WEAVING MECHANISM/FABRIC FORMATION
FREQUENTLY ASKED QUESTIONS (FAQs) and ANSWERS:
Dr.K.N. Chatterjee
Prof.,& Head of FAE department
Head, Research and Consulting
TIT Bhiwani
Haryana, India.
Q1: What is FABRIC?
Ans1: A fabric is a flat or tubular structure comprising of fibrous materials may be in the form of fibres (Nonwoven, Felt structures) or in the form of yarns (Woven, Knitted, Braided, crochet, etc.)
Q2: What is WOVEN FABRIC?
Ans2: A Woven fabric is a flat structure comprising of fibrous materials, either Natural, Man-made or Blended in the form of yarns. Woven fabrics are produced by interlacing together two elements: warp and weft.
Q3: What is WEAVING?
Ans3: Weaving is a process of interlacement of Warp and Weft. Weaving is generally carried out by a Weaving Machine, called Loom.
Q4: What is Warp?
Ans4: Warp is the longitudinal thread (Yarn) in a cloth or a Fabric. It is represented by the threads placed lengthwise in the fabric. The single Warp yarn is called End.
Q5: What is Weft?
Ans5: Weft is the transverse thread (Yarn) in a cloth or a Fabric. It is represented by the threads placed widthwise in the fabric. The single Weft yarn is called Pick.
Q6: What is Loom?
Ans6: A loom is a device that causes interlacement two sets of threads, namely, warp and weft threads, to form a fabric. The very first loom is the pit loom, followed by handloom, and then the power loom, of the non-automatic type, which had neither a positive let-off device nor warp stop mechanism or a weft changing mechanism. The semi-automatic loom was then developed which incorporated at least 2 above mechanisms. Then the automatic loom was developed which had all the three essential mechanisms, namely, positive let off the device, warp stop mechanism and weft replenishment mechanism. Latest developments took place as shuttleless weaving mechanisms, followed by Multi-phase looms.
Q7: What are Weaving Mechanisms?
Ans7: The basic weaving mechanisms in any type of loom can be classified as follows:
Primary motions (b) Secondary motions, and (c) Tertiary/Auxiliary motions.
The primary motions can be divided as:
Shedding Motion,
Picking Motion and
Beating- up motions.
Shedding Motion: The shedding opens the warp sheet into two layers to facilitate passage of shuttle, carrying the weft yarn. Shedding motions are generally carried out by three different shedding devices namely, Tappet Shedding, Dobby Shedding and Jacquard Shedding. Shedding motions can also be Negative or Positive shedding type.
Picking Motion: The Picking motion causes the shuttle carrying weft to be propelled from one end of the loom to another. Picking motions are generally carried out by Over-pick and Under-pick motions in Shuttle-looms and are carried out by gripper (in case of Sulzer looms), by rapier (in case of Dornier looms), by Air ( in case of Air-jet looms) and by Water ( in case of Water-jet looms). Picking motions can also be Negative or Positive picking type.
Beating- up motion: The beating- up motion lays the previously laid weft to the fell of the cloth. Beating-up motion is generally done by a Reed, which is generally mounted on a Sley. There are two types of Beat-up motions namely by CRANK-Beat -up (in case of Shuttle looms), and by CAM-Beat -up (in case of Shuttle-less looms like Sulzer, Dornier, Air-jet and Water-jet loom).
Secondary motions:
The secondary motions comprise of taking- up and let- off motions.
Take-up motions:
The take-up motion helps to wind the cloth on to the cloth roller and also influences the pick density in the cloth. There are many types of Take-up motions like 5-wheel, 7-Wheel and Continuous Type. Take-up motions can also be Negative or Positive type.
Let-off motions:
The let-off motion helps to let the warp from the weaver's beam at a uniform rate thus maintaining the warp tension constant throughout the weaving process. Let-off motions can also be Negative or Positive type.
