Shed
- Size of shed is very critical. It should allow a secured weft insertion.
- The requirements of shed opening are determined by - # The means of weft insertion. # Beat up motion.
- It is desirable to have a small shed opening in order to reduce the
left of heald frames and therefore to reduce the stress on the warp.
However, the magnitude of shed opening is mostly determined by the size of weft insertion device.
C = The depth of the shed at the front wall of the shuttle
A = The width of the shuttle
B = The distance from the cloth fell to the reed
D = The depth of the shed at the reed
During
the passage of the shuttle, B and D both vary because of the motion of
reed and D will also vary owing to the movement of the heald shafts
unless the passage of the shuttle coinsides with the period of dwell.
Types of shed:
Shed is broadly into two classes namely. They are…
- Open shed
- Closed shed
Closed shed: In the closed shed, the mechanism employed places all the warp threads level after the insertion of each pick of weft.
Open shed: In open shed, the warp is only moved when the pattern requires a change of position.
Closed sheds are classified into two types:
- Bottom closed shed
- Centre closed 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. This kind of shed is known as a stationary bottom
with a rising and falling top.
In
the figure A represents the bottom line of warp, B is the rising and
falling line and C an arrow showing the space passed through
Demerits:
- If takes a long time to produce a shed.
- Not suitable for high speed loom.
- Strain on warp is high.
Uses: Certain tappets, Single lift dobbies and jacquards produce bottom closed shed.
Center closed shed: Center
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 between the highest and lowest lines
of a divided warp.
In
the figure A is the closed warp line, B and C are respectively the
upper and lower lines of a divided warp and D and E are arrows which
show the movement of each thread to equal the depth of a shed, namely
half the distance in an upward and half in a downward direction.
Merits: Power consumption and wear and tear of loom parts is low.
Uses: Many woodcraft tappers and some hands looms use centre closed shed.
Semi open shed: This
is formed under both open and closed principles. Here a stationary
bottom line is refined, but threads from the top line either pass to the
bottom at one movement or carried out to the top. The remaining threads
move down.
In
the figure, A is the stationary bottom, B the top line and C the point
where downward movement ceases in threads that are to form part of the
top line for the succeeding pick. D and E are arrows showing the
movement of through healds and F that of threads which are to lift for
the next pick.
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.
Uses: Open shed is produced by the used of ordinary tappets.
