shedding
TRANSCRIPT
Shedding is one of the 5 loom motions
The others are weft insertion,beat-up,take-up,and let-off
Shedding separate the yarn sheet delivered from the
weavers beam into 2 levels to form a space(shed)to allow a
pick of weft yarn to be inserted.
The way the warp sheet is separated depends on the weave
design of the fabric.
WHAT DOES SHEDDING DO
Smallest possible: lowest possible additional warp tension
Clear enough: preventing faults
Proper shed dwelling: convenient for weft insertion
Smooth movement of the heald frames
(mechanism dependent):low noise, low parts damage, less
fabric faults, this also affect the loom speed
General requirements on shed
Choice of shedding mechanism determines the scope of fabric
that can be made
different types of shedding mechanism are available for looms:-
Tappet shedding mechanism
Dobby shedding mechanism
Jacquard shedding mechanism(most complex)
Types of shedding mechanisms
NEGATIVE Tappet is also known as cams.
BASIC PRINCIPLE: Cam followers follow the cam profiles.
In a tappet shedding mechanism, if the tappet controls only
one movement, either an upward or downward movement of
the heald shafts, then the shedding is known as ‘Negative
tappet shedding’
The heald shafts are returned by some external devices like
springs, dead weights and rollers…
How does tappet mechanism work
CONSTRUCTION:
A pair of tappets A and B are
fixed to the bottom shaft C at
180 degrees to each other.
Two treadle levers D and E are connected to the loom back-rail by a bracket F.
The bracket acts as a fulcrum for the levers. The two treadles have teeth to carry the lamb rods G and H respectively.
Two heald shafts J and K are connected to the lamb rods.
A top reversing roller shaft Q carries two rollers of different diameters.
Cont,. The roller of small diameter N
is connected to a leather strap
L to which the front heald
shaft J is connected.
The roller P of large diameter
is connected to a leather strap
M to which the back heald
shaft K is connected.
The tappets A and B touch the
anti-friction bowls or
followers R and S respectively,
which are fixed to the treadle
levers ̊.
The heald shafts have heald eyes T and U through which the warp
threads pass X is the war p sheet and Y is the cloth.
The odd ends are passed through one heald shaft while the even
ends are passed through the other heald shaft.
Timings and Setting:
1. Turn the crank to the top centre position.
2. Fix the anti-friction bowls to the treadle levers; they should move
freely in the slots.
3. Fix the treadle levers with a bracket to the back rail of the loom.
4. Set the grid and grid bracket to the front rail of the loom in the
slots of the grid.
5. Make sure that the tappet with the lower throw is fixed to the
bottom shaft at the starting handle side.
All negative shedding require the addition of parts known as over/under motion to reverse the direction of movement in healds.
The terms over and under refer to the position of these devise when in use.
Reversing motion may grouped as single acting reverse motions and compound reversing motions.
Negative tappet moves a shaft in one direction a reversing motion should act immediately a tappet eases and continue in action to the termination of the shafts journey where its maximum force should be excerted until the tappet is again brought into use.
REVERSING MOTION
THE GEOMETRY OF THE WARP SHED DEPTH OF SHED:
The depth of a shed is determined by the size of shuttle.
FELL OF THE CLOTH:
heald -1
heald-2
REED:
now, the distance between fell of cloth to reed=a
heald frame 1 to the fell=b1
heald frame 2 to the fell=b2
height of the reed=c
From the relation we get,
(b1/a)×c=x1
(b2/a)×c=x2
Here we find x2/x1
2.Next we find the dia of the reversing roller:
dia of the smaller roller subtracting the belt with we get=x cm
dia of the larger roller = y cm
Here we find y/x,
thus x2/x1≈y/x
POSITIVE SHEDDING MECHANISM
OBJECT:
This mechanism is to divide the warp sheet in to
two layers to form a warp shed for the passage of
shuttle.