Tertiary/Auxiliary motions:
The Tertiary/auxiliary motions consist of the warp stop motion, weft stop motion and warp protector motion. The warp stop motion is used to stop the loom in the event of warp breakages. This is necessary to prevent fabric defects such as missing ends and floats. The weft stop motion is used to stop the loom in the event of weft exhaustion or weft breakages. This is necessary to prevent missing weft threads called cracks, in the fabric. The warp protector is used to prevent multiple warp thread breakages in the event of shuttle getting trapped in the middle of the warp sheet.
Q8: What are the different types of Sheds?
Ans8: Shed is defined as the division of warp ends into two layers. The types of seeds are produced according to the position the ends assume between successive picks and the nature of the movement given to the ends. Types of Sheds are generally classified into two heads:Closed Sheds
Open Sheds
Closed Sheds are further classified into 3 heads:
Top Closed Shed
Bottom Closed Shed
Centre-closed shed
Open Sheds are further classified into 2 heads:
Semi-Open Shed
Open Shed
There are mainly four main types of sheds.
Bottom-closed Shed
In these types of sheds, all the ends return to the bottom position to form a closed shed after every pick. There is a great deal of wasted movement in this weave, as there would be in any other weave that required some ends to be lifted for. two or more picks in succession. Some early types of dobby produced this type of shed, but it is encountered nowadays only in handlooms and in single-lift jacquards.
Centre-closed Shed
In these types of sheds, all the ends return to the middle position to form a closed shed after every pick. Whatever the weave, every end must be either raised or lowered at every pick, so again there is a great deal of wasted movement. Centre-shed dobbies have been used for weaving some kinds of Reno fabrics in which the crossing of the ends was facilitated by the return of all the ends to the centre position after each pick. A simple adaptation of the Keighley dobby, by the addition of a 'shaker' motion, serves the same purpose, and the centre-shed dobby is now practically obsolete. It is still used in some jacquard machines because the movement is balanced and the wasted movement is reduced and distributed uniformly over all the ends, and it is, therefore, preferable in this respect to bottom closed shedding.
Semi-open Shed
As in bottom-closed shedding, in this types of the shed, the rest position is at the bottom, but an end required to stay up for two or more picks in succession drops only to the centre position between picks. There is less wasted movement than in the types of the shed. This type of shed is formed by double-lift jacquards. The ends do not form a single sheet between successive picks (i.e., the shed is not closed), but neither does it remain fully open, and hence we have the term 'semi-open'.
Open Shed
In these types of the shed, there is no rest position. Ends required to stay up or down for two or more consecutive picks simply stay up or down. There is no wasted movement, and the shed never closes, unless the weave is such that there is a complete interchange in the position of the ends between successive picks. This is an exceptional condition encountered in weaves such as plain, warp-rib, and matt weaves. For most purposes, this is the ideal type of shed. It is attained without difficulty in tappet shedding, especially with positive tappets. Double-acting dobbies give a close approximation to it, with a tendency for the shafts to drop slightly from their top position when up for two or more consecutive picks, particularly on crank-driven dobbies. This is because there must be some clearance between the edge of the giraffe and the hook at the time of selection, but even this slight drop is minimized by giving special attention to the cam profile on cam-driven dobbies so that the inner griffe starts to move out before the other one commences its inward movement. For all practical purposes, we can regard double- acting dobbies as producing an open shed.
Q9. What are the advantages of different types of Shed?
Ans 9. Advantages of the Different Types of Shed
It is much easier for the weaver to repair a broken end if all the ends are in one plane. With bottom-closed and centre-closed sheds, this happens after every pick, but, in general, it does not happen at all with semi-open and open sheds. It is therefore customary to fit a heald-levelling device to double-acting dobbies to enable the weaver to bring all the shafts into the bottom position quickly and easily while broken ends are being repaired.
