7 Dobby weaving

B. A. Muralidhar

1. INTRODUCTION:

Weave is the pattern (or) design with which the fabrics are manufactured, these weaves range from being very simple ones like the basic plain, twill (or) sateen weaves (or) could be more complicated like small geometric figures, patterns (or) motifs (or) further, it could be elaborative, artistic and decorative such as the damasks and brocades.

Weaving is the method of manufacturing woven fabrics by interlacing the warp and the weft yarns at right angle to each other. The process of weaving is carried out on a machine called ‘Loom’. All weaving machines control the warp threads to create a shed. The function of shedding mechanism is to make an opening for the shuttle to pass through and to change the warp threads according to the lifting plan for the next pick. This can be accomplished by tappet, dobby (or) jacquard shedding mechanism depending on the number of threads the weave pattern repeats on. Tappet and the dobby mechanism controls and lifts the heald shafts through which the warp yarns are drawn, whereas the jacquard machine controls the individual warp ends.

2. SCOPE AND USES

   2.1 Tappet shedding:

In this shedding mechanism the heald frames are operated by tappets which are mounted on the bottom shaft (or) a separate shaft known as tappet (or) counter shaft. Each tappet is designed according to the weave. For a plain weave repeating on two ends and two picks, two tappets are required and they are usually mounted on the bottom shaft. For all other weaves repeating on more than two picks, the tappets must be mounted on the tappet shaft. Tappets designed for a particular design cannot be used for any other design. One of the disadvantages of this type of shedding is the number of tappets that can be conveniently and economically used. It will be up to eight (or) maximum sixteen. So any weave repeating on more than sixteen ends and picks requires a versatile shedding mechanisms such as dobby (or) jacquard shedding.

2.2 Dobby shedding:

In dobby looms intricate designs can be woven. Dobby shedding mechanism is more complex than the tappet shedding system. In dobby shedding the heald frames are operated by jacks and levers,which are operated by wooden lags with pegs, corresponding to the lifting plan, rotated around the roller above the loom. Punched plastic paper pattern cards can also be used. Modern dobby looms are controlled via electronic system. Design possibilities are crepe, mock leno, honeycomb, double cloth etc. dobby looms can conveniently control heald frames from 6 to 40.

2.3 Jacquard shedding:

The jacquard shedding system is for creating complex design patterns in fabrics. In this shedding mechanism, the jacquard selects and lifts the warp ends individually. This shedding mechanism is suitable for large intricate design patterns where most of the ends in the repeat move independently. There are no heald frames as in other looms. Jacquards are either mechanical (or) electronic type capable of handling over 1200 harness cords which lift and lower the warp ends. The patterning potentials are virtually unlimited.

3. PRINCIPLES OF OPERATION

Dobbies are used in manufacture of complicated weaving patterns beyond the range of tappet shedding mechanism and is at the same time too small to be economically produced by a jacquard.

Dobbies are broadly classified as follows:

Positive and Negative

Depending on the cycle of knife movement, dobbies are further subdivided into

(a) Single lift, single jack (Or)

(b) Double lift, double jack

In a single lift dobby the knife movement is completed during one revolution of the main shaft. Whereas, in a double lift dobby it is performed during two revolutions of the main shaft.

Positive dobby:

The heald frames of a positive dobby are raised and lowered without the use of a reversing motion. These dobbies are suitable for weaving woollen, worsted and heavy cotton weaving on high speed looms.

Negative dobby:

In a negative dobby, the heald frame movement is controlled in one direction either to rise or to lower the heald frame. As most dobbies are mounted on top of the loom, the lifting of the heald shaft is carried out by them and the reversing is done using special reversing motion, springs or elastics.

Negative dobby’s are further classified depending on the nature of shed formed namely open, semi-open and closed shed dobby.

3.1 POSITIVE DOBBY

In the positive dobby shedding motion, the healds are usually pulled and lowered simultaneously to form the upper and the lower shed. As a general rule, while choosing a positive dobby the working speed of the loom has to be considered along with the type of fabrics. As such medium and heavy weight fabric are woven with a positive dobby.

Figure 1. Sectional view of positive dobby.