PRINCIPLE:
In this type of shedding, the heald shaft is raised
and lowered by the tappet
The tappet shaft carries another tappet which has a groove or track in which a bowl is placed.
The bowl is connected in turn to a tappet lever with link rods, links J and a heald shaft
Each tappet is separately connected to a heald shaft through link rods and tappet lever.
There are two fulcrums for tappet lever and linksA-tappet shaft; B-tappet
C-track; D-bowl
E-tappet lever; F-fulcrum for E
G-link rods;H-fulgrum for G
J-heald shaft links; K-heald shaft
CONSTRUCTION
At 00, the reed reaches the forward most
position and performs the beat up. On
the other hand, at 1800, the reed moves
to the backward most position.
The sley (and reed) moves forward and
backward continuously during the entire
3600. However, the healds do not move
continuously.
When the shed is completely open, the
healds remain stationary for certain time
so that shuttle can pass through the shed
without any obstruction.
This is called the ‘dwell’ period. Two
types of timing of shedding (early and
late) are generally used
• The shuttle enters and leaves the shed at around 1100 and 2400 respectively
• The timing of shuttle flight almost coincides with the dwell time. After 2400, the
healds start to move in opposite direction and the shed is again levelled at 0 0/3600.
Bottom - closed shed
TYPES OF SHED
In case of bottom closed shed, all the ends come to their lowest position after every pick to close the shed. This movement is unnecessary and thus there is lot of wasted movement.
MERITS:
The alternate tightening and slackening of threads produce a cloth
with good cover.
DEMERITS:
It takes a long time to produce a shed
It is unsuitable for high loom speeds
strain on the warp is high
wear and tear of the loom is high
power consumption of the loom is high
USE:
single-lift dobbies and single lift jacquards produce bottom closed
sheds.
Semi – open shed
In case of semi open shed, if the end has to be in up potion in two consecutive picks, then
it comes down up to the middle (warp line) and then again goes up. The wasted movement
is lower as compared to that of bottom closed shed.
MERITS:
It takes a minimum time to produce a shed
the loom can run at a high speed.
strain on the warp is low
wear and tear of the loom is low
power consumption of the loom is low
DEMERITS:
This shedding is troublesome to weavers because the two fixed lines make it more difficult to repair broke ends.
As the shed is always open, breakage may result especially the yarn is week.
when many of heald shafts are used, the strain on the warp yarns at the back heald shaft is increased and hence breakage may occur.
USE:
Many double lift dobbies and double lift jacquards form semi-open sheds.
Centre – Closed shed
In this case, the shed closes at the centre (warp line) after every pick. Therefore,
if the end has to be in up ( or down) position in two consecutive picks, it will
come to the middle position of shed depth between the two picks. The amount of
wasted movement is also very high.
MERITS:
It takes a minimum time to produce a shed
the loom can run at a high speed.
wear and tear of the loom is low
power consumption of the loom is low
DEMERITS:
Since every thread is moved to form shed, strain on the warp is
more than that open shed.
An unsteady movement is caused by the warp threads being in
constant motion.
USE:
centre shed dobbies, centre shed jacquards and hand loom form
centre closed shed. This formed in a plain loom while weaving plain
cloth.
Open shed
This is the ideal kind of shed and it minimizes the wasted movements of the ends (or
healds). If the end has to be in up position in two consecutive picks, then it remains
stationary in the up position between two picks. Similarly, if the end has to be in down
position in two consecutive picks, then it remains stationary in the down position
between two picks.
MERITS:
It takes a minimum time to produce a shed
the loom can run at a high speed.
wear and tear of the loom is low
power consumption of the loom is low
Strain upon the warp is low
Rising threads help to move the lowering threads
DEMERITS:
This shedding is troublesome to weavers because the two fixed lines
make it more difficult to repair broke ends.
As the shed is always open, breakage may result especially the
yarn is week.
when many of heald shafts are used, the strain on the warp yarns
at the back heald shaft is increased and hence breakage may occur.
