Yarn Twist on Machinery Spec Sheets: TPM, S/Z Direction and Twist Factor

What the twist figure actually counts

ISO 2061 defines twist as the number of turns about the yarn axis over a nominal gauge length before untwisting. It prefers turns per metre (tpm) and allows turns per centimetre. Many older specs and many buyers still use turns per inch (tpi). The conversion is fixed: 1 tpi = 39.37 tpm, and 1 tpm = 0.0254 tpi. A twister listed as "100–2,500 T/m" therefore covers about 2.5 to 63.5 tpi.

Two side effects matter when you buy machinery. First, twisting shortens the yarn, so the twisted product is heavier per metre than its components. The yarn count of a plied yarn is therefore not a simple sum of its singles. ISO 2061 includes a procedure for measuring the change in length on untwisting for this reason. Second, twist measured in the lab is twist in the yarn. The machine setting is a theoretical value calculated from speeds. The two can differ, which is why a factory acceptance test should name the method, for example ISO 2061 or ASTM D1423 (direct counting).

S or Z: direction is a separate specification

ISO 2061 describes the check. Hold a short length of yarn vertically. If the fibres, filaments or component yarns slope like the middle stroke of a letter S, the twist is S. If they slope like the middle of a Z, it is Z. The letters follow ISO 2. For a plied or cabled yarn, the final twist is the one designated. A full plied designation therefore usually gives two directions, one for the singles and one for the ply.

Ply twist commonly runs opposite to the singles twist, for example Z singles plied S, but this is a product decision, not a rule. Your product range may need both directions, so confirm that the machine can reverse direction and whether that is a control setting or a change of drive parts. If it means a parts change, put those parts on the spare-parts list in the contract.

Twist factor: the number that carries across counts

The same tpm makes a thick yarn hard and a fine yarn soft, because the fibre angle depends on yarn diameter as well as turns per length. The twist factor (also called twist multiplier, twist coefficient or alpha) corrects for count. ISO 2061 describes it as a measure of the spiral orientation of the fibres, related to the surface fibre angle and to how hard the yarn feels. University course material gives the working formulas. The UNSW wool course uses the tex form, and a polytechnic textile-testing lecture note gives both forms:

• Direct system (tex): α_tex = tpm × √tex
• Indirect systems: α = turns per unit length ÷ √count, for example α_e = tpi ÷ √Ne, or α_m = tpm ÷ √Nm

Because the count systems differ, the alphas differ too. Using tex = 590.5/Ne and 1 tpi = 39.37 tpm gives α_tex ≈ 956.7 × α_e. Using tex = 1000/Nm gives α_tex ≈ 31.62 × α_m. A supplier who writes "twist factor 3.8" without naming the system has not given you a number you can use.

Worked example. A cotton yarn of Ne 30 (19.68 tex) at α_e = 3.8 needs 3.8 × √30 = 20.8 tpi, or about 819 tpm. In tex terms, 819 × √19.68 ≈ 3,635, which equals 3.8 × 956.7. Keep the same α_e at Ne 60 and the twist rises to 29.4 tpi, or about 1,159 tpm. So keeping the same yarn character at a finer count needs more turns, and that cuts machine output (see below). A trial with your own yarn is the reliable way to confirm the twist factor that gives the hand you want. The Bhadrak polytechnic notes give cotton twist factors from about 3 (soft) to 8 (hard and lively) on the Ne basis. Crepe yarns sit at the top of that range.

Spindle speed, delivery speed and twist

For any spindle that inserts one turn per revolution, such as a ring spinning, ring twisting or uptwisting spindle:

twist (tpm) = spindle speed (rpm) ÷ delivery speed (m/min)

A two-for-one (TFO) spindle inserts two turns per revolution. The supply package stays still while the yarn passes through the rotating spindle and balloons around it, so each spindle revolution adds two turns. Patents for TFO twist-setting devices state the relationship directly. One gives the yarn speed as Y = 2S/T, where S is spindle rpm and T is twist per metre. Rearranged:

twist (tpm) = 2 × spindle speed (rpm) ÷ delivery speed (m/min)

Direct cabling and false-twist texturing work on different principles, covered in the guides to two-for-one twisting, cabling and doubling and to covered and textured yarn machines. The TZ618 direct cabling machine and the YQ800DS false-twist texturing machine are listed examples of those other routes.

