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The Influence and Adjustment of the Warp Let-Off Mechanism on Looms

The primary function of the warp let-off mechanism is to uniformly deliver warp yarn with consistent tension based on its consumption during weaving. To ensure continuous production, the mechanism must precisely control the length of unwound warp yarn to match the fabric’s take-up rate. In water jet looms, the let-off system dynamically adjusts warp feed according to weft density, maintaining stable tension for smooth operation.

(1) Working Principle of the Warp Let-Off Mechanism
Most domestic water jet looms employ continuous automatic tension-regulated let-off systems, comprising:
A movable back beam for tension detection and adjustment.
A stepless speed variator for controlled warp delivery.
Key Parameters & Effects:
Warp beam capacity and variator speed range determine weft density adaptability.
Adjustment stability: Achieved via change gears; improper settings cause uneven feed (“cloud weaving”) or length discrepancies.
Tension sensitivity: The movable back beam ensures high responsiveness.
Initial tension: Set by weighted levers; adjustable via counterweight mass/position.
Tension consistency: Properly adjusted systems maintain 3% tension variance from full to empty beam.
Critical Impacts of Poor Adjustment:
Irregular tension leads to defects (e.g., broken warps, fuzziness, shedding issues).
Accelerated mechanical wear and reduced first-quality yield.
Dynamic Compensation:
During beating-up/shedding, the swinging arm and damping springs absorb tension fluctuations, while a hydraulic damper suppresses vibrations, minimizing warp tension variations.

(2) Let-Off Mechanism Configuration
Warp Path Selection:
L-shaped path: For lightweight/fray-prone fabrics .
S-shaped path: For medium/heavy or textured yarns.
Change Gears: Selected based on weft density to stabilize feed rates. 
Transmission Arm Position: Must remain horizontal when the weighted lever is level for optimal sensitivity.
Variator Speed Axis: At the weighted lever’s lowest position, the axis should be at maximum speed ratio (fully clockwise). 
Damper Maintenance: Regularly inspect oil levels, corrosion, and functionality to protect the variator.

(3) Adjustment Procedures
While designs vary across loom models, core principles remain consistent. Key steps:

Tension Calibration:
Verify counterweight mass/hanging position combinations to achieve target tension.
Monitor let-off spring performance (fatigue or material defects affect tension stability).
Operational Optimization:
Adjustments must account for:
Shedding height
Warp beam diameter
Yarn path/thread thickness
Weft density
Critical Checks:
Spring adjustment: Compensates for tension spikes during shedding/beating. Poor tuning causes stop marks, start-up bars, or mechanical streaks.
Swing amplitude: Back beam should oscillate 5–10 mm synchronously with the loom.
Tension roller: Must rotate at warp speed (sync belts may be added).
Vibration/noise: Ensure weighted levers operate silently without abnormal impacts.
Outcome: Minimized start/stop defects and consistent fabric quality.

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