In the manufacturing of staple fiber yarns, a twist is inserted into the fine strand of fibers to hold the fibers together and impart the desired properties to the twisted yarns. Without a twist, the fine strand of fibers would be very weak and of little practical use. A change in the level of twist also changes many yarn properties, such as strength and softness.
What exactly does twist to a yarn?
The twist in a yarn binds the fibers together and helps to keep them in their respective positions. It thus gives coherence to yarn.
Twist gives sufficient strength to the yarn.
Twisting is also used to bring about novel effects that are prominently visible when the yarn is converted to fabric. This is achieved primarily by having a combination of yarns with different twist levels and twist directions in the fabric.
Effect of twist direction on sewing thread properties
Because of the twist in a yarn, the fibers on the yarn surface take a roughly helical configuration around the yarn. When the yarn is under tension, these surface fibers are also under tension. However, because of the helical configuration, part of the tension is diverted radially, which creates radial pressure.
The radial pressure tends to pack the fibers together, increasing the normal force between them, and so increasing their frictional resistance to slipping past each other. The more tension is applied to the yarn, the more it locks together, hence ‘self-locking’. An analogy is, when you wind a string around your arm, as you pull the string along the arm and away from each other, the string bites deeper into the flesh.
Without a twist, there won’t be any self-locking effect to prevent fiber slippage. Consequently, the yarn would have no strength.
When a twist is applied to a yarn torque is created. With a sewing thread, torque forces can be balanced by using opposite twists in the singles and ply twists. With a single yarn, required twist levels may create enough torque to require heat setting through a conditioning oven, autoclave, or even dye bath to minimize torque forces. Excessive torque can cause yarn or thread to kink excessively/snarl or “French-knot”, resulting in processing problems including snags, needle breaks, and skipped stitches.
How to perform a twist balance (TB) test.
Roll-off (do not off wind over the nose, this can add or subtract twist) approx 36”, pinching both ends between fingers to prevent twist from shifting, and move pinch points to approx. 6” apart allowing the thread to form a ‘U’.
Allow any movement of the yarn and wait until it comes to a stop. If there are no movement and vertical portions of ‘U’ remain parallel, then Twist Balance = 0.
If yarn twists upon itself, place two fingers through the final loop at the bottom of the “braid” and count the number of full 360° rotations it takes to get both ends back to parallel.
We find that a twist balance (TB) of five rotations or fewer works well for most applications. If the TB value is greater than five, an additional processing step called conditioning or heat setting is recommended to reduce the torque.