Worked example (illustrative figures). A TFO spindle at 12,000 rpm plying 2/40s cotton to 600 tpm delivers 2 × 12,000 ÷ 600 = 40 m/min. That is 2,400 m per hour. At a nominal 29.5 tex this is about 71 g per spindle-hour, before allowing for twist contraction or stoppages. A single-twist spindle at the same rpm and twist would deliver 20 m/min. Ask the same TFO for 900 tpm and delivery falls to 26.7 m/min.

Why maximum rpm is not achievable output

The formula shows the first reason. Spindle speed sets the rate at which turns are inserted, not the rate at which yarn comes out. Output depends on the twist your product needs, so the same machine produces very different kilograms on a low-twist knitting ply and on a high-twist crepe.

Second, the speed printed on a brochure is mechanical: the fastest the spindle and drive are built to run. Listed TFO data shows this. The catalogue for the filament twister HY310C-SF gives a 2,000–13,000 r/min spindle-speed range. The directory summary for the short-staple LL321G advises confirming a yarn-specific twist, spindle-speed and delivery combination, because its separate published maxima are not one guaranteed operating point. Rotor spinning shows the same pattern. The RFRS30D publishes a rotor-speed range of 45,000–110,000 r/min, but the maximum is a catalogue limit, and your yarn may run well below it. The speed you can actually run is set by the yarn. Yarn tension in the balloon rises with speed, so fine or weak yarns often have to run below the brochure figure. Coarse counts and large supply and take-up packages also raise the power needed.

Third, real output is net of doffing, piecing, breaks and package changes. Treat any kg/h figure as valid only for the yarn, twist and package it was calculated for.

What to specify in an enquiry

1. Twist range in tpm, plus any twist levels you must reach for your products.
2. Direction: S, Z, or both, and how direction is changed.
3. Your target twist factor, with its system named (α_tex, α_e or α_m).
4. Spindle type: single-twist or two-for-one, so speed claims can be compared configuration by configuration.
5. A production calculation from the supplier using your count, twist and package size, with the efficiency assumption stated. Some sheets do not support this yet. The JL628A short-staple twister, for instance, has no reliable model-specific parameter schedule in its current listing. When a sheet is that thin, check the supplier's manufacturer profile and ask for a corrected sheet. By contrast, the LB-192 covering machine publishes spindle, twist, draft and package data, which you can check against your own twist calculation.
6. The twist-test method for acceptance, for example ISO 2061 or ASTM D1423/D1423M.

Sources

1. ISO 2061:2015, *Textiles — Determination of twist in yarns — Direct counting method*, 4th edition, ISO, sample pages via iTeh Standards. (accessed 2026-11-04) View source
2. ASTM D1423, *Standard Test Method for Twist in Yarns by Direct-Counting*, ASTM International, catalogue page (current edition D1423/D1423M-16(2022)). (accessed 2026-11-04) View source
3. Auer, P., "Yarn Twist", lecture slides for the CRC for Premium Quality Wool undergraduate program, The University of New South Wales, published on Woolwise (Australian Wool Education Trust). (accessed 2026-11-04) View source
4. Upadhyay, S. S., *Lecture Note: Textile Testing-II* (Yarn Twist chapter), Government Polytechnic Bhadrak, Textile Engineering, 6th semester. (accessed 2026-11-04) View source
5. US 4,922,703 A, *Twist number setting device for a two-for-one twister*, published 1990-05-08, Google Patents. (accessed 2026-11-04) View source
6. US 5,174,102 A, *Twist number setting device for a two-for-one twister*, published 1992-12-29, Google Patents. (accessed 2026-11-04) View source