Study on Positive let-off motion.
The Let-off motion is an arrangement to let the warp from the weaver’s beam at uniform rate thus maintaining the appropriate warp tension throughout the weaving process.
We know, let-off is a device for controlling the delivery and tension of the warp during weaving. Let-off mechanism controls the rotation of the beam on a weaving, warp knitting or other fabric is forming machine where the beam is driven mechanically.
Positive let-off motion is defined as a motion in which the weaver’s beam is turned at a rate that maintains a constant length of warp sheet between cloth fell and weaver’s beam, the means of applying warp tension being separate from the beam-driving mechanism.
Types of let off motion:
- Positive let off motion.
- Negative let off motion.
- To know about the different parts formed in this mechanism.
- To learn the positive let off mechanism.
- To know the construction of the positive let off mechanism.
Main parts of positive let-off motion:
- Warp beam
- Floating back rest
- Driving rod
- Reciprocating collar
- Warm wheel
- Large beam wheel
- Adjusting rod
Positive let-off motion:
In the case of negative let-off mechanism the warp is pulled off the beam and the tension is regulated by slippage in a braking system. In a positive let-off motion, the beam is driven through a positive mechanism where no slippage takes place. This latter type of mechanism is seldom used and in most cases the tension is controlled by a mechanism driving the warp beam, which allows a certain loss of motion (or slippage) whenever the tension increases. Basically these are crude negative feed back automatic control systems which are related to controls such as are used in autoleveling during sliver production. These mechanisms are sometimes considered positive, but in reality they are semi-positive.
Above figure shows an example of a positive let-off motion. The warp beam is driven through a worm and wheel which are turned by a pawl and ratchet (R). The warp passes over a moveable back rest (B). If the warp tension is reduced, the back rest is moved up under the influence of the weighted lever (CD). The motion is transmitted through levers to a sheath which locks the movement of the ratchet wheel and disengages the pawl until the tension increases again to the normal level under the influence of the cloth take-up.
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If the tension increases beyond normal, the back rest is lowered, which causes the sheath to be moved away from the ratchet wheel; the let-off is restarted and overfeeds slightly, causing the tension to drop. The tension remains nearly constant between limits, and does not depend on the warp diameter on the beam.
In this positive let-off mechanism, constant tension can be maintained and any variation in tension can be detected. So it is used in modern power looms.
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