The warp beam stores the warp and pays it out as cloth is woven, and because tension equals brake torque divided by the beam’s radius, a fixed brake lets the tension rise as the beam empties: for a 100 m warp that fills a 100 mm core out to about 149 mm, tension rises by 49% by the end unless the brake load is cut in proportion to the radius. The warp is also sized before weaving to hold down the fuzz of soft yarn, using a starch paste that spoils within 2-3 days if left damp.
Part of Power Loom Construction
Storing, sizing and feeding the warp at constant tension from the back of the loom.
The Beam Changes Radius As It Pays Out
The overview: all the warp ends, potentially thousands, are pre-wound onto a large wooden cylinder, the warp beam, at the back of the loom, and warping is a separate careful operation. The brake that holds the beam back sets the warp tension T through the torque τ on the beam: T_total = τ ÷ R, where R is the radius of the warp layer on the beam (a standard torque relation).
How big does the beam get? On the reference loom, 2,000 ends of 100 m with an assumed yarn diameter of 0.2 mm (Ne 20; recalled), an assumed packing of 0.6 and a beam width of 1.1 m:
- yarn volume = 2,000 × 100,000 mm × (π/4 × 0.2²) = 6.28 × 10⁶ mm³
- beam volume = 6.28 × 10⁶ ÷ 0.6 = 1.05 × 10⁷ mm³; per 1,100 mm of width, an annulus of 9,500 mm²
- with a 100 mm core (R = 50 mm): π(R² − 50²) = 9,500, so R = 74.3 mm, a diameter of 149 mm
So R falls from 74.3 mm to 50 mm as the warp is used, and with a fixed brake torque the tension rises by 74.3 ÷ 50 = 1.49, a 49% increase. The cause is only geometry.
The overview’s wording on the weighted lever is the wrong way round and this article supersedes it: it says an “increasing torque arm” compensates as the beam gets smaller, but the torque must fall in proportion to the radius to hold tension constant, so the effective arm must shorten (or the weight be reduced) as the beam empties.
A Schedule For A Weighted Brake
To hold T constant the brake load must follow the radius. The radius when a fraction f of the warp remains is R = √(50² + f × (74.3² − 50²)):
| Warp remaining | R (mm) | Brake load as % of full |
|---|---|---|
| 100% | 74.3 | 100 |
| 50% | 63.3 | 85 |
| 25% | 57.1 | 77 |
| 0% | 50.0 | 67 |
So with a hung weight on a friction rope, take off weights in steps: roughly 15% off at half used, 23% at three quarters, 33% at the end. A positive let-off that measures tension and corrects it is better but needs more mechanism (Warp Tensioning and Let-Off).
Sizing Compensates For Soft, Hairy Yarn
A starch paste painted on the warp, or applied as the beam is wound, glues loose surface fibres to the strand and gives it a smooth skin that the reed and heddle can slide over, which is just what a soft, carded, low-twist warp lacks (Mechanical Carding, Twisting). It does not strengthen a weak core. The recipe is the flour paste of Flour Paste: full gelatinization at 80-95 C, never boiled hard. Cooking Paste says starch paste spoils within 2 to 3 days at room temperature and about a week with 5% vinegar, so a sized warp must be dried and used promptly, or stored dry; a damp sized warp moulds. The add-on is unsourced: weigh a skein before and after drying to measure it, and adjust against ends down per hour.
What You Need
- A beam of hardwood, true and balanced, with flanges, a core of the chosen diameter and journals (Woodworking).
- A friction brake with weights, or a ratchet and pawl (Floor Loom Construction describes both).
- A warping frame to wind the ends in order with the cross kept (Weaving).
- Size paste and a way to apply and dry it.
Method: Beaming And Setting The Brake
Step 1: Warp the ends in order on a frame, keep the cross, and tie it before removing.
Step 2: Size and dry if the yarn needs it.
Step 3: Beam the warp at even tension onto the beam, ends parallel, layers even.
Step 4: Mount the beam, thread heddles and reed.
Step 5: Set the brake for the full beam and mark the weight schedule above.
Step 6: Check tension at each marked stage (Warp Tensioning and Let-Off).
Worked Example
A brake load that gives 0.4 N per end on the full beam gives 0.4 × 1.49 = 0.60 N per end on the empty beam. At the 11% working fraction of the average end’s 3.5 N, that is 17%, and the weakest places are tested hardest at the end of the warp. Cutting the brake load by a third by then restores 0.4 N.
Common Mistakes
- Setting the brake once and weaving 100 m.
- A beam not true, so the tension surges once per turn.
- Storing a sized warp damp.
- Uneven beaming, which leaves some ends tighter than others.
Safety
A loaded beam stores energy
A beam with 800 N of warp pulling on it spins if the brake slips, and a snapped sheet whips. Lock the beam before servicing, release the brake in stages, and lift a full beam with a team (it carries 200,000 m of yarn). Stop the loom at the belt and know who reaches the water gate (Power Drive System). Children keep away from loaded beams and running looms (Task Assignment by Age).
What’s Next
- Learn next: Take-Up and Cloth Beam — receiving the finished cloth
- Combine with: Thread Quality Control — the yarn the beam carries