Q10. Describe the passage of warp yarns through a Loom
Ans 10. Passage of warp yarns through a Weaving Machine
The passage of warp through a loom is shown. The warp after leaving the weavers beam passes over two bars connected by a bracket at each end. One half of the warp end now passes under the back lease rod, and the other half passes over this rod. Those warp ends which pass under the back lease rod pass over the front lease rod and ends from over the back lease rod pass under the front lease rod. Therefore, the warp is completely divided as it passes through the lease rods, and facilitates the straightening of any warp ends which may break and become entangled before they reach the healds. The lease rods also assist in forming an even shed. Leaving the lease rods, the warp ends next pass through the heads. Odd numbered pass through the front heald, and the even numbered ends pass through the back heald . The heald consist of heald wires with eyes at the centre through which the warp ends are passed, the warp ends being thus controlled in their upward and downward movement. The warp ends next pass through the reed, this being comprised of a flat wire comb with the teeth secured at both ends. Usually, two ends pass between one tooth and the next this space being termed dent. In the figure, two warp ends are represented as being in the same dent.
At the point, Cloth fell. It may be considered as the point where the warp and weft become cloth because it is at this point where the last pick of weft, which was left by the shuttle, becomes beaten up. Passing forward, the cloth is held at each side by a temple which holds the cloth fell out to the width of the warp yarn, in the reed. From the temples, the cloth passes over the breast beam/front rest, partly around the sand or emery roller, over the steel roller, or tension rod, and then on to the cloth roller.
Q11: What do you mean by Heald Shafts?
Ans11: Heald shafts are related to the shedding mechanism. The heald shaft is made of wood or metal such as aluminium. It carries a number of heald wires through which the ends of the warp sheet pass. The heald shafts are also known as heald frames or heald staves. The number of heald shafts depends on the warp repeat of the weave. It is decided by the drafting plan of a weave. The main function of the heald shaft is as follows:
(i) It helps in shed formation
(ii) It is useful in identifying broken warp threads
(iii) It maintains the order or sequence of the warp threads
(iv) It determines the order of lifting or lowering the required number of heald for a pick. In other words, it helps in forming the design or pattern in a fabric.
(v) It determines the warp thread density in a fabric, i.e. the numbers of heald wires per inch determine the warp thread density per inch.
The heald shaft is a wooden or metal frame, the width of which is slightly greater than that of the warp, and it is usually between 36 and 48 cm deep. Healds were originally made of twisted cord, but, although these were cheap, they had a relatively short life and could not be used, dismantled, and reassembled for use in the production of another quality of the fabric. Twisted wire heald attached to a twisted cord around the top and bottom bars or staves of the heald frame lengthened the lifetime of the healds, twisted-wire or flat steel heald, which are free to move sideways on bars mounted just inside the framework of the heald frame, are now much more popular. Although they are more expensive, they have a much longer life and can be reassembled on the heald frames to suit any weaving requirements.
The heald shaft is a wooden or metal frame, the width of which is slightly greater than that of the warp, and it is usually between 36 and 48 cm deep. Healds were originally made of twisted cord, but, although these were cheap, they had a relatively short life and could not be used, dismantled, and reassembled for use in the production of another quality of the fabric. Twisted wire heald attached to a twisted cord around the top and bottom bars or staves of the heald frame lengthened the lifetime of the healds, twisted-wire or flat steel heald, which are free to move sideways on bars mounted just inside the framework of the heald frame, are now much more popular. Although they are more expensive, they have a much longer life and can be reassembled on the heald frames to suit any weaving requirements.
Q12: What do you mean by Sley?
Ans12: It is made of wood and consists of the sley race or race board, reed cap and metal swords carried at either end. The sley mechanism swings to and fro. It is responsible for pushing the last pick of weft to the fell of the cloth by means of the beat up motion. The sley moves faster when moving towards the fell of the cloth and moves slower when moving backwards. This unequal movement is known as the eccentricity of the sley. It is needed in order to perform the beat up and also to give sufficient time for passage of shuttle to pass through the warp shed. The beat up of the lastly laid pick of weft is accomplished through a metal reed attached to the sley.Q13: What is the Shuttle?
