WEAVING MECHANISM/FABRICFORMATION-II (Part 2)

WEAVING MECHANISM/FABRIC FORMATION-II (Part 2)

            FREQUENTLY ASKED QUESTIONS (FAQs) and ANSWERS:

Q18. What is Dobby Shedding? Mention the working principle of Dobby Shedding with a neat sketch. Also, show pegging plan of Climax Dobby with 3/3/1/1 twill weave.

      The cam shedding system has limitation in terms of a number of healds that can be effectively controlled during shedding. When a large number of healds is to be controlled by the shedding mechanism, Dobby system is preferred.
       In negative-dobby shedding, the shafts are raised by the dobby and lowered by some form of spring under-motion. In positive-dobby shedding, the dobby machine both raises and lowers the shafts. Negative dobbies tend to be simpler, and, because they are satisfactory except for heavy fabrics and high loom speeds, they are commoner than positive dobbies, except in woollen and worsted weaving and for high-speed unconventional looms.
       Keighley dobby is known to be a double acting dobby as most of the operations is done at half speed as compared to the loom speed (picks per minute). The basic components of Keighley dobby are as follows:

• Stop bars 
• Baulk  
• Hooks (two per heald)
• Knives (two for the entire dobby)
• Pegs on pattern chain 

                                             Figure 1: Working of Keighley dobby
The motion to the reciprocating knives (K1 and K2) originates from the bottom shaft of the loom. As one revolution of the bottom shaft ensures two picks, each of the two knives completes the cycle of inward (towards the left) and outward (towards the right) movements during this period. The two reciprocating knives are incomplete phase difference. When one knife is moving inward, the other knife is moving outward. In Figure 1, knife 2 (K2) has pulled hook 2 (H2) towards the right side. This has happened as there is a peg in the lag corresponding to the feeler 2 (F2). The peg has pushed the right end of the feeler 2 in the upward direction. Thus the left end of the feeler 2 has been lowered. So, the hook 2 was also lowered on knife 2 when the latter moved inward. So, the lower end of the baulk (B) moves away from the stop bar 2 (S2). Thus the heald shaft is raised as it is connected at the midpoint of the baulk.
In the next part of the cycle,w knife 2 will move inward and knife 1 (K1) will move outward. Now, there is no peg corresponding to the position of feeler 1 (F1). So, the right end of feeler 1 is lowered and the left end of it is raised. As a result, the connecting rod (R) has pushed the hook 1 in the upward direction. So, when the knife 1 will perform its outward movement, it will not be able to catch the hook 1. The top part of baulk will be resting on stop bar 1 and thus the heald will not be lifted for the next pick.
It is important to note here that when the heald is in a lower position for two consecutive picks, the top, as well as the bottom end of the baulk, will be resting on the respective stop bars i.e. S1 and S2. So, the midpoint of the baulk will not have any significant movement. On the other hand, if the heald is in the raised position for two consecutive picks, then one end of the baulk will move away from the stop bar and another end of the baulk will move towards the stop bar. Thus the middle point of the baulk will not experience any significant movement as schematically shown in Figure 2. Thus the amount of wasted movement is very nominal. Therefore, the system will produce an open shed.



Figure 2: Open shed formation in Keighley dobby



System of Pegging



Twill weave (3/3/1/1) which repeats on eight ends and eight picks (Figure 3 ) has been considered here for demonstrating the pegging plan. The system of pegging is depicted in Figure 4. This design can be produced by using eight healds and straight draft. The selection for heald movement is controlled by wooden pegs which can be inserted within the circular holes made on the wooden lags. The wooden lags inked together into a lattice which is mounted on the pattern wheel (or barrel). The pattern barrel is rotated by a certain degree once in two peaks. For example, if the barrel is hexagonal then it must rotate by 60° after every two picks. The presence of a peg within the hole results raised the position of the heald and vice versa. The position of two holes corresponding to the same heald is not on the same line. The lateral shifting of holes is done so that two adjacent feelers can be accommodated.







Figure 3: Point paper representation of 3/3/1/1 twill weave







Figure 4: Peg plan for 3/3/1/1 twill weave







Q19. What is Jacquard Shedding? Mention the working principle of Jacquard Shedding with a neat sketch. Also, show card cutting plan of Jacquard design with 2/2 twill weave.


Ans 19: With jacquard shedding system, individual ends can be controlled independently and thus large woven figures can be produced in fabrics.




