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Objectives ... CURRICULUM SL Topics Perio ds 1. Motion of Weaving 02 2. Loom 06 3. Shedding Mechanism 04 4. Tappet Shedding Mechanism 10 5. Picking Mechanism 09 6. Beating Up 02 7. Timing and Setting 02 8. Take Up Motion 06 9. Let-Off Motion 04 10 . Weft Fork Motion 02 11 . Warp Protecting Motion 03 Total 50 CONTENTS Topi cs Content Perio ds 01 Motion of Weaving 02 01.01 Principles and definition of fabric manufacture. 01.02 Motions in weaving: Primary Secondary and Tertiary motions. 02 Loom 06 02.01 Introduction. 02.02 Types of Loom: Handloom - brief idea of

Transcript of fabrication

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Objectives...

  CURRICULUMSL Topics Periods1. Motion of Weaving 022. Loom 063. Shedding Mechanism 044. Tappet Shedding Mechanism 105. Picking Mechanism 096. Beating Up 027. Timing and Setting 028. Take Up Motion 069. Let-Off Motion 0410. Weft Fork Motion 0211. Warp Protecting Motion 03  Total 50

 

 CONTENTS Topics  Content Periods01 Motion of Weaving 0201.01 Principles and definition of fabric manufacture.  01.02 Motions in weaving: Primary Secondary and Tertiary

motions.  

02 Loom 0602.01 Introduction.  02.02 Types of Loom: Handloom - brief idea of handloom,

Powerloom - default study of plain tappet looms.  

02.03 Various parts of loom and its functions.  02.04 Healds: Necessity of healds and types of healds.  03 Shedding Mechanism 0403.01 Definition.  03.02 Types of Shed: Closed shed (Bottom-closed and Centre-

closed), Open shed - Semi open shed.  

03.03 Merits, demerits and uses of each type of shed.  03.04 Shedding mechanisms and its kinds: Tappet, Dobby and  

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Jacquard shedding mechanism.03.05 The scope of Tappet, Dobby and Jacquard shedding.  04 Tappet Shedding Mechanism 1004.01 Tappets, cam and difference between cam and tappets.  04.02 Types of tappet shedding: Negative tappet shedding and

Positive tappet shedding.  

04.03 Various types of tappet shedding: Inside tappet shedding and Outside tappet shedding.  

04.04 Construction of cam and tappets for plain looms.  04.05 Condition of good shedding.  04.06 Early shedding and late shedding.  05 Picking Mechanism 0905.01 Methods of picking.  05.02 Types of picking mechanism: Introduction, Principles of

spring pick, Principles of Torsion-Picking, Weft control in the multiple-gripper weaving machine.

 

05.03 Conventional picking mechanism: Introduction, The cone-overpick mechanism, The cone-underpick mechanism, and other conventional picking mechanism.

 

05.04 Shuttle-checking devices: Conventional shuttle looms and Multiple-gripper weaving machines.  

05.05 Shuttle and its types.  05.06 Defects in shuttle and shuttle cop.  05.07 Defects in negative picking.  05.08 Essential feature to a good pick.  05.09 Comparison between underpick and overpick.  05.11 Early and Late picking.  05.12 Study of the following: Picker, Picking Band, Buffer, Check

Strap, Swell spring, Shuttle Guard, Shuttle flying, Shuttle trapping.

 

06 Beating up 0206.01 Introduction.  06.02 Construction and Mechanism.  06.03 Eccentricity of sley motion and its effect on loom working.  06.04 Factors affecting the sley motion.  07 Timing and Setting 0207.01 A Method of indicating loom-timing.  07.02 Timing of primary motions in tappet loom.  07.03 Setting sley, shedding and picking.  08 Take up Motion 06

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08.01 Introduction.  08.02 Classification of take up motion: Negative and positive take-

up motion.  

08.03 Five wheel take-up motion.  08.04 Seven wheel take-up motion.  08.05 Dividend of Loom.  08.06 Calculated dividend and practical dividend.  08.07 Calculation regarding dividends.  08.08 Changing the number of picks/inch.  09 Let-Off Motion 0409.01 Objects.  09.02 Types of Let-off motion: Negative and Positive Let-Off

Motions.  

09.03 Types of Negative Let-off motion: Frictional let-off motion; Chain, lever and weight let-off motion. Advantages and disadvantages of chain, lever and weight let-off motion.

 

09.04 Conditions to good Let-off motion.  10 Weft-Fork Motion 0210.01 Objects and principles.  10.02 Types of Weft Fork motion: Side Weft and Centre Weft fork

motion. Relative advantages and disadvantages between a single Weft fork and a centre weft fork motion.

 

10.03 Timing of side weft fork motion.  11 Warp Protecting Motion 0311.01 Introduction.  11.02 Types of Warp Protecting motion: Loose Reed, Fast Reed

and Electromagnetic Warp Protecting motion.  

11.03 Loom Knocking off or Banging off. Defects of knocking off.  

 

 Recommended BooksSL Title/Publisher Author1. Weaving Mechanism, Vol. I, II N.N. Banerjee2. The Mechanism of Weaving Fox3. Principles of Weaving Robinson and Marks4. Cotton Weaving and Designing J. B. Taylor5. Cotton Yarn Weaving A.T.A.6. Tappet and Dobby Looms T. Robberts7. Weaving, Machines, Mechanisms, Management Talukdar

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8. Weaving Technology Kulkarni

 

(Extracts from the book "Woven Fabric Production - I."published by the NCUTE, giving the technical information

about weaving, for the benefit of the members.)

