Woven and Knitted fabric formation
Description of the machines and processes
Name- Yeshanthi weeratunga
Part I : Primary and secondary motions of weaving
1)Explain briefly the function and importance of primary & secondary motions of weaving
The weaving process consists of two basic motions
Primary motions - shedding,
Secondary motions- left off
Separating the warp yarns into two layers by lifting and lowering the shafts, to form a tunnel known as the ‘shed’.
Passing the weft yarn (pick) across the warp threads through the shed.
Pushing the newly inserted weft yarn back into the fell using the reed.
Let off: The warp yarns are unwound from the warp beam during the above three processes.
Take up: The woven fabric is wound on the cloth beam during the above three processes.
The primary motions can further be divided as shedding, picking and beat up motions. The shedding opens the warp sheet into layers to facilitate passage of shuttle. The picking motion causes the shuttle carrying weft to be propelled from one end of loom to another. The beat up motion lays the previously laid weft to the fell of the cloth.
The secondary motions comprise of take up and let off motions. The take up motion helps to wind the cloth on to the cloth roller and also influences the pick density in the cloth. The let off motion helps to let the warp from the weaver’s beam at an uniform rate thus maintaining the warp tension constant throughout the weaving process. The auxiliary motions consist of the warp stop motion, weft stop motion and warp protector motion. The warp stop motion is used to stop the loom in the event of warp breakages. This is necessary to prevent fabric defects such as missing ends and floats. The weft stop motion is used to stop the loom in the event of weft exhaustion or weft breakages. This is necessary to prevent missing weft threads called cracks, in the fabric. The warp protector is used to prevent multiple warp thread breakages in the event of shuttle getting trapped in the middle of the warp sheet.
2) compare the different shedding mechanisms available most important 3) discuss the advantages & disadvantages in between shuttle weaving & shuttle less weaving techniques
The above operations must be synchronized to occur in the correct sequence and not interfere with one another. The full sequence is repeated for the insertion and interlacing of each weft yarn length with the warp yarns, and is therefore called ‘The Weaving Cycle’ Shedding Mechanisms:
All weaving machines control the warp yarns to create a shed. This can be accomplished with the following systems: • Crank shedding
• Cam shedding or tappet shedding
• Dobby shedding
• Jacquard shedding
Crank, cam and dobby mechanisms control the harnesses which lift the shafts. Jacquard machines control the individual warp yarns. Each system is outlined below: Crank Shedding:
Crank shedding mechanisms are simple and relatively cheap to use. However it can only be used for plain weave fabric constructions. In this system the harnesses are controlled by the crank shaft of the weaving machine. For each crank shaft revolution a wheel is rotated half a turn, which changes the harness position. This system is only used in air-jet and water-jet machines where high speed is achieved. Cam Shedding:
Cam shedding is also simple and inexpensive. A cam is a disk which has grooved or conjugated edges which corresponds to the lifting plan. The lifting plan controls which harnesses are lifted. The disadvantage of cam shedding is that when the woven design has to be changed the cams have to be rearranged to suit the new...
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