GATE Textile Engineering Yarn Manufacture, Yarn Structure and Properties Syllabus

Why is a closed-loop (output) auto-leveller commonly preferred on the draw frame's finisher passage?

Because the draw frame is the last sliver process before spinning preparation, accurate count control matters most here; a closed-loop measuring trumpet/transducer at the delivery measures the actual output sliver and corrects medium/long-term count deviation. Many modern draw frames combine open-loop short-term levelling with closed-loop monitoring/long-term correction.

On a draw frame auto-leveller, what device commonly senses sliver thickness at the input for an open-loop short-term system?

A spring-loaded tongue-and-groove (grooved) roller pair or a stepped/displacement sensing roller measures the instantaneous thickness of the incoming (combined) sliver; the displacement signal is converted electronically and used to vary the draft via a servo/variator acting on the drafting rollers, with timing matched to the transport delay.

List the sequence of operations in one complete combing cycle.

One combing cycle consists of: (1) feeding of the lap by the feed roller, (2) nipping of the fringe by the nipper, (3) combing of the leading fringe by the circular (cylinder) comb, (4) detaching — the detaching rollers grip the combed fringe and draw it back, while the top comb penetrates the trailing portion, (5) piecing — the new fringe is overlapped onto the previously detached web, and the cycle repeats.

What is the function of the top comb in the combing cycle, and when does it act?

The top comb is a stationary comb that descends into the trailing part of the fringe during the detaching phase, after the circular comb has combed the leading end. As the detaching rollers pull the fibres forward, they pass through the top comb, which removes short fibres, neps, and trash from the trailing (rear) portion that the circular comb could not reach.

Define 'noil' in combing and explain how noil percentage relates to yarn quality.

Noil is the waste fibre (short fibres, neps, trash) removed by the comber. A higher noil percentage removes more short fibres, giving longer mean fibre length, cleaner, stronger, more uniform combed yarn but lower yield; noil% is adjustable (e.g., via feed distance, detachment setting, type of feed) and is a key quality/economy trade-off.

How does the direction of feed (forward feed vs. backward feed) affect noil extraction in combing?

Backward feed (feed during the return of the nipper) extracts more noil and gives better combing quality, because the fringe is presented to the cylinder comb more times before detachment. Forward feed (feed during forward nipper motion) gives lower noil, higher production, and is used when less intensive combing is acceptable.

Name and describe two recent developments that have increased combing machine productivity.

(1) Increase in comber nips per minute (from ~250 to over 500–600 nips/min) through improved drive dynamics, lightweight balanced nipper motions, and better piecing systems. (2) Increase to 8 heads per comber, wider laps, robotic/automatic lap transport and changing, and continuous monitoring — together raising production while maintaining quality.

What lap-preparation development precedes the modern high-speed comber, and why is sliver-lap/ribbon-lap or lap former used?

Modern combing is preceded by a sliver-doubling lap former (e.g., draw-frame head + lap former producing a uniform, well-parallelized sheet of doubled slivers as a lap). Good lap preparation gives uniform fibre presentation, better fibre orientation, and uniform combing, which is essential for high-speed combers; trailing-hook orientation in the feed lap is preferred for effective combing.

Why must fibres be presented to the comber predominantly with trailing hooks, and how is this ensured?

Fibres should present trailing hooks so that during detachment the leading ends (which are combed by the cylinder comb) are straight, allowing the top comb to remove short fibres from the rear effectively. An even number of passages/reversals between card and comber is arranged so that the majority of hooks (which leave the card as trailing hooks) reach the comber as trailing hooks.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 $\phi \approx 0.5\text{-}0.6$.

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).

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 $$d \propto \sqrt{\frac{\text{tex}}{\rho\,\phi}}.$$

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 $\sqrt{2}$.

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 $\sqrt{N_{e}}$.

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.

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.

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 $\tan\theta_{r} = 2\pi r T$.

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 $\tan\theta_{surface} = \pi d T$.

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.

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.