SHEDDING

2.1 Object of Shedding Mechanism

     A shedding mechanism separates the warp threads into two layers or divisions to form a tunnel known as “shed”. The shed provides room for passage of the shuttle. A shed may be formed by means of tappets, cams, etc.

2.2 Types of Tappet Shedding Mechanism      Generally there are two types of shedding :      1. Negative shedding      2. Positive shedding       In plain looms, tappets are used to for m sheds.

2.2.1 Negative Tappet Shedding

     In a tappet shedding mechanism, if the tappet controls only one movement, either an upward or downward movement of the heald shafts, then the shedding is known as “negative tappet shedding”. The heald shafts are returned by some external devices like springs, dead weights, rollers, etc.

2.2.2 Positive Tappet Shedding

    In a tappet shedding mechanism, if the tappet controls both upward and downward movements of the heald shafts, then the shedding is known as positive tappet shedding.

Examples :

1. Jamieson’s tappet 2. Barrel tappet and 3. Ordinary tappet

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2.2.3 Tappets and Cams

     These are irregular metallic pieces used to produce an up-and-down motion in followers and levers. The up-and-down motion is obtained by giving rotary motion to these pieces. If the follower and lever are required to get a continuous up-and-down movement, a cam or wiper is used. If the follower and lever are required to produce up-and-down movement with regular intervals of rest, tappets are used. Figure 2.1 shows a pair of tappets and a cam. There are specific portions in tappets that correspond to “dwell” periods, i.e. regular intervals of rest for the major parts involved in the motion.

A pair of tappets                 Cam

Figure 2.1 Tappets and Cam

2.3 Negative Tappet Shedding Mechanism

2.3.1 Principle, construction and Working

Principle

A tappet is given a rotary motion so that it depresses a follower and a lever, known respectively as the anti-friction bowl and the treadle arrangement, by means of which the heald shaft is operated.

Construction

Figure 2.2 shows a negative tappet shedding mechanism. A pair of tappets A and B are fixed to the bottom shaft C at 180 degrees to each other. Two treadle levers D and E are connected to the loom back-rail by a bracket F. The bracket acts as a fulcrum for the levers. The two treadles have teeth to carry the lamb rods G and H respectively. Two heald shafts J and K are connected to the lamb rods. A top reversing roller shaft Q carries two rollers of different diameters. The roller of small diameter N is connected to a leather strap L to which the front heald shaft J is connected. The roller P of large diameter is connected to a leather strap M to which the back heald shaft K is connected. The tappets A and B touch the anti-friction bowls or followers R and S respectively, which are fixed to the treadle levers.

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A and B - TappetsC - Bottom shaftD and E - Treadle leversF - FulcrumG and H - Lamb rodsJ and K - Heald shaftsL and M - Leather strapsN - Top reversing roller (Smaller dia)P - Top reversing roller (Bigger dia)Q - Top reversing roller shaftR and S - BowlsT - Heald eyeU - Heald eyeV - Weft yar nW - Lease rodsX - War p sheet

Y - Cloth

Figure 2.2 Negative tappet shedding mechanism

The heald shafts have heald eyes T and U through which the war p threads pass X is the war p sheet and Y is the cloth. The odd ends are passed through one heald shaft while the even ends are passed through the other heald shaft.

Working

     When the bottom shaft is rotated in the clockwise direction as shown in the figure, the tappets are also rotated. The tappet will depress the anti-friction bowl and the treadle. Being fulcrumed at one end, the front portion of the treadle moves down. This action is transferred to the lamb rod, the heald shaft and the leather strap. So one heald shaft is lowered and the threads connected to this heald shaft are lowered and form the bottom layer of the shed.

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     The leather straps attached to the reversing rollers are connected in opposite directions, i.e. when leather strap is pulled down, it is unwound from its roller. The shaft therefore rotates in the clockwise direction and the other leather strap is wound on to its roller. The heald shaft is raised and therefore the lamb rod and treadle lever are also raised. The threads connected to the heald shaft are also raised and form the top layer of the shed.

     For the next shed, the other tappet works with the other set of bowl, treadle, lamb rod, heald shaft, strap and roller and the other heald shaft is lowered. The first heald shaft is raised by the top reversing rollers, and the positions of the healds shafts are thus interchanged. Thus, for one rotation of the bottom shaft, two sheds are formed.

     In this type of tappet shedding therefore, one tappet depresses the concerned treadle and the corresponding heald shaft is lowered. But the other heald shaft is raised by means of the top reversing rollers. So this type of shedding mechanism is known as “negative tappet shedding mechanism”

Timings and settings

1. Turn the crank to the top centre position.2. Fix the anti-friction bowls to the treadle levers; they should move

freely in the slots.

3. Fix the treadle levers with a bracket to the back rail of the loom.

4. Set the grid and grid bracket to the front rail of the loom in the slots of the grid.

5. Make sure that the tappet with the lower throw is fixed to the bottom shaft at the starting handle side.

6. Fix the top reversing rollers to the top reversing roller shaft to be equidistant from the ends and at the same time ensure that the connecting screws of the rollers are symmetrical about the central axis of the shaft when the heald shafts are at the same level. The roller of smaller diameter is always connected to front heald shaft.

7. The heald shafts are connected to the top reversing rollers by means of cords and leather straps. The leather straps are connected to the rollers, such that when one of them winds on its roller the other strap unwinds from its roller and vice versa.

