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    7.variance Curve

    Uploaded by

    Mugunthan Krishna
    - The variance-length curve is a graphical representation of the coefficient of variation (CV%) against reference cut length that is produced by evenness testers. It shows variations at different cut lengths and can identify where abnormal variations occur and what process may be responsible. - The CV is calculated and plotted for different cut lengths by adding together the mass of successive portions of material. This allows faults from inferior raw material or machinery to be identified by higher CV values appearing at longer cut lengths corresponding to where the fault originated in the process. - The variance-length curve of a faulty yarn will lie above the curve of an ideal, fault-free yarn.

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    7.variance Curve

    Uploaded by

    Mugunthan Krishna
    142 views15 pages
    - The variance-length curve is a graphical representation of the coefficient of variation (CV%) against reference cut length that is produced by evenness testers. It shows variations at different cut lengths and can identify where abnormal variations occur and what process may be responsible. - The CV is calculated and plotted for different cut lengths by adding together the mass of successive portions of material. This allows faults from inferior raw material or machinery to be identified by higher CV values appearing at longer cut lengths corresponding to where the fault originated in the process. - The variance-length curve of a faulty yarn will lie above the curve of an ideal, fault-free yarn.

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    7.Variance Curve

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    - The variance-length curve is a graphical representation of the coefficient of variation (CV%) against reference cut length that is produced by evenness testers. It shows variations at different cut lengths and can identify where abnormal variations occur and what process may be responsible. - The CV is calculated and plotted for different cut lengths by adding together the mass of successive portions of material. This allows faults from inferior raw material or machinery to be identified by higher CV values appearing at longer cut lengths corresponding to where the fault originated in the process. - The variance-length curve of a faulty yarn will lie above the curve of an ideal, fault-free yarn.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    142 views15 pages

    7.variance Curve

    Uploaded by

    Mugunthan Krishna
    - The variance-length curve is a graphical representation of the coefficient of variation (CV%) against reference cut length that is produced by evenness testers. It shows variations at different cut lengths and can identify where abnormal variations occur and what process may be responsible. - The CV is calculated and plotted for different cut lengths by adding together the mass of successive portions of material. This allows faults from inferior raw material or machinery to be identified by higher CV values appearing at longer cut lengths corresponding to where the fault originated in the process. - The variance-length curve of a faulty yarn will lie above the curve of an ideal, fault-free yarn.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 15

    Variance –Length Curve

    • The coefficient of variation at different cut lengths provided by the


    evenness testers provides invaluable information with regard to the
    variations prevalent at the specific cut lengths.

    • When the variations prevailing at different cut lengths are


    simultaneously represented graphically, it provides the possibility of
    segregating cut lengths at which abnormal variations occur and
    consequently identify the process stage which is most likely to be
    responsible.

    • This is made possible by the 'Variance Length Curve' which is a


    standard feature of most evenness testers.
    • In simple terms, a 'Variance Length Curve' is a graphical
    representation of the Coefficient of Variation (CV%) against
    the reference cut length.

    • The representation is explained with a simple hypothetical


    example as shown in Figure.

    • For constructing the variance length curve, the measuring


    field length is taken as the basic cut length at which the CV is
    calculated and plotted. For variations at other cut lengths, the
    mass of successive portion of material are added up and the
    CV calculated.
    Shape of Variance Length Curve
    • A variance-length curve can be set out in quite a simple
    manner by cutting a fiber assembly into pieces and
    determining gravimetrically the mass of these pieces.

    • The CV value is then calculated from each of these separate


    values. If this procedure is repeated for various cut lengths
    and the CV value recorded, one obtains the variance-length
    curve .

    5
    Identification of faulty yarn

    • When a yarn becomes faulty either due to an inferior raw


    material or due to improperly optimized process or due to
    faulty machinery then the coefficient of variation becomes
    higher at cut lengths corresponding to the source of the fault.

    • Therefore the variance length curve of a yarn with faults


    always lies above the curve for the ideal fault free yarn as
    shown in Figure.
    6
    3D V-L Curve

    • Different levels of long-term mass variation, which are


    also clearly visible in the corresponding cut length
    diagrams, appear as different inclinations of the
    variance-length-curves.

    • In a variance-length curve array it is very easy to spot


    exceptional samples.
    7
    How to calculate ideal V-L Curve?

    The minimum possible


    VL curve is plotted in Red Color by
    using the above formulae

    • number of fibres in the cross section(n)=117,

    • i =Mean Fibre length=22mm, L= Cut length = 100mm


    • CVlim(10cm)=(100/√117) X (√22/√100) = 4.33 8
    Location of Sources of Deviation

    • A fault created at any process stage creates higher CV


    values at longer cut lengths.
    • The cut length at which the maximum deviation occurs
    depends on the stage at which the fault originates since
    subsequent drafting increases the cut length at which the
    deviations are noticed.
    • For this purpose, the cut length ranges corresponding to
    the various departments should be identified. This is done
    by the following procedure.
    • Initial Cut Length l (cm) = Mean Fibre Length x K
    where K =1 + CV 2
    (CV - Coefficient of Variation of fibre length)
    • K ⋍ 1.18 for Cotton,
    K ⋍ 1.27 for Wool and
    K ⋍ 1.00 for Synthetic Fibres.

    • If the mean fibre length is not available, it can be estimated


    from the spectrogram, since the highest point of a
    spectrogram is given by

    • max = l x 2.82 (where l – Mean Fiber Length)


    • Consequently l = max / 2.82
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    THANK YOU

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