Mechanical jacquard systems can be classified into three categories:



· Single lift and single cylinder (SLSC)


· Double lift and single cylinder (DLSC)


· Double lift and double cylinder (DLDC)



Single-Lift Single-Cylinder (SLSC) Jacquard



Figure 1 shows the simplified side view of the SLSC jacquard. If the machine has the capacity to handle 300 ends independently, then it requires 300 hooks (one per end) which are vertically arranged and 300 needles (one per hook) which are horizontally arranged. For example, the needles can be arranged in six rows and each row will have 50 needles. In the side view, only six needles (one per horizontal row) are visible. Hooks, which are connected to individual ends through nylon cord (harness), are also arranged in six rows and each row is having 50 hooks. One knife is responsible for controlling the movement (lifting and lowering) of one row of hooks. However, whether a hook will be lifted or not will be ascertained by the selection mechanism which is basically a punched card system mounted on a revolving cylinder having a square or hexagonal cross-section. The needles are connected with springs at the opposite size of the cylinder. Therefore, the needles always exert some pressure in the right-hand side direction (Figure 1). So, if there is a hole in the punch card corresponding to the position of a needle, then the needle will be able to pass through the hole and thus the needle will remain in upright position thus making it accessible to the knife when the latter has started its upward movement after descending to the lowest height. On the other hand, if there is no hole, then the needle will be pressed towards the left side against the spring pressure. Thus the kink (which partially circumscribes the stem of a hook) present in the needle presses the hook towards the left side making the latter tilted enough from the vertical plane so that the knife misses it while moving upward. Therefore, the presence of a hole implies selection (ends up) and vice versa. A hole, in this case, is tantamount with a peg used on the lag of the dobby shedding system.



In case of SLSC jacquard, if the loom speed is 300 picks per minute, the cylinder will turn 300 times per minute (5 times per second) and the knives should also reciprocate (up and down) 300 times per minute. Thus it hinders the high loom speed. When a particular hook (and the corresponding end) has to be in up position in two consecutive picks, in between the two peaks, it descends to its lowest possible height (determined by the grate) and then moves up again. Thus it produces bottom closed shed. This happens as one end is controlled by a single hook.







Figure 1: Side view of single-lift single-cylinder jacquard



Features of SLSC Jacquard



· 500 end machine will have 500 needles and 500 hooks


· Cylinder should turn in every pick


· Knives must complete the cycle of rising and fall in every pick


· Bottom closed shed is produced



Double-Lift Single-Cylinder (DLSC) Jacquard



Double- lift single-cylinder (DLSC) jacquard is shown in Figure 2. In this case, one end is controlled by two hooks which are again controlled by a single needle. For example, hooks 1 and 2 control the end 1 and hooks 3 and 4 control the end 2. Two sets of knives are used in DLSC jacquard and they move up and down (rise and fall) in complete phase difference i.e. when one set of knives (K1 and K3) attain the highest position, the other set of knives (K2 and K4) attain the lowest position. At the given position, end 1 has been raised as the hook 1 has been lifted by the corresponding knife K1. However, end 2 has not been raised as hook 3 was not caught by the knife K3. In the next pick, end 1 will be lowered as the needle F has been pressed towards the left due to the absence of a hole in the punch card. So, hook 2 has become tilted and it will not be raised by the knife K2 when the latter will rise. Hook 1 will also descend along with Knife K1. Thus the end 1 will be lowered. On the other hand, end 2 will be raised in the next pick as there is a hole in the punch card corresponding to the position of the needle E. So, hook 4 is upright and it will be caught by knife K4 when the latter will move upward.















Figure 2: Side view of double-lift single-cylinder jacquard



In the case of DLSC jacquard, if the loom speed is 300 picks per minute then the cylinder will turn 300 times per minute but the knives will reciprocate (rise and fall) 150 times per minute. This is the advantage of DLSC jacquard over SLSC jacquard.



DLDC jacquard produces semi-open shed because if a particular end has to be in the raised position for two consecutive picks, it will descend up to the middle point of its vertical path and then move up. This will happen because one of the hooks will descend and the other hook will move up with their respective knives and they will cross at the middle of their vertical path. If the end has to remain in the bottom position for two consecutive picks, it will remain at the bottom without any intermediate movement.



