With a roller setting of, for example, 50 mm, a 40 mm fiber would be theoretically under control for 40/50 or 4/5 of its path and would be without control for only 1/5; a 10 mm fiber, on the other hand, would be controlled over only 1/5 and uncontrolled over 4/5. These floating fibers are the problem in drafting.
The ideal movement of the fibers would be achieved if the whole fiber strand moved with speed v2 into the nip region of the delivery roller pair without internal shifts, and if drawing- out of fibers first occurs here, and if only the nipped fibers were drawn out. In this case, each fiber would have either speed v2 or speed v1 at any given instant.
The fibers would be continuously guided under control. This is achievable to the maximum extent, however, only when the infed fiber mass is glued together (as in the former Pavil spinning system from Rieter), since fiber acceleration can then occur only when the fibers are gripped by the front rollers. Under normal circumstances, however, conditions are not nearly so favorable. The majority of floating fibers can take on any speed between v2 and v1 at any instant in their movement through the drafting zone, or can even change speed several times, which always leads to greater or lesser unevenness. Fortunately, however, there are a few helpful circumstances which reduce these adverse influences to some extent.
A certain additional guidance of floating fibers is achieved by:
- a sufficient number of longer fibers as carrier fibers for the shorter ones;
- guiding devices, such as rollers, needles, aprons, etc; and
- the friction field.
This last factor, which is extremely important for drafting behavior, will now be dealt with specifically.