Ans 13. The shuttle is basically a weft carrier and helps in interlacement of the weft with the warp threads to form cloth. The shuttle which is made of wood passes from one end of the loom to the other. It travels along with the wooden sley race and passes between the top and bottom layers of the warp sheet. The shuttle enters a shuttle box fitted at either end of the loom, after passing through the warp shed. A shuttle normally weighs about 0.45 kgs.Q14: What is Picker?
Ans 14: Picker is a piece made either of leather or synthetic material. It may be placed on a spindle or grooves in the shuttle box. It is used to drive the shuttle from one box to another. It also sustains the force of the shuttle while entering the box.Q15: What is Shuttle Box?
Ans 15: ShuttleBox is the housing for the shuttle and is made of wood. In the Over-pick loom, It has a spindle and a picker, in Under-pick loom is the only picker without a spindle. The top and side of the box towards the sley race are open. The shuttle dwells inside the box for the intermediate period between two successive picks.Q16: What is Reed?
Ans 16: Reed is a metallic comb which is fixed to the sley with a reed cap. The reed is made of a number of wires and the gap between wires is known as dents. Each tent can accommodate one, two or more warp ends. According to Stockport system, the count of the reed is decided by the number of dents in two inches.The reed performs a number of functions which are as follows:
(i) It pushes the lastly laid pick of weft to the cloth fell
(ii) It helps to maintain the position of the warp threads
(iii) It acts as a guide to the shuttle which passes from one end of the loom to the other.
(iv) It determines the fineness of the cloth in conjunction with the heald.
(v) It determines the openness or closeness of the fabric.
There are various types of reed such as ordinary reed, gauze reed, expanding reed, V reed etc.
Reed count is defined as the 40s, 60s, 80s and so on. By 2/80s Reed means there are 80 cents per 2 inch and 2 ends per dent which gives 80 ends per inch in a fabric. Similarly, 3/80s Reed means, there are 80/2 x 3 = 120 ends per inch in a cloth.
There are various types of reed such as ordinary reed, gauze reed, expanding reed, V reed etc.
Reed count is defined as the 40s, 60s, 80s and so on. By 2/80s Reed means there are 80 cents per 2 inch and 2 ends per dent which gives 80 ends per inch in a fabric. Similarly, 3/80s Reed means, there are 80/2 x 3 = 120 ends per inch in a cloth.
Q17: What is Weavers Beam?
Ans 17: Weavers beam is fixed at the back of the loom. The warp sheet is wound on to this beam. The length of warp in the beam generally depends on the capacity of a loom as well as per the order received by the buyer, may be more than a thousand metres. The number of warp ends accommodated in the Weavers beam depends on the Ends per inch and the width of the Reed/cloth including selvedge ends.Q18: What is Back Rest?
Ans 18. Back Rest is placed above the weaver's beam. This is also known as the back beam. It may be of the fixed or floating type. In the first case, the backrest merely acts as a guide to the warp sheet coming from the weaver's beam. In the second case, it acts both as a guide and as a sensor for sensing the warp tension. During Shedding, Back-rest moves forward to ease the warp tension while during Beat-up, Back-rest moves backwards to give higher tension to the warp sheet so as to make a fabric with good cover.Q19: What is Front Rest?
Ans 19. Front Rest is placed above the Cloth Roller. This is also known as the breast beam. It also acts as a guide for the cloth being wound on to the cloth roller. The front rest together with the backrest helps to keep the warp yarn and cloth in a horizontal position and also maintain proper tension to produce compact and good covered cloth.Q20: What is Cloth Roll?
Ans 20. It is also known as the cloth beam. The woven cloth is wound on to this roller. This roller is placed below the front rest. The amount of cloth wound depends on the capacity of a loom. In the case of the ordinary loom, it may be 80-100 meters, whereas in modern shuttle-less looms it may be 400-500 meters of cloth.Other types of questions:
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