YARN EVENNESS - 3
Neps can influence the appearance of woven or knitted fabrics quite considerably. Furthermore neps of a certain size can lead to processing difficulties, particularly in the knitting machines. Therefore the avoidance of neps in the production of spun yarns is a fundamental textile technological problem.
Neps can be divided, fundamentally , into two catergories:
-raw material neps
The rawmaterial neps in cotton yarn are primarily the result of vegetable matter and immature fibres in the raw material. The influence of the rawmaterial with wool and synthetic fibres in terms of nep production is negligible. Processing neps are produced at ginning and also in cotton , woollen and worsted carding. Their fabrication is influenced by the type of card clothing, the setting of the card flats, workers and strippers, and by the production speeds used.
"DIAGRAM" is a representation of the mass variations in the time domain. Whereas SPECTROGRAM is a representation of the mass variation in the frequency domain. Spectrogram helps to recognize and analyse the periodic fault in the sliver, roving and yarn.
For textile application, the frequency spectrum is not practical. A representaion which makes reference to the wavelength is preferred. Wavelength indicatres directly at which distance the periodic faults repeat. The more correct indication of the curve produced by the spectrograph is the wave-length spectrum.
Frequency and wavelength are related as follows
frequency = (wavelength)/(material speed)
In the SPECTROGRAM, the X-axis represents the wavelength. Inorder to cover a maximum range of wavelengths, a logrithmic scale is used for the wavelength representation. The y-axis is without scale but represents the amplitude of the faults in yarn.
The spectrogram consists of shaded and non-shaded areas. If a periodic fault passes through the measuring head for a minimum of 25 times, then it is considered as significant and it is shown in the shaded area. Wavelength ranges which are not statistically significant are not shaded. In this range the faults
are displayed but not hatched. This happens when a fault repeats for about 6 to 25 times within the tests length of the material.
As far as those faults in the unshaded area is concerned, it is recommended to first confirm the seriousness of the fault before proceeding with the corrective action. This can be done by testing a longer length of yarn. Faults which occur less than 6 times will not appear in the spectrogram.
A spectrogram starts at 1.1 cm if the testing speed is 25 to 200 m.min. It starts at 2.0cm if the testing speed is 400 m/min and it starts at 4 cm if the speed is 800 m.min. For spun material the maximum wavelength range is 1.28 km. Maximum number of channels is 80
Depending upon the wavelength of the periodic fault, the mass variations are classified as
- short-term variation( wavelength ranges from 1 cm to 50cm)
- medium-term variation( wavlength ranges from 50cm to 5 m)
- long-term variation(wavelength longer than 5 m)
- periodic variations in the range of 1 cm to 50 cm are normally repeated a number of times within the woven or knitted fabric width, which results in the fact periodic thick places or thin places will lie near to each other. This produces, in most cases, a "MOIRE EFFECT". This effect is particularly intensive for the naked eyes if the finished product is observed at a distance of approx. 50 cm to 1m.
- Periodic mass variations in the range of 50cm to 5m are
not recognizable in every case. Faults in this range are
particularly effective if the single or double weave width,
or the length of the stretched out yarn one circumference of
the knitted fabric, is an integral number of wave-lengths of
the periodic fault, or is near to an
integral number of wave-lengths. In such cases, it is to be expected that weft stripes will appear in the woven fabric or rings in the knitted fabric.
- Periodic mass variations with wave-lengths longer than
5m can result in quite distinct cross-stripes in woven and
knitted fabrics, because the wave-length of the periodic
fault will be longer than the width of the woven fabric or
the circumference of the knitted fabric. The longer the
wavelength, the wider will
be the width of the cross-stripes.Such faults are quite
easily recognizable in the finished product, particularly
when this is observed from distances further away than 1 m.
A periodic mass variation in a fibre assembly does not always result in a statistically significant difference in the U/V value. Nevertheless, such a fault will result in a woven or knitted fabric and deteriorate the quality of the fabric. Such patterning in the finished product can become intensified after dyeing. This is particularly the case with uni-coloured products and products consisting of
synthetic fibre filament yarns.
The degree to which a periodic fault can affect the finished product is not only dependent on its intensity but also on the width and type of the woven or knitted fabric, on the fibre material, on the yarn count, on the dye up-take of the fibre, etc. A considerable number of trials have shown that the height of the peak above the basic spectrum should not overstep 50% of the basic spectrum height at the wavelength position where the peak is available.
- CHIMNEY TYPE FAULTS:
The eccentricity roller results in a sinusoidal mass variation whereby the periodicity corresponds to full circumference of the roller. With one complete revolution of an OVAL roller, a sinusoidal mass variation also results, but 2 periodic faults are available. Chimney type of faults are mainly due to -mechanical faults -eccentric rollers, gears etc -improper meshing of gears -missing gear teeth -missing teeth in the timing belts -damaged bearings etc
- HILL TYPE FAULTS:
These faults are due to drafting waves caused by -improper draft zone settings -improper top roller pressure -too many short fibres in the material, etc Numerous measurements of staple-fibre materials have shown that there are rules for the correlation between the appearance of drafting waves in the spectrogram and the mean staple length. It is given below
-yarn : 2.75 x fibre length
-roving : 3.5 x fibre length
-combed sliver : 4.0 x fibre length
-drawframe sliver : 4.0 x fibre length
A periodic fault which occurs at some stage or another in the spinning process is lengthened by subsequent drafting.If the front roller of the second drawframe is eccentric, then by knowing the various drafts in the further processes, the position of the peak in the spectrogram of the yarn measurement can be calculated.
The wavelength of a defective part is calculated by multiplying the circumference of the part and the draft upto that part.
The wavelength of a defective part can be calculated if the rotational speed of the defective part and the production speed are known.
Doubling is no suitable means of eliminating periodic faults. Elimination is only possible in exceptional cases. In most cases, doubling can, under the best conditions, only reduce the periodic faults.
The influence of periodic mass variation is proportional to the draft.
Due to the quadratic addition of the partial irregularities, the overall irregularity of staple-fibre yarns increases due to the periodic faults only to an unimportant amount.
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