Dobby Shedding Mechanism | Classification | Types

Classification of Dobby:

INTRODUCTION: -

Dobby is a shedding device attached to the loom to increase the designing capacity. The dobby normally controls 12 to 24 hold shafts the dobbies are classified as negative or positive, single lift, double lift, crank driven, cam driven, etc.

The dobby forms a bottom closed, center closed, semi-open, and open type of shed. The dobby is also classified as left hand and right hand.

The right-hand dobby is one which is placed at the left-hand side of the left hand. It gets driven from the left-hand end loom.

The left-hand dobby is one placed at the right-hand side of the left-hand loom. Left-hand dobby gets its drive from the right end of the loom.

Difference between tappet and dobby shedding:

Tappet Shedding	Dobby Shedding
Tappet shedding is simple and cheap in construction.	Dobby shedding is relatively complicated.
Tappets produce simple and basic designs using a maximum of only eight heald shafts.	A dobby can produce small motifs using many more heald shafts up to a maximum of 40.
A tappet loom works efficiently and consumes less power.	A dobby loom consumes more power.
A tappet loom can be run at high speed with less machine vibration.	A dobby loom cannot be worked at high speed because of high machine vibration and excessive friction between moving parts.
Tappets are required to be change for altering the design gearing on the bottom and counter shafts are to be altered.	The designs can be modified easily change out is brought about quickly and does not require any major alternation
The number of heald shafts used and picks/repeat cannot be changed for a given set of tappets.	The number of healds and picks/repeats can be easily altered.
In a tappet, the shedding dwell period is maintained for the safe passage of the shuttle.	In swing lever dobbies there is no dwell period. In a cam dobby, a dwell is maintained.

Climax Dobby

The Climax Dobby is double-acting machine controlled from the bottom shaft, it forms the open shade. The lifting of heald is done by lifting of jacks. The healds are lowered because of springs used on the bottom side. It has 2 jacks. Dobby runs at half the speed. In the right-hand dobby, the cylinder rotates in a clockwise direction. In the left hand Dobby, cylinder rotates in an anticlockwise direction.

Working

Dobby gets its drive from the bottom shaft through a connecting rod. The T-lever gets oscillation and thereby knives move forward and backward in the framing machine slot. When the knives are towards the set of hooks selection of engagement hook and knife is made feller. The feelers are activated because of wooden pegs. when the hooks and knives are engaged, then the hook is pulled, while the other hook and the lever rest on the stop bar. now the heald is lifted up.

Cycle of lifting of healed into Climax Dobby:

A-The Healds shaft remains down no selection has taken place. knives K1 and K2 are about to move from extreme positions.

B-Heald is down and hook is not engaged with the knife on the second pick.

C-It represent the engagement of hook and knives is about to pull the hook to lift the shed.

D-Heald is lifted up on the pick.

E-The heald is again lifted up on next pick because of engagement of hook and knife.

Timing and setting of Climax Dobby:

1-At 270 degree The heald shaft is levelled, knives are mid way position.

2-The T- lever is horizontal. At 60 to 120 degree is the dwell Period of the dobby.

Heald are stationery
shed shed is open

3-At 90 degree, one arm of the t— lever is outside and another is inside.

4-The bottom shaft is at top or on bottom center

At 120 To 360 degree To 60 degree is the change period of the heald shaft.

Settings:

1- The driving rod connecting to the crank attached to the bottom shaft off the loop and T-lever must be set to appropriate length. If the rod is too short or long, one set of the hook will travelled then more distance than the other.

2- The length of driving rod is adjusted such that T-Lever is horizontal when the crank attached to the bottom shaft is in horizontal plane either in the front or back of rotary its stroke.

3- Crank and T-lever are kept in the horizontal position and now two knives are Adjusted by draw bolts, so that they are equidistance from their slots. The pattern cylinder is rotated by ratchet wheel. Cylinder is rotated by 1/8th revolution for every 2 pick. the cylinder is held steady buy flat steadied star wheel. The Pawl regulate through of the star wheel. The T-lever is lifting knives are extreme position there should be a quarter inch clearance between the tip of the pawl and the teeth of the cylinder ratchet.

For right hand Dobby visible outermost feeler which operate bottom drawer hook , pattern cylinder rotate clockwise.

In the left hand Dobby, the first feeler is a straight feeler acts on the top hook.

In pegging, the upper row of holes in the lug represents first pick.

Cross Border Fabrics:

Many fabrics needs border in the warp as well as in the weft way. for creating the weft way border design. The number of wooden leg required are high. so to create a border design, special cross border Dobby is used. the fabrics which need the cross border are sarees, carpets ,Chadar, tapestries, handkerchiefs, table covers.

Cross border Dobby

Cross border Dobby is used when two different weaves are required to woven using same number of healds.

Cross border Dobby eliminates high number of wooden legs which are required in normal dobbies.

Types of cross border dobbies:

1- Two cylinder cross border Dobby

2- Three cylinder cross border Dobby

Dobby Faults:

The common problem in the dobby is Jack missing. The heald shaft is not lifted when required and weft floats across the warp end giving rise the fabric effect called stitching.

Stitching is sometimes caused by shuttle flying. Warp thread get break occur because of shuttle fly causing a major fabric defect.

The causes of Jack missing (Heald not lifted) are:

1) Bent wooden peg in a lag.

2) Wooden peg is worn out due to frequent use.

3) The spacing between the lag should be uniform and cylinder should be positioned below the feeler in the perpendicular manner. if the lags are not spaced properly more feelers may be lifted causing lifting of improper jack.

4) The pattern barrel in which wooden lag rotate should not move laterally.

5) Needle operating hook may get stuck and jack is missed, good oiling is necessary.

6) Wooden peg will be broken and therefore jack is missed.

7) The hook may come off from the knife in the middle of stroke, causing missing jack.

Ruti Cam Dobby:-

The ruti cam dobby is negative in action i.e the heald are lowered because of the spring in this dobby. The knives are pushed forward by the pair of cams. The knive will come back and always in the contact with the cams because of the spring. Here dwell of about 1/3 pic is achieved. The selection of the hook are made by paper pattern instead of wooden pegs.

Positive Cam Dobby:-

The positive cam dobby is called positive because lifting and lowering of the heald is made positively. The dobby has cam which will give oscillation to the griffel and returning bar through the lever i.e when the griffel is engaged with the hook it will pull one side bulk and at the same time return bar push the other side of the bulk. Suppose heald is to be kept down on the next pick, the bottom hook will not engaged in the griffel, it is looked in the locking bar and thus engaged in the griffel, it is looked in the locking bar and thus other end of the bulk will not move even the upper return bar pushes to the bulk towards the stop and heald will be down. in short it return bar stop and locking bar together make the dobby positive along with grooved cam.