Direct Determination of Yarn Snarliness
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Indi an Journal of Fibre & Textile Research Vol. 28, March 2003, pp. 23-28 Direct determination of yarn snarliness A Primentas" School of Textile Industries, The University of Leeds, Leeds LS2 9JT, UK Received 5 October 2001; revised received and accepted J J March 2002 The formation of snarls that takes pl ace in the yarns in post-spinning processes, such as winding, warpin g, weaving and knitting, has been studied. This defect is due to the excessive yarn twist liveli ness (torsional buckling) and causes great trouble to thc manufacturers of yarns and fabrics. Biased testin g techniques are used for the measurement of this defect. A snarling testing apparatus PRIANIC and an unbiased testing technique have been developed and are reported. The investigation of factors such as yarn package form, yarn twist factor, time elapsed between production and processing stages of yarns and yarn conditi oning show the significant role th ey play in the reduction of yarn snarliness. Keywords: Knitting, Snarliness, Twist li ve liness, Warping, Weaving, Winding, Yarn package, Yarn twist factor 6 1 Introduction twist direction . Newly produced yams exhibit a great The twisted shape of the fibres forming a yarn tendency to untwist when freed from restraint. When blings to the fore two of their main physico-mechanical such a yarn is prevented from untwisting, by being held properties: torsional rigidity and torsional buckling. at both ends approaching each other, it forms an arc. The higher the torsional rigidity, the stronger is the tor The nearer to U-shape this arc gradually comes, the sional buckling effect, also known as yarn twist liveli easier is snarl formation (Fig. 1), whereas in highly ness. It is the tendency of a newly spun yarn to untwist twisted yarns, snarls are formed very fast before an arc spontaneousli, a detrimental yarn characteristic hav appearance (Fig. 2). Recent research7 has shown that ing two representatives - the snarliness that occurs in the torque required for a yarn to snarl is independent to the yarns and the spirality that appears in knitted fab the yarn twi st level. Snarls appear in thinner yarn parts rics. On the contrary, the crepe phenomenon, appears where twist accumulation occurs and/or in a likel y part in special woven fabrics, is considered as a beneficial of the yam when the numerical difference (yarn weight result of the yarn twist liveliness. minus untwisted stress) has a negative value. Yarn twist liveliness is affected by the twist factor, Many problems originate from the yarn snarliness yarn fineness2 and retractive forces, which, in turn, are and appear in post-spinning yarn processes. In yarn determined by the torsional and bending stresses in the winding, a snarly yarn might be accumulated behind 3 fibres , and the torque generated during yam twisting. the finger guide (knife) of an electronic slub catcher, The latter is constituted by three components, viz. tor resulting in yarn breakage and worsening its quality. 4 sion, bending and tension of the fibres , whereas its level depends on the degree of fibre migration that takes places. The twist liveliness of fal se-twisted and set yarns depends on their structure and, in particular, s on the properties of the helical parts of their filaments . 1.1 Snarl The snarl is due to yarn torque and takes place dur ing the torsional buckling effect, where the yarn retires into itself and simultaneously is twisted in the opposite a Present address: Department of Textile Engineering, Faculty of Applied Technologies, TEl of Piraeus, 250 Thivon & P. Ralli, Athens 12244, Greece Phone: + 3-2 10-4515137; Fax: + 3-2 10-4122977 ; E-mail: aprim@ teipir.gr Fig. I- Snarl formation of a normally twi sted yarn 24 INDIAN 1. FIBRE TEXT. RES. , MARCH 2003 midd le of a known length of yarn. As soon as the two yarn ends are brought together, a loop (s narl ) is formed as rotated unti l reaching the immobility state. The number of turns of the loop and/or th e measurement of the di stance between the two yarn ,s end s .' 6 give the yarn snarliness level. The drawbac k ! of th ese methods is th e constraint of the snarl j ! ~- ---------- formation in a specifi c yarn pl ace, a fact that does not permit the draw ing of true conclusions about the objecti ve snarling tendency of the yarn s. Furthermore, there is confusion about the mass of the hanged object and the length of the yarn sampl es. Also, th ere is no mention about yarn pretension. Al l th e above high lighted th e necessity of usin g a simple new method and an apparatus for the measurement of snarliness, overcoming most of the I sources of errors and shortcomings of th e earl ier ----~~- ........... - -.