Types of shedding
Shedding
Shedding is the raising of the warp yarns to form a shed through which the filling yarn, carried by the shuttle, can be inserted. The shed is the vertical space between the raised and upraised warp yarns. On the modern loom, simple and intricate shedding operations are performed automatically by the heddle or heald frame, also known as a harness. This is a rectangular frame to which a series of wires, called heddles or healds, are attached. The yarns are passed through the eye holes of the heddles, which hang vertically from the harnesses. The weave pattern determines which harness controls which warp yarns, and the number of harnesses used depends on the complexity of the weave. Two common methods of controlling the heddles are dobbies and a Jacquard Head.
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| Fig: shedding |
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Fig: Shedding in a air jet loom |
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Fig: Shedding in a rapier loom |
Types of Shed:
Shed is classified according to the position of the ends:1. Closed Shed
2. Open Shed.
1. Closed Shed is classified on two parts. A) Bottom Closed Shed, B) Centre Closed Shed.
2. Open Shed are classified on two parts. A) Semi-open Shed, b) Open Shed.
Bottom Closed Shed:
This kind of shed is produced by giving motion only to threads that are to form the upper line. Under this condition the warp is level on the bottom line. Hence in order to form a top shed, it is necessary to move some threads through a space equal to twice the depth of a shed, once up and once down, before a fresh selection is made. A shade of this kind is known as a stationary bottom with a rising and falling top.
Merits:
The altered lightening and slackering of threads produce a cloth with good cover.
Demerits:
- If takes a long time to produce a shed.
- Not suitable for high speed loom.
- Strain on warp is high.
Uses:
Single lift dobby’s and jacquard s produce bottom closed shed.
Centre Closed Loom Shed:
Centre closed shed is produced by imparting an upward movement to those threads which are to form the top line and a downward movement to the threads which are to form the bottom line, then after inserting a pick, both the lines meet at the centre. Here warp treads move in an upward and downward direction from a center line. The threads which to from top line makes upward and the other threads which are to form bottom line move downward. After inserting a pick both the lines meet at the centre line.
Merits:
Power consumption and wear and tear of loom parts is low.
Demerits:
Since every thread is moved to form each shed, strain on yarn is more.
Uses:
Some hands looms uses centre closed shed.
Semi open shed:
This is formed under both open and closed principles. Here a stationary bottom line is refined.
The top line is moveable one after inserting a pick, the top lines moves threads the bottom line. During moving down some threads which form once again at the top line are arrested midway and then carried out to the top. The remaining threads move down.
Merits:
1. Strain upon war is low.
2. Required minimum time for shed.
3. Loom can run at high speed
Demerits:
1. Strain on yarn is not equally distributed.
2. Difficult to find breakage yarn.
Uses:
Many double lift dobby and jacquard used this shed.
Open shed:
In this types of shed, the warp threads form two stationary lines, one at the top and another at the bottom .After inserting a pick change are made by carrying threads from one fixed line to the other. So some threads are lowered from the top line and some are raised from the bottom line during the change raising and lowering of the threads occurred simultaneously.
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