Yarn Twist Testing
category
Yarn twist testing measures twist per unit length (turns per metre tpm or turns per inch tpi), twist direction (Z or S), twist factor (αm = tpm / √Nm or αe = tpi × √Ne), and residual torque in spun and plied yarns — parameters determining yarn strength, elongation, handle, pilling propensity, and fabric stability. ISO 2061 untwist-retwist method: clamp yarn at gauge length (250 mm for staple, 500 mm for filament), apply standard pretension (0.5 cN/tex), untwist with motorised jaw until fibres lie parallel (untwist count T₁) then re-twist in original direction until original gauge length restored (retwist count T₂) — twist = (T₁ + T₂) / 2 / gauge length (m). Direct counting method (ISO 2061 optical): untwist under microscope counting individual twist turns over 25–50 mm gauge length — used for coarse yarns where untwist-retwist is inaccurate due to high fibre stiffness. Twist factor (αm = tpm × √tex / 10 for cotton Ne system): optimum twist factor for ring-spun weaving warp Ne 30: αe 3.8–4.2 (tpm ≈ 800–900); knitting weft Ne 30: αe 3.2–3.6 (tpm ≈ 670–760) — higher twist increases strength but reduces elongation and softness; lower twist improves handle but reduces warp breakage resistance by 15–25%. Residual torque (Zweigle G566, yarn loop test — loop length 500 mm, hold both ends, measure angle of rotation at equilibrium) — high residual torque (>45°) causes spirality in single jersey knitted fabric (ASTM D1422 spirality) and snarling during weaving.
Role
Yarn twist testing verifies the fundamental structural parameter of spun yarn that determines the strength-elongation balance required for warp weaving performance and the twist level that prevents residual torque spirality defects in knitted fabrics — two commercially critical quality requirements that diverge in optimal twist direction making twist measurement and control the central process variable in ring spinning mill management.