Weaving Preparation and Loom Mechanisms

Loom shedding and picking mechanisms

Every loom performs a fixed sequence of motions: shedding, picking, beat-up and take-up. Shedding separates the warp threads into two layers, the upper and lower sheds. It is produced by cam or tappet mechanisms, by dobby, or by Jacquard machines, where the Jacquard controls each warp end individually for figured fabrics. Picking inserts the weft through the shed by a shuttle, a projectile, a rapier, or an air or water jet. Beat-up drives the newly inserted pick against the fell of the cloth by the sley, and the sley is commonly driven by conjugate or matched cams. Take-up winds the finished cloth onto the cloth roller through a gear train, and the seven-wheel train is the classical arrangement.

Shedding timing is described as early or late. In late shedding, the shed is opened so that its dwell coincides with the passage of the shuttle through it, so the warp is rubbed less. Smooth filament yarns are easily damaged by abrasion, so late shedding is the preferred timing for filament weaving.

On a shuttle loom, the crankshaft turns once per pick while the shedding cam turns once per two picks. One degree of cam rotation therefore corresponds to two degrees of crankshaft rotation, and each shed dwell measured on the crankshaft must be converted through this 2:1 ratio before the cam dwells are added.

In a projectile loom, the torsion rod stores the energy that launches the projectile. For a rod of length L, shear modulus G and twist angle \theta, the stored energy is U = \frac{1}{2}\frac{GJ}{L}\theta^2. For a circular section, J = \pi r^4/2, so at a given twist the stored energy scales as r^4.

Heald staggering sets the relative positions of the heald frames and can be fixed or variable. Variable staggering shortens the sley dwell, whereas fixed staggering limits the shed height that can be formed. A high eccentricity ratio e = r/L in the crank drive raises the accelerations and increases vibration in the machine. Late shedding leaves little time for shed clearance, so it is unfavourable for heavy fabric.

A profile reed has dents whose cross-section is shaped to guide the air flow and the weft through the shed, and it is characteristic of the air-jet loom.

Each mechanical part has a specific role. A worm and worm wheel give a continuous positive take-up motion, a friction clutch drives the cloth roller with controlled slip, conjugate cams give the timed sley motion of a rapier loom, and a lease rod separates the warp threads into two sheets to form the shed.

Multiphase weaving inserts the weft in several phases within one shed opening. Because the phases constrain the interlacing, the range of weaves that can be produced is limited. The beat-up is applied in stages, so the force delivered at each beat-up is low, which suits fragile yarns. The method is adopted to raise output, so low productivity is not its characteristic, and the weft is laid with comparatively gentle tension rather than high tension.

Fabric cover factor

Cover factor describes how much of the fabric plane is covered by yarns. For a woven fabric, the warp fractional cover E_1 and the weft fractional cover E_2 combine as E = E_1 + E_2 - E_1E_2 = 1-(1-E_1)(1-E_2), because the overlapping area is counted only once.

This is Peirce's cover relation, and it holds for a single-layer plain interlacing structure. Double cloth, which consists of two separate layers, does not obey it. Terry fabric, whose pile loops add cover beyond the warp and weft crossings, and leno fabric, whose twisted warp pairs form a gauze interlacing, also fall outside this simple relation.

For a plain square fabric, the total cover is F = 1-(1-E)^2, where E = dn is the cover in one direction, d is the yarn diameter and n is the number of ends per unit length. From the total cover, E = 1-\sqrt{1-F}, and the spacing between adjacent ends is 1/n = d/E.

In the weft direction, the cover is proportional to the picks per unit length multiplied by the square root of the yarn linear density, because at constant packing density the yarn diameter scales with \sqrt{T}. To keep the cover constant when the weft yarn is changed, the picks per unit length must satisfy n\sqrt{T} = \text{constant}, so a finer yarn needs proportionally more picks.

In a jammed square plain fabric, a yarn of circular cross-section fills \pi/4 \approx 0.79 of its unit cell, so the fractional fabric cover of such a jammed fabric is about 0.79.

The maximum sett of a jammed plain fabric is set by the yarn diameter through Peirce's jamming relation, n_{max} = 25.4/(\sqrt{3}\,d) ends per inch with d in mm. The diameter is obtained from the cross-sectional area, which equals the linear density divided by the yarn density, with units converted consistently.

Winding package and traverse

Packages are built in several forms. In a parallel-wound package the coils of successive layers lie parallel, so the upper coils can slough off during unwinding.

In a ring bobbin, the main winding layers are separated by cross-winding layers. These cross-winding layers bind the coils, so slough-off is reduced compared with a parallel-wound package.

A cop is cross-wound, meaning successive layers of yarn cross each other at a helix angle and interlock. This interlocking prevents coils from slipping off the package tip during unwinding, so the yarn can be withdrawn smoothly at high speed.

In a drum-driven winder, the package is driven by surface contact with the drum. A diamond-grooved drum has a traverse groove cut in diamond shapes, and each diamond moves the guide across the full width of the package. For a single traverse, the drum therefore turns once for each diamond.

Winding time follows from yarn length and winding rate. Because linear density in tex is grams per kilometre, the yarn length equals the yarn mass divided by the tex value. The time equals that length divided by the effective speed, which is the machine speed multiplied by the efficiency.

The package and the drum surfaces move at the same linear speed. The number of package revolutions per drum revolution is therefore D/d, where D is the drum diameter and d is the package diameter. The wind per double traverse is directly proportional to D/d, not to d/D.

Shuttle loom picking mechanics

On a shuttle loom with a linear displacement profile for the picker, the nominal displacement of the shuttle is directly proportional to the angular movement of the crankshaft. The maximum actual velocity of the shuttle depends on the loom speed, and the maximum actual acceleration depends on both the loom speed and the alacrity of the picking system. The maximum acceleration and the maximum velocity do not occur at the same instant, because the acceleration peaks early in the stroke while the velocity is still building up.

Warp protection is a stop motion that acts when the shuttle is not delivered correctly. Entrapment of the shuttle inside the shed and flying of the shuttle out of the shed both trigger the warp protector. Warp breakage and warp slackening are sensed by the warp stop motion instead.

The beat-up force equals the sley mass multiplied by the sley acceleration at the front dead centre. For a slider-crank drive with crank radius r, angular speed \omega and eccentricity ratio e = r/L, where L is the connecting-rod length, the sley acceleration at that position is r\omega^2(1+e). At equal picks per minute and equal sley mass, the beat-up forces of two looms are therefore compared through the ratio of r(1+e).

Weaving resistance at beat-up

The weaving resistance is the force the reed must overcome to drive the pick into the cloth fell at the instant of beat-up. It depends on the elastic extension of the warp, which is governed by the free length of the warp and the elastic modulus of the warp yarn. It also depends on the compressibility of the loom-state fabric at the fell, which is governed by the elastic modulus of that fabric. The elastic modulus of the weft yarn does not directly set this resistance at the instant of beat-up.

Bumping is the shock and oscillation of the warp at beat-up, and it produces uneven cloth. Higher warp tension keeps the warp taut and limits its movement, which damps the impact. Higher pick density adds picks to be beaten up per unit length and raises the beat-up force. Reducing the shed height or the sweep of the sley does not lower the shock delivered at the fell.