Testing of yarn twist and factors affecting the test

Spinning short fibers into continuous yarns requires twisting, and the fibers are aggregated together to maintain a certain cohesive force, 

thereby giving the yarn a certain strength. The filaments in the composite yarn also need to be lightly twisted. If there is no cohesive force 

to hold them together, they will fall apart or be scratched during use. In addition, according to the needs of different uses, several single yarns

 can be combined and twisted to form strands and cables to make them more uniform and structurally stable to withstand higher loads. If two 

or more yarns are fed at different speeds and tensions during twisting, or different colors or fancy yarns are combined and twisted, fancy yarns

 can be formed. Twisting is a method of bringing fiber bundles, filaments or single yarns together so that textiles can withstand stress, strain 

and friction during manufacturing and use, and give textiles a unique appearance.

1. Concept of twist

In addition to affecting the tensile properties of yarn, twist also affects the diameter, specific volume, softness and stiffness of yarn, and affects

 the covering performance, warmth retention, crease recovery ability, dye absorption rate and permeability of fabric. Twisting also affects the

 hairiness on the surface of yarn, so twist is an important indicator of yarn structural characteristics. In addition to using twist and twist 

coefficient to represent the twisting characteristics of yarn, the twist direction in yarn is also very important. Twist direction refers to the

 tilt direction of the fiber in a single yarn or a single yarn in a ply yarn after twisting. There are two types of twist directions in yarn: S twist or 

clockwise twist; Z twist or counterclockwise twist. Single yarn generally uses Z twist, while S twist is often used for ply yarn.

2. Twist measurement method

The commonly used twist test methods in my country are direct counting method and untwist twisting method. The direct counting method is 

generally used for short fiber and ply yarn tests, while the untwist twisting method is used for spun yarn. In addition, there are secondary and

 tertiary untwist twisting methods, slip method, etc.

2.1. Direct counting method

One end of the sample is fixed, and the other end is rotated in the untwisting direction until the fibers in the yarn are completely straightened 

and parallel. The untwisted twist is the twist number of the sample length. The direct counting method is the most basic method for measuring 

the twist of yarn. The measurement results are relatively accurate and are often used as a standard for assessing the accuracy of other methods.

However, this method has low working efficiency. If the fibers in the yarn are tangled, the fibers are not easy to decompose and parallel, and 

the decomposition of fibers makes the yarn easy to break. The direct counting method is generally used to count the twist number of roving 

or ply yarn, while the black and white yarn counting method can be used to test the fine yarn, that is, a black roving and a white roving are fed 

into the same spindle to spin black and white fine yarn. The sample is clamped on the twist meter and untwisted in the same way as the direct 

counting method. As the untwisting increases, the distance between the black and white on the yarn becomes larger, and it is easier to count 

manually. Generally, the remaining twist number can be counted manually after untwisting to about half of the total twist number. The total

 twist number on the yarn is the sum of the untwisting number of the twist meter and the remaining twist number of the manual count.

2.2. Untwisting and twisting method

Untwisting and twisting method is a test based on the assumption that under a certain tension, the elongation of the yarn after untwisting is

 the same as the shortening of the yarn during reverse twisting. A typical test device is shown in the figure.

3. Factors affecting twist test

3.1. Pre-tension

There are different regulations for the pre-tension of untwisting and twisting method at home and abroad. Excessive pre-tension will make the 

pointer return to zero slowly during reverse twisting, thereby increasing the twist of reverse twisting, and causing fiber slippage in advance in

 the slip method; conversely, if the pre-tension is too small, the twist will be relatively reduced during reverse twisting, resulting in a low value 

during the untwisting and twisting method test. In the slip method and the secondary untwisting and twisting method, since the error caused

 by this tension change is taken into account, the method of subtracting the errors of the two samples is adopted, so the tension is less affected.

3.2. Allowable elongation value

The sample elongates after the yarn is untwisted, and the allowable elongation value set by the elongation limit prevents the slippage of the

 yarn fiber during the untwisting process. Experiments show that in the untwisting and twisting method test, the larger the allowable elongation

 value, the larger the measured twist value. At the same time, if the allowable elongation value is too large, the fiber slippage at the weak loop

 of the yarn will significantly affect the correctness of the experiment. The allowable elongation value is related to the twist coefficient. The

 larger the twist coefficient of the yarn, the larger the allowable elongation value can be selected; conversely, it should be smaller. Usually, the 

twist coefficient of the yarn does not vary much, so the difference in the allowable elongation value will not be too large.

3.3. Uneven yarn dryness

Uneven yarn dryness will affect the twist test. If there are defects such as cotton knots and bamboo knots on the yarn, the yarn will start to twist 

in the opposite direction before the original twist is completely withdrawn during the untwisting process, which will increase the test error. If 

there is a long uneven segment on the yarn, it will affect the correctness of the added tension and eventually cause additional test errors.

Conclusion:

In the yarn twist test, if the data deviates greatly, the above three factors affecting the test can be used as a reference to find out the reasons, 

and it is not ruled out that there are human factors that cause inaccurate data. Regardless of the type of test, inspectors must be careful and

 thorough to ensure the scientific accuracy of the data.