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• Ranganatham Ummadisetty

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Application Report Yarn Evenness CV

August 2003

Contents 1

Introduction ...............................................................................3

2

Reasons and effects of the yarn irregularity ...........................3

3

Definition of the coefficient of variation CV.............................4

4

Deviation of the CV mean value of the group (CV-MV) ...........5

5

Deviation of the CV of the winding position (CV-SP) ..............6

6

Settings ......................................................................................7

7

Result display .......................................................................... 10

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1

Introduction

The coefficient of variation CV is a well known value for the determination of the evenness of slivers, rovings and yarns. Each process in a spinning mill contributes a part to the evenness. The continuous determination of the quality parameters guarantees that all spinning positions produce the same quality. For the calculation of the yarn evenness CV, it can be selected between 2 measurements: · continuous over the whole bobbin length with selectable reference lengths or · starting from a bobbin change with selectable reference lengths. When a preset limit is exceeded, the system can provide an alarm for the respective spinning position and another one for the machine value derived from all producing spinning positions.

2

Reasons and effects of the yarn irregularity

The reason for yarn irregularity is based on the fact that it is not possible for staple fiber yarns to keep a constant number of fibers in the cross-section. Reasons can be divided into: · raw-material related faults, like e.g. the evenness of the fiber length, fiber adhesion or · process-related faults, caused by defect machine parts, like draw-box defects or the kind of roller coats. From these points is becomes clear, that the coefficient of variation is used as an efficient method for quality and process monitoring. In general it can be said: the lower the CV-value, the more even is the material and the more even it will look in the end-product. Fig. 2-1 shows a comparison between two fabrics with a low (CV = 11.48%) and a high coefficient of variation (CV = 17.76%).

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CVm = 11.48%

CVm = 17.76%

Fig. 2-1 Comparison of two fabrics with a different CVm

It is known, that the evenness is not constant over the whole bobbin length. It usually decreases from the tip to the base of a bobbin. This circumstance has to be taken into account when evaluating the values when setting in the setting of the monitoring limits.

3

Definition of the coefficient of variation CV

The coefficient of variation is given in percent; it is a measure for the yarn evenness and is defined as follows: CV% =

s ´ 100 x

mass/ diameter

+s _ x

-s length

Fig. 3-1 Graphical representation of the CV

With the help of the coefficient of variation, CVm as well as CVd, winding positions, which deviate in quality, can be monitored. CVm = Coefficient of variation based on the measurement of the yarn mass (capacitive sensor). CVd = Coefficient of variation based on the yarn diameter (optical sensor).

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4

Deviation of the CV mean value of the group (CV-MV)

The CV mean value of the group (CV-MV) is determined from all winding positions. As it is based on a large population, it does not show any erratic deviations. Erratic deviations can occur with individual winding positions. The upper alarm limit “CV-MV upper” and the lower alarm limit “CV-MV lower” can be set independent of each other. Compared to the CV of the winding position, this "alarm band" is set to a relatively high sensitivity because a mean value CV-MV which exceeds preset limits is usually an indication of serious problems. The CV-MV indicates important changes and trends of the yarn. In an initial test cycles, the settings of this alarm band should not be set too sensitive. After the CV mean value of the group has been determined over a certain time span (e.g. one shift or several doffings), then the upper and lower alarm limits can be set. If the upper or lower alarm limits are exceeded, then this will be indicated by an alarm. After a period of observation, the setting can then be adjusted according to the specific application. This is illustrated by the following Fig. 4-1: CV - MV Alarm is triggered

Upper alarm limit

CV - MV

Lower alarm limit

t

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Fig. 4-1 Schematic representation of the deviation behavior of the CV mean value of the group

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5

Deviation of the CV of the winding position (CV-SP)

The CV-MV is used as a basis for the CV value referred to a single winding position. The monitoring of the CV of the spinning position is carried out in relation to the current CV mean value of the group. As with the CV-MV, an "alarm band" can be set for the CV-SP value. The set value is effective in both the positive and the negative direction. If an alarm limit is exceeded, then this will be indicated by an alarm. Depending on the settings, the winding position can be blocked. Example: The percentage deviation (CV-SP), which is defined as the alarm limit, is calculated be means of the CV-MW, as shown in the following example: With a CV-MV of 14% and an alarm limit of ±20%, the effective range is between 11.2% and 16.8%. The deviation behavior of the CV of the winding position is shown schematically in the following Fig. 5-1: Alarm is triggered

CV

CV-SP

CV+ CV+CV-MW CV-

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CV+-

Fig. 5-1 Schematic representation of the deviation behavior of the CV of an individual winding position

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6

Settings

In the window "Q-Parameter" of the Control Unit, the following settings can be adjusted:

