SHEDDING MECHANISM

Presented by:

S.Rajesh kumar

PSG TECH

E-mail: srk3rajesh@gmail.com

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

Spring-easing motion Under motions

Four Rollers Two Rollers

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

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.

How does dobby mechanism work

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.

POSITIVE

Negative dobby shedding

Positive dobby shedding

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