Fiber Length

Fiber Length

The influence of fiber length

The length of the fiber is also one of the most important parameter for the spinning process as well as quality. It has a major influence on:

  • Yarn strength
  • Yarn evenness
  • Spinning limit and productivity
  • Yarn hairiness
  • Hand and luster of the fabric

The productivity is influenced by numbers of break, quantity of waste, required twist and general spinning conditions.

In general fibers < 5 mm will be lost as waste and fly during the processing. Fibers of 12 through 15 mm do not support the strength and only serves to “fill up” the yarn. Only fibers above this length (> 15 mm) supports the positive characteristics of the yarn.


It is not only the condition of fibers at purchase that is important in reference to the fiber length. Still more decisive is the length after carding. Processing conditions at the card must be such that the fibers survive carding without noticeable shortening. If there are a high proportion of unripe fibers this will not be the case.

The Staple Diagram

The fibers in the “cotton ball” do not show great differences in length. Noticeable shortening of many fibers occur before the spinning process due to mechanical working, for example by ginning and cleaning. In even the smallest tuft of cotton there will be all lengths from the absolute minimum to maximum.

By arranging the fibers from a tuft taken up in the hand, side by side with there ends aligned and sorted according to length in a coordinate system, a staple diagram is obtained; the so-called numerical diagram, view below picture. If created abstractly from the masses of length groups, then the “weight-balanced” graphic is obtained. This has a higher curve compared with the numerical diagram, because long fibers have more mass than short. This diagram compare well with the distribution of fibers in the yarn cross section. On the other hand, the numerical graphic emphasizes unduly the proportion of short fiber material.

Besides of the staple diagram, the use of Fibrogram is very common today. Whereas in the staple diagram the fibers are aligned at one end, in the Fibrogram they are arranged randomly by clamping on a line.

Actually the staple diagram represents an artificial picture, which does not occur anywhere in practice, the Fibrogram refer to the arrangement of the fibers at the “nip line” of rollers. It is created in the Digital-Fibrograph. The lengths are stated as “span lengths”; that is lengths which span a certain distance. A fiber beard of randomly clamped fibers is scanned optically across its length and the Fibrogram is derived from it. It gives a good representation of the drafting operation and of the arrangement of the fibers in the yarn.

Apart from above “classic” styles of staple diagram there are various of graphic showing different material and blend, for example, “Rectangular shape”, achievable only from synthetic fibers, “Triangular shape” including high numbers of short fibers, “Trapezoidal shape”, which would represent the ideal staple for processing, however a flat curve means a high price, “Stepped shape” representing fiber material of very different lengths mixed in wrong proportions.

Specification of fiber length

From the staple diagram we can access various measures of length as the maximum, minimum and average length of staple. Those numbers however are of less use for the yarn maker. They neither allow a statement of the product nor of the processing. The trade and processor therefore use more informative data, such as:

  • the classifying staple (called trade staple)
  • the hand staple (called spinner’s staple)

The trade staple is an important specification of length. It is established to 1 1/32” during classifying of the cotton and meet with the fiber length at about 25% in the weight based- and 15% in the numerical diagram. The hand staple is similar and established by the specialist in operation.

The following length groupings are used in stating the trade staple:

Staple diagram, specification of lengths

The specification of the trade staple alone is not enough as the slope of the curve is not taken into consideration. With the same trade staple length, the staple could approach either the rectangular or the triangular shape. The proportion of short fibers would then be high or low.

In order to determine how good the length is, a second point on the curve is to be used (e.g. 50% staple), the coefficient of variation or the proportion of short fibers.

Upper Half Mean Length and Uniformity Index

Above parameters, based on results from the Fibrograph, are used to evaluate higher numbers of cotton bales and for the processor to optimize the cotton material from point of price and processing.

The by weight measurement of the Upper Half Mean Length is calculated from the fibrogram. It corresponds to the “classer’s” length.

The Uniformity Index expresses the ratio of the Mean length to the Upper Half Mean Length. It is an indication of the distribution of the fiber length within the fibrogram.



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