing density vary across the radius of a typical staple-fibre yarn?

    Packing density is generally highest near the yarn core/centre and decreases toward the surface, because surface fibres are looser and protrude as hairs. Thus the yarn is denser at the centre and more open at the periphery; increasing twist tends to raise the overall packing density (up to a limit).

16. Give the relationship between yarn diameter $d$, yarn count, and fibre packing density.

    Assuming a circular cross-section, yarn specific volume relates diameter to linear density and packing. In tex with fibre density $\rho$ ($\mathrm{g/cm^{3}}$) and packing fraction $\phi$: $$d = \sqrt{\frac{4 \times \text{tex}}{\pi \rho \phi \times 10^{5}}}\ \text{cm}.$$ The key proportionality is $$d \propto \sqrt{\frac{\text{tex}}{\rho\,\phi}}.$$

17. Why is yarn diameter proportional to the square root of the yarn's linear density (tex)?

    Because linear density (mass per unit length) is proportional to cross-sectional area $A$, and $A = \tfrac{\pi}{4}d^{2}$. Therefore $\text{tex} \propto d^{2}$ (for fixed packing density and fibre density), which gives $$d \propto \sqrt{\text{tex}}.$$ Doubling the tex increases diameter by a factor of $\sqrt{2}$.

18. State the commonly used empirical relation between yarn diameter (in inches) and English cotton count $N_{e}$.

    A widely used empirical relation (Ashenhurst) is $$d = \frac{1}{28\sqrt{N_{e}}}\ \text{inches},$$ i.e. the yarn diameter in inches is approximately the reciprocal of $28\sqrt{N_{e}}$. The constant varies somewhat with packing/fibre type, but diameter is inversely proportional to $\sqrt{N_{e}}$.

19. Describe the general relationship between yarn twist and yarn strength, including the optimum.

    As twist increases from zero, yarn strength rises because greater fibre cohesion and transverse pressure reduce fibre slippage; strength reaches a maximum at an optimum twist multiplier, then decreases with further twist because the increasing fibre obliquity reduces the axial component of fibre strength (fibres lie more across the axis) and induces stress concentrations. This gives a characteristic strength-vs-twist curve peaking at the optimum twist.

20. Explain the two opposing mechanisms behind the strength-versus-twist curve of a staple yarn.

    (1) Cohesion mechanism: more twist increases inter-fibre friction/lateral pressure, reducing fibre slippage and letting more fibres carry load — increasing strength. (2) Obliquity mechanism: more twist increases the helix angle so fibres are more inclined to the yarn axis, reducing the axial contribution of each fibre's strength (factor $\cos^{2}\theta$). The balance of these gives a peak at optimum twist; beyond it, obliquity dominates and strength falls.

21. In the helical (idealized) model of yarn structure, relate the surface helix angle $\theta$ to twist $T$ (turns per unit length) and yarn diameter $d$.

    For a fibre on the yarn surface, in one turn it advances axially $\tfrac{1}{T}$ while wrapping a circumference $\pi d$, so $$\tan\theta = \pi d T.$$ The surface helix angle $\theta$ increases with both yarn diameter and twist; fibres at radius $r < d/2$ have a smaller local helix angle, with $\tan\theta_{r} = 2\pi r T$.

22. In the idealized helical yarn model, how does the local helix angle vary from the centre to the surface of the yarn?

    In the idealized (coaxial helix) model, a fibre at the yarn centre lies straight along the axis (helix angle $0$), and the helix angle increases with radius, being maximum at the surface. At radius $r$, $$\tan\theta_{r} = 2\pi r T,$$ so $\theta$ grows from $0$ at the core to $\theta_{surface}$ at $r = d/2$ where $\tan\theta_{surface} = \pi d T$.

23. Derive/state how twist contraction (retraction) affects yarn length, and define the twist contraction factor.

    Inserting twist makes surface fibres follow a helical path longer than the yarn axis, so the yarn contracts in length. The retraction (contraction) is expressed by a contraction factor or retraction coefficient, e.g. $$\text{retraction} = \frac{l_{0} - l}{l_{0}},$$ where $l_{0}$ is the untwisted fibre length and $l$ the twisted yarn length. Yarn count (tex) therefore increases slightly after twisting compared with the untwisted strand.

24. Using the helical model, express the relationship between twist multiplier and surface helix angle, showing why twist factor (not turns/length) characterizes the structure.

    Since $\tan\theta = \pi d T$ and $d \propto \sqrt{\text{tex}}$ (i.e. $d = k\sqrt{\text{tex}}$), we get $$\tan\theta = \pi k T \sqrt{\text{tex}} = \pi k\,\alpha_{tex},$$ where $\alpha_{tex} = T\sqrt{\text{tex}}$. Thus the surface helix angle depends only on the twist multiplier, not on count or turns/length separately, which is why yarns of equal twist factor have the same 'hardness'/helix geometry regardless of count.

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