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8. Lamb rods are connected to the heald shafts by cords.

9. Adjust the tappets on the bottom shaft and make sure of the following points :

i. The tappet with a bigger throw should be connected to the back heald shaft.

ii. The bowls should have perfect contact with the tappet surfaces.

iii. The treadles should be at the same level and parallel to each other at the top centre position.

iv. Heald shafts : The hook of the lamb rod of the front heald shaft should be connected to the first notch of the treadle lever while that of the back heald shaft should be connected to the third notch. If the depth of shed is altered, the connections of the hooks to the treadle levers can be changed.

Points to be observed

1. Turn the crank shaft through two revolutions and make sure that the bowls are always in contact with the tappets.

2. The heald shafts should not touch the side frames or the sley.

3. Turn the crank shaft to the bottom centre and check the size of shed. The bottom line of warp sheet or the heald eyes of the lowered heald shaft should have a clearance of 1 mm from the race board and the top.

2.3.2 Top Reversing Rollers

     The bigger reversing roller P is connected to the back heald shaft K and the smaller roller N is connected to the front heald shaft J. This is shown in Figure 2.3

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N - Roller of Smaller diameterP - Roller of bigger diameter

J and K - Heald shafts

Figure 2.3 Top reversing rollers

2.3.3 Identical Sheds

     In Figure 2.4, the broken lines represent the shed for the next pick. Z is the shuttle passing through the shed. The top and bottom lines of the shed are maintained at a certain angle to each other for the passage of the shuttle. This should be maintained for each shed. Referring to the figure, to get identical sheds (having a constant angle) it is clear that the distance h 2 moved by the back heald shaft must be greater than the distance h 1 moved by the front heald shaft. In figure 2.4, if the rollers N and P are of the same diameter, the distance moved by the back heald shaft will be same as that moved by the front heald shaft. But the sheds will not be identical. Therefore the back heald shaft is connected to the bigger roller and so an extra lift is obtained to get identical sheds.

Figure 2.4 Identical sheds

2.3.4 Tappets

Two tappets are connected to the bottom shaft at 180 degrees to each other because half a rotation of the bottom shaft is equal to one pick, and for each pick, one tappet will lower the heald shaft. The tappets

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have a portion corresponding to the dwell. This is used to arrest the movement of the heald shafts for a period of time. During this time, the shuttle is traversing from one box to the other. This period is usually 1/3 rd of a pick i.e. 120 degrees. See Figure 2.5.

Throw of a tappet

Referring to the figure, it is clear that the difference between the heal s 1 and toe s 2 of a tappet is equal to its throw. If the difference is high then the throw of tappet is also high. Higher-throw tappets apply more force to the treadle lever. A higher-throw tappet is always connected to the back heald shaft. This is mainly to compensate for the difference in leverage in the treadle levers.

Figure 2.5 Throw of tappet

2.3.5 Lift between the Back and Front Heald Shafts

This is due to the different connections of the heald shafts to the treadle levers. In Figure 2.6, ‘d’ represents the back shaft connection to one of the treadle levers and d 1 the front heald shaft connection to the other. Since d 1 is greater than d, the front heald shaft gets more lift than the back heald shaft.

Figure 2.6 Heald shafts connections

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2.3.6 Depth of Shed

Refer to Figure 2.7. By altering the positions of the lamb rod hooks on the treadle levers, the depth of shed is changed. By moving the lamb rod towards the fulcrum (distance d 1 ), the depth of shed is reduced and moving it away from the fulcrum (distance d), the depth of shed is increased. The depth of shed is altered when a shuttle of a different height is used.

                            Lamb rod hooks

Figure 2.7 Positions of lamb rod hooks

2.3.7 Advantages and Disadvantages of Tappet Shedding

Advantages

1. It is robust, simple and cheap.2. It is capable of lifting a heavy weight with less wear and tear

than other shedding mechanisms.

3. It can move heald shafts at great speeds.

4. It puts less strain upon the warp.

5. It consumes less power and gives greater output.

6. It requires less maintenance.

Disadvantages

1. If the weave is changed, it will be necessary to change the tappet and the change gear wheel in the counter shaft arrangement. So work involved in changing the weave is more.

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2. The capacity of a tappet to produce a pattern / weave is ver y much limited. A maximum of 8 or 10 tappets only can be used.

2.3.8 Faults that may Occur in Tappet Shedding Mechanism

1. If the tappet is faulty, it imparts a jerky movement to the heald shaft.

2. The tappet should always touch the bowls. Otherwise a severe blow is applied to the bowl and the vibration is transmitted to the heald shaft. End breakages may occur as a result of this.

3. Overshedding : If the depth of a shed is too much, strain on the warp will be more and end breakages may occur.

4. Undershedding : If the depth of shed is too low, the shuttle will not reach the other end and may be trapped in the shed or may fly out. Hence end breakages will occur.

5. Uneven shedding : Uneven shedding is caused by lifting one end of the heald shafts more than the other so the shuttle may move over some war p threads and fly out or get trapped in the shed.

6. If the shedding is mistimed, then other motions like picking and beat-up cannot be done smoothly and end breakages may occur.

2.4 Timing of Shedding

     The shedding mechanism is set according to the picking and beat-up mechanisms. Timing of shedding is set with respect to crank position.

