As fibers are carried along with the roller surfaces they are drawn apart. For this to occur, the fibers must assume the peripheral speed of the rollers. The transfer of the roller speed to the fibers represents one of the problems of drafting operations. The transfer can be effected only by friction, but the fiber strand is fairly thick and only its outer layers have contact with the rollers; furthermore, various non-constant forces act on the fibers.
For the purpose of illustration (Fig. 44), the forces acting on a fiber f in the drafting arrangement will be considered here. The fiber is bedded at its trailing end in a body of fibers (B1) which is moving forward slowly at speed v2. The leading end is already in a body of fibers (B2) having a higher speed v1. In this example, a tensile force FZ acts on the fiber f; this arises from the adjacent fibers of the body B2 already moving at the higher speed and the retaining force FR exerted by the fibers of the body B1. To allow acceleration of the fiber f and finally a draft, FZ must be greater than FR. Permanent deformation of the fiber strand could not be achieved if FZ is only slightly greater than FR. In this case, straightening and elongation of the fibers would produce a temporary extension, which would immediately disappear on removal of the extending force.
As already indirectly indicated, drafting takes place in three operating stages:
- straightening of the fibers (decrimping);
- elongation of the fibers;
- sliding of the fibers out of the surrounding fiber strand.
The effective drafting force can be represented by the curve form shown in Fig. 45. Up to point m, at which the fibers begin to slide apart, the curve climbs steeply. This is the straightening and extending stage. From point n onwards, by which stage many fibers are already sliding, the curve falls slowly with increasing draft. The reduction of the drafting force with the increasing extent of draft is easy to explain – there is a continuously declining number of fibers to be accelerated, i.e. to be drawn out of the slowly moving strand, since a higher degree of draft implies fewer fibers in the cross-section.
Besides the number of fibers in the cross-section, the drafting force is also heavily dependent upon:
- the arrangement of the fibers in the strand (parallel or crossed, hooks);
- cohesion between the fibers (surface structure, crimp, finish, etc.);
- nip spacing.