Dobby shedding mechanism-1

DOBBY SHEDDING

Limitation of cam shedding

The cam shedding has limitation in terms of no. of healds that can be effectively controlled during shedding. dobby is shedding mechanism is placed in the top of the loom i order to develop figured pattern or design.

in cam shedding problem is arises when the no of picks in the repeat of weave design is very high. Let us assume that the weave design is repeating on 10 picks (8/2 twill ). The no of heald frames is required in this case will be 10 and control these heald frames 10 cams will be required.
these cams will be mounted on the cam(tappet) shaft.which will be rotate at 1 tenth r.p.m. as compared to that of crank shaft.

when the large no of healds is to be controlled by the shedding mechanism dobby system is preferred.

TYPES OF DOBBY

1. according to lift:

        (a)  single lift

        (b)  double lift

    2. according to shed:

          (a) bottom close shed

          (b) center close shed

          (c) Semi open shed

          (d)  Open shed

     3. according to the drive of heald shaft:

           (a) positive

            (b) negative

        4.  according to no of jack lever:

            (a) single jack lever

            (b) double jack lever

        5   according to the position of heald shafts:

            (a) Vertical

            (b)  Horizontal

      6.  according to figuring capacity ( no. of heald shafts):

             45's, 36's,24's,20's,16's,12's

         7   Broadly dobby shedding can be classified as below:

             (a) Ordinary dobby(i.e. Single jack lever,Single lift Dobby)

             (b) Special dobby(i.e. Double jack lever, 48's dobby)

SCOPE OF DOBBY SHEDDING MECHANISM

the scope of dobby shedding is limited between tappet shedding and jacquard shedding. The no. of shafts that can be actuated by a dobby mechanism varies between 6 to 40 shafts.

when the pattern is beyond the range of a tappet either in the no. of shafts to be manipulated or in the picks to a repeat of the pattern and is of the same item too small to be economically produced by a jacquard.  a machine which is employed which is known as dobby. The no. of shafts it may be called shafts.

NO OF HEALD SHAFT REQUIRED

Tappet: up to 14 heald shafts

Dobby: up to 48 (theoretically)

            Piratically up to 36 (wool & allied)

            Piratically up to 24 (maximum)(cotton & allied)

 POSITIVE DOBBY

The dobby which can rises and lower the heald shafts or heald frames without the use of an additional reversing motion is called positive dobby. This type of dobby used for weaving heavy fabrics such as fancy woolen and worsteds.

NEGATIVE DOBBY

This type of dobby only rises the heald shafts or heald frames. Lowering is carried out by reversing under spring motion. Negative dobby are used for light to low and medium weight fabric. GSM of fabric is low to medium. 

DIFFERENCE BETWEEN POSITIVE AND NEGATIVE 

POSITIVE DOBBY	NEGATIVE DOBBY
1.     The dobby can raise and lower the heald shafts.	1.     The dobby can raise the heald shafts. Lowering is done by reversing spring mechanism.
2.     Additional arrangement is not required.	2.     Additional arrangement is required.(Dead weight/spring reversing)
3.     Heavy fabrics produced.	3.     Light weight fabrics produced.
4.     In this dobby loom is driven by crank shaft.	4.     In this dobby loom is driven by bottom shaft.
5.     Loom speed is lower.	5.     Loom speed is higher.
6.     Stress and strain on warp is lower.	6.     Stress and strain on warp is higher.
7.     Open shed is produced.	7.     Close shed and semi open shed is produced.

NEGATIVE DOBBY

When the connecting rods moves up and down and T-Lever gives outward and inward movement to the dobby. According to fabric design when comes in contact with feeler,then the right side or portion of feeler is raised and left portion of feeler being lowered.

Hooks lowered as hooks are supported with feeler.when the left end of the feeler is lowered then the upper hook come in contact with the upper knife. in this state when connecting rods moves down then the upper portion of T-Lever is gives outward movement. As a result, upper portion of S-Lever,bulk lever also moves to right side.thus a pull create on timber lever and link.
as shown in figure inside jack lever and outside jack lever are connected with timber lever and link. The lever moves up at the same time. Hence, the heald shift raise up.

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Similarly, When the left end of the feeler is lowered then the lower hook comes in contact with the lower knife.in this case, When connecting rod moves up and the bottom portion of the T-lever moves to the right side. As a result bottom portion of S-Lever moves to the right and the same process occurs.  i.e. top portion of bulk lever moves to the right. Here jack levers are moved at the same time. Hence the heald shafts again raise. Thus a heald shaft is alternatively raise by the lower knife and upper knife. The shaft will therefore be lowered with the help of return spring and will remain down for the next pick.