In general there are three timings. These are :

1. Normal shedding2. Early shedding

3. Late shedding

1. Normal shedding

     If the heald shafts are at the same level when the crank comes to the top center, that is, 0 degree or 360 degrees, then it is known as normal shedding. See Figure 2.8

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Figure 2.8 Timing of shedding

2. Early shedding

     A shed is said to be early when the pick insertion is effected after the shed is completely open. If the heald shafts are at the same level when the crank comes to 355 degrees or 5 degrees before the top centre, then it is known as early shedding.

3. Late shedding

     If the heald shafts are at the same level when the crank comes to 5 degrees after top centre, then the shedding is known as late shedding. See Figure 2.8.

Advantages of early shedding

1. Fabric cover is improved, because during beat-up, the warp yarns are evenly distributed.

2. It assists in clearing the shed while weaving fibrous war p like woollen and worsted. So entanglements are avoided.

3. Heavy weft yarn can be easily inserted. So hard weaves like corkscrew weaves can be woven.

Disadvantages

1. As the warp yarns are open widely during beat up, strain on the warp yarn is high.

2. Early shedding causes chaffing of war p yarn (yar n abrasion) and end breakages may occur.

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Advantages of late shedding

     Late shedding allows the weft to get beaten up before the shed has been properly crossed. During beat-up, opening of the shed is minimal. So strain on the war p yarn is low.

1. Late shedding can be used for weak warp yarn.2. It allows greater time for shuttle passage so it is suitable for

synthetic yarn.

3. It is useful if pick of low strength is used.

Disadvantages

1. It causes a very weak fell of the cloth because the weft may roll back.

2. It causes less distribution of yarn. So it is not suitable for fibrous warp.

3. As the warp yarns are not distributed evenly during beat-up, fabric cover is low.

2.5 Heald staggering and Asymmetric Shedding

Heald staggering

     In the formation of a shed, if all the war p yarns of one shed line go across those of the other shed line at the same time there will be much strain on the yarns. So there may be many war p yarn breakages. To avoid warp strain and end breakages, the movements of the heald shafts are staggered. By this arrangement, the warp yarns of one shed line will not cross the yarns of the other shed at the same time. For example, if four heald shafts are used and four tappets are used to control the heald shafts separately, the timings of the tappets are adjusted in such a way that the crossing of shed lines is altered. So end breakages can be reduced. Refer to Figure 2.9

A - Heald ShaftsB - ReedC - Lease rods

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Figure 2.9 Heald staggering

     During the weaving of heavy fabric like denim, canvas, satin duck, etc. the force required to push the weft is high. So there may be bumping of fabric. To avoid this weaving resistance, asymmetric shedding is used. In this shedding, the time and duration of movement of the top and bottom shed lines is so adjusted that they cross below the mid-lift position. At this point of crossing, the yarns at the bottom shed line are under higher tension than those at the top shed line.

     As the reed beats up the weft, the slack top shed line yields and permits the weft to be pushed to the fell of the cloth with relative ease. So it is easy to weave even heavy fabric.

2.6 Heald Shaft

A heald shaft consists of a wooden or metal frame carrying heald wires. The width of a heald shaft is slightly greater than that of the warp sheet and is usually 36 to 48 cm deep.

The functions of a heald shaft are :

1. To carry heald wires and maintain war p yarns in their correct positions

2. To form a shed line.

Types of heald wire

There are two types of heald wire. These are :

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1. Knitted heald wires2. Metal heald wires.

1. Knitted heald wires

     Heald wires were originally made of twisted cords. See Figure 2.10. These were ver y cheap but they had a relatively short life and could not be used in the production of high quality fabric. The life of the heald wire can be increased slightly by using the mail-eye and wire types.

d - Twisted wiree - Flat-steel (Simplex)

f - Flat-steel (Duplex)

 Figure 2.10 Knitted heald wires                  Figure 2.11 Metal heald wires

2. Metal heald wires

     In this type, the top and bottom wooden bars of the heald shafts have two steel strips attached to the heald wires that are inserted in between the steel strips and metal or wooden bar. Twisted wires or flat steel heald wires of the simplex or duplex type are free to move sidewards on the bars. These are more expensive when compared to knitted heald wires but they have a much longer life and can be assembled on the heald frames to suit any weaving requirements. See Figure 2.11.

2.7 Reversing Motions in Negative Tappet Shedding

     Negative shedding requires external devices known as over and under motions to reverse the direction of movement of the heald shafts. If the shedding tappets are placed under the heald shafts an over motion is required. If the tappets are placed over the heald shafts then an under motion is required. In over motion, bowls or pulleys are used. In under motion, springs or dead weights or elastic cords are used. These motions are classified as single acting and compound acting types. See Figures 2.12 and 2.13.

2.7.1 Over Motions or Top Reversing Motions

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Normally in negative tappet shedding motions, top reversing motions are used. The Following types of top reversing motion are used.

1. Roller reversing motions2. Spring reversing motions

                            Dead Weight              Elastic Cord                   Spring

Figure 2.12 Undermotions in negative shedding

2.7.2 Single and Compound Acting Types

Single acting type

     In this type, each heald shaft is connected to a roller and spring arrangement. Therefore each heald shaft is controlled separately.

Compound acting type

     In this type, rollers connect the heald shafts, which work complementary to each other. The rollers will not work independently of each other.

Examples

For two heald shafts

     Figure 2.13 shows the top-rollers arrangement for plain cloth using two heald shafts. Leather straps connected to the heald shafts pass over rollers A and B. When the tappet lowers one heald shaft, the other is raised.

