Yarn Strength | Measurement of Yarn Unevenness | Types of Yarn Irregularity
B.Tech, Dept. of Textile Engineering
Giani Zail Singh Punjab Technical University Campus,
Bathinda, Punjab, India
The requirements of yarn strength depend upon its end use. In cases, where yarn strength is not specified by the user.
Corrected strength, S2 = C1S1 – K (C2 – C1)/C2
= Corrected CSP / Nominal count
The single yarn strength tests are useful in the following situations:
- Assessing improvement in yarn strength and loss in elongation due to sizing.
- Planning to meet specifications of fiber strength.
1. Export Yarns:
Practically all European countries except U.K., stipulate yarn strength in terms of RKm in German and resistance kilometer in French and which means the breaking length in kilometers.
RKm = Nm × single thread strength(kg) (nm is metric count)
= Single thread strength (g)/tex (nm × tex = 1000)
The correction factor K for single yarn “strength” in grams and yarn count Ne is 70.9. the single thread is closely related to lea strength with correction coefficients of 0.90 – 0.99.
In the absence of a single thread tester, RKM can be calculated approximate. From lea strength as:
RKm = 1.7 C (4.0 S + 11.5)/ 1000
Where, C is cotton count and S is lea strength in lb
2. Assessing improvement in yarn strength and loss in extensibility due to sizing:
Single thread test becomes necessary to see that sizing has been properly carried out. An increase of 25 -30 % increase in strength and less than 2% loss in elongation is considered satisfactory
3. To produce specified fabric strength:
The fabric strength (strip test) is given by:
Fabric strength = (no. of threads/5 cm) × (single thread strength g) × F /1000
F = A constant which depends upon yarn quality, weave and thread density in fabric and lies generally b/w 0.95 and 1.05
Unevenness and Imperfections of Yarn:
The fabric appearance is also depending apron the uniformity of yarn thickness and blemishes, uniformity of cover b/w small areas say 5 sq. cm is used to determine fabric ‘s visual rating. Lack of this uniformity produced patchy fabric. An uneven yarn also performs poorly in spinning as well as in subsequent processes.
You may also like: Imperfection Index of Yarns (IPI)
Measurement and Assessment of Yarn Unevenness:
Since 1950’s the unevenness of yarn is being measured on electronic instrument such as Uster evenness tester. The variability measured on these instruments is that of voltage across the condensers which varies in proportion to the yarn mass. In effect, it comes to weighing 8mm pieces of yarn. The regularity is expressed either as PMD (U%) or CV%. These measures are related as;
CV% = 1.25 U%
However, this relationship holds good only for random, normally distributed variation and should be taken approx. only the constant 1.25 can vary from 1.1- 1.4 depending the type of yarn.
To assess avg. U% for a given yarn, 16 bobbins are collected equally from group of Frames. One test of U% at speed of 25m/ min (or 50m/min) is taken per bobbin for 4.0 mins.
Table: Norms for unevenness of cotton slivers and rovings
|Material||Count group||U% Sliver||Roving|
To achieve the above norms of evenness of yarns, it is necessary to ensure that the evenness of the rove and the silver are also maintained at the values given in above table.
Types of Yarns Irregularity:
The total yarn irregularity comprises of three basic types of variations
- Random irregularity
- Periodic irregularity
- Quasi-periodic irregularity
1) Random irregularity: This the min. possible variation expected. If variation in no. of fibers per cross section is the minimum most. This is achieved when the fiber and density follow a Poisson distribution. This variability of weight per unit length, both within and between fibers, and the minimum conceivable variability in the no. of fibers in the cross section of the yarn set the min. limit for yarn irregularity this can be calculated as:
CVl = 106/ root N for cotton
CVI = 100/ root N for manmade fibers
2) Periodic irregularity: Any deficiency in the roller or the gears, which is likely to affect constancy of roller speed, or which causes the distance between roller nips or between roller apron and roller nip to vary in a regular or periodic manner is likely to introduce thick and thin places at regular intervals.
When wound on this the yarn periodicity shows up as a U –shaped pattern formed by thick places. The wavelength of the periodicity is then given by:
Wavelength = 2 x Board width at the middle of the U-shape / no. of U-patterns (thick places) on both sides of the board at that width.
3) Quasi-irregularity: Such an irregularity but has varying wavelength and amplitude. Drafting waves which are inherent in the roller drafting system come under this category. The wavelength of drafting wave can be considered to have an avg. of 6-8cm. Its amplitude depends upon the short fiber content, roller setting, total draft and type of fiber control in the drafting system such as apron rollers. This type of irregularity is main cause of yarn unevenness.
You may also like:
- Reasons for Increasing Imperfection Index of Yarn
- Effect of Spacer on the Quality of Roving and Yarn
- Determination of Cotton Yarn Strength and C.S.P. by Lea Strength Tester
- Effect of Fiber Properties on Yarn Hairiness in Ring Spun Yarn
- Reasons for Poor Quality in Ring Spinning
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