Fig. 6-1 Setting window for the coefficient of variation at the Control Unit

Reference length: It is possible to set the reference length between 50 – 10'000 m. In winding, a reference length of 400 m has been accepted as the standard. This is a length which is necessary for a reliable CV-value. However, the setting of the reference length also depends on the objective when monitoring the coefficient of variation. · For data acquisition: For the monitoring of the CV it is recommended to select the reference length of 400 m starting from the bobbin tip (see "measurement"). As the yarn evenness increases from bobbin tip to bobbin base, it is guaranteed that results measured under the same circumstances (same yarn length) can better be compared with each other. A longer reference length is not recommended as the number of faults increases at the bottom part of the bobbin and thus, the CV-value is influenced. For pure data collection, no action is taken in case of exceeding limits. · For the selection of bad bobbins: The selection of the reference length depends on the quality requirements. The reference length must be derived from the possible CVdeviation in the yarn. The monitoring of faulty yarn must be carried out continuously (see "measurement"). This guarantees that bobbins which do not meet the quality requirements will be monitored and can be taken out of the winding process (action: block). Mainly in the production of compact yarns, faults which are formed in the compacting zone can influence the CV-value. Such faults can occur over the whole bobbin length.

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Measurement: The measurement can be carried out: · continuously

· at bobbin change

The following winding machines provide a bobbin change signal. This means that the winding position informs the clearer when a bobbin change is carried out: · Murata Mach Coner 7/5 · Murata Process Coner 21 · Savio Orion

· Schlafhorst Autoconer System 238 · Schlafhorst Autoconer 338

Alarm limit MV-monitoring This is an absolute monitoring of the CV-group mean value. The CV-MV alarm can only be deleted by increasing the alarm limits or when the CVMV decreases below the alarm limit. As no action is carried out in case of an exceeding limit, the alarm must be considered as a warning. If the alarm limit is set to 0, the monitoring is inactive. Monitoring of individual winding positions With the monitoring of the CV of a winding position, a relative deviation of the SP-MV from the CV-MV is set. The setting of the percent value must determined out for each individual application. Due to the diverse causes for the changes of the yarn evenness, it is not possible to give any recommendations for the settings. · The setting of a lower CV alarm limit serves for monitoring and the detection of yarns, which have too much twist caused by: – heavy ring travellers, – 2 ring travellers on one ring with different operating hours, i.e. the old traveller was not removed, – twisted drive belts. · The setting of a upper CV alarm limit which serves for the monitoring and detection of: – a general high CV, caused by diverse faults in the production process, – a rough ring, – slow spindles caused by loose drive belts. If the yarn evenness of a bobbin deviates from the SP ALARM LIMIT, a CVp- or CVm alarm is triggered. At the same time, this deviation from the mean value can be found on the window for "textile alarms" at the control unit.

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If information on the yarn evenness is desired only, there is the possibility to set the alarm limit, but without selecting any actions. In this case, the number of alarms is indicated in the shift report. If the alarm limit is set to 0, the monitoring is inactive. Action If the CV of a winding position exceeds one of the alarm limits, the iMK reacts according to the selected alarm, setting column ACTION. An entry is made in the logbook in all cases. There are three different possibilities: · none · cut · block If no action is chosen, the measurement with a set limit serves only for data collection to monitor the quality of the production. If USTER® QUANTUM EXPERT for winding is connected, the signal is transferred to this data system for alarm purposes. With the selection cut, a cut is triggered when a preset alarm limit is reached. CV alarms are quality parameters and indicate changes of the yarn quality. It does not make much sense to take the yarn off the cone since it causes damages in the packing and leads to weak places in the yarn. This setting should not be chosen. The action block can be recommended, if it is desired to take a bad bobbin out of the process. For this, trained personnel is necessary. Depending to the machine type, an automatic bobbin change is carried out or the bobbin must be changed manually.

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7

Result display

Fig. 7-1 shows the results of the CV-measurement of each winding position as well as the CV-mean value of the group and the absolute CV-alarm at the control unit.

Fig. 7-1 Display of the CV-value

· SP UPPER LIMIT The upper absolute CV-limit is calculated from the CV-mean value of the group and the set relative upper CV-alarm limit. · SP LOWER LIMIT The lower absolute CV-limit is calculated from the CV-mean value of the group and the set relative lower CV-alarm limit. If the CV of a winding position lies above or below the absolute SP ALARM LIMIT, a CVp- or CVm-alarm is triggered.

Uster Technologies AG Wilstrasse 11 CH-8610 Uster / Switzerland Phone +41 1 943 38 88 Fax +41 1 943 38 38 www.uster.com [email protected]

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