USE:
plain loom for producing twill and satin weaves
Dobby shedding
Negative dobby shedding:
In this type of shed, lowering of the heald frame occurred
mechanically.
In the negative shedding the heald frames are operated by the jack
& lever.
The levers are connected with the knife and the knifes are attached
with the driving rod by means of connecting needle.
A pattern is used here according to the weave plan.
When the teeth of chain is come to the contact of chain drum then
whole the arrangement moves together and lowering of the heald
frame is occurred by the spring.
The lowering of the heald frame is happens here by spring or jack
lever. When the pattern cylinder doesn’t find peg on the pattern
drum then bauck lever and jack lever bring the heald frame in
downward direction, therefore the lowering of the heald frame is
occurred by means of spring tension.
Positive dobby shedding:
In this type of shed lifting & lowering of the heald frame both is possible.
lifting is occurred by means of jack & lever, and lowering is occurred by
means of spring under tension.
Positive dobby shedding is the combination of three Cylinders, jack lever,
spring and a shaft.
Between the three cylinders a shaft is fulcrum in one side the upper
cylinder moves as the anti clockwise and lower cylinder moves as the clock
wise direction.
The main cylinder when get motion from the shaft when it found peg or
pattern plan then the cylinder attached with the upper half toothed disc.
So that the heald frame is up,& when the pattern cylinder doesn’t found peg
then the main cylinder attached with the lower half toothed disc and the
spring retains the heald frame to the downward direction.
DIFFIRENT MODES OF ACTION OF A DOBBY
The connection and disconnection of the hooks is caused by needles/feelers which contact the page in the pattern chain.
The knives being shown in each of the extreme positions, when both hooks are engaged, the baulk seesaws about point F and the harnesses are maintained in the up position.
When both hooks are disconnected, the baulk is held against both stops and the harnesses are maintained in the down position.
When one hook is disengaged, the harness will move up and down in sympathy with the other knife.
It is the practice to lift the rearward knife because at that time the appropriate portion of the baulk is held by the stop and the hook can be raised without damages.
A hook can be dropped at any time, but there may be a time lag before it latches into proper sequence.
By altering the distance X,it is possible to operate with semi-open sheds.
Therefore it is possible to obtain any desired shedding diagram by having 2 sets of pegs,hooks,and knives per harness.
This is known as double lift dobby.
Coarse pitch(English pitch)
200,400,600,800 needles
Fine pitch(french pitch)
Vincenzi:800,1320 needles
Verdol:896,1344,2688 needles
Types of jacquard
JACQUARD SHEDDING
• Pattern cards are presented to four-sided "cylinder" in
such a way that every card fits one side.
• Every card on the chain represents one pick in the weave
• Thus the cylinder speed of rotation is one-quarter that of
the loom crankshaft.
• The cylinder moves away from the needles, rotates one-
quarter of a revolution to present a new card and then
moves again towards the needles.
• A hole on the card indicates that the corresponding hook
has to be lifted.
• The movement of the needle through the hole, under
spring pressure, causes the hook to be ready to engage
with the knife.
• The knife therefore lifts the hook during its upward
movement, which in turn lifts the cord attached to the
hook.
• The ends passing through the needles attached to the
hook will then be lifted.
• When no hole faces the needle, it will be pushed by the
card against the spring and the hook will be kept away
from the knife.
HARNESS MANAGEMENT
• It is the system by which the ends are controlled during jacquard shedding with the help of nylon cords, heddles (heald eyes) and dead weights (lingoes).
• In the preceding part of discussion, it has been considered that the capacity of jacquard is 300 ends.
• Now, if the fabric has 3000 ends in its entire width then ten repeats of the design can be produced on the fabric.
Features of jacquard shedding
Capable of weaving all types of fabrics, from the
simplest to the most complex fabrics
Mostly used for weaving complex fabrics
Not economical for weaving simple fabrics
Posing speed limit on loom
Complex mechanism