Drawing rollers attenuate a fiber strand by delivering it faster than it enters. A rotating flyer then inserts twist and guides the yarn onto a slower bobbin. Three roller pairs draw out the strand before the flyer adds twist and guides it onto the bobbin. Adjust their speeds to compare draft, yarn linear mass, twist and take-up.
Drawing and Throstle Twisting Interactive Calculator
Follow the strand through three drafting nips to a rotating flyer and bobbin. Compare draft, delivered linear mass, ideal twist and winding speed.
Equation Used
- Equal drafting-roll diameters.
- Ideal mass conservation and no twist contraction.
- Fixed 50 mm winding diameter.
- Required bobbin speed, not a friction/tension prediction.
Equal roll diameters; no slip, loss or twist contraction. Winding diameter fixed at 50 mm; no package growth or builder traverse.
Draft, twist and winding are linked operations
The upper and lower rollers at each nip turn in opposite directions, so both contacting surfaces carry the strand forward. This model uses equal roll diameters; rotational-speed ratios therefore equal surface-speed ratios. The three entered speeds separately control their roller pairs.
Yarn leaving the front rollers passes an illustrative fixed guide and goes through the central flyer eye, along one flyer arm and through its guide to the bobbin. The flyer turns faster than the bobbin during normal winding. The historical construction used a loose bobbin and frictional restraint; the animation computes the bobbin speed required by the entered delivery and flyer speed, without solving friction or tension.
Rolls and flyer share one slowed time scale, displayed under the drawing. A fixed 50 mm winding diameter is assumed. Package growth and builder traverse are omitted, so the drawing shows a fixed winding band rather than claiming to model a complete package.
Compare the roller-speed progression
Change back, middle and front speeds to compare each drafting zone and the total draft. Feed linear mass changes delivered linear mass. Roll diameter changes delivery speed and twist per meter; flyer speed changes twist and the required bobbin speed.
If a downstream roll is slower than its preceding roll, the page flags the setting as unsuitable for ordinary drawing. The arithmetic ratio remains visible, but the animation does not certify that a compressive fiber process will operate.
Draft, linear mass and flyer winding
For equal roll diameters, stage drafts are Nm/Nb and Nf/Nm; total draft is Nf/Nb. With no loss, output linear mass is input linear mass divided by total draft.
For common roll diameter D in millimeters, front delivery is v=πDNf/1000 meters per minute. Ideal twist is Ns/v turns per meter, neglecting twist contraction and slip.
At the fixed 50 mm winding diameter, one relative flyer-to-bobbin turn winds π×0.05 meters. Required winding difference is ΔN=v/(π×0.05), and required bobbin speed is Ns−ΔN. A negative result means the entered speeds require counterrotation rather than the illustrated passive, slower co-rotating bobbin; that condition is explicitly flagged.
Example: 1, 4 and 20 rpm roller pairs
With equal 30 mm rolls, back speed 1 rpm, middle speed 4 rpm and front speed 20 rpm, the stage drafts are 4 and 5, and total draft is 20. An input of 4 g/m ideally delivers 0.200 g/m.
Delivery is 1.885 m/min. At 600 flyer rpm, ideal twist is 318.3 turns/m. A 50 mm winding diameter requires a 12 rpm flyer-to-bobbin difference, giving 588 bobbin rpm.
A kinematic study, not a spinning specification
The calculations assume equal roll diameters, no roller slip, no fiber loss, no twist contraction and constant winding diameter. Real drafting requires fiber-length-appropriate roll spacing and suitable tension. These are not solved.
Roll settings and operating speeds depend on the fiber and machine. The calculated linear mass and twist describe ideal process relationships; evaluating yarn quality also requires tension, slip and material behavior.
Drawing and throstle questions
Why do the rollers move more slowly than the flyer?
The same slowed time scale applies to every rotating part; their relative speeds follow the entered rpm.
Can the bobbin turn at the same speed as the flyer?
Not while winding positive yarn delivery at a finite diameter. Relative rotation provides the take-up.
What happens when a draft ratio is below one?
The page retains the ratio and flags the reversed speed progression. It does not depict that setting as ordinary successful fiber attenuation.
Does the bobbin grow?
No. Winding diameter is held at 50 mm and the illustrated yarn band is a steady reference, not accumulated material.
Historical construction reference
Gardner D. Hiscox, Mechanical Movements, Powers and Devices, printed page 344, figure 1437 (PDF page 354): drawing rolls, throstle and bobbin winding. The third roller pair, dimensions and ideal no-slip calculations are explicitly stated model choices.
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