What Is Twist?
Twist is the most important factor in how yarns behave in fabric. Almost all staple yarns and many filament yarns depend on twist to hold the fibers in their structure. Twist is the spiral pattern of the fibers around an axis in the yarn created by twisting one end of a fiber strand while the other end remains fixed (tension for stability).
The spiral pattern binds the fibers in place and provides the spun yarn with much of its tensile strength.
Twist is measured in terms of turns per meter (tpm), turns per centimeter (tpcm), or turns per inch (tpi). The exact level of twist is an important factor in the visual characteristics, performance, and longevity of the resulting yarn.
How Twist Changes Yarn Behavior
Twist level has a simultaneous effect on many properties within a yarn, especially with staple yarns. One of the most obvious effects is with bulk: as twist increases, the apparent diameter of the yarn decreases. Imagine twisting a loosely spun strand of cotton by hand - the more you twist, the more the strand tapers.
Strength is less predictable than fineness. Strength increases with the same amount of twist in a staple yarn until a little beyond the point of maximum strength, when further twist actually weakens the yarn, and very high twist can make yarns very brittle. Elasticity increases proportionally to the amount of twist.
Very highly twisted yarns are called crepe and are used for very fine yarns.
They have so much twist that they must be held under tension- for example, in a loom- to counteract their tendency to curl up on themselves.
It is very easy to identify a crepe yarn by drawing a few meters of it from the fabric and pulling it between two fingernails to remove the tensioning effect created by sizing. Crepe yarns are those that bouclé up into tight corkscrew curls-this curled effect continues once the yarn is in fabric. Articles woven from crepe yarns are less dimensionally stable than those woven from other yarns-crepe fabrics will stretch easily.
Twist degree also influences the fabric's susceptibility to soil and moisture. Since tightly twisted fibers tend to have a smoother outer surface and fewer loose fiber ends present for attachment, soil is more easily washed away. Moisture absorption is also decreased because a harder twist lessens the surface area of dry fiber.
Tightly twisted yarns are also more resistant to abrasion, because the continuous cycle between core and surface fibers ensures uniform wear.
With loose twist, thus, surface fibers are more available for irregular wear.
Twist also affects the appearance of fabric. Yarns spun with a high natural sheen display more reflectance at a light twist than a less lustrous yarn; relatively loose structures allow more of the fiber surface to reflect light, and the increased brightness begins to diminish as the twist is increased. Yarns spun specifically for a crepe fabric produce rough, crinkled finishes that directly result from the uneven surface created by crepe yarns, and slub yarns-deliberately spun at uneven levels of twist-bring their own characteristic rough-and-glossy quality to woven fabrics.
Twist Direction: S-Twist and Z-Twist
Fibers can be twisted to the right or to the left, and the textile industry has adopted convenient abbreviations for describing each: Z-twist and S-twist. Right twist, Z twist, means that the fibers are spiraled in the same diagonal direction of the center stroke of the capital Z; S twist, left twist, means the fibers follow the diagonal of the S.
Here’s a simple way of revealing the difference. Grab a small tuft of cotton or similar material and draw a straight line down one. Hold the fibers between the thumb and forefinger of each hand.
Turn the fingers on top to the right.
The line held by the fibers now travels diagonally in the same direction as the Z-bar. Now turn the fingers on top to the left. The line now follows the bar of the S.
Almost all commercial yarns are spun with a Z-twist, which is also more common in weaving yarns across the board. However, intentionally pairing S- and Z-twist yarns with each other-especially if such pairing occurs between two different yarns, or in the process of plying-is an effective way to create a distinctive look and feel that will not be as easy to find if working with single twist directions. Many hand spinners will be inclined to follow this same rule of thumb: spinning their singles with clockwise Z-twist, then plying the singles with an S-twist in the counterclockwise direction.
