GATE Textile Engineering Fabric Manufacture, Structure and Properties Flashcards

24 sample cards from the Fabric Manufacture, 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 three primary (essential) motions of a loom?

   Shedding (separating the warp into two sheets to form the shed), Picking (inserting the weft across the shed), and Beat-up (pushing the inserted weft to the cloth fell with the reed). Without these three no cloth can be woven.

2. What are the two secondary motions of a loom and their purpose?

   Take-up motion (winds the woven cloth forward at a controlled rate, fixing pick density/PPI) and Let-off motion (releases warp from the weaver's beam at uniform tension as weaving proceeds). They are necessary for continuous fabric formation but not for forming a single pick.

3. What are auxiliary (stop) motions on a loom and give examples.

   Auxiliary motions improve quality and prevent damage; they are not essential for weaving. Examples: warp stop motion (stops loom on warp breakage), weft stop/fork motion (stops on weft exhaustion or break), temple (holds cloth width), and protector/anti-crashing (loose-reed) motion to prevent shuttle trap damage.

4. Define shedding and the shed.

   Shedding is the operation of dividing the warp threads into two (or more) layers by raising and lowering the heald shafts. The triangular opening so formed between the upper and lower warp sheets, through which the weft is inserted, is called the shed.

5. What is positive shedding?

   A shedding mechanism that moves the heald shafts in both directions (up and down) positively by the mechanism itself — e.g., a matched/grooved tappet, a positive dobby, or jacquard with positive return. It gives precise control and is essential at high speed or with heavy healds, but is more complex.

6. What is negative shedding?

   A shedding mechanism that moves the heald shafts positively in one direction only (usually downward), with the return movement provided by an external element such as springs, dead weights, or top reversing/roller-and-strap. Ordinary tappet shedding with spring return is the common example.

7. Is ordinary tappet (cam) shedding positive or negative, and why?

   Ordinary single-tappet shedding is negative: the tappet lowers the heald shaft (positive in one direction), while the upward return is produced by springs, dead weights or a reversing roller. Using matched/conjugate (paired) cams makes the tappet shedding positive in both directions.

8. Distinguish dobby and jacquard shedding by capability.

   A dobby controls a limited number of heald shafts (typically up to ~24–30), so all ends on one shaft move together — suitable for small geometric/dobby designs. A jacquard controls warp ends individually (via hooks and harness), allowing very large, complex figured patterns with no shaft limit.

9. Describe an open shed and its advantages.

   In an open shed the warp threads are moved only when the pattern requires a change; threads not changing remain stationary in their up or down position. Advantages: least warp movement per pick, lower yarn strain, smoother running and higher loom speeds. Used with tappet/dobby looms.

10. Describe a (bottom) closed shed.

    In a closed shed all warp threads are brought back to a common level (the bottom level for a bottom-closed shed) after every pick, before the next shed is formed. This causes maximum thread movement and strain and limits speed, but the warp is level each pick (useful for hand-correcting picks).

11. What are semi-open and centre-closed sheds?

    Centre-closed shed: after each pick all threads are brought to a common central (mid) level, then divided up and down again. Semi-open shed: threads that must stay up drop only slightly and rise again, so they need not return fully to level — an intermediate between open and closed sheds with moderate strain.

12. Compare open shed and closed shed in terms of strain and speed.

    Open shed: threads move only on change, so minimum warp movement, lowest yarn strain, fewest end breaks, highest speed. Closed shed: every thread levels each pick, giving maximum movement, higher strain and end breakage, and lower achievable loom speed. Open sheds are therefore preferred for high-speed weaving.

13. List the principal methods of weft insertion (picking) in weaving.

    Shuttle picking (conventional, shuttle carries pirn) and shuttleless methods: projectile (gripper), rapier (rigid or flexible, single/double), air-jet, and water-jet. Shuttleless methods give higher speed, lower noise and supply weft from a stationary package via a measuring/accumulator system.

