Drafting thins a strand of fiber by feeding it through roller pairs where each pair turns faster than the one behind it, so the fibers slide past one another and the strand lengthens and lightens. The draft is the front roller surface speed divided by the back roller surface speed, and the way to check it is to weigh one metre of strand before and one metre after.
Part of Spinning Frame
Drawing a slightly twisted roving down to yarn thickness between roller pairs running at increasing surface speeds.
Draft Is A Ratio Of Surface Speeds, Set By Gearing
Fiber is not created or lost in the rollers, so what enters equals what leaves. If the front rollers deliver the strand 20 times faster than the back rollers feed it, the same fiber is spread over 20 times the length and each metre weighs one twentieth as much. The overview’s draft range is 10:1 to 30:1.
Its roller table gives surface speeds relative to the back rollers: 1× at the back, 3-5× at the middle, 10-30× at the front. Drafting is therefore done in two zones, and the zone drafts multiply:
- back to middle: 3-5
- middle to front: front ÷ middle, from 10 ÷ 5 = 2 up to 30 ÷ 3 = 10
- overall: for example 5 × 4 = 20, which equals front ÷ back = 20 ÷ 1
Splitting the draft between two zones is gentler on the fibers than one jump, because a short fiber is pulled from a slow pair by a fast pair that already grips its neighbours.
What You Need
- Roller pairs of fluted steel bottom roller and leather-covered top roller (Roller Construction).
- A gear train from the front roller to the others, whose ratio sets the draft.
- Roving of even weight and low twist (Roving Frame).
- A balance and a metre stick.
Method: Setting And Checking The Draft
Step 1: Choose the draft from the counts. Draft ≈ roving tex ÷ yarn tex. A 590.5 tex roving (Ne 1.0) drafted to 29.5 tex (Ne 20) needs 590.5 ÷ 29.5 = 20.
Step 2: Set the gears. With bottom rollers of equal diameter the draft is the gear ratio from front to back. With unequal diameters it is (front diameter × front rpm) ÷ (back diameter × back rpm).
Step 3: Run a measured length. Feed 10 cm through the back pair, catch the delivered strand and measure it. At draft 20 you should catch about 200 cm. Fibers that fail to draw smoothly and instead cling and then rush forward leave thick and thin places.
Step 4: Weigh. Weigh one metre of roving and one metre of yarn. The ratio is the real draft. A figure higher than the gears say means fiber is being lost as fly or waste; a lower figure means the strand is not being drawn as far as the gears intend, from slipping at a nip or in the drive. Weigh both samples at the same moisture, since damp fiber weighs more.
Worked Example
Front rollers 25 mm in diameter at 32.3 rpm deliver π × 25 mm × 32.3 = 2,537 mm per minute, 2.54 m per minute. A back pair of the same diameter drafting 20:1 turns at 32.3 ÷ 20 = 1.6 rpm. A feed of 0.127 m per minute becomes 2.54 m. The rollers turn slowly; it is the spindle that turns fast (see Twisting).
Common Mistakes
- One big draft instead of two zones. The fibers snap or float, giving thick and thin places.
- Roving twisted too hard. The fibers will not slide past one another and the roving drafts unevenly. The overview gives 2-5 twists per inch for roving.
- Roller spacing wrong for the fiber. See Roller Spacing and Gauging.
Safety
Roller nips draw fingers in
A nip pulls a thin strand and a finger with the same grip. Never put a hand near the front pair of a running frame, never clear a lap with your fingers, and never lean over rollers with loose hair or sleeves. The frame is stopped, and its drive disconnected, before any fault is cleared (Drive System). Adult work only; the corpus keeps children away from running machinery (Task Assignment by Age).
What’s Next
- Learn next: Twisting — how the drafted strand is held together
- Combine with: Roller Weighting — the pressure that stops fibers slipping at the nip