TheEffect OF Animal Fibre Diameter Distributions Within True FibreDiameter And Length On Yarn Unevenness
188.8.131.52Yarnunevenness prediction model
Fromdrafting theory the total relative variance (the square of thecoefficient of variation) of yarn is decomposed into threecomponents: roving variance, ideal variance and non-ideal variance(12):
Theideal unevenness is determined by the Martindale formula (13):
Theideal variance is the major component accounting for about 70% to 80%of the total variance. The formulae above is an indication that theappropriate unevenness result from random distribution of fibre whendrafting and exclusively depend on the amount of fibres in thecross-section of the yarn and the variability of diameter of thefibre. Roving variance is the total variance due to the inputcomponent and is usually as a result of top quality, the settings ofthe machine, the lubricant and the drawing system settings. Itseffect can be relatively small accounting for not less than 10% ofthe variance in total. Variance which is Non-ideal variance isusually created by drafting which is non-ideal in the spinningdrafting zone, and can be related to many factors such as fibrelength, crimp, spinning draft, roller setting, inter-fibre frictionand machine quality (12-16). This component accounts for about 15%-25% of the total variance. The relative importance of this componentincreases with increasing yarn linear density. Prediction of thenon-ideal unevenness is the key to the prediction of the totalunevenness. Fibre length and possible crimp effects are mainlyreflected through this component. The effect of machine quality canalso be incorporated into this component.
Yarnevenness can be improved either through improving roving quality orthrough reduction of the non-ideal unevenness, for example,optimising spinning draft and roller setting. In order to predict thenon-ideal unevenness it is important to have a good understanding ofthe mechanism of drafting which is not ideal. A mechanism that hasthe ability to move fibres in a non – random manner which isrelative to the array which is undrafted before accelerationcommences at a nip which is fixed leads to non-ideal drafting(17-18). The mechanisms of non-ideal drafting include the following:A) Sliver elasticity, which involves intermittent stretching of thesliver. The elasticity of sliver is accountable for only a negligibledegree of drafting irregularity in recent spinning systems (19). B)Roller eccentricity, which is machine dependent and generally hasonly a small effect on yarn irregularity. C) Floating fibres, thisrefers to fibres not gripped by either front or back rollers. This isa dominant cause of the additional yarn irregularity (12, 20-22).Fibre length makes a contribution to yarn unevenness mainly throughthis mechanism. As made known in Figure 1, the length of the fibrelength can influence the irregularity of the irregularity by a factorwhich is (L-H)/H, where (L-H) is the length of the zone which isfloating and (L-H)/H may be called the length of the floating zonewhich is normalised. Obviously, the smaller the length of the fibrethe lengthier the zone that is floating and the greater the chance ofthe fibre to change from the speed of a back roller to front rollerbefore it ranges the nip tip of the front roller.
Taylor(20) conducted detailed experiments, in a roller drafting system,showing that fibres change speed long before they reach the nip pointand that the acceleration point at which fibres change speed fromback roller speed to front roller speed is strongly correlated tofibre length, the shorter the fibre length, the greater is theprobability that it accelerates prior to that point. Fujino et al(21, 22) have further studied the regulation of movement of thefibres that float in apron of the drafting system. They suggestedthat the unsteady motion of floating fibres is the major cause of theadded variance during spinning drafting. However, it must be notedthat a fibre moving randomly out of turn does not increaseirregularity. Increased unevenness occurs only when fibre movementare correlated. Note that the perfect inequality is the dominantcomponent of the unevenness in total and how it relates with thefibre properties which have been described by the Martindale formula.The advantage of extrication of the non-ideal variance from the wholeis that it makes it easier to obtain a clear depiction of the effectsof the fibre length and other minor factors on yarn unevenness.