A yarn‘s count is the product of the degree of draft applied to a carded or drawframe sliver. This draft occurs on the rotor spinning machine between the feed roller (for sliver intake) and the delivery roller (for the yarn), and results from the speed ratio of the two drives. The draft can therefore be changed by altering either the sliver intake speed or the yarn take-off speed. However, since the take-off speed, i.e. delivery speed, is directly responsible for imparting twist to the yarn, and therefore must not be changed, the degree of draft can therefore only be changed by adjusting the sliver intake speed. The drafts used in mill practice are between 60 and 400- fold. Initially, however, much higher drafting occurs between the sliver feed and the fiber collecting groove, i.e. opening of the sliver down to the individual fibers. This corresponds to a maximum draft of up to 25 000-fold. The final yarn count is only formed from the individual fibers, i.e. fiber layers, in the collecting groove of the rotor. Only this ratio – yarn count to sliver count – corresponds to the degree of draft set at the machine control unit.

The degree of draft is calculated as follows:

draft = \frac {Nm_{yarn}}{Nm_{sliver}} = \frac {Ne_{yarn}}{Ne_{sliver}}= \frac {100}{(tex_{yarn}/ktex_{sliver})}

or, transferred to the machine:

draft = \frac {V_{yarn\ delivery\ m/min}}{V_{sliver\ intake\ m/min}}

Yarn and sliver count are thus calculated as follows:

Nm_{yarn} = Nm_{sliver} \times draft

Ne_{yarn} = Ne_{sliver} \times draft;

tex_{yarn} = 1000 \times \frac {ktex_{sliver}}{draft}

Nm_{sliver} = Nm_{yarn} / draft

Ne_{sliver} = Ne_{yarn} / draft

ktex_{sliver} = \frac {tex_{yarn} \times draft}{1000}

Drafts on the rotor spinning machine are much higher than in ring spinning, and also higher than in air-jet spinning, although spinning is also performed directly from a feed sliver in this latter spinning process. However, considerably finer slivers (1 - max. 3 ktex) must be fed to the air-jet spinning machine than to the rotor spinning machine, although fine slivers entail production drawbacks on the card and drawframe! The drafting range of up to 400-fold on the rotor spinning machine enables normal drawframe slivers in the range between 5 and 6 ktex (Ne 0.12 to 0.10) to be fed in, even when producing very fine rotor-spun yarns (see Fig. 89).

The results of mill trials have shown – by no means surprisingly – that high drafts, especially when processing cotton, have a positive impact on both yarn quality and spinning stability. The reason for this is that sliver intake speed is very low at high drafts, and the individual fibers therefore spend longer in the opening roller zone before being released from the fiber bundle that has been fed in. Fiber neps are opened more thoroughly, while dust and trash are more effectively released and removed from the fibers.

Fig. 89 – Maximum flexibility with drafts of up to 400-fold