How Much Twist Is Applied
The correct amount of twist for any given fiber is a factor of fiber length, yarn count and the ultimate end use of the yarn. Too little twist and the yarn will be weak, but as twist levels increase in the fiber to fiber interaction, the more the twist, the stronger the yarn. Increasing the twist from that optimum level (the maximum the fibers in the yarn will sit at right angles to the axis of the yarn) results in a shear interaction between the fibers in the yarn while decreasing the strength of the yarn.
Twist levels have an effect on much more than just strength.
They influence the hairiness, comfort, cost and linting possibilities of the yarn. The lower twist levels are more prone to lint, pill and have a rougher feel, however they are more comfortable.
Tube length. Combed yarns produced from longer staple need less twist than carded yarns obtained from shorter fiber since longer uninterrupted fibers have more contact points along their length and produce a stronger yarn at a lower twist level. Usually finer yarns demand a higher twist level compared to the coarser types, and knitting yarns are accorded a lower twist level than woven filling yarns.
In premium class yarns, twist is uniform down the fiber length with tpi close to the high end of the range typical of that yarn type.
Twist level, in a nutshell, is an important factor affecting both performance and cost.
Twist Categories, From None to Extreme
Monofilament and low-twist yarns
Monofilament yarns are composed of a single continuous fiber so have to be twisted at all as there is nothing to bind it. Multifilament yarns are usually twisted lightly to about 2 or 3 turns per inch so the separate filaments don’t slip apart but the overall yarn remains smooth.
Napping twist
Applied in a range of about 8 to 15 tpi, napping twist is a free-lofted spun yarn. Its purpose is to be used as the filling yarns in fabrics to be napped, a process that raises fibers from the surface of the yarn into a soft hand.
Average twist
With a count from 15 to 30 tpi, average twist is the most popular twist for staple-fiber yarns and is used very rarely with filament yarns. The hardest-wearing level of spun yarn is here. Combed, worsted and line (flax) yarns are in this level; it is made from long staple fibers in parallel, whereas carded, woollen and tow (flax) yarns use shorter fibers that aren't so well aligned.
Hard twist (or voile twist)
Hard twist, which has about 30 to 40 turns per inch (tpi), means that the yarn feels slightly stiffer and rougher as the tighter twist compresses the fibers more closely together. This stiffness is even more apparent with a twist-on-twist plied yarn, meaning that if a section of yarn is twisted in one direction and then twisted again in the same direction to ply it, the ply will add to the overall twist.
Crepe twist
At the far extreme of the range, crepe yarns have between 40 and 80 tpi, the greatest twist level of any type. Both spun and filament crepe yarns are (or are described as) unbalanced, because they kink and twist spontaneously immediately upon removal from a fabric.
Because of this lively performance, they have to be twist-set, a special finishing process, before they can be woven or knitted.
To tell that a yarn is crepe-twisted, stretch it out, then let go of one end; a genuine crepe yarn will pop into a corkscrew after you release it, often kinking or curling in on itself. Crepe yarns typically run horizontally in a fabric but can run vertically, and some fabrics use crepe yarns running both ways, but not all crepe fabrics include crepe yarn.
The more the twist level in the crepe yarns increased, or the faster the twist direction changed across the fabric, the more pronounced was the crinkled surface. A fabric woven from alternately six S-twist and six Z-twist filling yarns creates a much more dramatic crinkle than a similar fabric woven from alternate two S- and two Z-twist yarns.
Conclusion
Although it can be viewed as a detail of fiber construction, the twist of a yarn has a subtle influence on almost all the properties that are relevant to the finished fabric's performance - strength, bulk, elasticity, soil resistance, absorbency, abrasion resistance and visual texture all respond to changes in twist level.
It is also possible to vary in specific ways the twist direction, whether S or Z, and this provides another means of adjustment to the fabric's final appearance and use.
From zero twist in a monofilament to the surging energy of a crepe twist, the complete range has a purpose to meet various ends, and the emerging developments in spinning technology make twist one of the most basic means available to influence fabric qualities.
