Twist Per Metre and Twist Factor Measurement

Twist per metre (tpm) is measured by the untwist-retwist method (ISO 2061, SDL Atlas TW120, Zweigle D302) or direct counting, with the untwist-retwist method preferred for precision on 250 mm gauge length staple yarns. Procedure: clamp yarn under 0.5 cN/tex pretension between fixed jaw and rotating jaw (digital twist counter, ±1 revolution resolution) → rotate jaw to untwist until fibres lie parallel (detected by shadow of yarn projection at 0 twist — indicator mark on yarn or fibre parallelism by eye) → count rotations T₁ → re-twist in original direction to restore original 250 mm gauge length → count rotations T₂ → tpm = (T₁ + T₂) / (2 × 0.250 m). Precision: ±5 tpm at 800 tpm level (CV% ±0.6%) for experienced operator with proper parallelism detection. Twist factor calculation: αe (English twist factor) = tpi × √Ne = (tpm/39.37) × √(590.5/tex); αm (metric twist factor) = tpm / √(1000/Nm). Optimum twist factor by yarn application: ring-spun warp weaving Ne 30–40: αe 3.8–4.5; ring-spun knitting Ne 24–40: αe 3.2–3.8; ring-spun sewing thread Ne 40–80: αe 4.5–6.0; rotor yarn Ne 10–30: αe 3.0–3.5 (lower than ring at equivalent count because rotor false twist geometry produces different twist-strength relationship). Plied yarn twist: measured separately from single yarn twist — balanced ply (S-twist ply of Z-twist singles) has ply twist equal to 60–70% of single yarn twist tpm for standard 2-ply structure. Twist direction verification: Z-twist = right-hand helix (spiral follows right diagonal of letter Z), S-twist = left-hand helix — critical for ply balance and fabric hand: S/Z plied yarns produce balanced woven fabric, S/S or Z/Z causes weft spiralling.