Sectional view of the essential working parts of a positive dobby is represented in Figure1. This dobby is suitable for weaving heavy weight woollen and worsted fabrics. Heald frame top is connected to the upper arm of the jack lever A and the bottom to the lower arm. Each jack lever is connected at the centre to a vibrator gear B by means of a connector T. Above and below the central vibrator gear are two cylinder gears C1 and C2. The cylinder gears have teeth cut on only one half of their circumferences, the other half being blank. They are driven in opposite directions as shown by the direction of arrows and rotate continuously making one revolution every two picks. The vibrator gears made of steel discs 4.75 mm thick, and are cut with teeth to match those of the cylinder gears. However the entire surface of the vibrator gear is not covered with teeth. On one side a blank space of one tooth is left, and diametrically opposite side a blank space of three tooth is left.

The vibrator gear rotates freely on a pin O of the vibrator lever D which is fulcrumed on the heel pin P. The vibrator lever D which is resting on the pattern chain E moves round  the pattern cylinder F. the pattern chain consists of small rollers called risers and links called sinkers. When the pattern chain moves along the pattern cylinder either a riser or sinker, according to the lifting plan, is brought under the vibrator lever. A riser lifts its respective vibrator lever and brings the vibrator gear in contact with the top cylinder which is constantly rotating. When the teeth of the two mesh together, the cylinder gear C1 turns the vibrator gear B about half the revolution, that is until the blank spaces of 3 teeth is brought on the top. This movement of the vibrator gear causes the connector pin Q of the vibrator connector T to move from one dead centre to the other, with the result the corresponding heald frame lifted. The vibrator gear continues to keep the heald frame raised as long as there is roller the pattern chain. As soon as the tube comes under the vibrator lever, it brings down the vibrator gear in contact with the bottom cylinder gear C2 and again the vibrator gear turns half the revolution, this time lowering the heald frame.

A steadying pin S which is part of the vibrator gear, moves in a semi-circular slot of the vibrator gear and controls the extent of movement of the gear. Lock knife R locks the vibrator levers in position while the corresponding vibrator gears are in motion, this also prevents the vibrator being forced out of contact with cylinder gears. However, the lock knife is moved out of contact when the pattern chain brings a new pattern below the vibrator lever.

3.2 NEGATIVE DOBBY (Double lift single jack)

The essential parts of a double lift single jack dobby mounted on the top of the loom are shown in the Figure 2.

Figure 2. Double lift single jack – Negative dobby

The heald lifting jack (A) fulcrumed at O, Baulk lever (B) which holds the lifting jack

(A)   Draw hooks (C1 and C2) the knuckle end of each draw hook is held by the upper and lower ends of the baulk lever. Feelers D1 is a straight end feeler whereas feeler D2 is a curved end feeler. Both feelers are fulcrumed at (D) the back part the feelers are made heavy so that they remain on top of the wooden pattern cylinder E. The wooden pattern wheel placed beneath the feelers are given 1/8th turn every second pick. To accomplish this the cylinder is grooved lengthwise to enable the wooden lag to be housed properly during its rotation. The pattern chain F consists of number of lags linked to one other by a wire ring to form a continuous chain to run on the cylinder. Each lag is provided with two rows of holes and each row represents one pick. These lags are pegged using a small wooden peg according to the lifting plan. The needles G rests on the straight edge feeler D1 and support the top draw hooks C1 and the bottom draw hooks C2 are supported by the curved edges of the feelers D2. Draw knifes H1 and H2 extend the full width of the dobby and reciprocate in the slots of the side frame. Stop bars K1 and K2 also extend the full width of the dobby. The horizontal arm of the T-Lever L shown in Figure 3. is connected to a driving rod which is connected to a bracket on the bottom shaft whereas the two ends of the vertical arm are connected to draw bolts M1 and M2.

Figure 3. Pattern cylinder and T-lever

Double lift negative dobbies are of two types, single jack and double jack. With single jack dobby the disadvantage is getting a straight lift of the heald shafts. Various methods are used to prevent the lateral movement of the healds. The double jack dobby combines the two jacks by means of a single short link known as ‘C’ link as shown in Figure 4. The outer jack A is fulcrumed at O1 and controlled by the baulk lever B as with the single jack dobby. A short link L couples the outer jack A1 to the inner jack A2 fulcrumed at O2 and both the jacks are lifted together.