14. Explain the mechanics of weft insertion with a shuttle.

    The shuttle, carrying the weft pirn, rests in the box; the picking mechanism drives the picker, which propels the shuttle through the open shed across the warp. Weft is withdrawn over the pirn nose (over-end unwinding) during flight. The shuttle is decelerated and arrested (checked) in the opposite box. Kinetic energy imparted is $KE = \tfrac{1}{2}mv^{2}$, delivered in a very short time.

15. Define picking and distinguish over-pick from under-pick mechanisms.

    Picking is the propulsion of the shuttle through the shed. In an over-pick mechanism the picking stick is pivoted at the top and swings from above — suited to lighter, higher-speed looms. In an under-pick mechanism the picking stick/lever is pivoted at the bottom and driven from below — suited to heavier, slower looms and drop-box weaving.

16. Name common shuttle-loom picking mechanisms.

    Cone over-pick mechanism, cone under-pick mechanism, side-lever under-pick mechanism, and parallel-pick. Each uses a picking tappet/cone or nose to suddenly accelerate the picking stick and picker that drives the shuttle.

17. What is shuttle checking and how is it achieved?

    Checking is the controlled deceleration and arrest of the fast-moving shuttle in the receiving box without excessive rebound. It is achieved by the picker, the spindle, and frictional grip of the swell/binder (and a buffer/spring), which together absorb the shuttle's kinetic energy and hold it in position for the next pick.

18. Define beat-up.

    Beat-up is the third primary motion in which the reed, carried on the sley, pushes (beats) the last-inserted pick of weft up to the cloth fell, positioning it against the previously woven cloth. It establishes the weft (pick) density of the fabric, expressed as picks per inch/cm.

19. What is the cloth fell and the role of the reed in beat-up?

    The cloth fell is the boundary line where the newly inserted weft meets the woven cloth. The reed (a comb of wires/dents fixed in the sley) both spaces the warp ends and, on its forward stroke, beats the weft up to the fell; the reed also helps determine cloth width and ends-per-unit-width.

20. Describe the drive (kinematic) arrangement that gives the sley its motion.

    The sley (lay) is mounted on swords pivoted on the rocking shaft and is driven from the crank (bottom) shaft through a crank arm and connecting rod (pitman). This forms a slider-crank/four-bar linkage, converting the crank's rotation into the reciprocating (oscillating) forward-back motion of the sley with approximately simple-harmonic motion.

21. What is the sley (crank-to-connecting-rod) ratio, and how does it affect dwell?

    It is $n = l/r$, the ratio of connecting-rod length $l$ to crank radius $r$ (the eccentricity ratio is its inverse $r/l$). A larger $n$ (long connecting rod) makes the sley motion closer to true SHM and gives a longer dwell near back centre, allowing more time for the shuttle to traverse the shed.

22. Give the approximate displacement and velocity of the sley as a slider-crank mechanism.

    With crank angle $\theta$ measured from front centre and $n=l/r$, displacement $x \approx r\left(1-\cos\theta + \tfrac{1}{2n}\sin^{2}\theta\right)$ and velocity $v \approx \omega r\left(\sin\theta + \tfrac{\sin 2\theta}{2n}\right)$, where $\omega$ is the crank angular speed. As $n \to \infty$ these reduce to pure SHM.

23. What is a loom timing diagram, and what is its reference?

    A loom timing (cycle) diagram shows the relative phasing of the primary and secondary motions over one complete pick, expressed as crankshaft angle from $0^{\circ}$ to $360^{\circ}$. The usual reference is beat-up at front centre ($0^{\circ}/360^{\circ}$), with back centre at $180^{\circ}$; shedding, picking, take-up and let-off are timed relative to it.

24. State the essential timing relationships shown in a shuttle-loom timing diagram.

    Beat-up occurs at front centre ($0^{\circ}$). The shed must be fully open (and dwelling) while the sley is near back centre so the shuttle can cross safely; picking is timed (roughly $90^{\circ}$–$130^{\circ}$) so the shuttle enters and reaches the opposite box before the next beat-up; the healds level (shed change/crossing) shortly before beat-up so the weft is bound at the fell.

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