The bobbin must lag the flyer by exactly the number of revolutions per minute that makes its wound circumference equal the yarn delivered, which is about 32.3 rpm on a 25 mm barrel and about 18.0 rpm on a 45 mm full bobbin when 2.54 m of yarn arrives each minute. A fixed pair of whorl sizes gives a constant lag, so it overwinds as the bobbin fills, and a friction brake on the bobbin, which adjusts itself, is the simpler cure but lets the yarn tension fall by the ratio of the diameters.
Part of Spinning Frame
The speed arithmetic of a flyer frame’s bobbin: how fast it must lag the flyer, how that changes as it fills, and how whorls and brakes deliver it.
The Rule Is Geometry, Not A Tolerance
This article is derived from first principles and the overview’s description, not from a sourced tolerance. The overview says the flyer runs slightly faster than the bobbin, the difference controls winding, and equal speeds add twist without winding. The size of “slightly” follows from the following.
Yarn delivered per minute is v. Each revolution of lag takes up one wound circumference, π × D, where D is the diameter of the layer being laid. So
(flyer rpm − bobbin rpm) × π × D = v, or lag = v ÷ (π × D).
Take the delivery for 20 twists per inch at an assumed 2,000 rpm: v = 2.54 m per minute (see Twisting). The bobbin dimensions are assumed: barrel 25 mm, full 45 mm.
| Layer diameter | Lag needed |
|---|---|
| 25 mm | 2,540 ÷ (π × 25) = 32.3 rpm |
| 35 mm | 2,540 ÷ (π × 35) = 23.1 rpm |
| 45 mm | 2,540 ÷ (π × 45) = 18.0 rpm |
The lag falls to 18.0 ÷ 32.3 = 56% of its starting value, the inverse of the diameter ratio 45 ÷ 25 = 1.8. Twist is unaffected, since it depends only on flyer speed and delivery.
A Fixed Whorl Ratio Overwinds As The Bobbin Fills
With a double band, the flyer and the bobbin each have a whorl, and the bobbin speed is a fixed fraction of the flyer’s. The lag is therefore fixed. Set it for the empty barrel, 32.3 rpm of 2,000, and the full bobbin winds at π × 0.045 × 32.3 = 4.57 m per minute against a delivery of 2.54: 1.8 times too fast. The yarn is stretched until it breaks or until the band slips. A fixed lag is correct at one diameter only.
It Is Also Sensitive To Whorl Size
The bobbin must turn at 2,000 − 32.3 = 1,967.7 rpm, a ratio of 0.9838 to the flyer. With a 25.00 mm flyer whorl the bobbin whorl must be 25.00 ÷ 0.9838 = 25.41 mm. An error of 0.1 mm in either whorl changes the speed ratio by 0.1 ÷ 25 = 0.4%, which is 8 rpm, a quarter of the 32.3 rpm needed. The band’s own thickness or a worn groove does the same. The overview names pulley size or brake tension as the control; this shows why a brake is easier to run.
A Brake Self-Adjusts But Changes Tension
With a Scotch-tension brake (the arrangement in Spinning Wheel) the flyer drives the yarn round and the bobbin is held back by a friction torque τ. The bobbin turns only as the yarn tension pulls it, so lag adjusts itself to the delivery. The yarn tension that overcomes the brake is T = τ ÷ R, where R is the radius of the layer. For an assumed tension of 0.30 N on the full bobbin (R = 22.5 mm), τ = 0.30 × 0.0225 = 6.75 mN·m, so on the empty barrel (R = 12.5 mm) T = 6.75 ÷ 12.5 = 0.54 N. The tension is 1.8 times higher when the bobbin is empty, again the diameter ratio. A progressive brake that eases as the bobbin fills evens it out.
The tension must stay below the breaking load of the weakest thin place in the yarn. A recalled figure of 12 cN per tex for cotton gives about 3.5 N for 29.5 tex (not from the corpus; measure yours as in Twist Testing); thin places are weaker, and warp yarn meets more load later on the loom (Warp Tensioning and Let-Off).
What You Need
- Bobbins with a smooth barrel and true flanges, a groove for the whorl.
- Whorls turned to matched diameters, or a brake band and weight.
- A tachometer substitute: a revolution count over a timed minute using a mark on the bobbin.
Method: Setting The Winding
Step 1: Compute the lag for the empty and full diameters, as above.
Step 2: Fit the brake lightly and run with the yarn. The path between the roller and the eye should be smooth and slightly curved.
Step 3: Count the lag at start, middle and end with a mark on the flyer and bobbin.
Step 4: Adjust until the lag at the empty and full bobbin approaches the table.
Common Mistakes
- Choosing whorls for the empty bobbin and ignoring the fill.
- Setting the brake only for a full bobbin, so the empty barrel pulls the yarn hard.
- Mixed tension across spindles, which makes bobbins of different density.
Safety
Bands and brakes sit beside a spinning arm
Adjust a band or brake only with the frame stopped. A band re-seated on a turning whorl pulls fingers in. See Doffing for the stopping rule.
What’s Next
- Learn next: Drafting Rollers — the part that sets the delivery speed used above
- Combine with: Winding On — the practical side of the same balance