A - Roller A,B,C - Rollers

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B - Roller D - Strap

1,2 - Heald shafts

1,2,3 - Heald shafts

Figure 2.13 Two heald shafts                    Figure 2.14 Three heald shafts

     For three heald shafts     Figure 2.14 shows the top-rollers arrangement for three heald shafts. The diameter of roller B must be twice that of A; the diameter of C is immaterial. The reason for having the diameters of B and A in the ration 2:1 is that when the first heald shaft is taken down, either the second or the third one must be taken up the same distance.     Suppose the first heald shaft is taken down by a distance of 4”, the strap D will be taken up only by 2”. This is due to roller A being half the size of B. The tappets are so constructed as to allow only one heald shaft to go up for each pick. If this heald shaft is the second one and the third is immovable, the second will be taken up 4” or the same distance as the first was taken down. If the strap D is fastened to the heald shaft it would be taken up 8” instead of 4”. This arrangement of rollers is suitable for 2/1 twill or 1/2 twill weaves.

     For four and five heald shafts     Figure 2.15 shows the arrangement used for four heald shafts and Figure 2.16 that used for a five heald-shaft design.

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1,2,3,4 - Heald Shafts                                        1,2,3,4,5 - Heald Shafts

Figure 2.15 Four heald shafts                         Figure 2.16 ve heald shafts

     Lacey top reversing roller motion     This is of novel design and was patented by a Canadian inventor. In Figure 2.17, the reversing motion consists of an arm A, connected to one end of the loom frame. A stud B is fixed on A. A lever C is loosely mounted on the stud. The length of the top arm of this lever is twice that of the bottom arm. In the top arm a stud D carries stepped rollers E. The rollers carrying straps F and F 1 are used to raise two heald shafts.

     The lower arm of C also carries a stud G which carries a quadrant lever H that is freely mounted. The curved face of the top of the quadrant lever is used to connect one heald shaft through strap I. The bottom of the quadrant lever carries a stud J which supports a second pair of stepped rollers K and from these rollers two straps are connected to raise two heald shafts.

     The mechanisms may be used for the following weaves with simple adjustments.

a. Plain : To weave plain weave two heald shafts are necessary. Hence the rollers E or K are used.

b. Three heald-shaft twill weave : To weave 1/2 or 2/1 designs, the stepped rollers K and the quadrant lever H are used.

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c. Four heald-shaft twill weave : To weave 1/3 or 2/2 or 3/1 designs, the stepped rollers K and E are used.

d. Five heald-shaft weave : All the rollers and top curved surface of quadrant are used to raise five heald shafts.

A - ArmB - StudC - LeverD - Stud in the top armE - Stepped rollersF and F 1 - Leather strapsG - StudH - Quadrant leverI - Leather strapJ - Stud

K - Stepped rollers

Figure 2.17 Lacey top reversing roller motion

Advantages

1. The Lacey motion is connected to one end of the loom. So there is sufficient light for the weaver to work.

2. It controls up to a maximum of five heald shafts.

3. It is connected to the side frame at one end; oil drops will not fall on the fabric.

4. According to the given weave, the arrangement of the rollers can be changed easily.

2.7.3 Spring Reversing and Easing Motions

     A simple type of spring reversing motion is shown in Figure 2.18 Heald shaft A is connected to leather strap B wound on a disc C which in turn is connected to a spring D.

     When the heald shaft is lowered, the disc rotates in the anti-clockwise direction and the spring is pulled. After inserting the pick,

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the disc rotates in the clockwise direction and the spring is released. Hence the heald shaft is reversed.

     In modern looms, the spring type reversing motion is modified by changing the disc into a quadrant or clock spring type as shown in Figure 2.19.

A - Heald shaftB - Leather strapC - Disc

D - Spring

Figure 2.18 Spring reversing motion                Figure 2.19 Spring reversing motion

     Spring-easing motion

     When spiral springs are used in under motions, their stretch will be more than the movement of heald shafts. Hence the springs will not exert the right force consistently. Also the life of the springs may be reduced.

     A simple and effective arrangement, as shown in Figure 2.20, reduces the stretch on the spring. A quadrant of this kind stretches the spring gradually when the heald shaft is raised.

     Also the spring is released gradually when the heald shaft is lowered.

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Figure 2.20 Spring-easing motion

(to be continued in the next issue)

2.8 Positive Tappet Shedding Mechanism

Principle

In this type of shedding, the heald shaft is raised and lowered by the tappet.

Construction

     Figure 2.21 shows a positive tappet shedding mechanism. The tappet shaft A carries tappet B that has a groove C or track in which a bowl D is placed. The bowl is connected in turn to a tappet lever E, link rods G, links J and a heald shaft K. Each tappet is separately connected to a heald shaft through link rods and tappet lever. F and H are fulcrums for tappet lever and links G respectively.

A - Tappet shaftB - TappetC - TrackD - BowlE - Tappet leverF - FulcrumG - Link rodsH - FulcrumI - Heald wireJ - Heald shaft linksK - Heald shaft

Figure 2.21 Positive tappet shedding mechanism

Working

When the tappet is rotated, the bowl is also rotated. According to the shape of the groove, the bowl is moved up or down or is still. If the bowl is moved up, the tappet lever moves to the right through the links G and J and the heald shaft is lowered. If the bowl is moved down, the tappet lever moves to the left and the heald shaft is raised. Since the heald shaft is raised and lowered by means of the mechanism, this

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tappet shedding is known as positive tappet shedding mechanism. When the bowl stands still, the heald shaft is in the “dwell” stage.

2.9 Types of Shed

1. Open shed

There are two types of open shed.

