Friday, December 6, 2019

DOBBY MACHINE ( A SHEDDING MOTION MECHANISM), DIFFERENT TYPES OF DOBBIES,


DOBBY MOTION: ( A SHEDDING MOTION MECHANISM) :

Dobby is a machine that performs shedding motion in the loom. If the weave pattern has more number of ends and picks in the weave repeat, then it is not possible to weave on the tappet looms. Dobby shedding motion is used to weave these kinds of weaving patterns. Every heald shaft can be raised and lowered in different styles in dobby shedding. Normally 20 heald shafts are used maximum in dobby but in some dobbies, 28 to 48 jack levers are used maximum. Small motifs and geometrical weave patterns can be woven on dobby looms. Those weave patterns, which are not possible to weave on tappet can be woven on dobby but all weave patterns woven on dobby cannot be woven on tappet looms.

CLASSIFICATION OF DOBBY:

Dobby can be classified in following categories according to different bases:

·       * According to shed closing.

·      * According to reversing motion used.

·      * According to selection mechanism used.

·    *According to lifting action in respect of loom’s revolution.

According to shed closing:

According to shed closing position, a dobby can be classified in the following categories:

BOTTOM CLOSED DOBBY:

In this kind of dobby, all the heald shafts rest on the bottom position. Only those heald shafts are raised which are needed in the weave pattern at a particular pick position. After insertion of pick, all the raised heald shafts come again at their bottom position. Thus shed closing (shed levelling) takes place at the bottom position of the shed layer.

CENTRE CLOSED DOBBY:

In this kind of dobby, the shed closing (levelling) takes place in the middle of the shed. When some heald shafts start to raise and some heald shafts start to lower at a particular pick position in a weave pattern, they cross to one another at the centre position of the shed.

According to reversing motion used:

According to reversing motion mechanism used in the dobby, it can be classified into the following categories:

Negative dobby:

The dobby which is not able to bring down the heald shaft itself is called a negative dobby. This dobby lifts the heald shaft only. The extra attachment is used to bring down the heald shaft. This attachment is called reversing motion. A deadweight or pulling spring is used to bring down the shaft. In this dobby, all the raised heald shafts are levelled after the insertion of each pick.

Positive dobby:

In this kind of dobby, only those heald shafts have come at shed levelling position which changes their position at the particular pick. Heald shafts are raised and lowered by their internal mechanism, there is no need of reversing motion attachment to bring down the heald shaft. In this type of dobby, the heald shaft remains in rest position at top or bottom position until the weave pattern does not demand to change the position of the heald shaft.
Positive dobby further can be classified into two categories:

Semi-positive dobby:  

In this type of dobby, those heald shafts which need to become in rest position at top as per weave pattern come slightly down and lift again. This kind of dobby is known as semi-positive dobby.

Fully positive dobby:  

In this kind of dobby, the heald shafts remain at their complete rest position until the weave pattern does not require changing the position of the heald shaft.

According to selection mechanism used

According to the selection mechanism used in the dobby, a dobby can be classified as follows:

Mechanical dobby: 

In his kind of dobby, the selection of raising and lowering is performed mechanically. It can be further classified as follows:

Peg and lattice-type selection mechanism:

In this kind of dobby, wooden pegs are used to select the lifting plan of dobby. The wooden pegs are fitted on the lattice according to the weave pattern.

Selection through punched plastic paper:

A plastic paper is used to select a lifting pattern in this kind of dobby. This paper is punched according to the lifting plan. A hole in the card denotes that the particular heald shaft will raise at the predefined pick. A blank portion means the heald shaft will be lowered.

Electronic dobby:

In this kind of dobby, the selection of lifting plan is selected electronically. An electromagnet is used to perform a lifting plan. This electromagnet is energized and de-energized automatically as per the lifting plan fed into the control computer.

According to lifting action in respect to loom’s revolution

On this basis the dobby can be classified as follows:

Single-acting dobby:  it runs at the same revolution equal to loom. One lifting cycle completes in one complete round of dobby.

Double-acting dobby:

In this type of dobby, the revolution per minute of dobby is just half of the revolution per minute of the loom. One complete lifting cycle is completed in half revolution of dobby.

According to the mounting position of dobby:

According to the mounting position of dobby, a dobby can be classified as follows:

·       • Top-mounted dobby.

·      •  Semi top-mounted dobby.

        •   Right hand mounted dobby.

·       • Left hand-mounted dobby.

             

You may also be interested in the following articles:


Let - off motion, secondary loom motion

Take - up motion, a secondary loom motion

Shedding motion, a primary loom motion

Picking motion, a primary loom motion

Beating motion, a primary loom motion

Schematic diagram of loom

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