There are two main purposes of introducing knots during winding of yarns:
Continuity knots determined by the ring bobbin capacity are inevitable and these must go into the fabric.
The introduction of a knot for clearing purposes, however, is decided based on the end use and productivity considerations.
The cost of elimination, along with the cost of proportional failures in the fabric production machinery increases linearly with each knot inserted.
It is therefore essential to know the incidence of disturbing faults in the yarn before wasting time in applying clearing limits to a yarn.
A knot should satisfy the following requirements for efficient usage in the textile industry:
- Be easy to tie.
- Have good resistance to slippage.
- Be of a size and shape that gives it little chance of catching or jamming in narrow openings i.e. every knot must support the full weaving stress as it makes the journey from warp beam to cloth fell.
It is unfortunate that with just one of the most commonly used knots (Figure), (dog knot, fisherman’ s knot and weaver’ s knot) it is difficult to meet all the above requirements satisfactorily.
The tying of dog knot is simple and quick, but it is bulkier than weaver’ s and fisherman’ s knot. It is only suitable for creeling and these knots will not be incorporated into fabrics.
Fisherman’ s knots are most popular but it is not at all convenient to tie by hand. They have a diameter 3-4 times that of the parent yarn and are popular for filaments, cotton, woolens, worsteds, and blends.
Weaver’ s knots are most popular in the cotton industry and to some extent in the worsted industry. The diameter of weaver’ s knot is 2 to 3 times that of the parent yarn.
The weaver’ s knot may be single or double depending upon slippage property of yarn. It has been observed that the fisherman’ s knot, weaver’ s knot and dog knot rank in the decreasing order of resistance to slippage.
These knots can be tied by hand or by hand knotter or automatic mechanical knotter. Use of mechanical knotters also offers other advantages like the uniformity of knot size and tail ends.
Factors Determining the Quality of Knots
1 Resistance to slippage
Most of the knot failures in the downstream process are due to slippage only. Degree of slippage depends on fibre type, blend composition, yarn linear density, twist, number of plies and flexural rigidity, etc.
2 Size of the Knots
A bigger knot causes a serious obstruction as it passes through a small path like heald eye, reed dent, and a knitting needle, etc. It leads to uneven tension and results in end breakage in the process.
3 Knot tail ends
In the practical weaving situation, initial knot tail lengths are common of a length where slippage to failure is dominated by one path i.e. by changing the configuration.
The tail length should basically be judged from the linear density and flexural rigidity. For yarn with lower linear density or lower flexural rigidity (or both), failure of knot begins at shorter initial tail lengths and vice-versa for yarns with greater linear density and flexural rigidity.
The tail length should be of about 5 – 10 mm in a weaver’ s knot and about 6 – 12 mm in fisherman’ s knot depending on knot slippage resistance.
The direction of the tail also plays an important role in end breakage in knitting. Investigation showed that under every knitting condition the end breakage due to a fisherman’ s knot is always more than that due to a weaver’ s knot.
In weavers knot both tails point in the same direction, whereas in the fisherman’ s knot the direction of the tails is opposite to each other.
In a practical situation, the thickness of a weaver’ s knot is that of the knot itself whereas the thickness of a fisherman’ s knot is the sum of the diameter of the yarn and the knot.