The winding process serves to achieve additional objectives made necessary by the requirements of the subsequent processing stages.
The length of yarn in a ring bobbin is not adequate to run the warping process efficiently i.e. the warping machine has to stop frequently to change the cop in the creel.
For efficient warping, long continuous lengths of yarn of the order of fifty thousand meters or more are required. The winding process, therefore, has the basic function of obtaining a larger package from several small ring bobbins.
This conversion process provides one with the possibility of cutting out unwanted and problematic objectionable faults. The process of removing such objectionable faults is called as yarn clearing.
It is important to note that, when such objectionable faults are cut out, to maintain continuity of the yarn in the wound package, the two ends have to be joined together by knotting or other means just as we join ends when a ring bobbin is exhausted.
In other words, we are trying to replace a more serious fault with a relatively less serious one. Therefore the clearing function has to be discrete in terms of what faults have to be removed and what not to be removed.
The quality of the knot is also important in terms of the performance of the yarn in subsequent processing stages. Of late, splicing of the yarn ends has become quite popular and is gradually replacing knotting by way of its better appearance while at the same time retaining sufficient strength.
The efficiency with which the processes of yarn clearing, knotting and splicing are carried out, therefore, decides the success of realizing the objectives of the winding process.
Yarn clearing is the process of removing objectionable yarn faults at winding.
The various types of yarn faults are represented in Figure.
The faults can generally be categorized in terms of its cross-sectional size and length. In the winding process, only those faults need to be removed, which if allowed to pass into the cone, would show up as a fabric defect or will cause an end break in the subsequent processes.
Faults with cross-sectional size less than 2 times the mean yarn diameter (< + 100%) are generally too frequent, and cannot be minimized by clearing in winding; they have to be minimized only by adopting a better optimization of the processes of spinning and spinning preparation.
These as well as the other faults which neither causes an end break nor produces a fabric defect are not yarn faults which need removal by clearing.
Fault Removal by Mechanical Slub Catchers
Practically, any fault in the yarn can be classified into one of three types
(i) Thick places such as slub spun in fly, bad piecings, etc.
(ii) Weak thin places and
(iii) Thin places that are not weak.
The original method of removal of thin faults is by mechanical means, – thick places by introducing a slub catcher in the path of the yarn and weak thin places by adequately tensioning the yarn.
With such mechanical methods, the two parameters that define the efficiency of clearing are the ‘ slub catcher setting ’and the ‘unwinding tension’.
Slub Catcher Settings
The slub catcher settings are to be determined based on the end use requirements with regard to the acceptability of the various sizes of thick objectionable faults.
A closer setting i.e., the lesser distance between the slub catcher plates results in the removal of more faults. Recommendations on the optimum value of the slub catcher setting are provided by many sources.
One such recommendation states that the optimum setting for slub catchers with fixed blades is 1.5 times the yarn diameter for combed yarns and twice the yarn diameter for carded yarns.
Recommendations are also available for slub catchers with oscillating serrated and non-serrated blades.
Weak thin places can be removed by providing adequate tension in the yarn. Tensioning of yarn is also required for obtaining a firm package.
In addition to the tensions available through the design of the winding path, it is usually necessary to apply additional tension by using tension discs.
The weight of such tension discs is usually decided based on the strength of the yarn being wound. A typical recommendation is provided as follows:
Tension Weight in gms = 0.571 x Lea Strength in kg + 1.8
Clearing Efficiency and Cuts-to-Faults Ratio
The performance of a yarn clearer can be judged by means of its ‘ Clearing Efficiency’. Clearing Efficiency is defined as:
Clearing Efficiency = 100 * [No. of expected faults to be removed – No. of faults remaining in the yarn] / (No. of expected faults to be removed)
Under ideal circumstances, a cut by the yarn clearer has to be initiated only to remove an objectionable fault.
But in practical situations, the number of cuts executed is always relatively higher when compared to the number of faults removed. To estimate this additional number of cuts, the Cuts-to Faults Ratio expression is used.
Cuts-to Faults Ratio = [No. of Cuts Expected / No. of Faults Removed]
The Cuts-to-Faults Ratio is also sometimes referred to as the ‘ Knot Factor’. The Clearing Efficiency and the Cuts-to-Faults Ratio together characterize the performance of a yarn clearer.
The most efficient clearer is the one with a Clearing Efficiency of 100 and a Cuts-to-Faults Ratio of 1. This ideal situation is however never realized in practice.
For further understanding refer to the article on “Electronic Yarn Clearer“.