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Sunday, October 19, 2014

Tappet, Dobby and Jacquard Shedding | Dwell period | Advantages and Disadvantages of Tappet and Dobby shedding

Definition of Tappet, Dobby and Jacquard Shedding | Definition of Dwell period | Advantages and Disadvantages of Tappet and Dobby shedding

Tappet shedding
          By strongly beating the anti-friction bowl which is attached with cradle bowl in order to form shed for running arrangement of shuttle by the control of heald shaft is called tappet shedding. I.e. which types of shed are formed in tappet loom is called tappet shedding.

Tappet shedding
Rapier tappet shedding loom

Dobby shedding
          The dobby is a shedding device placed on the top of a loom in order to produce a pattern by using a large number of healds than the capacity of a tappet. In fancy weaving the dobby is used to produce small pattern by means of warp threads and healds. The shed which is produced by a dobby is called dobby shedding.

Dobby shedding
Rapier loom machine with dobby
          Dobby is generally used to control 8-36 number of heald shaft for shedding.
Jacquard shedding
          The jacquard is a shedding device placed on the top of the loom to produce large patterns by using a large number of warp threads separately by means of harness cords, hooks and needles. But no heald shaft is used here. The shed which is produced by jacquard is called jacquard shedding.

Jacquard shedding
Rapier Jacquard Loom Machine
          When more than 36 no. of warp required for shedding of a particular design, is done by jacquard shedding. It can be used to operate up to 2200 individual yarn for shedding.
Dwell period
          The tappet should be so made that headls will remain stationary while the shuttle passes through the shed. This stationary period is known as dwell of healds or dwell period or pause.
          Or, Dwell is the stationary period when the heald frames do not change their position and shed remains open to allow the passage of the shuttle from one shuttle box to another.
          The dwell period depend upon the following factors –
        The width of the fabrics. In narrow fabrics dwell will be short.
        In widths loom the dwell period will be long.
        In widths loom the dwell period will be long.
        In narrow R.S loom dwell period will be short and vice versa.
        For fine and tender warp dwell period will be short.
        For coarse, strong and elastic warp, dwell period will be long.
        If the picking force is high the dwell period will be short.
Advantages and disadvantages of tappet shedding
Advantages
        Simplest.
        Cheapest of all shedding motions.
        If conveniently employed, it gives the best result within its capacity.
        Action is certain.
        Less wear and tear.
        It can move healds at high speed.
        Dwell period may be adjusted to suit the type of fabric to be woven.
        Puts less strain upon the threads.
        Consumes less power and gives greater output.
        It causes fewer defects to the fabric.
Disadvantages
        Over shedding strains and breaks the warp threads.
        Under shedding does not permit the passage of the shuttle through the shed.
        Sometimes unequal shedding by lifting one end of the shaft more than other.
        Missed shedding.
        Unsuited dwell period.
        May impart jerky motion.
Advantages and disadvantages of dobby shedding
Advantages
   In this loom many numbers of healds can be used for weave a figured fabrics.
   Particularly 12-24 healds can be used in a cotton industry to weave fabrics.
   In which fabrics are not possible to weave in tappet shedding loom and jacquard loom for increasing the production cost, to weave this kind of fabrics dobby are used extensively.
Disadvantages
   Comparatively cost is high than tappet loom.
   Less productivity than tappet loom.
   Less speed.
   The adjustment of dwell is complicated.
   Maintenance cost is so high.

Wednesday, April 30, 2014

Harness mounting system of jacquard loom

Harness Mounting:
            The jacquard harness is the system of cords, healds and lingoes that transmit the movement of the hooks to the individual warp threads. A simple form of harness is represented in the fig. below:
A simple form of harness is represented in the figure.
There are two main types of harness ties.
1.      Norwich system.
2.      London system.
Norwich system:
            When jacquard mechanism is mounted with the parallel of the loom is called Norwich Jacquard. In this jacquard the long rows of needles are parallel to the comber board & at the right angle to the warp. The cords will be at the front or at the back or at both front & back of the loom.
London system:
            When the jacquard mechanism is mounted at one end of the loom and at 90˚ angle of the loom is known as London Jacquard. In this system the jacquard is turned through a right angle the cords stay at the side of the loom.
The both tie systems are divided into the following four types.
1.      Single tie-up.
2.      Repeating tie-up.
3.      Pointed tie-up.
4.      Mixed tie-up.
Single tie-up: (For asymmetric design)
            In this system only one harness cord is attached to one neck cord. There are must be as many hooks as there are threads in the width of the fabric. If the design is required to be woven 400 ends there are 400 hooks in the jacquard.
No. of neck cord = No. of harness cord = No. of warp in a repeat
Scope: This tie is used to produce a fabric containing only one repeat across the width of the fabric.
Repeating tie-up:
            This is the most common design tie used for both Norwich and London harness ties. In this tie there must be as many harness cards tied to each neck cord as there are 4 repeats in the full width of the fabric. If there are 4 repeats of the pattern then there will be 4 harness cords tied with each neck cord.
No. of neck cord = No. of jacquard
Scope: This type of tie is used to produce a fabric having more than one repeat across the width of the fabric.
Pointed tie-up: (Single repeat)
            This tie up is widely used where pointed draft is used and for symmetrical design if turned over its centre line. Suppose a design has 800 ends. Then it is divided in to two groups i.e. 400 ends in each group. So there are required 400 ends in each group. So there are required 400 Neck cords. In this system two harnesses is attached with one neck cord.
Scope: This tie up is used to produce symmetrical design which turned over its center line.
Mixed tie-up:
In this tie up a large number of harness cord is used. A great skill is required of designer because the design required a greater capacity of the jacquard that has been actually used.
Scope: This tie is used if a large repeat pattern is to be produced with the existing capacity of the jacquard.

