MOTION IN WEAVING:-

Earlier I have explained about the first secondary motion. Now here is the second Secondry motion is as under;

B)  LET-OFF MOTION:-

The main objective of let-off motion is to release the warp after beating every pick and to regulate the warp tension. The motion which delivers the the warp yarns from the warp beam (weaver:s beam) at required rate that a suitable constant tension by unwinding it from a flanged roller called let-off motion. When the fabric weavning remains continue, the take-up motion pulls the woven fabric continuously to maintain the fell of the cloth position.

Function of let-off motion:-

1) To maintain and control the necessary tension upon warp.

2) To unwind and deliver the necessary amount of warp sheet from the warp beam with the simultaneous and continuously to the take-up motion.

3) To impact the number of pick per unit space in the fabric being subsidiary to take-up motion.

Condition of good let-off motion:-

A good let-off motion must posses certain features which are given below;

(1) Uniform and equal tension upon the warp.

(2) Provision to regulate decreasing diameter of warp on the beam during the weaving.

(3) Arrangement for taking the warp back .

(4) It should also be sensitive, any little variation in the diameter of yarn and tension upon the warp.

Types of Let-off Motion:-

There are three types of let- motion, they are

1- Positive let-off motion,

2- Semi-positive let-off motion,

3- Negative let-off motion.

Main parts of let-off motion (mechanism):-

1. Warp beam.

2. Floating back rest.

3. Feeler.

4. Spring.

5. Worm.

6. Ratchet.

7. Driving rod.

8. Collar.

9. Reciprocating collar.

10. Worm wheel.

11. Large bream wheel.

Positive let-off motion (mechanism):-

The beam turning mechanism (let-off) is shown in the figure. The beam is driven by ratchet on a short vertical shaft which also carries , the the worm , and drives the worm wheel A pinion is on the same shaft as the worm wheel drives the large beam wheel, which is fixed to one of the beam flange.

Diagram of positive let-off mtion.

A pawl operater turns the ratchet wheel by the driving rod that get motion of the sley sword. Each time the sley moved forward the oscillating collar is connected to fix collar and there is engagements of pawl with ratchet. As the tension in warp sheet is increased, the floating rest will moved downwards and the rods carrying the fix collar will move to right and the rod will move the driving rod to the left. This will bring the fixed collar to the oscillating collar. As a result, the force of imparted oscillating collar and fixed collar is more. The pawl drives more ratchet wheel teeth. So the beam motion is more and more warp is withdrawn to the increased tension.

SEMI POSITIVE LET-OFF MOTION:-

The delivery of warp is controlled in such way that and adjustment can be made to maintain tension or path length by slippage. Some jobs are done by manually and some mechanically.

CONSTRUCTION:-

In the semi-positive let-off motion, a pushing pawl present in ratchet wheel. Ratchet wheel is connected by a shaft in worm C with the worm wheel B and worm wheel rotates clock wise direction. Weight fulcrumed at H and floating back rest K is present downward and the other end of weight lever to pushing pawl.

(schematic diagram of Semi-positive let-off motion.)

MAIN PARTS:-

A. Warp beam 

B. Worm wheel.

C. Worm.

D. Shaft.

E. Ratchet wheel.

F. Pawl.

G. Weight lever.

H. Fulcrum.

I. Floating roller.

Working principle:-

The warp beam is driven through a worm and worm wheel are turn by a pawl and ratchet. When warp tension more, the floating back rest is lowered and this causes downward movement of weight lever at the back side and upward movement of connecting rod. The out end of pawl move upward and the other end comes to the contact of ratchet teeth and worm and worm wheel. A worm and worm wheel are on the same shaft. So the warp beam rotates highly and causes let-off motion and uniform tension. When warp tension is less, the floating back rest moves upward and this causes downward movement of the out end of pawl be disengaged with ratchet teeth and no let-off motion.

Ref:-https://textilelearner.net