Attenuating Shear Force
Viscous and frictional drag exerted axially on fibrous bundles by differential surface velocities of successive roller pairs within a drafting zone constitutes the primary force governing fiber slippage. In flax wet and dry spinning mills, roller drafting stress regulates the sliding of individual elementary fibers along the pectin-rich middle lamella during roving attenuation. This mechanical stress arises from the gripping pressure of weighted top cots, the velocity ratio between entry and exit roller pairs, and the cohesive strength of inter-fiber friction.
Mill spinning technicians measure this drafting resistance indirectly through motor power consumption on spinning frames or directly through force transducers fitted to drafting saddles, logging the parameters on spinning setup sheets. Buyer specifications focus on the resulting count variation coefficient and mass unevenness recorded on yarn test certificates according to GB/T 398. The physical boundary of this drafting stress stops precisely at the nip point of the delivery roller pair, where downstream flyer or ring twisting begins.
Rheological Attenuation
Wet-spun linen requires precise control over roller drafting stress because rovings pass through a hot-water trough to soften intra-fiber pectins immediately before reaching the drafting unit. Softened pectins act as a viscous lubricant, demanding higher mechanical control over drafting roll pressures to avoid uncontrolled slippage or thick-and-thin drafts. Roller drafting stress must overcome the internal cohesion of the softened fiber bundles without exceeding the tensile strength of the individual elementary fibers.
If drafting stresses climb excessively high due to cold water baths or insufficient nip distances, fibers fracture instead of sliding, reducing fiber length and generating high levels of short-fiber waste. Conversely, insufficient stress lets fibers draft in cohesive clusters, producing severe cross-sectional irregularities that translate directly into yarn imperfections.
Process Equilibrium
Setting drafting roller nip pressures requires a steady balance between rubber cot hardness and spring or pneumatic loading systems. Roller drafting stress increases when high-density slivers enter narrow roll gaps, inducing accelerated wear on polyurethane roll covers and generating localized heat. Technicians calibrate roller gauge settings against the fiber bundle length distribution, preventing two ends of a single fiber bundle from being gripped simultaneously by consecutive roller pairs.
When gauge lengths fall below maximum fiber lengths, draft stresses spike sharply, resulting in broken fibers, rapid fiber lapping on draft rollers, and spinning ends down. Correctly calibrated draft stresses promote uniform bundle sliding, lowering unevenness and supporting smooth downstream winding. Mechanical stress controls cease once the yarn exits the front nip, where twisting forces convert parallel drafting flow into rotational yarn geometry.