Tuesday, April 22, 2014

Note on shedding mechanism of single lift single cylinder jacquard

Introduction:
Jacquard is a shedding device placed on the top of the loom to produce large figure patterns by controlling a very large no. of warp threads separately by means of harness cords, hooks and needles. Here no heald shaft is used. Jacquard may be controlled by pegged card, punched card, punched tape etc. or by electrical means.
Construction:
Single Lift Single Cylinder Jacquard is the original and the simplest type of jacquard. It works on bottom closed shed type of shedding mechanism. Here one set of knife with griffe controlled the hooks. The hooks are controlled by needles of one cranked eye and every needle is selected one hook in that pick. One neck cord with harness cord is controlled by this mechanism.
Mechanism:
          During the cycle of operation one of the faces of the cylinder together with a card is brought against the needle board. If a hole is punched in the card the corresponding needle will project through in the cylinder. the hook controlled by that needle will remain is such a position that its upper hooked end un punched will be caught by the raising knife. The un punched position of the card will press back the needle & consequently the hook controlled by that will be away from the path of the using knife. Thus the hook gets selection according to the design cut for a particular card.
            When the hooks are lifted by the knives the cylinder moves out a limited distance. A catch holds it against the top corner of the cylinder. The cylinder is turned about its axis & new card is presented to the needles during its next cycle.
By this time the griffe along with its knives descend to lower the warp threads to the bottom shed line for a fresh warp threads to the bottom shed line for a fresh selection of the hook at the next pick.
Single Lift Single Cylinder Jacquard is the original and the simplest type of jacquard. It works on bottom closed shed type of shedding mechanism.
Conclusion:
By this mechanism we learn how jacquard mechanism work and how it produce decorate design by using yarn individually. Moreover it is a very helpful and important practical for our textile life.

Friday, February 7, 2014

Definition | Types | Position | Construction of Tappet Shedding

Shedding tappet | Types | Position of fixing shedding tappet | Construction of tappet for Plain weave | Conditions of good shedding

Shedding tappet: Tappet, Cam are the names given indiscriminately to those irregular pieces of mechanism to which a rotary motion is given for the purpose of producing reciprocating motion in the rod and levers for shedding.
Types: We can divide it as following two types-
     1.     Positive: These give both upward and downward movements to healds without the aid of any additional mechanism. But pullers, stocks and bowls are based on compensating principles.
Lifting and lowering →Mechanically. Which are-
a)     Wood crofts section tappet.
b)    The oscillation tappet.
c)     Nutralls chain tappet.
d)    Scroll tappet.
     2.     Negative: These require the addition of pullers, stocks, and bowls, springs or dead weights to reserve either pulls the head down or lifts it. Which are-
a)     Ordinary plain tappet.
b)    Jamiesons tappet.
c)     The barrel tappet.
Position of fixing shedding tappet:
          a)     Over or under the centre of heald shaft.
          b)    Over or under one end of the heald shaft.
          c)     Outside of the loom frame.
Over or under the centre of heald shaft: When the device is placed over the centre of heald shafts, then the oil, grease etc applied to tradle bowl for antifriction property may fall on the fabric and contaminate it when placed under the heald shafts, some difficulties may arise while cleaning.
Over or under one end of the heald shafts: When the device is placed over or under one end of the heald shaft then pressure is applied at any one side of the heald shaft. Hence connecting cards should be used on two sides.
Outside of the loom frame: In this system, it is easy to clear the tappets and to apply grease, oil to tradle bowl and easy maintenance can be done. But it requires more space.
Construction of tappet for Plain weave:
To design a shedding tappet for plain weave, the following particulars have been taken into consideration-
                           i.            Lift if tappet-4”
                         ii.            Distance from the centre of the driving shaft to the nearest point of contact with the trade bowl-2”
                      iii.            Dwell of tappet one third of pick.
                      iv.            Diameter of the trade bawl-2”
Tappet, Cam are the names given indiscriminately to those irregular pieces of mechanism to which a rotary motion is given for the purpose of producing reciprocating motion in the rod and levers for shedding.
At a radius of 2 inches the circle – A. This circle represents the distance from the centre of the driving shaft to the nearest point of contact with the tradle bowl. For plain weave tappet, the bottom shaft is used as a driving shaft. At a radius of 3 inches the circle – B and one inch is added for the radius of tradle.
At a radius of 7 inches the circle –C. Four inch is added for lift of tappet. The circle represents the centre of the tradle bowl, when the inner circle of it is acting upon the bowl. The circle ‘C’ represent the centre of the bowl when pressed down by the device. Divide each half circle into three equal parts by lines FK and GH. Divide FH and GK each into six equal parts and divide the space between the circles B and C into six unequal parts. To find out the six unequal parts, describe a semi-circle L between B and C. Now divide its circumference into six equal parts and then draw perpendicular line from them on the line GH. This gives six unequal divisions on the lift to obtain the desired eccentric shape of the tappet. From the corner of these unequal spaces and with the tradle bowl describes circles representing the position of the tradle bowl at different parts or its movement. Now draw the curved lines touching the extremities of the tradle bowl. This gives the outline of the plain tappet.
Conditions of good shedding:
              i.            A shed should be as small as the nature of the work to be done.
           ii.            Healds should be placed at right angles with the warp threads.
         iii.            Exact vertical movement of the heald shafts is required.
         iv.            Perfect leveling of the headl shaft.
           v.            The heald shafts must begin to move slowly.