1. Fully-open shed2. Semi-open

2. Closed Shed

There are two types of closed shed.

1. Centre-closed shed2. Bottom-closed shed

2.9.1 Open Shed

The shed is always in the open position in this type of shed

(i) Fully-open shed

In this type of shed, the warp threads form two stationary lines, one at the top and the other at the bottom. after inserting a pick, changes are made by carrying threads from one fixed line to the other, so some threads are lowered from the top line and some threads are raised from the bottom line. During this change, the raising and lowering of threads occur simultaneously. Therefore the shed is formed in a minimum period of time. As the falling threads help the rising threads to move, strain upon the war p yar n is low. Figure 2.22 shows a fully-open shed. In the figure, A and B are the stationary bottom and top lines respectively. The arrows C and D show the movements of the falling and rising threads respectively. Full lines show that the shed is always in an open position only. So this shed is known as fully-open shed.

Merits

1. Rising threads help to move lowering threads.2. Strain upon the warp is low, so it requires a minimum period of

time to form a shed.

3. The loom can run at a high speed.

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4. Power consumption is low.

5. Wear and tear of the loom parts is low.

Demerits

1. This type of shedding is troublesome to weavers because the two fixed lines make it more difficult to repair broken ends. Therefore a levelling mechanism is added to all looms using this type of shedding mechanism. While repairing broken ends, this levelling mechanism is brought in to operation.

A - Bottom line of war pB - Top line of warpC - Movement of falling threadsD - Movement of rising threads

2. Figure 2.22 Fully-open shed

3. As the shed is always open, breakages may results especially when the yarn is weak.

4. When many heald shafts are used, the strain on the warp yarns in the back heald shafts is increased and hence warp breakages may occur.

Uses

This type of shed is used in

1. Plain loom for producing twill and satin weaves and2. In double-lift dobby and in double-lift jacquards.

(ii) Semi-open shed

This is formed under both closed and open principles. In this shed, a stationary bottom line is retained. The top line is a movable one. After inserting a pick, the top line moves towards the bottom line. When the threads are moving down, some of the threads which are to form once again at the top line are arrested midway and are then carried to the top line. The remaining threads move down. Similarly the threads which are to be at the top line also move up and are carried to the top line. Figure 2.23 shows a semi-open shed. In the figure, A is the bottom stationary line. B is the top line. Arrow D, shows the movement of

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rising threads from the bottom to the top line. Arrow E shows the movement of rising threads from the bottom to the top line. Arrow F shows the movement of the arrested threads at the midway position C. From the midpoint C these threads are carried to the top line.

The full lines indicate the positions of shed lines after inserting a pick. They are in a semi-open state. So this type of shed is known as semi-open shed.

A - Bottom line of war pB - Top line of war pC - Point where some of the downward movement of threads is arrestedD - Movement of rising threadsE - Movement of falling threadsF - Movement of arrested threads

Figure 2.23 Semi-open shed

Merits

1. In a semi-open shed, the strain upon the warp is low.2. It requires minimum time to form a shed.

3. The loom can run at a high speed.

4. Power consumption is low.

5. Wear and tear of the loom parts is low.

Demerits

1. This shedding is troublesome to weavers because the two fixed lines make it more difficult to repair broken ends. Therefore a levelling mechanism is added to all the looms using this type of shedding motion. While repairing broken ends, this levelling mechanism is used.

2. As the shed is always open, breakages may result, especially when the yarn is weak.

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3. When many heald shafts are used, the strain on the warp yar n at the back heald shafts is increased and hence warp breakages may occur.

4. In a fully-open shed, the strain on the rising and falling threads is equally distributed. But in a semi-open shed, since some of the threads are coming from the bottom line and some threads are arrested midway and again carried to the top, the strain is not equally distributed.

Uses

Many double-lift dobbies and double-lift jacquards form semi-open sheds.

2.9.2 Closed Shed

This type of shed closes after ever y pick is inserted. So all the war p threads come to the same level after each pick is inserted.

(i) Centre-closed shed

In this type of shed, war p threads move in an upward and downward direction from a centre line. The threads which are to form the top line move upwards and the threads which are to form the bottom line move to bottom line. After inserting a pick both the lines meet at the centre-line.

Figure 2.24 shows a centre-closedshed. A is the centre-line. B and C are the top and bottom lines respectively. D and E are the arrows showing the movements of the rising and falling threads respectively.

A - Bottom line of war pB - Top line of war pC - Point where some of the downward movement of threads is arrestedD - Movement of rising threadsE - Movement of falling threads

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Figure 2.24 Centre-closed shed

Merits

1. A rising thread is partially balanced by a falling thread.2. The machine can run at high speed.

3. Power consumption and wear and tear of the loom pards are low.

Demerits

1. Since every thread is moved to form each shed, strain on the war p is more than that for the open shed.

2. An unsteady movement of threads is caused by the warp threads being in constant motion.

Uses

Centre-shed dobbies, centre-shed jacquards and handlooms form centre-closed sheds.

(ii) Bottom-closed shed

This kind of shed is formed by giving motion to only those threads that form the top line. Under this conditions, after inserting a pick, all the warp yarns come to the bottom line. Figure 2.25 shows a bottom -closed shed. A represents the bottom stationary line, B the top line and C is the arrow showing the movement of threads.

A - Bottom stationary line of warpB - Rising and falling threadsC - Arrow showing the movement of threads

Figure 2.25 Bottom closed shed

Merit

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The alternate tightening and slackening of threads produces a cloth with good cover.

