Rieter

With a small amount of draft, namely with V between 1 and 2, the drafting forces are often inadequate to induce permanent relative fiber shifts. In this region, the socalled critical drafting region, extremely disruptive stickslip effects are often observed.

Here, the drafting force has to take the fibers from a static condition (motionless coherence of the fibers in a compact strand) to a dynamic condition, that is, to set the fibers in motion relative to their neighbors. As often also found in other fields, this mechanical operation not only requires considerable force, but also does not always occur without disturbance. In the critical region, the drafting force may suffice to overcome the frictional coherence instantaneously, but not to maintain acceleration.

The fibers are therefore braked and again take on the speed of their slowly moving neighbors.

The drafting force will again take effect and accelerate the fibers but will not be able to maintain the acceleration. Thus, there is a continual changing of conditions between acceleration and standstill, i.e. a kind of stop-and-go movement, with often disastrous consequences for the evenness. In the force-draft diagram (Fig. 46), this is clearly recognizable as greater or smaller deviations.

For cotton sliver, the critical drafting region lies somewhere between V=1.15 and 1.4, and, for cotton roving (on the ring spinning machine), it is between V=1.3 and 1.7. For man-made fibers, for which the stick-slip effect is usually more strongly marked, the range lies somewhat higher, depending upon friction between fibers e.g., delustering, spin finish, etc. Operating in the critical drafting region can be risky.