Timing of Shedding
Timing of Shedding
The shedding mechanism is set according to the picking and beat-up mechanisms. Timing of shedding is set with respect to crank position.
In general there are three timings. These are :
1. Normal shedding
2. Early shedding
3. Late shedding
1. Normal shedding
If the heald shafts are at the same level when the crank comes to the top center, that is, 0 degree or 360 degrees, then it is known as normal shedding. See Figure 1
2. Early shedding
A shed is said to be early when the pick insertion is effected after the shed is completely open. If the heald shafts are at the same level when the crank comes to 355 degrees or 5 degrees before the top centre, then it is known as early shedding.
3. Late shedding
If the heald shafts are at the same level when the crank comes to 5 degrees after top centre, then the shedding is known as late shedding. See Figure 1.
Advantages of early shedding
1. Fabric cover is improved, because during beat-up, the warp yarns are evenly distributed.
2. It assists in clearing the shed while weaving fibrous war p like woollen and worsted. So entanglements are avoided.
3. Heavy weft yarn can be easily inserted. So hard weaves like corkscrew weaves can be woven.
Disadvantages
1. As the warp yarns are open widely during beat up, strain on the warp yarn is high.
2. Early shedding causes chaffing of war p yarn (yar n abrasion) and end breakages may occur.
Advantages of late shedding
Late shedding allows the weft to get beaten up before the shed has been properly crossed. During beat-up, opening of the shed is minimal. So strain on the war p yarn is low.
1. Late shedding can be used for weak warp yarn.
2. It allows greater time for shuttle passage so it is suitable for synthetic yarn.
3. It is useful if pick of low strength is used.
Disadvantages
1. It causes a very weak fell of the cloth because the weft may roll back.
2. It causes less distribution of yarn. So it is not suitable for fibrous warp.
3. As the warp yarns are not distributed evenly during beat-up, fabric cover is low.
Heald staggering and Asymmetric Shedding
Heald staggering
In the formation of a shed, if all the war p yarns of one shed line go across those of the other shed line at the same time there will be much strain on the yarns. So there may be many war p yarn breakages. To avoid warp strain and end breakages, the movements of the heald shafts are staggered. By this arrangement, the warp yarns of one shed line will not cross the yarns of the other shed at the same time. For example, if four heald shafts are used and four tappets are used to control the heald shafts separately, the timings of the tappets are adjusted in such a way that the crossing of shed lines is altered. So end breakages can be reduced. Refer to Figure 2
A - Heald Shafts |
B - Reed |
C - Lease rods Figure 2: Heald staggering During the weaving of heavy fabric like denim, canvas, satin duck, etc. the force required to push the weft is high. So there may be bumping of fabric. To avoid this weaving resistance, asymmetric shedding is used. In this shedding, the time and duration of movement of the top and bottom shed lines is so adjusted that they cross below the mid-lift position. At this point of crossing, the yarns at the bottom shed line are under higher tension than those at the top shed line. As the reed beats up the weft, the slack top shed line yields and permits the weft to be pushed to the fell of the cloth with relative ease. So it is easy to weave even heavy fabric. |
2 comments:
keep it up
its too interesting..if possible please give an explanation about shed angle
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