Demerits

1. It takes a long time to produce a shed since it is necessary to move the threads a space equal to twice the depth moved in other types of sheds.

2. It is unsuitable for high loom speed.

3. Strain on the warp is high.

4. Wear and tear of the loom parts is high.

5. Power consumption of the loom is high.

Uses

Single-lift dobbies and single-lift jacquards produce bottom-closed sheds.

2.10 The Geometry of a Shed

Depth of shed

The depth of a shed is determined by the size of the shuttle. In Figure 2.26, A is the shuttle width, B is the distance from the cloth fell to the reed, C is the depth of the shed at the front wall of the shuttle and D is the depth of shed at the reed. During the passage of the shuttle, the distance B and D will vary because of the motion of the reed. But the important parameter to consider is C, the depth of the shed at the front wall of the shuttle. There must be a minimum of 2 mm gap between the top line of the shed and the shuttle. The depth of the shed is obtained by providing the required stroke of tappet.

A - Shuttle widthB - Distance from cloth fell to reedC - Depth of shed at reedD - Depth of shed at reed

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Figure 2.26 Geometry of shed

Stroke of Tappet

The stroke of a tappet is decided from the following particulars. Refer to Figure 2.27

1. The distance of the heald shaft connection on the treadle lever from the fulcrum of the treadle lever.

2. The distance of the centre of the treadle bowl from the fulcrum of the treadle lever.

3. The position of the heald shafts in relation to the fell of the cloth.

4. Height of the front side of the shuttle inside the warp shed.

5. The sweep of the sley.

In the figure, the following numerical values are assumed.

1. The distance of the front heald shaft from the fell of the cloth is 32 cm.

2. The distance of the back heald shaft from the fell of the cloth is 36 cm.

3. The distance of the front of the shuttle from the fell of the cloth is 11.0 cm.

4. The height of the shuttle at the front is 3.5 cm.

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5. A clearance of 5 mm is given between the top front edge of the shuttle and the top warp line.

6. The angle between the reed and the race board, which is known as the bevel, is taken as 90o

7. The sley sward moves through 15o from the vertical beat-up position to its backward most position.

From the data given above, it is possible to calculate the total movement of the front and back heald shafts, for the same angle of shed.

Calculations of heald shaft movement

From the above data, the calculations of the heald shafts movements are given below :Angle AOB is the shed angle; OA (OD) is the top warp line;OB(OE) is the bottom warp line;CO is the horizontal line joining the front and back rests.

Figure 2.27

Bevel of the reed (angle between the race board and the reed is 90o)

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Since the sley has moved 15o from the front position to the backward position, COB is 15o (see Figure 2.26)

Calculations of the shed angle AOB

Tan AOB = [Height of the shuttle + Clearance between the shuttle and the top line of the warp]_________________________________________________________________(Distance of the shuttle from the fell of the cloth)

   3.5 + 0.5 =-------------- = 0.3636      11.0

Angle AOB = 190 59’;     Angle AOC = 190 59’ - 150 = 40 59’In triangle COA, CA = tan angle AOC x OC = 0.0872 x 32 = 2.79 cm.In triangle COB, CB = tan COB x OC = 0.2679 x 32 = 8.57 cm.                           AB = CA + CB = 2.79 + 8.57 = 11.36 cm.The triangles AOB and DOE are similar. So, (DE/AB) = (OF/OC) = 36/32                       36DE = 11.36 x ----  = 12.78 cm.                      32

Thus, the front heald shaft has to move through a distance of 11.36 cm. and the back heald shaft has to move through a distance of 12.78 cm.

Calculation of the stroke of the shedding tappets and relative diameters of the two top rollers

From the above calculations the following data are obtained

L1 = the distance of the front heald shaft from the fulcrum of the treadle = 50 cm.L2 = the distance of the back heald shaft from the fulcrum of the treadle = 46 cm.d1 = the diameter of the top roller that controls the front heald shaft;d2 = the diameter of the top roller that controls the back heald shaft;S1 = stroke of the tappet that controls the front heald shaftS2 = stroke of the tappet that controls the back heald shafth1 = vertical movement of front heald shaft = 11.36 cm.h2 = vertical movement of back heald shaft = 12.78 cm.

 S2        h2       L1 ------ = ------ * ------  S1       h1       L2

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    12.78         50   =----------   * ------ = 1.22    11.36         46

Thus the stroke of the tappet operating the back heald shaft should be 22% greater than that operating the front heald shaft.

The relative diameters of the top rollers should be :

    d2            h2           12.78  --------  =  -------  =   ----------  =    1.125     d1           h1            11.36

Hence the diameter of the top roller of the back heald shaft reversing mechanism should be about 12.5% greater than that controlling the front heald shaft.

Since the treadle levers controlling the healds shafts are fulcrummed at the back of the loom, the actual leverage of the treadle lever operating the back heald shaft is less than that of the front heald shaft. Because of the shorter leverage, the back heald shaft will move a shorter distance compared to the movement of the front shaft, whereas, as per the calculation shown above, the back shaft should move a greater distance to maintain the same depth of shed. Therefore, the tappet operating the back heald shaft should have a greater throw (or stroke) than the front tappet.

2.11 Importance of Simple Harmonic Motion in Plain Power Looms

The design of the shedding mechanism should be such that :

i. at the start of the movement of the heald shaft, the velocity of the heald shaft should be less than normal;

ii. about the middle of its movement, its velocity is at the maximum;

iii. at the end of its movement, its velocity is again low.

