Rieter

Variation in yarn twist

Index

The equation

$Twist/m\ = \frac{Spindle\ speed\ (rpm)}{Delivery\ speed\ (m/min)}$

is generally used to calculate the number of turns in the yarn. As just established, this is not wholly accurate, since the turns arise from the traveler and not from the spindle. In the given example, 104 turns per minute are missing at the base of the winding on the cop (larger diameter), and 191 turns per minute at the tip (smaller diameter). However, these missing turns are a theoretical rather than a practical problem for two reasons. Firstly, the inaccuracy of measurement in the estimation of yarn twist by test instruments is greater than this twist variation.

Secondly, the yarn finally receives its full twist in any case. This happens as soon as the yarn is drawn off the cop over the end, since each rotation of the yarn around the tube (1 wrap) leads to the insertion of an additional turn in the yarn. The compensation of the missing turns can then be explained easily.

If 191 turns per minute are missing at the tip, and 15 m of yarn has to be wound up in this period, the result is

Tm (missing)= 191 turns/min / 15 m/min = 12.73 turns/m

During unwinding, each yarn wrap on the cop (one circumference) produces one additional turn. At the tip (cop diameter 25 mm), we have:

Ta (additional)= 1 000 mm/m / 25 mm × π = 12.73 turns/m

that is exactly the number of turns previously missing. It must, however, be ensured that cops are always unwound over end, even during twist tests.