Friday, March 13, 2020

Mechanism and working procedure of dobby

Principle of Negative Dobby Shedding:
The main function of a dobby is to raise and lower the heald shaft according to weave pattern. In negative dobby, the heald shafts are raised with the help of jack lever. The lowering of heald shafts are lowered by extra attachment called reversing motion. Dead weight of pulling springs are used to bring down the heald shaft. The dobby is driven by bottom shaft. Bevel gears are used to transfer the motion from bottom shaft to connecting rod (vertical shaft). Knife makes to and fro motion. One or two knives are used according to single or double lift dobby. One or two knives are used as per lifting mechanism for each heald shaft.

The selection of hooks are controlled by lattice pegs directly or indirectly. The lattice rotates on lattice cylinder intermittently. When the hook falls on the moving knife, it is pulled by knife. Since the hook is connected to the jack lever by suitable mechanism, so that the jack lever lifts the heald shaft connected to jack lever. When the knife returns into its back position, the reversing motion brings the heald shaft at bottom position. The jack lever and hook also return with heald shaft. Negative dobby has many limitations:
Negative dobby has speed limitations.
Reversing of heald shaft is jerkier.
Weaving of heavy weight fabrics is very difficult.
In this type of dobby, heald shaft missing defect is possible.

There are two types of negative dobbies in used. One is top mounted dobby, in which the over motion is used. In this kind of dobby, jack lever lifts the lowers the heald shaft only. The heald shaft comes in its original position with the help of reversing motion.
Other is bottom mounted dobby. In this kind of dobby, under motion is used. In this dobby, jack lever lifts the heald shaft only. The heald shaft returns in its original position with the help of reversing motion.

Positive dobby:
The positive dobby is very efficient dobby. In this dobby, the raising and lowering of heald shafts are performed by its own mechanism(positive cames fitted in dobby). There is no need of extra reversing motion to lower the heald shaft. Today rotary dobby is mostly used in modern looms.

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