Any one of the following motions can obtain the above type of movement :

i. Simple Harmonic Motion (SHM)

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ii. Parabolic Motion

iii. Polynimial Motion and

iv. Cycloidal Motion.

     The kinematic characteristics like displacement, velocity and acceleration are shown in Figure 2.28 a, b, c and d.

     To get smooth movements of heald shafts without any jerks during their raising and lowering, the simple harmonic motion (SHM) is essential in any loom. The velocity and acceleration of movements of the heald shafts are uniformly increased, then decreased.

     SHM is the most commonly used motion for shedding in non-automatic shuttle looms. In this kind of motion, the amplitude of acceleration is comparatively low, but at the beginning and the end of the traverse, the movement of the heald shafts shows sudden changes in acceleration which leads to jerky movement; it is not therefore suitable for high-speed looms.

     There is a constant positive or negative acceleration with the tappets imparting parabolic motion. The velocity of the heald shaft increases and decreases at a constant rate, first up to half the lift and then during the second half lift respectively. This helps in getting approximately 15% higher crossing velocity, but the motion displays sudden change in acceleration at the beginning and end of the motion. This results in a considerable number jerks to the heald shafts and is therefore unsuitable for high-speed looms.

     Polynimial and cycloidal motions have higher amplitudes of acceleration, but the acceleration changes gradually throughout the traverse without any sudden changes. These two kinds of motion give raise to finite jerks. So they are suitable for high-speed looms.

  _______  Displacement X - Angular movement of crank  -.-.-.-.-.- Velocity Y - Lift of follower  ----------- Acceleration

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Figure 2.28 Motions of cams and followers

2.12 counter Shaft Arrangement

2.12.1 Objects

The objects of counter shaft arrangement are :

1. To control a maximum of eight or ten shedding tappets.2. To change the speed of the shedding tappets so that fabric

design repeating up to eight or ten picks per repeat can be woven.

2.12.2 Need for Counter Shaft Arrangement

Before dealing with the need for the counter shaft, let us have an idea about the basic weaves produced in a plain power loom.

Plain Weave

In a plain weave, each warp yarn passes over alternate weft yarns. Neighboring warp yarns pass over the adjacent weft yarns. See Figure 2.29

Figure 2.29 Plain Weave

Twill Weave

This construction makes a pattern of diagonal lines. Each warp yarn passes over (and/or under) more than one weft. adjacent warp yarns follow the same pattern, but are displaced by one cell. This is shown in Figure 2.30

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Figure 2.30 Twill Weave

Satin Weave

In a satin weave, the warp yarn floats over four or more weft yarns and passes under only one. Adjacent warp yarns have their floats arranged as randomly as possible, so no twill line is generated, as seen clearly in Figure 2.31.

Satin Weave

In plain looms, two tappets are used for weaving plain and rib weaves. When the bottom shaft rotates once, the two shedding tappets form two sheds. Two picks are thus inserted for each complete rotation of the bottom shaft. Hence two picks per repeat of the design can be woven easily.

Now let us consider a design with three-picks per repeat. Here three shedding tappets are used. If we fix three shedding tappets to the bottom shaft, then three sheds are formed for each rotation of the bottom shaft. During this time, only two picks are inserted. But we have to insert three picks. So we have to change the speed of the bottom shaft. It is practically impossible to do so. So another shaft known as counter shaft is used. The shedding tappets are fixed to this shaft and the speed of the counter shaft is reduced.

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2.15 Electronic Shedding Mechanism

Toyoda (loom manufacturer) has introduced the latest electronic technology in shedding. Figure 2.37 shows a schematic diagram of an electronic shedding motion. Each of the heald shafts A is connected via links B to cam C. Each cam is driven by a servo motor D and gears E and F. The servo motors are connected to an E-shed controller, a 32-bit main CPU and a function panel, as shown in the diagram.

In the function panel, the timing for shedding, dwell and shedding pattern are set according to the weaving design. The heald shafts are moved up and down, according to the settings, positively, precisely and smoothly by the servo-motors.

The E-shedding mechanism has the following features :

1. Ultimate flexibility

The E-shed combines the optimum cross-timing and dwell angle for each shedding curve. This makes it easy to weave various kinds of fabrics such as high-density fabrics, traditionally woven by cam shedding; or complex fabrics with a dooby and even fabrics difficult to weave with conventional shedding motions because of loose warp yarn or incomplete shedding.

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A - Heald shaft                   D - Serve-motorB - Links                            E & F - Gears

                                  C - Cam

Figure 2.37 Electronic shedding mechanism

2. Better Fabric QualityUnlike existing shedding devices, the E-shed allows separate upper and lower dwell angles to be set, which improves beating-up performance and fabric quality. Meanwhile, combined variations in cross-timing prevent warp entanglement and minimise problems caused by warp yarn. And since the shedding and beating-up motions are not synchronized, there are fewer stop marks during star-up.

3. High Efficiency

Freely controllable warp leads to increased warp shedding ability and minimal mispicks in weft insertion. This obviously means greater efficiency.

4. Easy Operation

Settings can be changed by a touch of the function panel. This makes it much quicker and easier to be flexible when manufacturing many different kinds of textile articles in small quantities.

5. Not Affected by Upper/Lower frame Imbalance

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The E-shed is not affected by any difference between the numbers of upper and lower heald frames unlike in dobby shedding, which enables wider range of patterns to be woven with great ease.

(To be continued - Par t II)