It is a shedding device placed on top of the loom. It provides figuring effects and produces smaller motives. It is normally built to control 12, 16, 20, 24, 28 and up to 30 heald frames. A dobby may be provided with single or 2 or 3 cylinders.
It is used for weaving from finest silk to heaviest upholstery fabrics.
Capacity of dobby:
Capacity of dobby expressed in terms of Jacks.
Also known as figuring capacity of dobby indicates the lifting of the no. of ends per repeat. Following are capacities of dobby found.
Classification of dobby:
1.Based on no. of picks/crank rotation/bottom shaft rotation classified into 2 types:
Single lift dobby:
All elements perform their action for every weaving cycle to open a shed, and comes to original position once the cycle is completed.
One pick for crank revolution.
After every weft insertion the shed is closed and the pick is beaten up at the closed shed.
The drive is from crank shaft.
Pattern chain length is long.
The speed is limited to 160 to 180 rpm.
Double lift dobby:
Shed opens for every pick but the all the elements performed for every 2 weaving cycles.
Two picks are for bottom shaft rotation.
Drive is from bottom shaft.
Pattern chain length is short.
It is suitable for high-speed weaving operations.
2.Based on the position of starting handle towards:
Right hand dobby:
Starting handle towards right side.
Dobby unit will be on left hand side.
Left hand dobby:
Starting handle towards left hand side.
But dobby unit is on right hand side.
3.Based on shed forms:
Bottom Closed Dobby:
In this kind of dobby, all the heald shaft rest on bottom position. Only those heald shafts are raised which are needed in 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 bottom position of shed layer.
Center Closed Dobby:
In this kind of dobby, the shed closing (levelling) takes place at the middle of shed. When the 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 center position of shed.
Based on action:
Negative dobby indicates, the use of dobby for lifting the Healds and use of springs for lowering the Healds.
A dobby is said to be positive if the heald shaft movement is controlled in both the direction.