Working of the Double lift single jack dobby:

When the loom starts working the T-lever swings, and reciprocates the knives through draw bolts, and the knives complete one reciprocation every two picks because they are  driven from the bottom shaft. With the movement of the knives the pattern cylinder with the continuous lags rotate 1/8th of a turn bringing a lag with pegs and blanks beneath the feelers. A peg in the lag will lower the corresponding hook which will engage with the draw knife. For instance, consider the straight edge feeler is lowered by a peg on the lag, then the top draw hook C1 is also lowered to engage with the top knife H1. Similarly if the curved edge feeler is lowered, the bottom draw hook C2 is lowered to engage with the bottom knife H2. Then the draw hook C1 and C2 engage with the knife and will be drawn forward along with its baulk lever by the knife during the sweep of the T-lever. If the top part of the baulk lever is pulled forward the bottom part rests solidly against the stop bar K2. Thus the stop bars K1 and K2 act as fulcrum for the forward moving baulk levers, which in turn lifts the jack lever and the heald frame. However a blank in the lag would keep the respective draw hook raised above the knife and so the heald frame is not raised.

Figure 4. Negative dobby (Double lift double jack)

Working of the Double lift double jack dobby:

In a double lift double jack dobby as shown in Figure 4. The two jacks are connected by a short link known as C-link. The short link L combines the outer jack A1, fulcrumed at O1 to the inner jack A2, fulcrumed at O2 and both the jacks are lifted together without any additional aid.

4. EXTRA WARP AND EXTRA WEFT DESIGNS

The formation of a figure by means of an extra thread could be accomplished by either an extra warp or extra weft, or the two methods may be employed in combination. When extra warp is introduced then a separate warp beam is required for each warp because of the different take-up rates between the ground and extra threads. The form of the design may render it necessary for the extra threads to be inserted in intermittent or continuous order with the ground threads. The arrangement of the figuring and ground threads may be 1 Figure X 1 Ground, 1 Figure X 2 Ground, 1 Figure X 3 Ground so on, according to the solidarity of the structure required. However for extra weft figuring, the loom must have the capability to insert more than one kind of weft. For looms with changing boxes at one side may be used to produce similar effects to the one by one order (pick at will) by wefting 2 Figure and 2 Ground.

The extra warp motif and the ground weaves are shown in Figure 5 and 6 respectively. Whereas, the full construction of the extra warp motif with ground weave introduced is represented in Figure7. The solid marks on the extra ends, indicate warp up. While the lifts of the ground ends are represented by a cross marks.

Figure 7. Method of constructing a squared paper design

Extra weft figured fabrics may be formed by only one series of warp thread along with 1, 2 or more extra weft picks in addition to the ground weft. The complete structure in  Figure 10 shows the figuring picks (solid marks) arranged in alternate order with ground picks (cross marks). In the figures 8-10 the design convention has been reversed, i.e. solid marks and cross marks indicate warp down and blank marks warp up. This help designers visualise the figure formed on design paper.

Figure 10. Method of constructing a squared paper design.

5. Methods for securing the extra warp and weft threads:

One of the following methods may be used in securing the extra warp and extra weft threads, in the portion of the cloth where they are not required to form figure.

1. The extra yarn is allowed to loosely float on the back of the cloth.
2. The extra yarn is allowed to loosely float on the back, and is afterwards cut away.
3. The extra threads are bound in on the underside of the cloth, by stitching threads.
4. The extra threads are interwoven on the face of the cloth in the form of small figures.

Conclusion:

In this module on dobby weaving, we have introduced the viewers to the different shedding mechanism, followed by the principles of operations of the dobbies. Then the working of positive dobby, negative dobby and the construction of both extra warp and extra weft designs have been discussed.

References

1. Talukdar M. K., Sriramulu P. K., Ajgaonkar D. B. 1998. Weaving-Machines-Mechanisms-Management, Mahajan Publishers Private Limited, Ahmedabad, INDIA.
2. Grosicki Z.  J.  Reprinted  (2004).  Watson’s  Textile  Design  and  Colour  Woodhead Publishing Limited, Cambridge, England.

Web links

• http://nptel.ac.in/courses/116102005/26