FABRIC MANUFACTURING ll - PowerPoint Presentation

Slide1

FABRIC MANUFACTURING ll

DOBBY SHEDDING

JACQUARD SHEDDING

SIZING

MULTIPLE BOX MOTION

DOBBY SHEDDING

What

is dobby

?

A dobby is a shedding device attached to a loom for producing a variety of small motif.

Design Possibilities of a Dobby System

Twill

Satin

Crepe

Honey Comb

Huck-a-Back

Mock leno

Bedford Cord

Double Cloth

Difference between dobby and tappet shedding

Dobby shedding is complicated

than tappet shedding.

A dobby can produce small motifs using many more heald shafts up to maximum of 40.

A dobby loom consumes more power.

A dobby loom cannot be worked at high speed because of high speed machine vibration and excessive friction between moving parts.

Tappet shedding is simple and cheap in construction.

Tappets produce simple and basic designs using a maximum of only eight heald shafts.

A tappet loom works efficiently and consumes less power.

A tappet loom can be run at high speed with less machine vibration.

Classification of dobby

lever

and cam

,

right hand and left hand

,

single lift

and double lift

,

positive and negative.

NEGATIVE AND POSITIVE DOBBY 

A

NEGATIVE DOBBY

activates

the

heald shafts in

one direction only , that means the upward or downward movement of the heald shafts. The

reverse direction is done by

using some additional device like

dead wt

.,

spring

elastic

bands etc.

A negative dobby is used for producing light or medium weight fabrics like cotton, silk or synthetic.

A

POSITIVE DOBBY

activates

the

heald shafts

in both the directions , upward and downward . It is used for producing heavy fabrics like woollen and worsted .

LEFT HANDED AND RIGHT HANDED DOBBY 

A

RIGHT HANDED DOBBY

is

the one which is placed at the left side of a right hand loom. It gets its drive from the left hand end of the loom.

A

LEFT HANDED DOBBY

is

the one which is placed at the Right Hand end of a Left Hand

Loom . It

gets its drive from right hand end of the loom.

SINGLE LIFT AND DOUBLE LIFT DOBBBY

In a

Single Lift Dobby

the selection of hooks takes place once for

every pick

. Each

heald shaft

has its own individual hooks that can be used for raising the

heald shafts

.

The single lift dobby is generally used in the weaving of silk and rayon fabrics and also certain gauge fabrics. In this dobby the weft is beaten up in a closed shed which is essential for silk weaving because it ensures minimum friction in the warp.

Characteristics of single lift dobby are

–

It forms bottom closed shed.

The weft is beaten up in closed shed.

Each heald shaft is controlled by its individual hooks in every picks.

Drive from the crank shaft of the loom activates the dobby.

Characteristics of double lift dobby are-

A double lift dobby forms an open shed.

The weft is beaten up in crossed shed.

Each heald frames are controlled independently by pairs of hooks, one at the top and other at the bottom.

Drive from the bottom shafts of the loom activates the dobby.

Comparison of Single Lift Dobby and Double Lift Dobby

SINGLE LIFT

The dobby operates once in every pick.

The Dobby is connected to the crank shaft.

A single lift dobby gives a stationary bottom closed shed.

The Single lift dobby is very slow and causes more wear and tear and vibration of the machine parts.

Single lift dobby is used for producing light and medium weight fabrics like Silk, Rayon, etc. and also for special fabric like gauge.

DOUBLE LIFT

A double lift dobby operates once for every two picks

.

The Dobby is connected to the bottom shaft.

A double lift dobby gives an open shed

Double lift dobby system operates faster with less wear and tear and less vibration of the parts.

Double lift dobby is used for weaving cotton fabrics.

Advantages of double lift dobby over single lift dobby.

In a double lift dobby all the moving parts are involved in a series of movements during which two successive picks are inserted. Hence the time available for selecting any given hooks is twice that available in single lift system.

A shed is produced in less time than in single lift dobby and with the least amount of strain on the warp

.

There is less wear and tear of working parts in double lift dobby than single lift.

The rising and falling healds work in better equilibrium in a double lift dobby system.

CLIMAX DOBBY 

Climax dobby can be classified as a horizontal, negative, double-lift, double jack, lever dobby. It is placed at one end of the loom at a suitable height for the conventional handling by the weaver.

Timing and setting of climax dobby

0

o

-360

o

– Healds are level, the draw knives on the middle position, t lever will in horizontal position.

150

o

-210

o

- Heald shafts are stationary, shed is open, one arm of the t-lever is outside and other arm is inside extremely.

180

o

- The shed is open, the bottom shaft, crank or L-lever is in top or in bottom centre.

210

o

-150

o

- It is the change period of heald shaft.

DEFECTS OF DOBBY

A

defect related to all the dobby system and a common defect is

Jack Missing

.

It is the failure of a

heald shaft

to lift as

and

when required.

When

Jack Missing

occurs

the weft may be seen as a float yarn running across the warp and gives rise to a fabric defect called stitching.

Advantages of modern cam dobby

The elements of a cam dobby constitute a precise assembly of moving parts that work smoothly and accurately. They work with much less vibration than those in a climax dobby.

The dobby is placed at one side of the loom and hence allows maximum light on the fabric. The problem of oil drops falling on the cloth, found commonly in the climax dobby is not encountered in a modern cam dobby.

In modern cam dobby systems the drive is derived from the crank shaft by mean of a suitable reduction gear arrangement like a chain sprocket or a bevel and bevel wheel combination, an arrangement that makes for smooth running.