Features of DLSC Jacquard



· 500 end machine will have 500 needle and 1000 hooks


· Two sets of knives rise and fall in opposite phase


· Cycles of movement (rise and fall) of each set of knives spans over two picks


· Cylinder should turn in every pick


· Semi-open shed is produced



Card-cutting Instructions:


In jacquard weaving, instructions are passed to the card-cutter according to the design, i.e., pick wise lifting pattern. In Fig. , the cylinder is assumed to be at the back of the loom, so we see the front of the card being presented to the cylinder in Fig. Fig. shows the first two cards in the set and that part of the design from which they have been partly cut. Fig. indicates that the warp has been drawn in from left to right and from back to front-a straight S draft. The first pick is at the bottom of the design, and the card-cutter has read from left to right. The solidly filled squares in the design correspond to the holes that have been cut in the cards. This will produce the correct orientation of the pattern in the fabric. The same result can be obtained by rotating the design through 180 degrees so that the square on the design corresponding to the first end and first pick will be in the top-right-hand corner. The card-cutter will then read from right to left and from top to bottom. This is a customary and more convenient method. When this method is used, the design is rotated before the picks are numbered, so that the numbers will be the right way up for the card-cutter to read. The conditions outlined above would normally be agreed and established once and for all in any jacquard-weaving shed, and the card-cutter would know how to proceed without further instructions.















Q20. What is Positive and Negative Picking? Mention the working principle of Picking mechanism with a neat sketch. Calculate the picking force required to accelerate a picker assuming own data.

Ans 20:
Shuttle picking mechanisms are broadly classified as cone over-pick and cone under-pick mechanisms. Several modifications of cone under-pick mechanism manifest as parallel pick and link pick.

Cone Over-Pick Mechanism

The one-over pick mechanism is shown in Figure 1. A picking cam attached to bottom shaft displaces the cone (picking cone) which is attached with the upright picking shaft. This causes rotation of the picking shaft. As a result, the picking stick, which is attached to the uppermost end of picking shaft, swing in a horizontal plane over the loom and transmits the motion to shuttle through picking strap and picker guided by a spindle. Picking strap is a leather or polymeric belt which is flexible. Here picker is constrained by the spindle to move in a straight line which otherwise would have followed a path of the arc. Obviously, this restriction of the path is achieved at the expense of some energy. Moreover, pairs of picking cam and follower installed at either end of the loom have seldom ensured picking is having varying elastic behaviour (one of them is attached through a “stiff” short shaft while that at the further end through a long “flexible” one). All these warrant frequent adjustment of picking-strap or picking cam and nose settings. A system where a different cam and follower pairs are used for each end makes the matter work enduringly with standardized settings.






Fig. 1: Cone-overpick loom



A slightly simplified perspective view of the mechanism is shown in Figure. It is robust, easy to adjust and maintain, and comparatively gentle in action, so that shuttles and pickers, as well as parts of the mechanism itself. have a long life. These facts account for its former popularity, which began to wane with the growth of automatic weaving, for which it is not suitable. chiefly because the picking stick and spindle at one side of the loom occupy the position in which the battery or magazine of an automatic loom must be placed. This spindle is situated over the shuttle-box and is essential to guide the shuttle along the correct path. It is normally set slightly up and slightly towards the front of the loom at its inner end. The back end of the shuttle will thus receive a similar lift at the end of the stroke so that its leading end will receive correct delivery down and into the shed.


The mechanism is also unsuitable for standardized settings, which are being increasingly used for automatic looms, because of the tendency of the flexible leather picking strap to stretch slowly in use and to vary with regard to its elastic properties as atmospheric conditions change. The mechanism incorporates several adjustments for the timing and strength of the pick. The timing of the pick (i.e., the crankshaft position at which the picker begins to move) can be altered by turning the picking tappet on its boss. The common method of altering the strength of the pick (Le., the shuttle speed) is by altering the length of the picking strap. Shortening it removes some of the slack and increases the distance moved by the picker and hence increases the shuttle speed, but it also advances-the timing slightly. Lowering the picking cone in its slot in the picking shaft also increases the shuttle speed by altering the leverage of the system and thus increasing both the extent of the picker's movement and its velocity. In this case, however, an increase in shuttle speed is accompanied by slight retardation of the timing because the tappet nose will strike the cone slightly later. This adjustment is seldom used. It is also possible to alter the position of the picking stick at the point of attachment to the picking shaft. This has a somewhat unpredictable effect on shuttle speed and timing. Not only has the mechanism a surfeit of points of adjustment, but none of them alters the shuttle speed without also altering the timing to some extent. Standardized settings are virtually impossible in these circumstances. Large changes in shuttle speed, such as might be required for looms of different width or for weaving widely different types of fabric, can be made by changing the nose bit of the picking tappet for one with a different profile. In this connection, it is interesting to note that certain makers of high-speed automatic looms offer a range of interchangeable picking cams for their under pick looms for weaving different widths and types of fabric.




