Principle of Negative Let-off Mechanism

Last Updated on 03/04/2021

Experiment Name:
Study on Negative Let-off Mechanism.

During each cycle of weaving, the warp sheet has to be released from weaver’s beam in order that the woven fabric can be rolled up on a cloth roller without changing the location of the fell of the cloth. This operation is called let-off mechanism.

We can say it briefly, Let-off is to supply warp thread in the weaving zone at a predetermined rate. If let-off is not uniform, the fabric shall be uneven with variations in pick density and firmness. Negative let-off is a mechanism for controlling the rotation of the beam on a weaving, warp knitting or other fabric forming machine where the beam is pulled round by the warp against a breaking force applied to the beam.

Main parts of negative let-off motion:

  1. Warp beam.
  2. Beam ruffle.
  3. Chain.
  4. Machine frame.
  5. Weight lever.
  6. Pivot.
  7. Fulcrum.
  8. Weight.

Principle of Negative Let-off Mechanism:
In a negative system of warp let off (below figure), tension in the warp is regulated by friction between a chain or rope of let-off motion (6) and the beam ruffle (4). This friction depends on pull applied to the chain or rope by the weighted lever (7), the coefficient of friction between the chain or rope and the ruffle and the arc of contact between the chain or rope and ruffle. Because tension on the slack side of chain is Ts, and the frictional force F between the chain and rope opposing rotation of the beam must balance the tension on the tight side Tt, the static condition of equilibrium is given by,

F = Tt − Ts

principle of Negative Let-off Mechanism
Figure: Principle of negative let-off mechanism

In this system the tension of the warp is regulated by the friction between chain and the beam ruffle. The friction is controlled by dead weight on the weight lever and the distance of deadweight from the pivot. Heavier the dead weight and longer the distance of it from the pivot lesser the let-off.

The warp beam dia gradually decreases as weaving proceeds. So it’s necessary to increase the let-off rate. If the dead weight is kept on the same place, the let-off rate will remain unchanged. So an experienced worker is required to change the dead weight gradually with the change of the warp beam dia. As a result irregular tension occurs and the rate of yarn breakage may increase.

Negative let-off mechanism is a very simple and manual mechanism. It is suitable for light and medium weight fabrics. It is mainly used in old looms and for weaving of plain cotton fabric.

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