--- ........ ~. -- -.- -- - . meth ods. Therefore, in the present work, a snarl in g testing apparatus PRIANIC and an unbi ased testin g technique have been developed and are reported. Fig.2-Snarl fo rmatio n of a hi ghl y twisted yarn 2 Materials and Methods 2.1 Snarling Testing Apparatus PRIANIC Due to lack of tensions, the snarl s are fo rmed in sin gle The apparatus shown in Figs 3 and 4 consists of a yarns as they are with drawn over the top of bobbins base where two stands are firmly fixed. A th readed during the fo lding-twisting processes and especially rod with two ball bearings is placed on th e top of in two-for-one twisting where thi s problem becomes these stands. A motor on the base drives the threaded more serious. In weaving, snarl s in weft yarns appear rod where beneath the latter there is a scale with when the shuttle returns to the shed from the box. li near metre (l00 cm). On th e right side of the Some of these snarl s are entrapped into the cloth and apparatus, a movable part (J) with clamp jaw and a do not open out even wh en th e weft is sub seq uentl y pointer is in contact with th e threaded rod , whereas a tensioned as picking proceeds, makin g necessary th e stationary jaw (2) with a special yarn pretension cloth mending for their removal. As the hanks made system ex ists on the left side. The pretension system from hi ghl y twisted yarns are removed from the reel consists of an adjustable co unterbalance and a swift , th ey shrin k and fo rm numerous snarl s, causing graduated rod (0-200 g) that is based on the TEX great trouble during their positioning on the rods system , with tension uni t of tex/2 ± 10% g. (buttons) of the hank-dyeing machi ne. Moreover, during the winding of these dyed yarn hanks to cones, 2.1.1 Principle the formed snarls can cause snatching, tension peaks The operation principle of the dev ice is simple. A and yarn breakage. yarn length of one metre is clamped on the two jaws It is a virtue for a yarn, with its optimum twist of the dev ice. The mo vab le part approaches th e factor, to have the minimum snarliness level that ca n stati onary one as it is driven by the rotat ing threaded be achieved by several ways such as storin g9 at rod. The yarn figures a U-shape loop and according to adequately high temperature and rel ativ e humidity its twi st li ve liness, a snarl is fo rmed. (70-75 %) or by steaming lO th at resu lts in a fast yarn relaxati on. 2.1.2 Testing Procedure The device is set to its ini tial condit ions. The 1.2 Existing Methods for Testing Yarn Snarliness movable part (1) is returned to the ri ght side of the Several wo rkers3.11. 14 , in an endeavour to measure device, where the pointer in dicates th e va lue 100 on yarn snarliness, have described various methods that the scale. As the lin ear density of the yarn is known, are more or less based on the same principle. In th ese the appropriate adjustmenr is made on th e pretension methods, a li ght mass object is suspended fro m th e system. After discarding a substanti al quantity of yarn PRIMENTAS: DIRECT DETERMINATION OF YARN SNARLINESS 25 Fi g.3-Snarling testing apparatus "PRIANIC" Guide Movable Jaw (1) Pulley· Threaded Rod \ / Fig.5-Effect of yarn unwinding method on yarn twi st (a-over Yarn Specimen end, b-sideways, and c-under-end) Scale standard operation speed. The movable jaw moves towards the stationary jaw at a constant speed of 0.5 cmls, giving the total maximum time of 200 s for testing a dead yarn. In the existing various apparatus and methods, there is no reference about the speed of the movable jaw. Moreover, th e testing time using the PRIANIC apparatus has been reduced remarkably in Pointer rel ation to the time of the other methods that ranged approximately between 10 min and 15 min. Fig.4-Schematic presentation of the snarling testing apparatus "PR1ANIC" Furthermore, the apparatus is suppli ed with a proper yarn pretension system (tex/2 ±1O% g) that is strongly from a bobbin, one metre yarn specimen is fastened in recommended for testing yarn characteristics under the grip of the stationary j aw (2) while holding the dynamic conditions. The most important feature of other end of the yarn to prevent the loss of twist. The the PRIANIC device and the accompanied method is yarn is stretched until no contact of the graduated rod that the yarn forms freely a snarl in the most likely pointer of the pretension system can be detected and part of it, confirming its unbiased testing principle.