Cone Under-Pick Mechanism



Cone under-pick mechanism is depicted in Figure. Here also a picking cam attached to bottom shaft displaces the cone turning the picking shaft (side shaft) located horizontally. The other end of the picking shaft is connected with an upright picking stick through the picking strap (lug strap). This causes the picking stick to move in a vertical plane and transmits the motion to shuttle by the picker attached at the upper end of it. In this system, the picking stick and other appendages are located below the shuttle trajectory while picking cams and follower, as usual, below the loom and driven from the bottom shaft. The system is naturally suitable for automatic looms. Here picker slides over its spindle and picking timing are regulated by cam adjustment like in over pick motion. An almost inextensible lug strap allows shuttle speed adjustment by either raising or lowering it around picking a stick. Absence of stretchable parts in the under-pick system ensures the retention of correct setting over a long period in contrast with the cone over-pick mechanism. A cone under-pick mechanism on a loom is depicted in Figure.










When the stiffness of a picking mechanism is known, and it is easy to measure it under static conditions, it can be used in conjunction with the nominal and actual displacement curves to estimate the forces exerted by the picker on the shuttle during picking and to compare these with the constant force that would produce the same final shuttle speed.


Since a force of3 ·65 kN acting on the picker parallel to the axis of the shuttle produces a deflection of 1 m, the force required to produce a deflection of O·075 m. is 0·075 x 3650 N = 274 N. This is the peak force exerted by the picker on the shuttle during picking. Now, the uniform force that would produce the same final - speed over the same distance is given, as before, by:


F = 0·32 X (12.22)² /2 x 0·20 =119N.


The actual peak force (274 N) includes the force required to overcome friction. An example in the preceding section gave a typical value of 32·5 N for this, so we may reasonably assume that, of the actual peak force of 274 N, a force of about 240 N was used to overcome inertia. This is almost exactly twice the uniform force (119 N) calculated above.


Q21. What is Eccentricity of sley? Mention the working principle of Beat-up motion with a neat sketch.




Ans 21: The objectives of beat-up motion are as follows:



· To push the newly inserted pick up to the cloth fell


· To ensure uniform pick spacing in the fabric



Sley Motion



Beat-up is done by the reed which is carried by the sley. Sley derives its rectilinear reciprocating motion from the rotating crankshaft through the connections of crank and crank arm which makes a four-bar linkage mechanism. This is illustrated in Figure 1.







Figure 1: Sley motion





Sley Eccentricity



As sley motion deviates from SHM, during its backward journey sley covers more displacement from the rotation of crankshaft than the rotation of the crankshaft. Similarly, during its forward journey, sley covers less displacement from the rotation of crankshaft than the rotation of the crankshaft. This difference in the sley displacement during its backward and forward movement is termed as sley eccentricity. In the case of SHM, the displacement is the same from 0°-90°, 90°-180°, 180°-270° and 270°-360°. are











Q22. What do you mean by FABRIC ANALYSIS? Mention the different parameters that are measured, calculated during fabric analysis.


Ans 22: Fabric Analysis means analyzing fabric parameters from a given sample. Basically, for the reproduction of fabrics from a given swatch, fabric analysis is done. Fabric analyzing also done to gather Information from a woven fabric. The following parameters are analysed, measured and calculated:

1. Weave (structure)- Design, Drafting plan, Lifting plan, Denting plan and sectional views
2. Order of colouring in warp and weft
3. Set – ends (warp) and picks (weft) per cm or Ends per inch (EPI) and Picks per inch (PPI)

• Yarn particulars
• Counts (tex/English count/denier)
• Twist per cm/meter or TPI (Turns per inch)
• S or Z twist
• Single or fold yarn (2-ply, 3-ply, 4-ply, etc)

1. Crimp% in warp and weft
2. Width of warp in reed (in cm or in meter or an inch
3. Warp length for a given finished fabric length (meter/yards)
4. Weight of fabric per unit area/GSM (gm/sq. m or Ounces/sq. yard)
5. Type of raw material for both warp and weft (like cotton, polyester or blended material)
6. Type of finishing applied
7. Other factors: tensile strength, resistance to abrasion, drape, shower resistance, colourfastness
8. To define Face or Back of the fabric

• In defining which is the face or back of the woven fabric, it is mainly accorded to the type of material, yarn count, arranging of yarn, dyeing and printing, weave pattern, finishing effect. Typical for fabric surface are:


– Smoother;


– Soft handle;


– The face with solid jacquard pattern, pattern weave or printed weave


– It is always with higher warp float proportion of fabric


– Fabric with special effects, the effects usually appears on the face.