Norms for Lea Strength


Ramandeep Singh
B.Tech, Dept. of Textile Engineering
Giani Zail Singh Punjab Technical University Campus,
Bathinda, Punjab, India

Yarn Strength:
The requirements of yarn strength depend upon its end use. In cases, where yarn strength is not specified by the user, the values shown in Tables 6.3 and 6.4 should be aimed at.

The average lea strength must be corrected for any deviation of through actual yarn count from the nominal by using the following formula:

Corrected strength S2= C1S1 – K (C2 – C1)/C2

                                 = corrected CSP / Nominal count

The single yarn strength tests are useful in the following situations:
  1. Export
  2. Assessing improvement in yarn strength & loss in elongation due to sizing.
  3. Planning to meet specifications of fibro 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)
= g/tex

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 appro. 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 & 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

Where, 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 depend 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.
Unevenness of yarn
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 (or50m/min) is taken per bobbin for 4.0 mins. The average is then compared with the norm given in the tables 6.3 & 6.4.

Count group
UPTO 15s


Above 30s

Above 50s
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 table 6.5a .

Types of Yarns Irregularity:
The total yarn irregularity comprise of three basic types of variations
  1. Random irregularity
  2. Periodic irregularity
  3. Quasi-periodic irregularity
1) Random irregularity: this the min. possible variation expected. If variation in no. of fibbers per cross section is the minimum most. this is achieved when the fiber and density follows a Poisson distribution . this variability of weight per unit length , both with in 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 =
       = 100/ root N for man made 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* 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 a 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. it’s 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.


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