The warp is held at a steady tension for the whole length of a cloth and is flexed and rubbed on every pick, so it needs yarn that is strong, evenly twisted and smooth. On the reference loom 2,000 ends at an assumed 0.4 N each pull with 800 N, and yarn with one weak place per 1,000 m would break 6 ends an hour, so the weak places, not the average strength, decide how well the loom runs.
Part of Power Loom Construction
Keeping the warp at an even tension from the warp beam to the cloth beam, and what that asks of the yarn the spinning frame made.
The Warp Is Where The Yarn Is Tested
On the loom the warp is a sheet of ends held between the warp beam at the back and the cloth beam at the front. It is let off the warp beam (Warp Beam) as cloth is taken up (Take-Up and Cloth Beam), and the overview wants a consistent tension so that the cloth is even. Too slack, and the shed does not clear, ends tangle and the shuttle rubs; too tight, and ends break. Weft is used once; warp is stretched at the shed on every pick (0.66% on the assumed geometry in Shedding), rubbed through a heddle eye and the reed (Beating Up), and kept under tension for the full length. That is why the spinning tree sets a separate specification for warp: 18-25 twists per inch at about Ne 20, against 12-18 for weft (Thread Quality Control).
The Numbers The Warp Must Meet
Take the reference loom of Weaving Mechanics: 2,000 ends of Ne 20 (29.5 tex).
- Load. An assumed tension of 0.4 N per end gives 2,000 × 0.4 = 800 N across the sheet, about 82 kg-force on the beams and frame.
- Strength. A recalled figure for cotton yarn is about 12 cN per tex (not from the corpus; measure yours as in Twist Testing), so a 29.5 tex end breaks near 12 × 29.5 = 354 cN = 3.5 N. The working tension is 0.4 ÷ 3.5 = 11% of that on the average end, and a weak place, say half as strong, sits at 23% before the shed stretch and the abrasion are counted.
- Breaks. The warp moves forward at the cloth speed, 3 m an hour, on each of 2,000 ends: 6,000 m of yarn passes the heddles every hour. If the yarn has one weak place that will fail per 1,000 m, the loom breaks 6,000 ÷ 1,000 = 6 ends an hour, one every 10 minutes. For one break an hour the yarn needs one such place per 6,000 m. Illustrative values; count ends down on your own loom.
A bench test pulling 20 pieces of half a metre checks 10 m of a 200,000 m warp (0.005%, Thread Quality Control), so the real measure is the loom’s ends-down rate. A soft, low-twist or fuzzy yarn fails on all three counts: it breaks at lower load, it abrades against reed and heddles, and it catches on neighbouring ends.
What Sizing Is For
Warp sizing, a thin paste applied to the warp before weaving (Warp Beam), is there to make up for a yarn that is hairy and soft: it glues the loose fibres to the strand and gives it a smooth skin that resists the reed. It does not repair a yarn with a weak core. A card-and-roving spun yarn needs it more than a combed one.
What You Need
- Warp yarn that meets the specification.
- A warp beam, cloth beam and tension control (the following pages).
- A spring balance for checking tension.
Method: Setting And Checking Warp Tension
Step 1: Set the let-off so the warp is held tight without stretching.
Step 2: Measure total tension. Press a straight bar across the whole sheet at mid-span until it deflects δ and read the force F on a spring balance. For a tight sheet of length L the tension is T = F × L ÷ (4 × δ) (small-deflection result for a taut string, recalled mechanics). With L = 0.7 m and δ = 10 mm, 800 N gives F = 4 × 800 × 0.01 ÷ 0.7 = 46 N, about 4.7 kg-force.
Step 3: Divide by the number of ends to get the tension per end: 800 ÷ 2,000 = 0.4 N.
Step 4: Record ends down per hour and adjust.
Common Mistakes
- Setting tension by feel only.
- Using the mean yarn strength to size the tension, not the weak places.
- Treating sizing as a substitute for twist.
Safety
800 N of stored tension, and a drive the weaver cannot stop from the loom
A snapped sheet of warp, a slipping brake or a released beam whips and recoils, and the beams and ratchets pinch. Slacken the let-off before working on a beam, never reach into a running loom to tie a broken end, and know where the stop and the water gate are (Power Drive System). Children keep away from running machinery (Task Assignment by Age).
What’s Next
- Learn next: Warp Beam — storing and feeding the warp
- Combine with: Take-Up and Cloth Beam — the other end of the same tension